CHAPTER 28 THE NERVOUS SYSTEM. EVOLUTION OF THE ANIMAL NERVOUS SYSTEM The nervous system consists of neurons and supporting cells. Association neurons

Chapter 28

The nervous system

Evolution of the Animal Nervous System

• The nervous system consists of neurons and supporting cells.• Association neurons (or interneurons) are

located in the brain and spinal cord of vertebrates, together called the central nervous system (CNS).• They help provide more complex reflexes and,

in the case of the brain, higher associative functions, including learning and memory.


• Sensory neurons that carry impulses from sensory receptors to the CNS.

• Motor neurons that carry impulses away from the the CNS to effectors (muscles and glands).

• Together, the motor and sensory neurons constitute the peripheral nervous system (PNS) of vertebrates.

1

2

3

Associationneuron

Cellbody

Motorneuron

Axon

Dendrites

Sensoryneuron

Direction ofconduction

Cell body

Dendrites

Cell body

Axon

Axon

Organization of the vertebrate nervous system

Centralnervous system

“Integration center’’

(Associationneurons of brain and

spinal cord)

Motornervous system

(Efferentnerves)

Peripheralnervoussystem

MusclesOrgansGlands

Sensory receptors

Sensorynervous system

(Afferentnerves)


• Sponges are the only major phylum of animals that lack nerves.• Sponges respond minimally to stimuli and do

not send messages from one part of the body to another.


• The simplest nervous systems occur among cnidarians.• Cnidarian neurons are

linked to one another in a web, or nerve net.

• There is no associative activity and little coordination.

• Any motion that results is called a reflex because it is an automatic consequence of stimulation.


• The first associative activity in nervous systems is seen in free living flatworms, phylum Platyhelminthes.• These animals have a

ladderlike nervous system with two nerve cords.

• The two cords converge at an enlarged association area that functions as a primitive brain.


• A series of evolutionary changes lead from the flatworms to the vertebrate nervous system:• More sophisticated sensory

mechanisms.• Differentiation into central and

peripheral nervous systems.• Differentiation of sensory and

motor areas.• Increased complexity of

association.• Elaboration of the brain.

1

2

3

4

5

Cnidarian

Nervenet

Associativeneurons

Nervecords

Flatworm

Brain

Ventralnervecords

Arthropod

Central nervoussystem

Peripheralnerves

Giantaxon

Brain

Mollusk

Earthworm

Neurons Generate Nerve Impulses

• All neurons have the same basic structure: cell body, dendrites, and axon.

Cell body NucleusDendrites

Schwanncell

Schwanncell

Axon

Nucleus

Schwanncell

Axon

Myelin sheath(b)

Node ofRanvier

Myelinsheath

(a)

Axon


• Most neurons depend upon support from neuroglial cells.• Schwann cells and oligodendrocytes are

important supporting cells that envelop the axons of many neurons with a sheath of fatty material called myelin.

• Myelin acts as an electrical insulator.


• Schwann cells produce myelin in the PNS while oligodendrocytes produce myelin in the CNS.• The myelin is wrapped around the axon as a

myelin sheath comprised of multiple layers.• The myelin sheath is interrupted at intervals,

leaving unmyelinated gaps called nodes of Ranvier.• Nerve impulses jump from node to node, greatly

increasing the speed of transmission.• In multiple sclerosis and Tay-Sachs disease, the myelin

sheath degenerates.


• When a neuron is “at rest,” active transport channels (sodium-potassium pumps) in the neuron transport Na+ out of the cell and K+ ions in.• The result is to make the outside of the

membrane more positive than the inside, a condition called the resting membrane potential.


• Neurons are constantly expending energy to maintain the resting membrane potential.• The voltage difference between the neuron

interior and exterior is –70 millivolts.• The resting potential is the starting point for a

nerve impulse.


• A nerve impulse travels along the axon and dendrites as electrical current caused by ions moving in and out of the neuron through voltage-gated channels.• These membrane channels open and close in

response to electrical voltage changes.

• The impulse starts when pressure or other sensory inputs disturb a neuron’s plasma membrane, causing Na+ channels to open.


• When Na+ channels open, Na+ floods into the neuron from the outside.• For a brief moment, the inside of the neuron is

“depolarized,” becoming more positive.• The open Na+ channels in the small patch of

depolarized membrane remain open for only one half of a millisecond.

