cellular and molecular neurobiology

8/14/2019 Cellular and Molecular Neurobiology

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NEUROSCIENCE

LECTURE

SUPPLEMENT

Nachum Dafny, Ph.D., Professor

Department of Neurobiology and Anatomy

University of Texas Medical School at Houston

A. Cellular and Molecular Neurobiology


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TABLE OF CONTENTS

Page

Overview of the Nervous System ...........................................................................1

Organization of Cell Types....................................................................................17

Resting Potentials and Action Potentials...............................................................32

Propagation of the Action Potentials ....................................................................38

Synaptic Transmission at Skeletal Neuromuscular Junction ................................43

Synaptic Transmission in the Central Nervous System

and Synaptic Plasticity...........................................................................................57


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OVERVIEW OF THE NERVOUS SYSTEM

Nachum Dafny, Ph.D.

The human nervous system is divided into the central nervous system (CNS) and the peripheral

nervous system (PNS). The CNS, in turn, is divided into the brain and the spinal cord, which lie

in the cranial cavity of the skull and the vertebral canal, respectively. The CNS and the PNS,

acting in concert, integrate sensory information and control motor and cognitive functions.

The Central Nervous System (CNS)

The adult human brain weighs between 1200 to 1500g and contains about one trillion cells. Itoccupies a volume of about 1400cc - approximately 2% of the total body weight, and receives

20% of the blood, oxygen, and calories supplied to the body. The adult spinal cord is

approximately 40 to 50cm long and occupies about 150cc. The brain and the spinal cord arisein early development from the neural tube, which expands in the front of the embryo to form

the main three primary brain divisions: the prosencephalon (forebrain), mesencephalon(midbrain), and rhombencephalon (hindbrain) (Figure 1A). These three vesicles further

differentiate into five subdivisions: telencephalon, diencephalon, mesencephalon,

metencephalon, and the myelencephalon (Figure 1B). The mesencephalon, metencephalon, andthe myelencephalon comprise the brain stem.

Figure 1. Schematic lateral view drawing of human embryo at the beginning of the 3rd (A) and 5th (B)

week of gestation.


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descending nerve fibers. The spinal cord is an essential link between the peripheral nervoussystem and the brain; it conveys sensory information originating from different external and

internal sites via 31 pairs of spinal nerves (Figure 5). These nerves make synaptic connections

in the spinal cord or in the medulla oblongata and ascend to subcortical nuclei.

The central nervous system, which includes the spinal cord and the brain, is the most protectedorgan in the human body. It is protected from the external environment by three barriers: skull,

meninges, and CSF.

The meninges are composed by three fibrous connective tissues(Figure.6). The most external

is a dense collagenous connective tissue envelope known as the dura mater (Latin for hardmother). The second, or the intermediated membrane, is a delicate non-vascular membrane of

fine collagenous layer of reticular fibers forming a web-like membrane, known as the arachnoid(Greek for spider). It is separated from the inner pia layer by subarachnoid space, which is

filled with cerebrospinal fluid. The inner most delicate connective tissue membrane of

collagenous is the pia mater, a thin translucent elastic membrane adherent to the surface of the

brain and the spinal cord. Blood vessels located on the surface of the brain and the spinal cordare found on top of the pia matter. The meninges are subject to viral and bacterial infection

known as meningitis, a life-threatening condition that requires immediate medical treatment.

Figure 6. Schematic drawing of the brain and spinal cord meninges.

The space between the skull and the dura is known as the epidural space. The space betweenthe dura and the arachnoid is known as the subdural space. The space between the arachnoid


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and the pia is known as the subarachnoid space. In this space, there is a clear liquid known asthe cerebrospinal fluid (CSF). The CSF serves to support the CNS, and to cushion as well as

protect it from physical shocks and trauma. The CSF is produced by the choroid plexus which

is composed of a specialized secretory ependymal layer located in the ventricular system.

The ventricular system is a derivative of the primitive embryonic neural canal. This system isan interconnected series of spaces within the brain containing the CSF (Figure 7).

Figure 7. Schematic drawing of ventricular system in four different angles.

In general, the CNS can be divided into three main functional components: the sensory system,the motor system, and homeostasis and higher brain functions. The sensory system consists of

the somatosensory, viscerosensory, auditory, vestibular, olfactory, gustatory, and visualsystems. The motor system consists of motor units, and the somatic (skeletal muscle) system,

the spinal reflexes, the visceral (autonomic) system, the cerebellum, several subcortical andcortical sites, as well as the brain stem ocular motor control system. The homeostasis and

higher functional system includes the hypothalamus, cortical areas involved in motivation,

insight, personality, language, imagination, creativity, thinking, judgment, mental processing,and subcortical areas involved in learning, thought, consciousness, memory, attention,

emotional state, sleep and arousal cycles.


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The Cerebral Hemispheres

The telencephalon is the largest and most obvious parts of the human brain are the cerebral

hemispheres. The cerebrum has an outer layer - the cortex, which is composed of neurons and

their supporting cells, and in fresh brain, has a gray color thus called the gray matter. Under the

gray matter there is the white matter, which is composed of myelinated ascending anddescending nerve fibers, and in fresh brain have a white color. Embedded deep within the white

matter are aggregation of neurons exhibiting gray color and known as subcortical nuclei. Thecerebral hemispheres are partially separate from each other along the midline by the

interhemispheric fissure (deep groove) the falx cerebri (Figure 8A); posteriorly, there is a

transverse fissure that separates the cerebral hemisphere from the cerebellum, and contains thetentorium cerebellum. The hemispheres are connected by a large C-shaped fiber bundle, the

corpus callosum, which carries information between the two hemispheres.

Figure 8. Schematic drawing of six cortical lobes: Dorsal view (A), Lateral view (B), Mid-sagittal section

showing the limbic lobe (in green) (C), and Horizontal section showing the insular cortex (D).

For descriptive purposes each cerebral hemisphere can be divided into six lobes. Four of theselobes are named according to the overlying bones of the skull as follows: frontal, parietal,

occipital and temporal (Figures 8A and 8B), the fifth one is located internally to the lateral

sulcus the insular lobes (Figure 8B and 8D), and the sixth lobe is the limbic lobe (Figure 8C)which contains the limbic system nuclei. Neither the insular lobe nor the limbic lobe is a true

lobe. Although the boundaries of the various lobes are somewhat arbitrary, the cortical areas in

each lobe are histologically distinctive.

The surface of the cerebral cortex is highly convoluted with folds (gyri), separate from each

other by elongate grooves (sulci). These convolutions allow for the expansion of the corticalsurface area without increasing the size of the brain. On the lateral surface of the cerebralhemisphere there are two major deep grooves-sulci (or fissure), the lateral fissure (of Sylvian)

and the central sulci (of Rolando), these sulci provide landmarks for topographical orientation

(Figure 9A). The central sulcus separates the frontal lobe from the parietal lobe and runs from

the superior margin of the hemisphere near its midpoint obliquely downward and forward untilit nearly meets the lateral fissure (Figures 8A and 8B). The lateral fissure, separating the frontal

and parietal lobes from the temporal lobe, begins inferiorly in the basal surface of the brain and

extends laterally posteriorly and upward, separating the frontal and parietal lobes from thetemporal lobe (Figure 9A). The frontal lobe is the portion which is rostral to the central sulcus


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and above the lateral fissure, and it occupies the anterior one third of the hemispheres (Figures8 and 9). The boundaries of the parietal lobe are not precise, except for its rostral border the

central sulcus. The occipital lobe is the portion which is caudal to the parietal lobe (Figures 8

and 9). Along the lateral surface of the hemisphere, an imaginary line connecting the tip of theparietal-occipital sulcus and the preoccipital notch (Figure 9A); separate the parietal lobe from

the occipital lobe. On the medial surface of the hemisphere (Figure 9B), parieto-occipital sulcusforms the rostral boundary of the parietal lobe. The temporal lobe lies ventral to the lateralsulcus, and on its lateral surface, it displays three diagonal oriented convolutions-the superior,

middle, and inferior temporal gyri (Figure 9A). The insula lies in the depths of the lateral

sulcus. It has a triangular cortical area with gyri and sulci (Figures 8B & 2D, and Figure 9A).

The limbic lobe consists of several cortical and subcortical areas (Figure 9B).

Figures 9A and 9B. Lateral schematic drawing of the different cortices, sulci and gyri (A) and mid-sagittal

drawing emphasizes the limbic lobe (in green) (B).

