Fundamentals of the Nervous System and Nervous Tissue

Chapter 11

Introduction The nervous system is the master

controlling and communicating system of the body

It is responsible for all behavior Along with the endocrine system it is

responsible for regulating and maintaining body homeostasis

Cells of the nervous system communicate by means of electrical signals

Nervous System Functions

The nervous system has three overlapping functions Gathering of sensory input Integration or interpretation of sensory input Causation of a response or motor output

Introduction Sensory input

The nervous system has millions of sensory receptors to monitor both internal and external change

Integration It processes and interprets the sensory input

and makes decisions about what should be done at each moment

Motor output Causes a response by activating effector

organs (muscles and glands)

Organization of the Nervous System

Organization There is only one nervous system;

however, for convenience the nervous system is divided into two parts The central nervous system

• Brain and spinal cord

• Integrative and control centers The peripheral nervous system

• Spinal and cranial nerves

• Communication lines between the CNS and the rest of the body

Organization The peripheral nervous system has two

fundamental subdivisions Sensory (afferent) division

• Somatic and visceral sensory nerve fibers

• Consists of nerve fibers carrying impulses to the central nervous system

Motor (efferent) division• Motor nerve fibers

• Conducts impulses from the CNS to effectors– (glands and muscles)

Organization The motor division of the peripheral

nervous system has two main subdivisions The somatic nervous system

• Voluntary (somatic motor)

• Conducts impulses from the CNS to skeletal muscle The autonomic nervous system (ANS)

• Involuntary

• Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

Innervation of Visceral Organs

Organization The autonomic nervous system has two

principle subdivisions Sympathetic division

• Mobilizes body systems during emergency situations

Parasympathetic division• Conserves energy

• Promotes non-emergency functions The two subdivisions bring about opposite

effects on the same visceral organs What one subdivision stimulates, the other

inhibits

Peripheral Nervous System Visceral organs are

served by motor fibers of the autonomic nervous system and by visceral sensory fibers

The somata (limbs and body wall) are served by motor fibers of the somatic nervous system and by sensory somatic sensory fibers

Arrows indicate the direction of impulses

Histology of the Nervous Tissue Nervous tissue is highly cellular

Less that 20% of the CNS is extracellular space Cells are densely packed and tightly

intertwined Nervous tissue is made up of two cell types

Neurons• Excitable cells that transmit electrical signals

Support cells• Smaller cells that surround and wrap the delicate

neurons These same cells are found within CNS and

PNS

Supporting Cells All neurons associate closely with

nonnervous support cells of which there are 6 types Support cells of the CNS

• Astrocytes

• Microglial

• Ependymal

• Oligodendrocyte Support cells of the PNS

• Schwann cells

• Satellite cells

Supporting Cells in the CNS The supporting cells of the CNS are

collectively called neuroglia or simply, glial cells

Like neurons, glial cells have branching processes and a central cell body

Neuroglia can be distinguished by their much smaller size and by their darker staining nuclei

They outnumber neurons in the CNS by a ration of 10 to 1

Make up half of the mass of the brain

Astrocytes Star shaped Most abundant type

of glial cell Radiating projections

cling to neurons and capillaries, bracing the neurons to their blood supply

Astrocytes play a role in exchanges between capillaries and neurons

Astrocytes Cells function as

antigen presenting cells of the immune response

Control chemical environment around neurons, recapturing potassium ions and released neuro- transmitters

Astrocytes signal each other via intracellular calcium pulses

Microglial Small ovid cells with

relatively long “thorny” processes

Their branches touch nearby neurons to monitor health of the neuron

Microglial migrate toward injured neurons

Microglial Small ovid cells with

relatively long “thorny” processes

Their branches touch nearby neurons to monitor health of the neuron

Microglial migrate toward injured neurons

Microglial When invading micro-

organisms are present or damaged neurons have died, the micro- glial transforms into a special type of macro- phage that protects the CNS by phagocytizing the microorganisms or neuronal debris

Important because cells of the immune system can enter CNS

Ependymal Range in shape from

squamous to columnar and many are cilated

Line the central cavities of the brain and spinal cord

Form a fairly permeable barrier between cerebrospinal fluid of those cavities and the cells of the CNS