• If the voltage change of the depolarization is great enough, it causes nearby voltage-gated Na+ and K+ channels to open.


• The Na+ channels open first, which starts a wave of depolarization moving down the neuron.• This moving local reversal of

voltage is called an action potential.• An action potential follows an all-

or-none law: a large enough depolarization produces either a full action potential or none at all.

Na+ Na+

K+

As the action potential travels farther down the axon, voltage-gatedNa+ channels close and K+ channels open, allowing K+ to flow outof the cell and restoring the negative charge inside the cell.Ultimately, the sodium-potassium pump restores the restingmembrane potential.

In response to a stimulus, the membrane depolarizes: Voltage-gated Na+ channels open, Na+ flows into the cell, and the insidebecomes more positive.

The local change in voltage opens adjacent voltage-gated Na+

channels, and an action potential is produced.

At the resting membrane potential, the inside of the axon isnegatively charged because the sodium-potassium pump keepsa higher concentration of Na+ outside. Voltage-gated ion channelsare closed, but there is some leakage of K+.

K+

K+

Na+

Na+

Na+

Na+

Na–Kpump

K+

K+K+

+ + + + + +

+ + + + + +

– – – – – –

– – – – –

+–

–

– – + + + + + +

– – – + + + + + +

– – – – – –

– – – – – –

–+

+

+ +

+ +

K+

K+

+ – – – + + + +

+ + – – – + + + +

– – – –

– – –

+–

+Na+

– + + +

+ –– – +

K+

+ + + – – – + +

+ + + + – – – + +

– –

– –

+–

–

–Na+

– + +

+ +– – +

+–

–

1

2

3

4

Na+ channel

Voltage-gatedchannels

K+ channel

Na+


• The K+ voltage-gated channels open after a slight delay, causing K+ to flow out of the cell• this makes the interior of the neuron more

negative, causing the voltage-gated Na+ channels to close.

• The period of time after an action potential has passed but before the resting potential is restored is called the refractory period.

http://youtu.be/XdCrZm_JAp0

The Synapse

• Axons do not actually make direct contact with other neurons or with other cells.• A narrow gap called the synaptic cleft,

separates the axon tip and the target neuron or tissue.

• This junction of an axon with another cell is called a synapse.• The membrane on the axon side is called the

presynaptic membrane.• The membrane on the receiving side is called

the postsynaptic membrane.

A synapse between two neurons

© Dennis Kunkel/Phototake

Axon ofpresynapticcell

Synapticcleft

Postsynapticcell

Presynapticvesicles

Presynapticmembrane

Postsynapticmembrane

The Synapse

• Signals from an axon are carried across the synapse by chemical messengers called neurotransmitters.• These chemicals are packaged in tiny sacs, or

vesicles, at the tip of the axon.• When a nerve impulse reaches the end of an

axon, it causes the vesicles to release the neurotransmitters into the synaptic cleft.• Chemically-gated channels on the post-

synaptic membrane then respond by allowing ions to enter.

Events at the synapse

Neurotransmitter

Axonterminal

Mitochondria

Synapticvesicles

Synapticcleft

Postsynapticcell skeletalmuscle)

Chemically-gatedion channel

Gate closed

(b)

(a)

The Synapse

• The vertebrate nervous system uses dozens of different kinds of neurotransmitters.

• They fall into two classes, depending on whether they excite or inhibit the postsynaptic cell.• In an excitatory synapse, the chemically-gated

channel is usually a Na+ channel.• This can lead to an action potential.

• In an inhibitory synapse, the chemically-gated channel is usually a K+ or Cl– channel.• This prevents an action potential.

The Synapse

• An individual nerve cell can possess both excitatory and inhibitory synapses.• Integration must occur in which the various

excitatory and inhibitory electrical effects tend to cancel or reinforce one another.• If the result of the integration is a large

enough depolarization, an action potential will fire.

Integration

A single motor neuron in the spinal cord may have as many as 50,000 synapses!

AxonAxon hillock

(right): © E.R. Lewis, YY Zeevi, T.E. Everhart, U. of California/Biological Photo Service

The Synapse

• Examples of neurotransmitters:• Acetylcholine (Ach)

• Used at the synapse between a nerve and a muscle fiber, called a neuromuscular junction.