The cerebral cortex is a functionally organized organ. A functional organized system is a set of

neurons linked together to convey a specific type(s) of information to accomplish a particular

task(s). It is possible to identify on the cerebral cortex primary sensory areas, secondarysensory areas, primary motor area, premotor area, supplementary motor area and association

areas, which are devoted to the integration of motor and sensory information, intellectual

activity, thinking and comprehension, execution of language, memory storage and recall.

The frontal lobe is the largest of all the brain lobes and is comprised of four gyri, precentralgyrus that parallels to the central sulcus, and three horizontal gyri: the superior, middle, and

inferior frontal gyri. The inferior frontal gyrus is comprised of three parts: the orbital, the

triangular and opercular. The term opercular refers to the lips of the lateral fissure. Finally,


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the straight gyrus (gyrus rectus) and the orbital gyri form the base of the frontal lobe (Figure9B). Four general functional areas are in the frontal lobe. They are the primary motor cortex,

where all parts of the body are represented, the premotor and supplementary motor areas. A

region concerned with the motor mechanisms of speech formulation comprised of the opercularand triangular parts of the inferior frontal gyrus are known as Brocas speech area, and the

remainder of the prefrontal cortex is involved in mental activity, personality insight, foresight,and reward. The orbital portion of the prefrontal cortex is important in the appropriate

switching between mental sets and the regulations of emotion.

The parietal lobe is comprised of three gyri:postcentral gyrus, superiorand inferior parietalgyri (Figure 9A). The postcentral gyrus is immediately behind the central sulcus which forms

its anterior boundary. The postcentral gyrus comprises the primary somatosensory cortex whichis concerned with somatosensory reception, integration and processing sensory information

from the surface of the body and from the viscera, and is important for the formulation of

perception. Caudal to the postcentral gyrus is the inferior parietal gyrus. The intraparietalsulcus separates the posterior parietal gyrus from the inferior parietal gyrus. The inferior

parietal gyrus represents the cortical association area which integrates and processes sensoryinformation from multiple modalities such as auditory and visual information. The inferiorparietal gyrus, which is known as Wernicke's area, is also important for language and reading

skills, whereas the superior parietal gyrus is concerned with body image and spatial

orientations.

The temporal lobe is formed by three obliquely oriented gyri: the superior, middle, andinferior temporal gyri (Figure 9A). Inferomedial to the inferior temporal gyrus are the

occipitotemporal and the parahippocampal gyri, which are separated by the collateral sulcus.

The upper surface of the superior temporal gyrus, which extends into the lateral fissure, iscalled the transverse temporal gyrus (of Heschl) and is the primary auditory cortex. The caudal

part of the superior temporal gyrus, which extends up to the parietal cortex, forms part ofWernickes area. Wernickes area is concerned, in part, with processing the auditoryinformation and is important in the comprehension of language. The inferior part of the

temporal lobe (i.e., the occipitotemporal gyri) is involved in visual and cognitive processing.

More medially is the parahippocampal gyrus, which is involved in learning and memory.

Portions of the frontal, parietal, and temporal lobes, which are adjacent to the lateral sulcus andoverlie the insular cortex, are known as the operculum. The inferomedial surface of the

temporal lobe is made up of the uncus and the parahippocampal gyrus medially. The inferior

surface of the temporal lobe rests on the tentorium cerebelli.

The occipital lobe is the most caudal part of the brain, lies on the tentorium cerebelli (Figure

9A) and is comprised of several irregular lateral gyri. On its medial surface, there is a

prominent fissure the calcarine fissure and parieto-occipital sulcus. The calcarine fissure(sulcus) and the parieto-occipital sulcus also define a cortical region known as the cuneus. The

cuneus sulcus divides the occipital lobe into the cuneus dorsally and ventrally into the lingual

gyrus. The occipital lobe contains the primary and higher-order visual cortex.

The insula lobe is located deep inside the lateral fissure and can be seen only when the

temporal and the frontal lobes are separated. The insula is characterized by several long gyriand sulci, the gyri breves and gyri longi. There is some evidence that the insular cortical areas

are involved in nociception and regulation of autonomic function (Figures 8B and 8D).


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The limbic lobe is not a true lobe and is comprised of several cortical regions such as thecingulate and parahippocampal gyri, some subcortical areas like the hippocampus, amygdala,

septum, and other areas with their respective ascending and descending connections (Figures

8C and 9B). The limbic lobe is involved in memory and learning, drive related behavior, and

emotional function.

There are subcortical areas in the telencephalon like the basal ganglia and the amygdaloid

nucleus complex. The corpus callosum is a collection of nerve fibers which connect the twohemispheres. The corpus callosum is divided into rostrum (head), body, the most rostrally part

is the genu (knee) with connecting the rostrum and the body, and the splenium at the caudal

extremity (Figure 10). Behavioral studies have shown that the corpus callosum play an

important role in transferring information from one hemisphere to the other.

Figure 10. The corpus callosum and its different parts.

The Diencephalon

The second major derivative of the prosencephalon is the diencephalon. The diencephalon isthe most rostral structure of the brain stem; it is embedded in the inferior aspect of the

cerebrum. The posterior commissure is the junctional landmark between the diencephalon andthe mesencephalon. Caudally, the diencephalon is continuous with the tegmentum of the

midbrain. During development the diencephalon differentiates into four regions: thalamus,

hypothalamus, subthalamus and epithalamus (Figure 11). The epithalamus comprises the stria

medullaris habenular trigone, pineal gland and the posterior commissure (Figure 11).


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Figure 12. Midsagittal drawing of the brain stem.

Pons. The pons is continuous with the midbrain and is composed of two parts, the pontine

tegmentum (located internally) and the basilar pons. At the level of the pons, the cerebralaqueduct has expanded to form the fourth ventricle (Figure 12). The cerebellum is situatedposterior to the pons and forms part of the roof (tectum) of the forth ventricle. The pons

contains nuclei that receive axons from various cortical areas. Projections from the axons of

these pontine neurons form large transverse fiber bundles which traverse the pons and ascend to

the contralateral cerebellum via the middle cerebellar peduncles. Also, within the pons base andtegmentum are longitudinally ascending and descending fibers. The nuclei of the 5th

(trigeminal), 6th (abducens), 7th (facial) and the 8th (vestibulocochlear) nerves are located in

the pons tegmentum.

Medulla Oblongata (myelencephalon is also known as the medulla). The medulla lies between

the pons rostrally and the spinal cord caudally. It is continuous with the spinal cord just aboveto foramen magnum and the first spinal nerve. The posterior surface of the medulla forms thecaudal half of the fourth ventricle floor and the cerebellum, its roof (Figure 12). The base of the

medulla is formed by the pyramidal-descending fibers from the cerebral cortex. The medulla

tegmentum contains ascending and descending fibers and nuclei from the 9th

(glossopharyngeal), 10th (vagus), 11th (accessory) and the 12th (hypoglossal) nerves. Thecorticospinal fibers (pyramid) are alongside the anterior median fissure, and decussate (cross

the midline) to the contralateral side on their way to the spinal cord. Other prominent structures

in the medulla are the inferior olive, and the inferior cerebellar peduncle. The medulla containsnuclei which regulate respiration, swallowing, sweating, gastric secretion, cardiac, and

vasomotor activity.

Thearterial blood supply to the brain is derived from two arterial systems: the carotid systemand the vertebrobasilar system. A series of an anastomotic channels lying at the base of the

brain, known as the circle of Willis, permits communication between these two systems (Figure

13).


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the ventral (anterior) root. The spinal nerve is formed by the joining of the dorsal and theventral roots. The cranial nerves leave the skull and the spinal cord nerves leave the vertebrae

through openings in the bone called foramina (Latin for opening).

Figure 15. Schematic drawing of the peripheral nervous system.

The PNS is divided into two systems: the visceral system and the somatic system. The visceral

system is also known as the autonomic system. The autonomic nervous system (ANS) is oftenconsidered a separate entity; although composed partially in the PNS and partially in the CNS,

it interfaces between the PNS and the CNS. The primary function of the ANS is to regulate and

control unconsciousness functions including visceral, smooth muscle, cardiac muscle, vessels,

and glandular function (Figure 16). The ANS can be divided into three subdivisions:

1. The sympathetic (or the thoracolumbar) subdivision associated with neurons located inthe spinal gray between the thoracic and the upper lumbar levels;

2.