Beating cilia circulates cerebrospinal fluid

Oligodendro- cytes

Fewer branches than astrocytes

Cells wrap their cytoplasmic extensions tightly around the thicker neurons in the CNS

Produce insulating coverings called myelin sheaths

Supporting Cells of the PNS There are two supporting cells in the

PNS Satellite cells Schwann cells

These cells are similar in type and differ mainly in location

Satellite Cells

Somewhat flattened satellite cells surround cell bodies within ganglia

Thought to play some role in controlling the chemical environment of neurons with which they are associated, but function is largely unknown

Schwann Cells

Surround and form myelin sheaths around the larger nerve fibers in PNS

Similar to the oligodendrocytes of CNS Schwann cells are vital to peripheral nerve

fiber regeneration

Neurons Neurons are the structural units of the

nervous system Neurons are highly specialized cells that

conduct messages in the form of nerve impulses from one part of the body to another

Neuron Characteristics Extreme longevity

Live and function optimally for a lifetime Amitotic

As neurons assume their role in the nervous system they lose their ability to divide

Neurons cannot be replaced if destroyed High metabolic rate

Require continuous and abundant supplies of oxygen and glucose

Homeostatic deviations often first appear in nervous tissue which has specific needs

Neurons The plasma membrane of neurons is the

site of electrical signaling, and it plays a crucial role in most cell to cell interaction

Most neurons have three functional components in common A receptive component A conducting component A secretory or output component

Each component is associated with a particular region of a neuron’s anatomy

Neuron structure Typically large, complex cells, they all

have the following structures Cell body

• Nuclei

• Nissl bodies

• Axon hillock Cell processes

• Dendrites

• Axon

• Myelin sheath or neurilemma

Neuron Cell Body

Neuron Cell Body The cell body consists

of a large, spherical nucleus with a prominent nucleolus surrounded by cytoplasm

The cell ranges from 5 to 140m in diameter

The cell body is the biosynthetic center of the neuron

Neuron Cell Body The cell body contains

the usual organelles with the exception of centrioles (not needed in amitotic cells)

The rough endoplasmic reticulum or Nissl bodies is the protein and membrane making machinery of the cell

The cell body is the focal point for neuron growth in development

Neuron Cell Bodies Clusters of cell bodies in the CNS are

called nuclei The relatively rare collection of cell

bodies in the PNS are called ganglia

Neuron Processes Cytoplasmic

extension called processes extend from the cell body of all neurons

The CNS contain both neuron cell bodies and their processes

The PNS consists chiefly of processes

Motor neuron

Neuron Processes Bundles of neuron

processes are called tracts in the CNS

Bundles of neuron processes in the PNS are called nerves

Two types of neuron processes

Dendrites Axons

Motor neuron

Note: Convention of “typical” neuron

Dendrites Dendrites are short, tapering diffusely

branching extensions Motor neurons have hundreds of

dendrites clustering close to the cell body Dendrites are receptive to input and

provide an enormous surface area for the reception of signals

In many areas of the brain the finer dendrites are highly specialized for information collection

Dendrites Dendritic spines

represent areas of close contact with other neurons

Dendrites convey information toward the cell body

These electrical signals are not nerve impulses but are short distance signals call graded potentials

Axons Each neuron has a

single axon The axon arises

from the cone shaped axon hillock

It narrows to form a slender process that stays uniform in diameter the rest of its length

Length varies; short or absent to 3 feet in length

Motorneuron

Axonhillock

Axons Each axon is called

a nerve fiber Each neuron has

only one axon but may possess a collateral branch

It branches profusely at its end to form more than 10,000 telodendria

Motorneuron

Axonhillock

Myelinated Axon Many nerve fibers,

particularly those that are long or large in diameter, are covered with a whitish, fatty segmented myelin sheath

Myelin protects and electrically insulates fibers from one another

Myelinated Axon Myelin increase the

speed of transmission of nerve impulses

Myelinated axons transmit nerve impulses rapidly; 150 meters/second

Unmyelinated axons transmit quite slowly; 1 meter/second

Myelinated Processes Myelin sheaths are associated only with

axons and their collaterals as these are impulse conducting fibers and need insulation

