CHAPTER 48 Nervous System

Nervous System

Function: coordinates and controls bodily functions with

nerves and electrical impulses

The human brain

Contains an estimated 100 billion nerve cells, or neurons

Each neuron

May communicate with thousands of other neurons

Communication between neurons can be long-distance

electrical signals or short-distance chemical signals

Nervous System

In all vertebrates, the nervous

system shows a high degree of

cephalization and distinct CNS

and PNS components

The brain provides the integrative power that

underlies the complex behavior of vertebrates

The spinal cord integrates simple responses to

certain kinds of stimuli and conveys

information to and from the brain

Figure 48.19

Central nervous

system (CNS) Peripheral nervous

system (PNS)

Brain

Spinal cord

Cranial

nerves

Ganglia

outside

CNS

Spinal

nerves

Information Processing

Nervous systems process information in three stages

Sensory input, integration, and motor output

Figure 48.3

Sensor

Effector

Motor output

Integration

Sensory input

Peripheral nervous

system (PNS)

Central nervous

system (CNS)

Anatomy

The central canal of the spinal cord and the four

ventricles of the brain are hollow, since they are

derived from the dorsal embryonic nerve cord

4 characteristics of chordates: notochord, dorsal hollow

nerve cord, pharyngeal slits, and post-anal tail

Gray matter

White

matter

Ventricles

Figure 49.5

Anatomy

Gray matter – no myelin sheath

Located on outside in brain and inside in spinal cord

White matter – has myelin sheath

Located on outside in spinal cord and inside in brain

Peripheral Nervous System

The PNS transmits information to and from the CNS

Plays a large role in regulating a vertebrate’s movement and internal environment

Divisions of PNS:

Sensory and Motor division – sends signals to and from the CNS

Motor divides into the Somatic nervous system and the Autonomic nervous system

Autonomic nervous system divides into Parasympathetic, Sympathetic, and Enteric divisions

Peripheral Nervous System

Somatic nervous system

Carries signals to skeletal muscles

Autonomic nervous system

Regulates the internal environment, in an involuntary manner

Carries signals to cardiac muscle, smooth muscle, and

glands

Sympathetic and parasympathetic divisions

Antagonistic effects on target organs

Parasympathetic division Sympathetic division

Action on target organs: Action on target organs:

Location of

preganglionic neurons:

brainstem and sacral

segments of spinal cord

Neurotransmitter

released by

preganglionic neurons:

acetylcholine

Location of

postganglionic neurons:

in ganglia close to or

within target organs

Neurotransmitter

released by

postganglionic neurons:

acetylcholine

Constricts pupil

of eye

Stimulates salivary

gland secretion

Constricts

bronchi in lungs

Slows heart

Stimulates activity

of stomach and

intestines

Stimulates activity

of pancreas

Stimulates

gallbladder

Promotes emptying

of bladder

Promotes erection

of genitalia

Cervical

Thoracic

Lumbar

Synapse

Sympathetic

ganglia

Dilates pupil

of eye

Inhibits salivary

gland secretion

Relaxes bronchi

in lungs

Accelerates heart

Inhibits activity of

stomach and intestines

Inhibits activity

of pancreas

Stimulates glucose

release from liver;

inhibits gallbladder

Stimulates

adrenal medulla

Inhibits emptying

of bladder

Promotes ejaculation and

vaginal contractions Sacral

Location of

preganglionic neurons:

thoracic and lumbar

segments of spinal cord

Neurotransmitter

released by

preganglionic neurons:

acetylcholine

Location of

postganglionic neurons:

some in ganglia close to

target organs; others in

a chain of ganglia near

spinal cord

Neurotransmitter

released by

postganglionic neurons:

norepinephrine

Figure 49.8

Autonomic Nervous System

The sympathetic division

Correlates with the “fight-or-flight” response

The parasympathetic division

Promotes a return to self-maintenance functions

Resting and digesting

The enteric division

Controls the activity of the digestive tract, pancreas, and gallbladder

Controlled by other 2 divisions

Neuron Structure

Most of a neuron’s organelles are located in the cell

body with cytoplasmic projections off the cell body

Figure 48.4

Dendrites

Cell body

Nucleus

Axon hillock

Axon Signal

direction

Synapse

Myelin sheath

Synaptic

terminals

Presynaptic cell Postsynaptic cell

Neuron Structure

Most neurons have dendrites

Highly branched extensions that receive signals from other

neurons

The axon is typically a much longer extension

Transmits signals to other cells at synapses

May be covered with a myelin sheath – fatty cell that

wraps around the axon

Types of Neurons

Neurons have a wide variety of shapes that reflect

their input and output interactions

Figure 48.5

Axon

Cell

body

Dendrites

(a) Sensory neuron (b) Interneurons (c) Motor neuron

Types of Neurons

Sensory neurons transmit information from sensory

receptors to the CNS

Detects external stimuli and internal conditions

Interneurons integrate the information in the CNS

Motor output leaves the CNS via motor neurons and

travels to the PNS

Neuron communicates with effector cells (muscles and

glands)

Three stages of information processing

Reflex – body’s automatic responses to certain stimuli

Figure 49.3

Sensory neurons

from the quadriceps

also communicate

with interneurons

in the spinal cord.

The interneurons

inhibit motor neurons

that supply the

hamstring (flexor)

muscle. This inhibition

prevents the hamstring

from contracting,

which would resist

the action of

the quadriceps.

The sensory neurons communicate with

motor neurons that supply the quadriceps. The

motor neurons convey signals to the quadriceps,

causing it to contract and jerking the lower leg forward.

4

5

6

The reflex is

initiated by tapping

the tendon connected

to the quadriceps

(extensor) muscle.

1

Sensors detect

a sudden stretch in

the quadriceps.

2 Sensory neurons

convey the information

to the spinal cord.

3

Quadriceps

muscle

Hamstring

muscle

Spinal cord

(cross section)

Gray matter

White

matter

Cell body of

sensory neuron

in dorsal

root ganglion

Sensory neuron

Motor neuron

Interneuron

Supporting Cells (Glia)

Essential for the structural integrity of the nervous

system and for the normal functioning of neurons

CNS

Astrocytes – nutrients to neurons in CNS

Oligodendrocytes – protection

Ependymal cells – lines ventricles and has cilia to move

cerebrospinal fluid

Microglial cells – protection against microorganisms

PNS

Schwann cells – protection

Astrocytes

Provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters

Helps create the blood-brain barrier that regulates substances entering the brain tissue

Figure 49.6 50 µ

m

Oligodendrocytes and Schwann Cells

Glia that form the myelin sheaths around the axons of

many vertebrate neurons

Helps to transmit signals faster down the axon

Myelin sheath Nodes of

Ranvier

Schwann

cell Schwann

cell

Nucleus of

Schwann cell

Axon

Layers of myelin

Node of Ranvier

0.1 µm

Axon

Figure 48.12

Nerve Physiology

Across the plasma membrane, every cell has a voltage

called a membrane potential

The inside of a cell is negative relative to the outside and

is measured using a voltmeter

The resting potential is the membrane potential of a

neuron that is not transmitting signals

Resting membrane potential = - 70mV

Resting Membrane Potential

In all neurons, the resting potential depends on the

ionic gradients that exist across the plasma membrane

Ion pumps and ion channels maintain the resting potential

of a neuron

CYTOSOL EXTRACELLULAR

FLUID

[Na+]

15 mM

[K+]

150 mM

[Cl–]

10 mM

[A–]

100 mM

[Na+]

150 mM

[K+]

5 mM

[Cl–]

120 mM

–

–

–

–

–

+

+

+

+

+

Plasma

membrane

Figure 48.6

Resting Membrane Potential

The concentration of Na+ is higher in the extracellular

fluid than in the cytosol while the opposite is true for K+

A neuron that is not transmitting signals contains many

open K+ channels and fewer open Na+ channels in its

plasma membrane

The diffusion of K+ and Na+ through these channels

leads to a separation of charges across the membrane,

producing the resting potential

Why is charge -70 mV?