• It is used in excitatory synapses in skeletal muscle and in inhibitory synapses in cardiac muscle.

• Glycine and GABA• These are inhibitory neurotransmitters,

especially in neural controls of body movements.

The Synapse

• Biogenic amines are a group of neurotransmitters including:• Dopamine, important in controlling body

movements.• Norepinephrine and epinephrine, which are

involved in the autonomic nervous system.• Serotonin, which is involved in sleep

regulation and other emotional states.

Addictive Drugs Act on Chemical Synapses

• Emotional states (mood, pleasure, pain, etc.) are determined by particular groups of neurons that use special sets of neurotransmitters and neuromodulators.• Many researchers think that

depression results from a shortage of serotonin.

• Prozac, an antidepressant, inhibits the reabsorption of serotonin.

Prozacblocksreabsorption

Serotonin

Receptor


• Nerve cells are particularly prone to the loss of sensitivity when exposed to a chemical signal for a long time.• If receptor proteins within synapses are

exposed to high levels of neurotransmitters for prolonged periods, the nerve cell often responds by inserting fewer receptor proteins into the membrane.


• When receptor proteins in the pleasure pathways of the brain are exposed to high levels of dopamine due to cocaine, for example, the nerve cells respond by lowering the number of receptor proteins.• With so few receptors, the drug user needs the

drug to maintain even normal nerve activity levels.

• This is addiction, the physiological adaptation of the nervous system due to drug abuse.

Key Biological Process: Drug addiction

At a normal synapse, neuro-transmitters are quicklyrecycled by transporterproteins, so the firing rate ofreceptor proteins stays low.

Drug molecules like cocainebind to the transporters andblock recycling, so the levelof neurotransmitters rises,and the firing rate increases.

The receiving neuron “turnsdown the volume” by loweringthe number of receptors,so the firing rate returns tonormal.

If the cocaine is removed,the level of neurotransmittersfalls to normal, too low tofire the reduced number ofreceptors.

1 2 3 4Neurotransmitter

Transporterprotein

Synapse

Receptor protein Drug molecule

Evolution of the Vertebrate Brain

• The brain is the most complex vertebrate organ ever to evolve, and it can perform a bewildering variety of complex functions.


• The basic organization of the vertebrate brain can be seen in the brains of primitive fishes.

ThalamusCerebrum

Olfactory bulb

Optic chiasm

Hypothalamus

Forebrain(Prosencephalon)

Midbrain(Mesencephalon)

Hindbrain(Rhombencephalon)

Medulla oblongata Pituitary

Optic tectum

Cerebellum

Spinal cord


• The brain is divided into three regions that are found in differing proportions in all vertebrates:• 1) The hindbrain, or rhombencephalon - The

largest portion of the brain in fishes.• 2) The midbrain, or mesencephalon - Devoted

primarily to processing visual information in fishes.

• 3) The forebrain, or prosencephalon - Concerned mainly with olfaction (smell) in fishes.• Plays a far more dominant role in neural

processing in terrestrial vertebrates than in fishes.

The evolution of the vertebrate brain

Shark

Frog

Crocodile

Human

Cat

BirdOlfactory tract

Cerebrum

Midbrain

Optic tectum

Cerebellum

Spinal cord

Medulla oblongata


• In sharks and other fishes, the hindbrain is predominant, and the rest of the brain serves primarily to process sensory information.

• In amphibians and reptiles, the forebrain is far larger, and it contains a larger cerebrum devoted to associative activity.

• In birds, which evolved from reptiles, the cerebrum is even more pronounced.

• In mammals, the cerebrum is the largest portion of the brains.

How the Brain Works

• The cerebrum is the center for thought and association and makes up about 85% of the weight of the brain.• The cerebrum is divided into

right and left halves, called cerebral hemispheres.

• Much of the neural activity of the cerebrum occurs within a thin, gray outer layer called the cerebral cortex.

Corpus striatumThalamus

Cerebralcortex

CorpuscallosumSkullPinealglandPons

CerebellumSpinalcord

HypothalamusPituitarygland

ReticularformationMedulla

oblongata

How the Brain Works

• The cerebral cortex is gray because it is densely packed with cell bodies.• The wrinkles on the surface of the cerebral

cortex increase its surface area (and number of cell bodies) threefold.