Dendrites which carry only graded potentials are always unmyelinated

Myelination of an Axon Myelin sheaths in the

PNS are formed by Schwann cells

The cells first become indented to receive the axon and then wrap themselves around it in a jelly roll fashion

Initially the wrappings are loose, but the cell cytoplasm is squeezed out between layers

Myelination of an Axon When the wrapping

process is complete many concentric layers wrap the axon

Plasma membranes of myelinating cells have less protein which makes them good electrical insulators

Myelinated Axons The nucleus and most

of the cytoplasm of the Schwann cell is located just beneath the outer layer of the plasma membrane

The outer layer is called the sheath of Schwann

Gaps, called Nodes of Ranvier, occur between Schwann cell

Myelinated Axons Nodes of Ranvier

occur at regular intervals along the axon

Since the axon is only exposed at these nodes nerve impulses are forced to jump from one node to the next which greatly increases the rate of conduction

Myelinated Axons Schwann cells that

surround but do not coil around peripheral fibers are considered unmyelinated

Each axon occupies a separate tubular recess

Fibers are typically thin

CNS Axons Oligodendrocytes form

the CNS myelin sheaths In contast to Schwann

cells, oligodendrocytes can form the sheaths of as many as 60 processes at one time

Nodes are spaced more widely than in PNS

Axons can be myelinated or unmyelinated

CNS Axons Regions of the brain containing dense

collections of myelinated fibers are referred to as white matter and are primarily fiber tracts

Gray matter contains mostly nerve cell bodies and unmyelinated fibers

Classification of Neurons Neurons can be classified structurally or

functionally Both classifications are described in the

text Functional classification is usually used to

describe how the neurons work within us

Structural

Classification Multipolar - many

processes extend from cell body, all dendrites except one axon

Bipolar - Two processes extend from cell, one a fused dendrite, the other an axon

Unipolar - One process extends from the cell body and forms the peripheral and central process of the axon

Multipolar Neurons Multipolar

neurons have three or more processes

Most common type in humans

Major neuron of the CNS

Most have many dendrites and one axon, some neurons lack an axon

Bipolar Neurons Bipolar neurons are

rare in the human body Found only in special

sense organs where they function as receptor cells

Examples include those found in the retina of the eye and in the olfactory mucosa

Unipolar Neuron Unipolar neurons have a single

process that emerges from the cell body

The central process is more proximal to the CNS and the peripheral is closer to the PNS

Unipolar neurons are chiefly found in the ganglia of the peripheral nervous system

Function as sensory neurons

Functional Classification The functional classification scheme

groups neurons according to the direction in which the nerve impulse travels relative to the CNS

Based on this criterion there are three neurons Sensory neurons Motor neurons Association neurons

Sensory Neurons

Neurons that transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS are called sensory or affective neurons

Virtually all primary sensory neurons of the body are unipolar

Sensory Neurons: Bipolar Bipolar sensory

neurons are only found in the special sensory organs of the eye or olfactory mucosa

Nuerons convey sensory input to higher CNS levels (eye to occipital lobe)

Sensory Neurons: Bipolar

Motor Neurons

Neurons that carry impulses away from the CNS to effector organs (muscles and glands) is called a motor or efferent neuron

Upper motor neurons are in the brain

Lower motor neurons are in PNS

Association Neurons or Interneurons These neurons lie

between the motor and sensory neurons

These neurons are found in pathways where integration occurs

Confined to CNS Make up 99% of the

neurons of the body and are the principle neuron of the CNS

Turn to Basic Concepts of Neural Integration

Page 419

Neural Integration The organization of the nervous system is

hierarchical The parts of the system must be

integrated into a smoothly functioning whole

Neuronal pools represent some of the basic patterns of communication with other parts of the nervous system

Neuronal Pools Neuronal

pools are functional groups of neurons that process and integrate incoming information from other sources and transmit it forward

One incoming presynaptic fiber synapses withSeveral different neurons in the pool. WhenIncoming fiber is excited it will excite somePostsynaptic neurons and facilitate others.