Figure 48.7

Inner

chamber Outer

chamber Inner

chamber

Outer

chamber –90 mV +62 mV

Artificial

membrane

Potassium

channel

K+ Cl–

150 mM

KCL

150 mM

NaCl 15 mM

NaCl

5 mM

KCL

Cl–

Na+

Sodium

channel

+ –

+ –

+ –

+ –

+ –

+ –

(a) Membrane selectively permeable to K+ (b) Membrane selectively permeable to Na+

Action Potential

Gated ion channels open or close in response to the

binding of a specific ligand or a voltage change

The response is a change in the membrane potential

When ion channels open, two different responses can

occur: hyperpolarization or depolarization

Both are called graded potentials because the magnitude

of the change in membrane potential varies with the

strength of the stimulus

Cell Responses

Some stimuli trigger a

hyperpolarization

An increase in the magnitude of

the membrane potential

Figure 48.9

+50

0

–50

–100

Time (msec) 0 1 2 3 4 5

Threshold

Resting

potential Hyperpolarizations

Me

mb

ran

e p

ote

ntial (m

V)

Stimuli

(a) Graded hyperpolarizations

produced by two stimuli that

increase membrane permeability

to K+. The larger stimulus produces

a larger hyperpolarization.

Cell Responses

Other stimuli trigger a

depolarization

A reduction in the magnitude of

the membrane potential

Figure 48.9

+50

0

–50

–100

Time (msec)

0 1 2 3 4 5

Threshold

Resting

potential Depolarizations

Me

mb

ran

e p

ote

ntial (m

V)

Stimuli

(b) Graded depolarizations produced

by two stimuli that increase

membrane permeability to Na+.

The larger stimulus produces a

larger depolarization.

Cell Responses

A stimulus strong enough to

produce a depolarization that

reaches the threshold triggers

a different type of response,

called an action potential

Threshold = membrane

voltage amount that causes an

action potential

- 55 mV Figure 48.9

+50

0

–50

–100

Time (msec)

0 1 2 3 4 5 6

Threshold

Resting

potential

Me

mb

ran

e p

ote

ntial (m

V)

Stronger depolarizing stimulus

Action

potential

(c) Action potential triggered by a

depolarization that reaches the

threshold.

Action Potential

An action potential is a brief all-or-none

depolarization of a neuron’s plasma membrane that

carries information along axons

Both voltage-gated Na+ channels and voltage-gated

K+ channels are involved in the production of an action

potential

Action Potential

When a stimulus depolarizes the membrane Na+

channels open, allowing Na+ to diffuse into the cell

As the action potential subsides K+ channels open, and

K+ flows out of the cell

A refractory period follows the action potential during

which a second action potential cannot be initiated

Conduction of Action Potentials

An action potential can travel long distances by

regenerating itself along the axon

At the site where the action potential is generated,

usually the axon hillock an electrical current

depolarizes the neighboring region of the axon

membrane

The speed of an action potential increases with the

diameter of an axon

In vertebrates, axons are myelinated causes the speed of

an action potential to increase

Conduction of Action Potentials

Action potentials in myelinated axons jump between

the nodes of Ranvier in a process called saltatory

conduction

Cell body

Schwann cell

Myelin

sheath

Axon

Depolarized region

(node of Ranvier)

+ + +

+ + +

+ + +

+ +

– �–

– �–

– �–

– – –

–

–

–

Figure 48.13

Synapse

Synapse connects one cell to another

Space between the two cells is called the synaptic cleft

In an electrical synapse electrical current flows directly

from one cell to another via a gap junction

The vast majority of synapses are chemical synapses

Synapse

In a chemical synapse, a presynaptic neuron releases

chemical neurotransmitters, which are stored in the

synaptic terminal

Figure 48.14

Postsynaptic

neuron

Synaptic

terminal

of presynaptic

neurons

5 µ

m

Synapse

When an action potential reaches a terminal the final

result is the release of neurotransmitters into the

synaptic cleft

Figure 48.15

Presynaptic

cell

Postsynaptic cell

Synaptic vesicles

containing

neurotransmitter Presynaptic

membrane

Postsynaptic

membrane

Voltage-gated

Ca2+ channel

Synaptic cleft

Ligand-gated

ion channels

Na+ K+

Ligand-

gated

ion channel

Postsynaptic

membrane

Neuro-

transmitter

1 Ca2+

2

3

4

5

6

Direct Synaptic Transmission

The process of direct synaptic transmission involves the

binding of neurotransmitters to ligand-gated ion

channels

Neurotransmitter binding causes the ion channels to

open, generating a postsynaptic potential

Postsynaptic potentials fall into two categories:

Excitatory or Inhibitory

Direct Synaptic Transmission

After its release, the neurotransmitter diffuses out of

the synaptic cleft

May be taken up by surrounding cells and degraded by

enzymes

Neurotransmitters

Chemical messengers that act on cells to create a

response

The same neurotransmitter can produce different

effects in different types of cells

Types:

Acetylcholine, biogenic amines, various amino acids and

peptides, and certain gases

Neurotransmitters

Acetylcholine is one of the most common

neurotransmitters in both vertebrates and invertebrates

Can be inhibitory or excitatory

Used in muscle contraction

Biogenic amines: include epinephrine, norepinephrine,

dopamine, and serotonin

Are active in the CNS and PNS

Neurotransmitters

Various amino acids and peptides are active in the

brain

Gases such as nitric oxide and carbon monoxide are

local regulators in the PNS

Animal Nervous Diversity

All animals except sponges have some type of nervous

system

What distinguishes the nervous systems of different

animal groups is how the neurons are organized into

circuits

Animal Nervous Diversity

The simplest animals with nervous systems, the cnidarians have neurons arranged in nerve nets

Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

Figure 49.2

Nerve net

(a) Hydra (cnidarian)

Nerve

ring

Radial

nerve

(b) Sea star (echinoderm)

Animal Nervous Diversity

In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident

Annelids and arthropods have segmentally arranged clusters of neurons called ganglia

These ganglia connect to the CNS and make up a peripheral nervous system (PNS)

Figure 49.2

Eyespot

Brain

Nerve

cord

Transverse

nerve

(c) Planarian (flatworm)

Brain

Ventral

nerve

cord

Segmental

ganglion

Brain

Ventral

nerve cord

Segmental

ganglia

(d) Leech (annelid) (e) Insect (arthropod)

Anterior

nerve ring

Longitudinal

nerve cords

Ganglia

Brain

Ganglia

Figure 49.2 (f) Chiton (mollusc) (g) Squid (mollusc)

Animal Nervous Diversity

Nervous systems in molluscs correlate with the animals’

lifestyles

Sessile molluscs have simple systems while more complex

molluscs have more sophisticated systems

Animal Nervous Diversity

In vertebrates, the central nervous system consists of a

brain and dorsal spinal cord

The PNS connects to the CNS

Figure 49.2

Brain

Spinal

cord

(dorsal

nerve

cord)

Sensory

ganglion

(h) Salamander (chordate)

Embryonic Development of the Brain

In all vertebrates, the brain develops from three

embryonic regions: the forebrain, the midbrain, and

the hindbrain

By the fifth week of human embryonic development,

five brain regions have formed from the three

embryonic regions

As a human brain develops further the most profound

change occurs in the forebrain, which gives rise to the

cerebrum

Functional magnetic resonance imaging

Technology that can reconstruct a three-dimensional map of brain activity

The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks

Figure 48.1

Brainstem

The brainstem consists of three parts:

medulla oblongata, pons, and midbrain

The medulla oblongata contains centers that control

heart rate, blood pressure, breathing, swallowing, and

vomiting

The pons controls breathing

The midbrain contains centers for the passing

ascending and descending signals

Arousal and Sleep

A diffuse network of neurons called the reticular formation is present in the core of the brainstem

A part of the reticular formation, the reticular activating system (RAS) regulates sleep and arousal

Figure 49.10

Eye

Reticular formation

Input from touch,

pain, and temperature

receptors

Input from ears

Cerebellum

The cerebellum is important for coordination and

balance

Also involved in learning and remembering motor skills

Diencephalon

The embryonic diencephalon develops into three adult brain regions:

epithalamus, thalamus, and hypothalamus

The epithalamus includes the pineal gland (releases melatonin) and the choroid plexus (capillaries that produce cerebrospinal fluid)

The thalamus sends sensory information to the cerebrum and sends motor information from the cerebrum

Diencephalon

The hypothalamus regulates homeostasis

Basic survival behaviors such as feeding, fighting, fleeing,

and reproducing

Also known as the limbic center

The hypothalamus regulates circadian rhythms

Such as the sleep/wake cycle (biological clock)