The parasympathetic (or craniosacral) subdivision is associated with the 3rd, 7th, 9thand the 10th cranial nerves as well as with the 2nd, 3rd, and 4th sacral nerves;

3. The enteric subdivision is a complex neuronal network within the walls of thegastrointestinal system and contains more neurons than the spinal cord. The visceral(autonomic) system regulates the internal organs outside the realm of conscious control.

The PNS component of the somatic system includes the sensory receptors and the

neurons innervating them and their nerve fibers entering the spinal cord. The visceraland the somatic nervous system are primarily concerned with their own functions, but

also work in harmony with other aspects of the nervous system.


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Figure 16. Schematic drawing showing the autonomic nervous system. C, T, L and S indicate thecervical, thoracic, lumbar and sacral levels of the spinal cord, respectively. III, VII, IX, X indicate

the 3rd, 7th, 9th and 10th crania nerves respectively.

Orientation to the Central Nervous System

In this section you will be introduced to representative sections through the human CNS. They

will acquaint you with prominent structures that will help you to recognize the level and

orientation of the section and provide landmarks for locating nuclei and tracts involved insensory and motor functions. Directional terms are used in describing the locations of structures

in the CNS.

Figure 17. Orientation of the central nervous system of the spinal cord and different brain sections.

Keep in mind that certain terms were developed to describe the nervous system of quadrupeds

and may have a slightly different meaning when applied to bipeds. For example, the ventralsurface of the quadruped spinal cord is comparable to the anterior surface of the biped (Figure


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18). In the following descriptions, the terms are applied to a standing human. The terms rostraland anterior refer to a direction towards the face/nose. The terms caudal and posterior refer to a

direction towards the buttocks/tail. The terms inferior and superior generally refer to spatial

relationships in a vertical direction (Figure 18). A coronal section is parallel to the verticalplane and a midcoronal section would divide the head into anterior and posterior halves (Figure

19). The sagittal section is also parallel to the vertical plane, but a midsagittal section woulddivide the head into right and left halves. The horizontal (axial) section is parallel to thehorizontal plane and a mid horizontal section would divide the head into superior and inferior

halves. Transverse or cross sections of the spinal cord of humans are taken in a plane

perpendicular to the vertical, i.e., in the horizontal plane of the head. Most electromagnetic

imaging techniques produce images of the brain in the coronal, horizontal (axial) and sagittalplanes. The representative sections are transverse sections through the spinal cord and brain

stem and coronal sections through the telencephalon and diencephalon (Figure 17).

Figure 18. A schematic illustration showing the brain direction.

Figure 19. A schematic illustration showing the three planes of brain section.

Transverse Section through the Spinal Cord. This section was taken at the level of the thoracicspinal cord (Figure 17A). The spinal cord neuron (gray matter) form a central core taking a

butterfly configuration that is surrounded by nerve fibers (white matter). In the left and right

halves of the spinal cord, the gray matter is organized into a dorsal horn and ventral horn with

the intermediate gray located between them. In the thoracic spinal cord, which is illustrated inthis figure, a lateral horn extends laterally from the intermediate gray (Figure 17A). The spinal

cord white matter is subdivided into the posterior white column, the anterior white column and

the lateral white column. The anterior white commissure joins the two halves of the spinal cordand is located ventral to the intermediate gray. The dorsal root fibers enter the spinal cord at the


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ORGANIZATION OF CELL TYPES

Jack C. Waymire, Ph.D.

The human nervous system is estimated to consist of roughly 90 billion nerve cells.There are hundreds of different types of neurons based on morphology alone. Further

neurons that look similar may have strikingly different properties- for example the

neurotransmitter(s) they respond to or utilize. This lecture reviews the cellular

components of nervous tissue. You should be able to describe neurons and glia, their

morphological components, as seen with the light and electron microscope, and some of

the fundamental functional roles these cell types play in the nervous system.

I NEURONS

A. Structure of a model neuron. The morphology of an individual neuron isintimately related to its particular role in processing and transmitting information.Neurons interact with sensory receptors (which themselves may be neurons or epithelial

derivatives), with other neurons, and with effectors such as skeletal muscle fibers and the

smooth muscle components of the viscera. Ignoring the wide variety of neurons that

exist in the nervous system we can describe a generalized neuron which has features

shared by most neurons, as seen in the light microscope. This neuron is diagrammed in

Fig. 1. It has the basic subcellular organelles needed to carry out metabolic functions.

Figure 1. Diagrammatic representation of a neuron: This neuron makes functional contacts with

other neurons via axon collateral and with muscle fibers via the motor endplates. Neurons may

contact blood vessels, neurons, glands and muscles.

1. Cell Body. The region of the neuron containing the nucleus is known as the cellbody, soma, or perikaryon. The cell body is the metabolic "center" of the neuron.

Structures situated toward this center are designated proximal, while structures away

from the cell body are said to be distal. A membrane surrounds the entire neuron,

including the cell body and the cytoplasmic processes (dendrites and axon).


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Figure 2. Diagrammatic representation of the neuron cell body or perikaryon emphasizing the

endoplasmic reticulum, Golgi apparatus and cytoskeleton.

The metabolic center of a neuron is the cell body, or soma illustrated in Figure 2. The

interior of the soma, consists of cytoplasm, a gel within a microtrabecular lattice formed

by the microtubules and associated proteins that make up the cytoskeleton. These

microstructures of the cytoplasm are described in greater detail in the Glossary.

Energy producing metabolism and the synthesis of the macromolecules used by the cell

to maintain its structure and execute its function are the principal activities of the

neuronal soma. As described in Dr. Byrnes lectures, it also acts as a receptive area for

synaptic inputs from other cells. Embedded within the neuronal cytoplasm are the

organelles common to other cells, the nucleus, nucleolus, endoplasmic reticulum,

Golgi apparatus, mitochondria, ribosomes, mitochondria lysosomes and

peroxisomes. Many of these cell inclusions are responsible for the expression of genetic

information controlling the synthesis of cellular proteins and enzymes involved in energy

production, growth, and replacement of materials lost by attrition.

2. Dendrite. The membrane of the neuron can function as a receptive surface over

its entire extent; however, specific inputs (termed afferents) from other cells are received

primarily on the surface of the cell body and on the surface of the specialized processes

known as dendrites (as shown in Figure 3). The dendritic processes may branch

extensively and is often covered with projections known as dendritic spines. Spines

provide a tremendous increase in the surface area available for synaptic contacts. The

dendritic processes of neurons are essentially expansions of cytoplasm containing most

of the organelles found in the cell body. Dendrites contain numerous orderly arrays of

microtubules and fewer neurofilaments (see below). The microtubules associated

proteins (MAPs) in the dendrite are higher in molecular weight than those found in the


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axon. An example is MAP2. In addition microtubules in dendrites have their positive

ends toward the cell soma. Mitochondria are often arranged longitudinally. Rough

endoplasmic reticulum and ribosomes are present in large, but are not small dendrites.

The shape and extent of the "dendritic tree" of an individual neuron is indicative of the

quantity and variety of information received and processed by that neuron.

Figure 3. Diagrammatic representation of the neuron dendrite, emphasizing the areas of contact by

other afferent inputs to the neuron.

Information is received by the dendrite through an array of receptors on dendrite surfaces

that react to transmitters released from the axon terminals of other neurons. Dendrites

may consist of a single twig-like extension from the soma or a multi-branched network

capable of receiving inputs from thousands of other cells. For instance, an average spinal

motor cell with a moderate-sized dendritic tree, both in the number of branches and their

length, may receive 10,000 contacts, 2,000 on the soma and 8,000 on the dendrites.

3. Axon. The other type of process in our idealized neuron is the axon shown in Figure

4. Each neuron has only one axon and it is usually straighter and smoother than the

dendritic profiles. Axons also contain bundles ofmicrotubules and neurofilaments and

scatteredmitochondria

. The MAPs in axon have a lower molecular weight than those inthe axon. A predominant MAP in axons is tau. Beyond the initial segments, the

axoplasm lacks rough endoplasmic reticulum and free ribosomes. The branches of axons

are known as axon collaterales. The axon itself is often surrounded by a membranous

myelin sheath formed by non- neuronal cells. The myelin sheath acts to insulate the

plasmalemma of the axon in a way that necessitates the saltatory spread of the

depolarization of the plasmalemma and increases the speed of conduction of the nerve

impulse.


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Figure 4. Diagrammatic representation of the neuron axon emphasizing the areas of microtubules,

neurofilaments coursing within the cytoplasm.