• Underneath the cortex is a solid white region of myelinated nerve fibers that shuttle information between the cortex and the rest of the brain.

How the Brain Works

• Different areas of the cerebral cortex control different body activities.

Speech Taste

Face Face

Hand Hand

Arm Arm

Trunk Trunk

Leg Leg

Motor

Parietal lobe

Association

Hearing

Cerebrum

Frontallobe

Visual association area

Occipitallobe

Primary visual area

Cerebellum

Spinal cord

Medulla oblongata

Temporallobe

Smell

Balanceand

coordination

Vision

Sensory

How the Brain Works

• The right and left cerebral hemispheres are linked by bundles of neurons called tracts.• The tracts serve as information highways telling

each half of the brain what the other half is doing.

• These tracts cross over so that each half of the brain controls muscles and glands on the opposite side of the body.

How the Brain Works

• Beneath the cerebrum are the thalamus and hypothalamus.• The thalamus is the

major site of sensory processing in the brain.

• The hypothalamus integrates internal activities.

• It controls centers in the brain stem that regulate the internal environment of the body, such as heartbeat, temperature, blood pressure, and respiration rate.

• It also directs secretions of the pituitary gland.

Hypothalamus Thalamus

Hippocampus

Amygdala

How the Brain Works

• Extending back from the base of the brain is a structure known as the cerebellum.• It controls balance, posture, and muscular

coordination.• It is best developed in birds to control the

complicated balance and coordination associated with flight.

How the Brain Works

• The brain stem connects the rest of the brain to the spinal cord and includes:• the midbrain• pons• medulla oblongata

• The brain stem contains nerves that control functions, such as breathing, swallowing, digestion, heartbeat, and diameter of blood vessels.

How the Brain Works

• The two hemispheres of the cerebrum are responsible for different activities.• Language is lateralized to one dominant

hemisphere, usually the left.• The nondominant hemisphere is adept at

spatial reasoning.

Different brain regions control various activities

The Spinal Cord

• The spinal cord is a cable of neurons extending from the brain down through the backbone.• The spinal cord is surrounded and protected by a

series of bones called the vertebrae.• Spinal nerves pass out to the body from between

the vertebrae.

The vertebrate nervous system

Greymatter

Whitematter

Sacralnerves

Lumbarnerves

Thoracicnerves

Cervicalnerves

Cerebrum

Cerebellum

Thalamusandhypothalamus(surroundedbythe cerebrum)

Spinalcord

Tibialnerve

Sciaticnerve

Femoralnerve

Voluntary and Autonomic Nervous Systems

• The motor pathways of the PNS can be further divided into:• The somatic (voluntary) nervous system

• Relays commands to the skeletal muscles

• The autonomic (involuntary) nervous system

• Relays commands to the smooth muscles of the body and to cardiac muscle

The divisions of the vertebrate nervous system

Centralnervoussystem

Nervous system

Peripheralnervoussystem

Motorpathways

Sensorypathways

Spinalcord

Brain

Somatic(voluntary)

nervous system

Autonomic(involuntary)

nervous system

Parasympatheticdivision

Sympatheticdivision


• Motor neurons of the voluntary system stimulate muscles to contract in two ways.• First, muscles are stimulated to contract in

response to conscious commands.• Second, muscles can be stimulated as part of

reflexes (does not require conscious control.


• A reflex produces a rapid motor response to a stimulus because a sensory neuron passes information directly to a motor neuron.• Useful for the body to

react particularly quickly in time of danger.

• Many reflexes never reach the brain.

Graymatter

SPINAL CORDWhitematter

Dorsalrootganglion

Stretchreceptor

Patella

Patellartendon

Quadricepsmuscle(effector)

Motorneuron

Sensoryneuron

Monosynapticsynapse


• Neurons of the autonomic nervous system carry messages from the CNS that keep the body going even when it is not active.• The CNS uses this system to maintain the

body’s homeostasis.• It is comprised of two elements that act in

opposition to one another:• Sympathetic nervous system that

dominates in times of stress.• Parasympathetic nervous system that

does the opposite and conserves energy.

Documents

CHAPTER 28 THE NERVOUS SYSTEM. EVOLUTION OF THE ANIMAL NERVOUS SYSTEM The nervous system consists of neurons and supporting cells. Association neurons