Neuronal Pools Neurons most

likely to generate impulses are those most closely associated with the incoming fiber because they receive the bulk of the synaptic contacts

These neurons are in the discharge zone

Discharge Zone

Neuronal Pools Neurons farther

away from the center are not excited to threshold by the incoming fiber, but are facilitated and can easily brought to threshold by stimuli from another source

The periphery of the pool is the facilitated zone

Facilitatedzone

Neuronal Pools Note: The illustrations presented are a

gross oversimplification of an actual neuron pool

Most neuron pools consist of thousands of neurons and include inhibitory as well as excitatory neurons

Types of Circuits Individual neurons in a neuron pool send

and receive information and synaptic contacts may cause either excitation or inhibition

The patterns of synaptic connections in neuronal pools are called circuits and they determine the functional capabilities of each type of circuit

There are four basic types of circuits Diverging, converging, reverberating, and

parallel discharge circuits

Diverging Circuits In diverging circuits

one incoming fiber triggers responses in ever-increasing numbers of neurons farther and farther along in the circuit

Diverging circuits are often called amplifying circuits because they amplify the response

Diverging Circuits These circuits are

common in both sensory and motor systems

Input from a single receptor may be relayed up the spinal cord to several different brain regions

Impulses from the brain can activate a hundred neurons and thousands of muscle fibers

Converging Circuits The pattern of

converging circuits is opposite to that of diverging circuits

Common in both motor and sensory pathways

In these circuits, the pool receives inputs from several presynaptic neurons, and the circuit as a whole has a funneling or concentrating effect

Converging Circuits Incoming stimuli

may converge from many different areas or from the same source, which results in strong stimulation or inhibition

Reverberating (oscillating) Circuits In reverberating

circuits the incoming signal travels through a chain of neurons, each of which makes collateral synapses with neurons in the previous part of the pathway

As a result of this positive feedback, the impulses reverberate through the circuit again and again

Reverberating circuit

Reverberating (oscillating) Circuits Reverberating circuits

give a continuous signal until one neuron in the circuit is inhibited and fails to fire

These circuits are involved in the control of rhythmic activities such as the sleep-wake cycle and breathing

The circuits may oscillate for seconds, hours, or years

Parallel After-Discharge Circuits The incoming fiber

stimulates several neurons arranged in parallel arrays that eventually stimulate a common output cell

Impulses reach the output cell at different times, creating a burst of impulses called an after discharge that may last 15 ms after initial input ends

Parallel After-Discharge Circuits This circuit has no

positive feedback and once all the neurons have fired, circuit activity ends

These circuit may be involved with complex problem solving activities

Patterns of Neural Processing Processing of inputs in the various circuits

is both serial and parallel In serial processing, the input travels

along a single pathway to a specific destination

In parallel processing, the input travels along several different pathways to be integrated in different CNS regions

Each pattern has its advantages The brain derives its power from its ability

to process in parallel

Serial Processing In serial processing the whole system

works in a predictable all-or-nothing manner

One neurons stimulates the next in sequence, producing a specific, anticipated response

Reflexes are examples of serial processing but there are others

Reflexes Reflexes are rapid, automatic responses

to stimuli, in which a particular stimulus always causes the same motor response

Reflex activity is stereotyped and dependable

Some your are born with and some you acquire as a consequence of interacting with your environment

Serial Processing: A Reflex Arc

Reflexes occurs over neural pathways called reflex arcs that contain five essential components

Receptor Sensory neuron CNS integration center Motor neuron Effector

Parallel Processing In parallel processing inputs are

segregated into many different pathways Information delivered by each pathway is

dealt with simultaneously by different parts of neural circuitry

During parallel processing several aspects of the stimulus are processed Barking dog

The same stimulus can hold common or unique meaning to different individuals

Parallel Processing Parallel processing is not repetitious

because the circuits do different things with more information

Each parallel path is decoded in relation to all the others to produce a total picture of the stimulus

Parallel Processing Even simple reflex arcs do not operate in

complete isolation As an arc moves through an association

neuron this activates parallel processing of the same input at higher brain levels

The reflex arc may cause you to pull away from a negative stimulus while parallel processing of the stimulus initiates problem solving about what need to be done

Parallel Processing Parallel processing is extremely

important for higher level mental functioning

An integrated look at the whole problem allows for faster processing

Parallel processing allows you to store a large amount of information in a small volume

This allows logic systems to work much faster

Chapter 11: Fundamentals of the Nervous System and

Nervous Tissue

End of Chapter