Biological clocks usually require external cues to remain

synchronized with environmental cycles

Cerebrum

The cerebrum contains right and left cerebral

hemispheres

Each consist of cerebral cortex overlying white matter and

basal nuclei (regions of gray matter inside brain) – centers

for planning and learning movement sequences

Left cerebral

hemisphere

Corpus

callosum

Right cerebral

hemisphere

Basal

nuclei

Figure 49.13

Cerebrum

A thick band of axons, the corpus callosum provides

communication between the right and left cerebral

cortices

In humans, the largest and most complex part of the

brain is the cerebral cortex, where sensory information

is analyzed, motor commands are issued, and

language is generated

Cerebrum

Each side of the cerebral cortex has four lobes

Frontal, parietal, temporal, and occipital

Frontal lobe

Temporal lobe Occipital lobe

Parietal lobe

Frontal

association

area

Speech

Smell

Hearing

Auditory

association

area Vision

Visual

association

area

Somatosensory

association

area

Reading

Speech

Taste

Figure 48.27

Cerebrum

In the somatosensory cortex and motor cortex neurons

are distributed according to the part of the body that

generates sensory input or receives motor input

Figure 48.28

Tongue

Jaw Lips

Primary

motor cortex Abdominal

organs

Pharynx

Tongue

Genitalia

Primary

somatosensory

cortex

Toes

Parietal lobe Frontal lobe

Lateralization

The left hemisphere becomes more adept at language,

math, logical operations, and the processing of serial

sequences

The right hemisphere is stronger at pattern recognition,

nonverbal thinking, and emotional processing

Language and Speech

Studies of brain activity have mapped specific areas

of the brain responsible for language and speech

Figure 48.29

Hearing

words

Seeing

words

Speaking

words

Generating

words

Max

Min

Emotions

The limbic system is a ring of structures around the

brainstem

Figure 48.30

Hypothalamus Thalamus

Prefrontal cortex

Olfactory

bulb Amygdala Hippocampus

Emotions

This limbic system includes three parts of the cerebral

cortex: amygdala, hippocampus, and olfactory bulb

These structures attach emotional “feelings” to

survival-related functions

Structures of the limbic system form in early

development and provide a foundation for emotional

memory, associating emotions with particular events or

experiences

Memory and Learning

The frontal lobes are a site of short-term memory

Interact with the hippocampus and amygdala to

consolidate long-term memory

Many sensory and motor association areas of the

cerebral cortex are involved in storing and retrieving

words and images

Neural Stem Cells

The adult human brain contains stem cells that can

differentiate into mature neurons

The induction of stem cell differentiation and the

transplantation of cultured stem cells are potential

methods for replacing neurons lost to trauma or

disease

Figure 49.24

Nervous Disorders

Unlike the PNS, the mammalian CNS cannot repair itself

when damaged or assaulted by disease

Current research on nerve cell development and stem

cells may one day make it possible for physicians to

repair or replace damaged neurons

Mental illnesses and neurological disorders take an

enormous toll on society, in both the patient’s loss of a

productive life and the high cost of long-term health care

Schizophrenia

About 1% of the world’s population suffers from

schizophrenia

Schizophrenia is characterized by hallucinations,

delusions, blunted emotions, and many other symptoms

Available treatments have focused on brain pathways

that use dopamine as a neurotransmitter

Depression

Two broad forms of depressive illness are known

Bipolar disorder and major depression

Bipolar disorder is characterized by manic (high-mood) and

depressive (low-mood) phases

In major depression patients have a persistent low mood

Treatments for these types of depression include a

variety of drugs such as Prozac and lithium

Alzheimer’s Disease

Alzheimer’s disease (AD) is a mental deterioration

characterized by confusion, memory loss, and other

symptoms

AD is caused by the formation of neurofibrillary

tangles and senile plaques in the brain

A successful treatment for AD in humans may hinge on

early detection of senile plaques

Parkinson’s Disease

Parkinson’s disease is a motor disorder caused by the

death of dopamine-secreting neurons in the substantia

nigra

Characterized by difficulty in initiating movements,

slowness of movement, and rigidity

There is no cure for Parkinson’s disease although

various approaches are used to manage the symptoms