4. Initial Segment and Axon Hillock. The cone-shaped region of the cell body from

which the axon originates in an idealized neuron is the axon hillock. This areae is free of

ribosomes and most other cell organelles, with the exception of cytoskeletal elements andorganelles that are being transported down the axon. The neurofilaments become

clustered together as fascicles. The region between the axon hillock and the beginning of

the myelin sheath is known as the initial segment. This is the anatomical location for the

initiation of the action potential. The area under the axolemma in this region has material

that stains darkly when viewed by EM. This is shown in Figure 5. At the distal-most end

of the axon or its collaterales are small branches whose tips are button-shaped

cytoplasmic enlargements, called terminal boutons or nerve endings.


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Figure 5. Diagrammatic representation of the neuron initial segment emphasizing the areas where

the action potential is initiated.

5. Nerve Ending. That part of the plasma membrane of the nerve ending which is

specialized to form functional contacts with other cells is termed the synapse. When

neurons interact with muscle fibers, the region of functional contact is called theneuromuscular junction or motor endplate. According to the classical definition of

synapses, when a nerve ending synapses on a dendrite or soma of a second neuron they

are termed an axodendritic or axosomatic synapse, respectively. However, almost all

possible combinations of pre- and postsynaptic elements have been found in the central

nervous system. These different types of synapse are designated by combining the name

of the structure of the presynaptic element with that of the postsynaptic structure. For

example, when the transfer of information occurs from an axon to axon or from one

terminal to another, the synapse involve is called an axo-axonic synapse.

Cellular elements at the typical nerve ending Regions of functional contacts between

neurons (synapses) have distinct morphological characteristics. Though a great deal ofvariation exists in the size and shape of boutons of individual neurons, synapses can be

identified by the presence of the following: (1) A presynaptic complement of

membrane-bound synaptic vesicles exists. These are spherical vesicles in excitatory

nerve ending, shown in Figure 6. In inhibitory neurons the synaptic vesicle are often

flattened. (2) The nerve ending often has aggregations of dense material in the

cytoplasm immediately adjacent to the membrane on one or both sides of the junction

(these are known as pre- or postsynaptic densities-although this is not an absolute


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criterion. This dense material on the presynaptic side is thought to be the site of vesicle

attachment preceding neurotransmitter release. (3) There is a distinct synapticcleft or

intercellular space of approximately 20-40 nm; and (4) Many mitochondria are present,

especially in the nerve terminal.

Figure 6. Left This is a diagrammatic representation of an excitatory synapse, showing the spherical

vesicles and the presynaptic density. Right This is a diagrammatic representation of an inhibitory

synapse, showing the flat vesicles and the presynaptic density.

B. Variations in neuronal structure.. Numerous variations of the "model" neuron

described above exist. An important modification, which occurs especially in receptor

neurons, involves the designation of a neuronal process as a dendrite or as an axon.

Classically, the axon has been identified as the myelinated or unmyelinated process that

transmits signals away from the cell body. The classical view of the dendrite is that of an

unmyelinated tube of cytoplasm which carries information toward the cell body.However, this distinction does not hold for ALL neurons. Some cells have a myelinated

process that transmits signals toward the cell body. Morphologically the "dendrite" and

the "axon" may, therefore, be indistinguishable. Neither the position of the cell body nor

the presence or absence of myelin is always a useful criterion for understanding the

orientation of the neuron. The region of impulse initiation is more reliable guide to

understanding the functional focal point of the cell. This region is analogous to the initial

segment of the model neuron, discussed above. Routinely the fiber or process, which

contains the initial segment or trigger zone, is referred to as an axon. Note that the

trigger zone does not have to be immediately adjacent to the cell body.

One of the most important variations is the differences in the ways neurons make

synapses with other neurons. This will be covered by Dr. Byrne when he discusses the

fact that synapses can be on any element of the postsynaptic cell. As we will discuss in

the next lecture, the nature of the synapse varies also. Some nerve endings such as those

of monoamine neurons make very diffuse synaptic contacts and have no classic synaptic

cleft.


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Figure 7. Comparison of variations in the structure of neurons.

C. Naming neuronal types. A number of conventions have evolved to classify and

name neurons. One of the oldest, devised by Golgi in the late 1800s,.is based on the

complexity of the dendritic tree of the neuron. Through this approach cells are classified

as unipolar, bipolar and multipolar neurons as shown in Figure 8. Unipolar cells haveonly one cell process. These are primarily found in invertebrates but a form of this type

of cells is the vertebrate sensory neurons. Because these cell start out developmentally as

bipolar neurons and then become unipolar as they mature, they are called pseudo-

unipolar cells. Bipolar cells are present in the retina and the olfactory bulb. Multipolar

cells make up the remainder of neuronal types and are, consequently, the most numerous

type. These have been further sub-categorized into Golgi type I cells that are small

neurons, usually interneurons, and Golgi type II cells that are large multipolar neurons.

Cells are also named for their shape (e.g. pyramidal cells) or for the person who first

described them (e.g. Purkinjecells). And more recently cells have been named for their

function or the neurotransmitter they contain (e.g. CNS norepinephrine cells groups are

designated A1-A12). This is possible because of the development of histochemical and

immunocytochemical methods to specifically identify the neurotransmitter type used by

neurons.

Figure 8. Two variations in cell morphology: Left is the pyramidal cell named for its characteristic

pyramid shape. This cell is prominent in the cerebral cortex. Right is the cell soma and dendrites of

the Purkinje cell found in the cerebellum and named for the scientist, Purkinje.


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composition in neurons is different than found in other cells. They are composed of three

subunits that are arranged to form a 10-nm diameter tubule. It is the neurofilament that

stains with heavy metal to permit the visualization of neuronal shape. Neurofilaments

run in loose bundles around the cell nucleus and other organelles and funnel into the base

of the axonal and dendritic processes where they form parallel arrays distributed

longitudinally. Neurofilaments are more abundant than microtubules in axons whereasmicrotubules are more abundant than neurofilaments in dendrites. It is the

neurofilaments that undergo modification in the Alzheimer's disease to form

neurofibrillary tangles, as will be discussed later in the Neuroscience course.

Nucleolus is in the center of the nuclei of all neurons. It is a prominent, deeply stained

spherical inclusion about one-third the size of the nucleus. The nucleolus synthesizes

ribosomal RNA, which has a major role in protein synthesis.

Nucleus of the neuron is large and round and is usually centrally located. In some cells,

masses of deeply staining chromatin are visible in the nucleus. The nuclear membrane of

neurons is like that of other cells - a double membrane punctuated by pores (nuclearpores) which are involved in nuclear-cytoplasmic interactions. The nucleus in neurons is

spherical and ranges in diameterfrom3 and 18 micrometers depending on the size of the

neuron. Neurons with long axons have a larger cell body and nucleus. As in other cells,

the principal component of the nucleus is deoxyribonucleic acid (DNA), the substance of

the chromosomes and genes.

Peroxisomes are small membrane bounded organelles that use molecular oxygen to

oxidize organic molecules. They contain some enzymes produce, and others that degrade

hydrogen peroxide.

Plasma membrane of the neuron appears in the electron microscope as a typical bi-layered cellular membrane, approximately 10 nm thick.

Postsynaptic density is darkly staining material of postsynaptic cell adjacent to the

synapse. Receptors, ion channels and other signaling molecules are likely bound to this

material.

Presynaptic density is the region of darkly staining material adjacent the synapse where

synaptic vesicle are hypothesized to dock prior to fusion with the presynaptic membrane.

Ribosomes are particles composed of ribosomal RNA and ribosomal protein that

associates with mRNA and catalyzes the synthesis of proteins. When ribosomes areattached to the outer membranes of the ER the organelle is termed rough ER. The rough

ER, in laminae with interspersed ribosomes is visible with the light microscope as Nissl

substance. In light microscopic preparations, the appearance of Nissl substance varies in

different types of neurons. It may appear as densely stained ovoids or as finely dispersed

particles or aggregations of granules.


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Synaptic vesicles are small membrane bound spherical organelles in the cytoplasm of

neurons that contain neurotransmitter and various proteins necessary for neurotransmitter

secretion. Vesicle containing inhibitory neurotransmitter are often flat or elliptical

whereas vesicles that contain excitatory neurotransmitter are usually more spherical.

Synapse is the cell-cell communication junction that allows signals to pass from a nervecell to another cell. The synaptic cleft is the gap between the pre- and postsynaptic cells'

membranes. In a chemical synapse the signal is carried by a diffusable neurotransmitter.

The cleft between the presynaptic cell and the postsynaptic cells is 20 to 40 nm wide and

may appear clear or striated. Recent studies have indicated that the cleft is not an empty

space per se, but is filled with carbohydrate-containing material.

II. Glial cells and function

The most numerous cellular constituents of the central nervous system are the non-

neuronal, neuroglial ("nerve glue") cells that occupy the space between neurons. The

estimate is that there are roughly 360 billion glial cells. These cells comprise 80-90% ofthe cells in the CNS. We will cover the general classifications of the neuroglial cells and

describe some of the general properties that distinguish neuroglia from neurons.

Neuroglia differ from neurons in several general ways:

(1) do not form synapses,

(2) have essentially only one type of process,

(3) retain the ability to divide, and

(4) are less electrically excitable than neurons.

Neuroglia are most reliably classified and distinguished from neurons, at the lightmicroscopic level, with alkaline (basic) dyes showing nuclear morphology. In addition,

several metal stains are used show the shape of the cell and cytoplasmic architecture.

Characteristics of nuclei, including size, shape, staining intensity and distribution of

chromatin, are used to distinguish cell types in pathological material. Cell body

characteristics, including size, shape, location, and branching pattern and density of

processes, are also used.

Neuroglia are divided into two major categories based on size, the macroglia and

the microglia. The macroglia are of ectodermal origin and consist of astrocytes,

oligodendrocytes and ependymal cells. Microglia cells are probably of mesodermal

origin. A comparison of the various neuroglial types is shown in Figure 9.


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Figure 9. Types of neuroglia.

A. Macroglia.

Protoplasmic Astrocytes Protoplasmic astrocytes are found primarily in gray matter.

With silver or glial specific stains, their cell bodies and processes are very irregular. The

processes may be large or very fine, sometimes forming sheets that run between axons

and dendrites, and may even surround synapses. These fine sheet-like processes give the

protoplasmic astrocyte cell body a "fuzzy" or murky appearance under the light

microscope. Within the cytoplasm bundles of fine fibrils may be seen. The nucleus of a

protoplasmic astrocyte is ellipsoid or bean-shaped with characteristic flecks of chromatin.

Specific types of intercellular junctions have been noted between the processes of

protoplasmic astrocytes. These probably mediate ion exchange between cells.

Fibrous Astrocytes Fibrous astrocytes are found primarily in white matter, have a

smoother cell body contour than do protoplasmic astrocytes as seen with glial-specific


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bodies of oligodendroglia are often located near capillaries, but they lack the definite

perivascular end feet characteristic of astrocytes. In white matter, oligodendroglia are

often located as rows of cells between groups of neuronal processes. These are termed

interfascicular oligodendroglia and are involved in the formation and maintenance of

the myelin surrounding the neuronal processes nearby.

The processes of oligodendrocytes are fewer and more delicate than astrocytes, and

the cell body shape is polygonal to spherical. Utilizing alkaline dyes to stain the nucleus,

the oligodendrocyte nucleus is smaller than that of the astrocyte, eccentrically located in

the cell body, and contains clumps of chromatin. The cytoplasm of oligodendrocytes

tends to be darker than that of astrocytes with silver stains, and does not contain glial

fibrils (although they do contain microtubules).

Figure 11. Diagram showing an oligodendrocyte (left) and the fact that one oligodendrocyte

myelinates several internodal regions (right).

The role of oligodendroglia in the central nervous system, particularly of the

interfascicular oligodendrocytes, is the formation and maintenance of myelin. Myelin is

the sleeve of membranous material, that wraps the neuronal axon to facilitate the

conduction of the action potential through saltatory conduction. Myelin is composed of

concentric layers of membranes compacted against one another with an internal (i.e.

against the nerve fiber) and an external collar of cytoplasm. It is estimated that a single

oligodendrocyte contributes to the myelination of several adjacent nerve processes.

Moreover, more than one oligodendrocyte contributes to the myelination of a single

internode of an axon. The lamellae of myelin membranes result from the spiral wrapping

of the axon by cytoplasmic processes of interfascicular oligodendroglia. As will be

covered in more detail by Dr. Byrne, along the length of the nerve process the myelin

sheath is interrupted at intervals by nodes of Ranvier. Each segment of myelin betweenthe nodes is formed by more than one glial cell. Also the oligodendrocyte forming a

particular myelin internode (i.e. the myelin between two nodes) is seldom seen directly

adjacent to the myelin-wrapped process. This is because thin cytoplasmic bridges connect

the region of the oligodendrocyte cell body to the external wrap of myelin. It is

important to note that the region of the axon exposed at the node of Ranvier is not bare.

It may be the site of branching of the axon, the site of synaptic contacts, or it may be


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covered with various glial processes. The axon in this region usually contains

concentrations of organelles, especially mitochondria.

In the PNS Schwann cells are responsible for the myelin formation. These cells

myelinate axons differently than the interfascicular oligodendroglia. They migrate

around the axon, laying a membrane covering around the axon by squeezing out thecytoplasm of the Schwann cell. Also every internode of a PNS axon represents a single

Schwann cell. In addition unmyelinated axons in the PNS are also inclosed by in

channels formed by Schwann cells.

Figure 12. Diagrammatic representation of how single Schwann cells myelinate each internodal

region.

Ependyma Ependymal cells are derived from the early germinal epithelium lining the

lumen of the neural tube and thus are also ectodermal derivatives (along with neurons,

astrocytes, and oligodendrocytes). Ependymal cells line the ventricles of the brain and

the central canal of the spinal cord. They are arranged in a single-layered columnar

epithelium, and have many of the histological characteristics of simple epithelium,

varying from squamous to cuboidal, depending upon their location. The ependyma

forming the ventricular lining do not connect to a basal lamina, but rest directly uponunderlying nervous tissue. The surface facing the ventricle contains many microvilli and

cilia. These cilia move cerebrospinal fluid (CSF) in the ventricles. The lateral borders of

the ependymal cells are relatively straight and form junctions with adjacent cells.

Figure 13. Diagrammatic representation of the arrangement of ependymal cells to form the ciliated

lining of the ventricles.


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Ependymal cells are modified in various regions of the ventricles into layers of

cuboidal epithelium, which do lie on a basement membrane (formed by an outgrowth of

the pia) over a rich bed of vasculature and connective tissue. This is the choroid plexus

we studied in the Laboratory that is responsible for the secretion, uptake and transport of

substances to and from the CSF.

B. Microglia

Microglia, in contrast to the other types of glial cells, originate from embryonic

mesoderm. They are present throughout the central nervous system, but tend to be

inconspicuous in mature normal tissue and are difficult to identify with the light or

electron microscope. They are more abundant in gray matter, and may compromise up to

5-10% of the neuroglia in the cerebral cortex.

The general appearance of microglia is similar to oligodendrocytes although they are

smaller, and have undulating processes with spine-like projections. Microglial nuclei are

elongated or triangular and stain deeply with alkaline dyes.

Following damage to nervous tissue, microglia proliferate and migrate to the site ofinjury where they clear cellular debris by phagocytosis. The reacting microglia have a

swollen form with shortened processes and are difficult to discriminate from phagocytes

from the periphery or migrating perivascular cells. It is estimated that at least one third

of the phagocytes appearing in the area of a lesion are of CNS origin.


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Resting Potentials and Action Potentials

Despite the enormous complexity of the brain, it is possible to obtain an understanding of its

function by paying attention to two major details:

First, the ways in which individual neurons, the components of the nervous system, arewired together to generate behavior.

Second, the biophysical, biochemical, and electrophysiological properties of theindividual neurons.

A good place to begin is with the components of the nervous system and how the electrical

properties of the neurons endow nerve cells with the ability to process and transmit information.

Introduction to the Action Potential

Figure 1.1

Important insights into the nature of electrical signals used by nerve cells were obtained morethan 50 years ago. Electrodes were placed on the surface of an optic nerve of an invertebrate eye.

(By placing electrodes on the surface of a nerve, it is possible to obtain an indication of thechanges in membrane potential that are occurring between the outside and inside of the nerve

cell.) Then 1-sec duration flashes of light of varied intensities were presented to the eye; first dimlight, then brighter lights. Very dim lights produced no changes in the activity, but brighter lights

produced small repetitive spike-like events. These spike-like events are called action potentials,

nerve impulses, or sometimes simply spikes. Action potentials are the basic events the nerve

cells use to transmit information from one place to another.


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Features of Action Potentials

The recordings in the figure above illustrate three very important features of nerve action

potentials. First, the nerve action potential has a short duration (about 1 msec). Second, nerve

action potentials are elicited in an all-or-nothing fashion. Third, nerve cells code the intensity of

information by the frequency of action potentials. When the intensity of the stimulus isincreased, the size of the action potential does not become larger. Rather, the frequency or the

number of action potentials increases. In general, the greater the intensity of a stimulus, (whetherit be a light stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a

muscle receptor) the greater the number of action potentials elicited. Similarly, for the motor

system, the greater the number of action potentials in a motor neuron, the greater the intensity of

the contraction of a muscle that is innervated by that motor neuron.

Action potentials are of great importance to the functioning of the brain since they propagate

information in the nervous system to the central nervous system and propagate commands

initiated in the central nervous system to the periphery. Consequently, it is necessary to

understand thoroughly their properties. To answer the questions of how action potentials areinitiated and propagated, we need to record the potential between the inside and outside of nerve

cells using intracellular recording techniques.

Intracellular Recordings from Neurons

The potential difference across a nerve cell membrane can be

measured with a microelectrode whose tip is so small (about a

micron) that it can penetrate the cell without producing any

damage. When the electrode is in the bath (the extracellularmedium) there is no potential recorded because the bath is

isopotential. If the microelectrode is carefully inserted into the

cell, there is a sharp change in potential. The reading of thevoltmeter instantaneously changes from 0 mV, to reading a

potential difference of -60 mV inside the cell with respect to the

outside. The potential that is recorded when a living cell isimpaled with a microelectrode is called the resting potential, and

varies from cell to cell. Here it is shown to be -60 mV, but can

range between -80 mV and -40 mV, depending on the particular

type of nerve cell. In the absence of any stimulation, the resting

potential is generally constant. Figure 1.2


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Figure 1.3

It is also possible to record and study the action

potential. Here the cell has already been impaled

with one microelectrode (the recording electrode)which is connected to a voltmeter. The electrode

records a resting potential of -60 mV. The cell has

also been impaled with a second electrode calledthe stimulating electrode. This electrode isconnected to a battery and a device that can

monitor the amount of current that flows through

the electrode. Changes in membrane potential areproduced by closing the switch and by

systematically changing both the size and polarity

of the battery. If the negative pole of the battery isconnected to the inside of the cell, there will be an

instantaneous change in the amount of current that

flows through the stimulating electrode, and the

membrane potential becomes transiently morenegative. This result should not be surprising. The

negative pole of the battery makes the inside of the

cell more negative than it was before. A change inpotential that increases the polarized state of a

membrane is called a hyperpolarization. The cell

is more polarized than it was normally. Use yet alarger battery and the potential becomes even

larger. The resultant hyperpolarizations are graded

functions of the size of the stimuli used to producethem.

When the positive pole of the battery is connected to the electrode, the potential of the cell

becomes more positive when the switch is closed. Such potentials are called depolarizations.

The polarized state of the membrane is decreased. A larger battery produces even largerdepolarizations. Again, the magnitude of the response is proportional to the size of the stimulus.

However, an unusual event occurs when the size of the depolarization reaches a level of

membrane potential called the threshold. A totally new type of signal is initiated; the actionpotential. Note that if the size of the battery is increased even more, the amplitude of the action

potential is the same as the previous one. If the suprathreshold stimulus was long enough,

however, a train of action potentials would be elicited. The action potentials would continue tofire as long as the stimulus continues, with the frequency of firing being proportional to the

magnitude of the stimulus.

Action potentials are not only initiated in an all-or-nothing fashion, but they are also propagated

in an all-or-nothing fashion. An action potential initiated in the cell body of a motor neuron inthe spinal cord will propagate in an undecremented fashion all the way to the synaptic terminals

of that motor neuron.


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Figure 1.4

An experiment to test Bernstein's

hypothesis that the membrane potential is

equal to the Nernst Equilibrium Potential

(i.e., Vm = EK) is illustrated to the left.

The K+ concentration outside the cell was

systematically varied while the membrane

potential was measured. Also shown is theline that is predicted by the Nernst

Equation. The experimentally measuredpoints are very close to this line. Moreover,

because of the logarithmic relationship in

the Nernst equation, a change inconcentration of K+ by a factor of 10

results in a 60 mV change in potential.

Note, however, that there are some deviations in the figure at left from what is predicted by theNernst equation. Thus, one cannot conclude that Vm = EK. Such deviations indicate that another

ion is also involved in generating the resting potential. That ion is Na+. The high concentration of

Na+

outside the cell and relatively low concentration inside the cell results in a chemical(diffusional) driving force for Na+ influx. There is also an electrical driving force because the

inside of the cell is negative and this negativity attracts the positive sodium ions. Consequently,

if the cell has a small permeability to sodium, Na+ will move across the membrane and the

membrane potential would be more depolarized than would be expected from the K

+

equilibriumpotential.

Goldman-Hodgkin and Katz (GHK) Equation

When a membrane is permeable to two different ions, the Nernst equation can no longer be used

to precisely determine the membrane potential. It is possible, however, to apply the GHKequation. This equation describes the potential across a membrane that is permeable to both Na +

and K+.


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Note that is the ratio of Na+

permeability (PNa) to K+

permeability (PK). Note also that if thepermeability of the membrane to Na+ is 0, then alpha in the GHK is 0, and the Goldman-

Hodgkin-Katz equation reduces to the Nernst equilibrium potential for K+. If the permeability of

the membrane to Na+

is very high and the potassium permeability is very low, the [Na+] terms

become very large, dominating the equation compared to the [K+] terms, and the GHK equation

reduces to the Nernst equilibrium potential for Na

+

.

If the GHK equation is applied to the same data

in Figure 1.4, there is a much better fit. The value

of alpha needed to obtain this good fit was 0.01.

This means that the potassium K+

permeability is100 times the Na+ permeability. In summary, the

resting potential is due not only to the fact that

there is a high permeability to K

+

. There is also aslight permeability to Na+, which tends to make

the membrane potential slightly more positive

than it would have been if the membrane werepermeable to K+ alone.

Figure 1.5

Membrane Potential Laboratory

Clickhere to go to the interactive Membrane Potential Laboratory to experiment with the effectsof altering external or internal potassium ion concentration and membrane permeability to

sodium and potassium ions. Predictions are made using the Nernst and the Goldman, Hodgkin,

Katz equations.


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Propagation of the Action Potential

Changes in the Spatial Distribution of Charge

Once an action potential is initiated at one point in the nerve cell, how does it propagateto the synaptic terminal region in an all-or-nothing fashion?

This figure shows a schematic diagram of an

axon and the charge distributions that would be

expected to occur along the membrane of thataxon. Positive charges exist on the outside of the

axon and negative charges on the inside. Now

consider the consequences of delivering some

stimulus to a point in the middle of the axon. Ifthe depolarization is sufficiently large, voltage-

dependent sodium channels will be opened, andan action potential will be initiated. Consider forthe moment "freezing" the action potential at its

peak value. Its peak value now will be about +40

mV inside with respect to the outside. Unlike charges attract, so the positive charge willmove to the adjacent region of the membrane. As the charge moves to the adjacent region

of the membrane, the adjacent region of the membrane will depolarize. If it depolarizes

sufficiently, as it will, voltage-dependent sodium channels in the adjacent region of the

membrane will be opened and a "new" action potential will be initiated. This chargedistribution will then spread to the next region and initiate other "new" action potentials.

One way of viewing this process is with a thermal analogue. You can think of an axon as

a piece of wire coated with gunpowder (the gunpowder is analogous to the sodiumchannels). If a sufficient stimulus (heat) is delivered to the wire, the gunpowder will

ignite, generate heat, and the heat will spread along the wire to adjacent regions and

cause the gunpowder in the adjacent regions to ignite.

Determinants of Propagation Velocity

There is a great variability in the velocity of the propagation of action potentials. In fact,

the propagation velocity of the action potentials in nerves can vary from 100 meters per

second (580 miles per hour) to less than a tenth of a meter per second (0.6 miles per

hour). Why do some axons propagate information very rapidly and others slowly? Inorder to understand how this process works, it is necessary to consider two so-called

passive properties of membranes, the time constant and the space or length constant. Why

are these called passive properties? They have nothing to do with any of the voltage-dependent conductances discussed earlier. They have nothing to do with any pumps or

exchangers. They are intrinsic properties of all biological membranes.

Figure 3.1


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Time Constant. First, consider a thermal analogue. Place

a block of metal at 10oC on a hotplate at 100oC. How

would the temperature change? It will increase from itsinitial value of 10oC to a final value of 100oC. But the

temperature will not change instantly. In fact, it would

change as an exponential function of time. An analogoussituation occurs in nerve cells, when they receive aninstantaneous stimulus. The figure at right represents an

idealized nerve cell. The recording electrode initially

measures a potential of -60 mV (the resting potential).At some point in time (time 0), the switch is closed. The

switch closure occurs instantaneously and as a result of

the instantaneous closure, instantaneous current flowsthrough the circuit. (This is equivalent to slamming the

block of metal on the hotplate.) Note that despite the fact

that this stimulus changes instantly, the change in

potential does not occur instantaneously. It takes time forthe potential to change from its initial value of -60 mV to

its final value of -50 mV. There is a total of 10 mV

depolarization, but the change occurs as an exponentialfunction of time.

Figure 3.2

There is a convenient index of how rapidly exponential functions change with time. Theindex is denoted by the symbol and called the time constant. It is defined as the

amount of time it takes for the change in potential to reach 63% of its final value. In thisexample, the potential changes from -60 to -50 and the 63% value is -53.7 mV. Thus, the

time constant is 10 msec. The smaller the time constant, the more rapid will be the

change in response to a stimulus. Therefore, if this neuron had a time constant of 5 msec,then in 5 msec the membrane potential would reach -53.7 mV. The time constant is

analogous to the 0 to 60 rating of a high performance car; the lower the 0 to 60 rating, the

faster the car. The lower the time constant, the faster or more rapidly a membrane will

respond to a stimulus. The effects of the time constant on propagation velocity will

become clear below.

The time constant is a function of two properties of membranes, the membrane resistance

(Rm) and the membrane capacitance (Cm). Rm is the inverse of the permeability; thehigher the permeability, the lower the resistance, and vice versa. Membranes, like the

physical devices known as capacitors, can store charge. When a stimulus is delivered, it

takes time to charge up the membrane to its new value.

Space Constant. Consider another thermal analogue. Take a long, metal rod that is again

initially at 10oC and consider the consequences of touching one end of the rod to a


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hotplate which is at 100oC (Assume that it is placed there for a certain amount of time to

allow the temperature changes to stabilize). How would the temperature be distributed

along the length of the rod? There would be a gradient that could be described by an

exponential function because of the physical processes involved.

Figure 3.3

An analogous situation occurs in nerve cells. The figure atleft represents an idealized nerve cell in which recordings are

made from different regions along the axon at 1 mmincrements. The cell body is impaled with a stimulating

electrode connected to a battery, the value of which changes

the potential of the cell body to -50 mV (the equivalent ofputting a 10oC rod on a 100oC hot plate). This axon, even

though it initially had a spatially uniform resting potential of

-60 mV, now has a potential of -50 mV in the soma becausethat is the region in which the stimulus is applied. However,

the potential is not -50 mV all along the axon; it varies as a

function of distance from the soma. One mm away thepotential is -56 mV; at 2 mm away it is even closer to -60mV; and far enough along the axon, the potential of the axon

is -60 mV, the resting potential. Just as there is an index for

how a change in potential changes with the time (the time

constant), there is also an index denoted by the symbol

(called the space constant or the length constant) which is anindication of how far a potential will spread along an axon in

response to a subthresholdstimulus at another point. In

Figure 3.3, the space constant or length constant is 1 mm. In

1 mm the potential will change by 63% of its final value. If

was greater than 1 mm, the potential would spread a greaterdistance. If was 1/2 mm, the potential would spread lessalong the axon. Thus, whereas the time constant is an indexof how rapidly a membrane would respond to a stimulus in

time, the space constant is an index of how well a

subthreshold potential will spread along an axon as a function

of distance.

The length constant can be described in terms of the physical parameters of the axon,

where d is the diameter of the axon, Rm is, as before, the membrane

resistance, the inverse of the permeability, and Ri is the internal resistance(resistance of the axoplasm). Ri is an indicator of the ability of charges to

move along the inner surface of the axon.

Propagation Velocity. How are the time constant and the space constant related to

propagation velocity of action potentials? The smaller the time constant, the more rapidly


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a depolarization will affect the adjacent region. If a depolarization more rapidly affects anadjacent region, it will bring the adjacent region to threshold sooner. Therefore, the

smaller the time constant, the more rapid will be the propagation velocity. If the space

constant is large, a potential change at one point would spread a greater distance alongthe axon and bring distance regions to threshold sooner. Therefore, the greater the space

constant, the more rapidly distant regions will be brought to threshold and the more rapidwill be the propagation velocity. Thus, the propagation velocity is directly proportional tothe space constant and inversely proportional to the time constant. There are separate

equations that describe both the time constant and the space constant. The insight above

allows us to make a new equation that combines the two.

The equation provides insights into how it is possible for different axons to have differentpropagation velocities. One way of endowing an axon with a high propagation velocity is

to increase the diameter. However, there is one serious problem in changing the

propagation velocity by simply changing the diameter. To double the velocity, it is

necessary to quadruple the diameter. Clearly there must be a better way of increasing

propagation velocity than by simply increasing the diameter.

Another way to increase the propagation velocity is to decrease the membrane

capacitance. This can be achieved by coating axons with a thick insulating sheath known

as myelin. One potential problem with this approach is that the process of covering theaxon would cover voltage-dependent Na+ channels. If Na+ channels are occluded, it

would be impossible to generate an action potential. Instead of coating the entire axonwith the myelin, only sections are coated and some regions called nodes are left bare.

Propagation in Myelinated Fibers

Propagation in myelinated fibers works as illustrated in Figure

3.4. Start with an action potential at a node on the left. In the

absence of myelin, the action potential would propagate

actively through the simple mechanisms discussed above, but

the myelin occludes all the voltage-dependent sodium channels.(In fact, myelinated axons do not even have sodium channels in

the internodal region.) Rather, the potential change at one nodespreads in the internodal region along the axon passively just as

the temperature would spread along a long metal rod. The

potential spreads, but gets smaller (decrements), just as atemperature change induced at one end of a rod would get

smaller as it spreads along a rod.

Figure 3.4


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Now consider the point at which the passively spreading potential reaches the next node.A "new" action potential will be initiated. The stimulus for this action potential is the

depolarization that emerges from the end of the myelin. Think of the gunpowder

analogue again, but this time coat the rod with some insulation and put gunpowder onlyat the bare regions. Because of the insulation, a temperature change produced by the

ignition of the gunpowder will spread effectively along the metal rod. The temperaturewill be sufficient to ignite the gunpowder at the next region and the process will repeat

itself.


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Synaptic Transmission at the Skeletal

Neuromuscular Junction and Mechanisms of

Neurotransmitter Release

The synapse is a specialized structure that allows one neuron to communicate with

another neuron or a muscle cell. There are billions of nerve cells in the brain and each

nerve cell can make and receive up to 10,000 synaptic connections with other nerve cells.

Also, the strength of the synapse is modifiable. Changes in the strength of synapses

endow the nervous system with the ability to store information.

Anatomy of the Neuromuscular Junction

The synapse for which most is known is the one formed between a spinal motor neuron

and a skeletal muscle cell. Historically, it has been studied extensively because it isrelatively easy to analyze. However, the basic properties of synaptic transmission at theskeletal neuromuscular junction are very similar to the process of synaptic transmission

in the central nervous system. Consequently, an understanding of this synapse leads to an

understanding of the others. Therefore, we will first discuss the process of synaptic

transmission at the skeletal neuromuscular junction.


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Figure 4.1

The features of the synaptic junction at the neuromuscular junction are

shown in the figure at left. Skeletal muscle fibers are innervated by mo

neurons whose cell bodies are located in the ventral horn of the spinalcord. The terminal region of the axon gives rise to very fine processes

that run along skeletal muscle cells. Along these processes are speciali

structures known as synapses. The particular synapse made between aspinal motor neuron and skeletal muscle cell is called the motor endpla

because of its specific structure.

The synapse at the neuromuscular junction has three characteristic

features of chemical synapses in the nervous system. First, there is adistinct separation between the presynaptic and the postsynaptic

membrane. The space between the two is known as the synaptic cleft.

The space tells us there must be some intermediary signaling mechanisbetween the presynaptic neuron and the postsynaptic neuron in order t

have information flow across the synaptic cleft. Second, there is a

characteristic high density of small spherical vesicles. These synapticvesicles contain neurotransmitter substances. Synapses are also associawith a high density of mitochondria. Third, in most cases, there is a

characteristic thickening of the postsynaptic membrane, which is due a

least in part to the fact that the postsynaptic membrane has a high densof specialized receptors that bind the chemical transmitter substances

released from the presynaptic neuron. Additional details on the

morphological features of synaptic junctions is provided in Chapter 8,Part 7 and Chapter 11, Part 4.

Physiology of Synaptic Transmission at the Neuromuscular Junction

Figure

4.2

This figure illustrates in a very schematic way how it ispossible to study the physiology of synaptic transmission

at the skeletal neuromuscular junction in great detail. A

piece of muscle and its attached nerve are placed in asmall experimental chamber filled with an appropriate

Ringer solution. The resting potential of the muscle cell

is recorded with a microelectrode. Electrodes are also

placed on the surface of the nerve axon. Brief electricshocks cause action potentials to be initiated, which

propagate to the synaptic terminal.


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Figure 4.3

The figure above illustrates two types of potential changes that were recorded in such an

isolated nerve-muscle preparation. The experiment also illustrates the properties of apowerful drug, curare, which has proven to be very useful in studying the process of

synaptic transmission at the skeletal neuromuscular junction. Part A illustrates thesequence of potential changes recorded in the muscle cell as a result of stimulating the

motor axon. The arrow indicates the point in time when the shock is delivered to the

motor axon. Note that there is a quiescent period of time after the shock. The delay is due

to the time it takes for the action potential in the motor axon to propagate from its site ofinitiation. After the delay, there are two types of potentials recorded in the muscle cell.

First, there is a relatively slowly changing potential that will be the focus of the following

discussion. If that slow initial potential is sufficiently large, as it normally is in skeletal

muscle cells, a second potential, an action potential, is elicited in the muscle cell.

Action potentials in skeletal muscle cells are due to ionic mechanisms similar to those

discussed previously. Specifically, there is a voltage-dependent change in Na+

permeability followed by a delayed increase in K+ permeability. (For smooth muscle cells

and cardiac muscle cells the ionic mechanisms are different, however.)

The underlying event that triggers the action potential can be revealed by taking

advantage ofcurare, an arrow poison used by some South American Indians. A low dose

of curare (Part B) reduces the underlying event, but it is still not sufficiently reduced tofall below threshold. If a somewhat higher dose of curare is delivered (Part C), the slow

underlying event becomes subthreshold. The underlying signal is known as the endplate

potential (EPP) because it is a potential change recorded at the motor endplate. Generally,

it is known as an excitatory postsynaptic potential (EPSP).

Curare blocks the endplate potential because it is a competitive inhibitor of acetylcholine

(ACh), the transmitter released at the presynaptic terminal. Curare does not block the

voltage-dependent Na+

conductance or the voltage-dependent K+

conductance thatunderlies the muscle action potential. Curare affects the stimulus (the EPSP) which

normally leads to the initiation of the muscle action potential. An animal that is poisoned


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with curare will asphyxiate because the process of neuromuscular transmission at

respiratory muscles is blocked.

Normally, the magnitude of the endplate potential is quite large. It more than reaches

threshold; it goes beyond threshold. In fact, the amplitude of the endplate potential is

about 50 mV, but only about 30 mV is needed to reach threshold. The extra 20 mV iscalled the safety factor. Note that even if the endplate potential were to become

somewhat smaller, the EPP would reach threshold and there would still be a one-to-onerelationship between an axon potential and the motor axon and an action potential in the

muscle cell.

Propagation of the EPP

What are the properties of the EPP and how does it

compare with the properties of the action potential?

Is the endplate potential due to a voltage-dependentchange in Na

+and K

+permeabilities like the action

potential?

Is the endplate potential propagated in an all-or-

nothing fashion like the action potential?

The figure on the right illustrates an experiment thatexamines the propagation of the endplate potential. The

muscle fiber is impaled repeatedly with electrodes at 1

mm intervals. (Note that the endplate potential is small

because this experiment is done in the presence of alow concentration of curare so the endplate potential

can be recorded without the complications of triggeringan action potential.) The endplate potential is not

propagated in an all-or-nothing fashion. It does spread

along the muscle, but it does so with decrement. Thus,

the spread of the endplate potential from its site ofinitiation to other sites along the muscle cell occurs

passively and with decrement, just as a subthreshold

potential change in one portion of the axon spreadsalong the axon, or just as a change in temperature at

one point on a metal rod spreads along the rod.

Figure 4.4


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Overview of the Sequence of Events Underlying the EPP

Figure 4.5

What are the other steps in the process of chemical synaptic transmission? Figure 4.5provides an overview. A nerve action potential that is initiated in the cell body of a spinal

motor neuron propagates out the ventral roots and eventually invades the synapticterminals of the motor neurons. As a result of the action potential, the chemical

transmitter acetylcholine (ACh) is released into the synaptic cleft. ACh diffuses across

the synaptic cleft and binds to special receptors on the postsynaptic or the postjunctional

membrane. The binding of ACh to its receptors produces a conformational change in amembrane channel that is specifically permeable to both Na+ and K+. As a result of an

increase in Na+

and K+

permeability, there is a depolarization of the postsynapticmembrane. That depolarization is called the endplate potential or more generally the

EPSP. If the EPSP is sufficiently large, as it normally is at the neuromuscular junction, it

leads to initiation of an action potential in the muscle cell. The action potential initiates

the process of excitation contraction coupling and the development of tension. The

duration of the endplate potential is about 10 msec.

Two factors control the duration of the EPSP at the neuromuscular junction. First, ACh is

removed by diffusion. Second, a substance in the synaptic cleft,

called acetylcholinesterase (AChE), hydrolyzes or breaks down ACh.


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Role of AChE

Figure 4.6

An important family of substances,

one of which is neostigmine, inhibitsthe action of AChE. Neostigmine

blocks the action of AChE, andthereby makes the endplate potentiallarger and longer in duration. This

figure illustrates two endplate

potentials. One was recorded in

saline and curare and a secondrecorded after neostigmine was

added to the solution. (Curare is

added so that the properties of theEPP can be studied without

triggering an action potential in the

muscle cell.) After applyingneostigmine the endplate potential is

much larger and longer in duration.

Myasthenia Gravis

Myasthenia gravis is associated with severe muscular weakness because of a decrease inthe number of acetylcholine receptors in the muscle cell. If the endplate potential is

smaller, the endplate potential will fail to reach threshold. If it fails to reach threshold,

there will be no action potential in the muscle cell and no contraction of the muscle,which causes muscular weakness. Neostigmine and other inhibitors of AChE are used to

treat patients with myasthenia gravis. These agents make the amount of acetylcholine thatis released more effectively reach the remaining acetylcholine receptors.

Iontophoresis of ACh

Iontophoresis is an interesting technique that

can be used to further test the hypothesis thatACh is the neurotransmitter substance at the

neuromuscular junction. If ACh is the

transmitter that is released by this synapse, one

would predict that it should be possible tosubstitute artificial application of the transmitter

for the normal release of the transmitter. Since

ACh is a positively charged molecule, it can beforced out of a microelectrode to simulate the

release of ACh from a presynaptic terminal.

Figure 4.7


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Figure 4.8

Indeed, minute amounts of ACh can beapplied to the vicinity of the

neuromuscular junction. This figure

compares an EPP produced bystimulation of the motor axon and theresponse to ejections of ACh. The

potential change looks nearly identical to

the endplate potential produced by thenormal release of ACh. This experiment

provides experimental support for the

concept that ACh is the naturaltransmitter at this synapse.

The response to the ejection of ACh has

some other interesting properties that areall consistent with the cholinergic natureof the synapse at the skeletal

neuromuscular junction. Neostigmine

makes the response to the iontophoresisof ACh longer and larger. Curare reduces

the response because it competes with the

normal binding of ACh. If ACh is ejected

into the muscle cell, nothing happens

because the receptors for acetylcholine

are not in the inside; they are on the

outside of the muscle cell. Application ofacetylcholine to regions of the muscle

away from the end-plate produces no

response because the receptors for theACh are concentrated at the

cellular and molecular neurobiology

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