Chapter 48:Nervous System

Command and Control Center •The human brain contains an estimated 100 billion nerve cells, or neurons•Each neuron may communicate with thousands of other neurons

Figure 48.1

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

Nervous systems consist of circuits of neurons and supporting cells•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

Organization of Nervous Systems• The simplest animals with nervous systems, the

cnidarians have neurons arranged in nerve nets

Figure 48.2a

Nerve net

(a) Hydra (cnidarian)

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

Figure 48.2b

Nervering

Radialnerve

(b) Sea star (echinoderm)

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

Figure 48.2c

Eyespot

Brain

Nerve cord

Transversenerve

(c) Planarian (flatworm)

• 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)

Brain

Ventral nervecord

Segmentalganglion

Brain

Ventralnerve cord

Segmentalganglia

Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod)

Anteriornerve ring

Longitudinalnerve cords

Ganglia

Brain

Ganglia

Figure 48.2f, g (f) Chiton (mollusc) (g) Squid (mollusc)

• Nervous systems in molluscs correlate with the animals’ lifestyles

• Sessile molluscs have simple systems while more complex molluscs have more sophisticated systems

• In vertebrates the central nervous system consists of a brain and dorsal spinal cord

• the PNS connects to the CNS

Figure 48.2h

Brain

Spinalcord(dorsalnervecord)

Sensoryganglion

(h) Salamander (chordate)

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 nervoussystem (PNS)

Central nervoussystem (CNS)

• Sensory neurons transmit information from sensors that detect external stimuli and internal conditions

• Sensory information is sent to the CNS where interneurons integrate the information

• Motor output leaves the CNS via motor neurons which communicate with effector cells

• The three stages of information processing are illustrated in the knee-jerk reflex

Figure 48.4

Sensory neurons from the quadriceps also communicatewith 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 detecta sudden stretch in the quadriceps.

2 Sensory neuronsconvey the information to the spinal cord.

3

Quadricepsmuscle

Hamstringmuscle

Spinal cord(cross section)

Gray matter

White matter

Cell body of sensory neuronin dorsal root ganglion

Sensory neuron

Motor neuron

Interneuron

Neuron Structure• Most of a neuron’s organelles are located in the

cell body

Figure 48.5

Dendrites

Cell body

Nucleus

Axon hillock

AxonSignal direction

Synapse

Myelin sheath

Synapticterminals

Presynaptic cell Postsynaptic cell

• Most neurons have dendrites– Highly branched extensions that receive signals from

other neurons

• The axon is typically a much longer extension– That transmits signals to other cells at synapses– That may be covered with a myelin sheath

• Neurons have a wide variety of shapes that reflect their input and output interactions

Figure 48.6a–c

Axon

Cell body

Dendrites

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

Supporting Cells (Glia)• Glia are non-neuronal, supporting cells which

surround neurons and their axons to provide support, nutrition, and electrical insulation.

• In the CNS, astrocytes provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters

Figure 48.7 50 µ

m

• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) are glia that form the myelin sheaths around the axons of many vertebrate neurons

Myelin sheathNodes of Ranvier

Schwanncell Schwann

cellNucleus of Schwann cell

Axon

Layers of myelin

Node of Ranvier

0.1 µm

Axon

Ion pumps and ion channels maintain the resting potential of a neuron•Across its plasma membrane, every cell has a voltage called a membrane potential•The inside of a cell is negative relative to the outside

• The membrane potential of a cell can be measured

Figure 48.9

APPLICATIONElectrophysiologists use intracellular recording to measure the

membrane potential of neurons and other cells.

TECHNIQUE A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.

Microelectrode

Referenceelectrode

Voltage recorder

–70 mV

The Resting Potential• The resting potential is the membrane potential

of a neuron that is not transmitting signals• = -70mV

• In all neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane

CYTOSOL EXTRACELLULARFLUID

[Na+]15 mM

[K+]150 mM

[Cl–]10 mM

[A–]100 mM

[Na+]150 mM

[K+]5 mM

[Cl–]120 mM

–

–

–

–

–

+

+

+

+

+

Plasmamembrane

Figure 48.10

• The concentration of Na+ is higher in the extracellular fluid than in the cytosol while the opposite is true for K+

• By modeling a mammalian neuron with an artificial membrane we can gain a better understanding of the resting potential of a neuron

Figure 48.11a, b

Inner chamber

Outer chamber Inner

chamberOuter chamber

–92 mV +62 mV

Artificialmembrane

Potassiumchannel

K+

Cl–

150 mMKCL

150 mMNaCl

15 mMNaCl

5 mMKCL

Cl–

Na+

Sodium channel

+ –

+ –

+ –

+ –

+ –

+ –

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

• A neuron that is not transmitting signals contains has 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

Gated Ion Channels• Gated ion channels open or close in response to

a change in the membrane potential or binding of a ligand

Action potentials are the signals conducted by axons•If a cell has gated ion channels its membrane potential may change in response to stimuli that open or close those channels

• Some stimuli trigger a hyperpolarization an increase in the magnitude of the membrane potential

Figure 48.12a

+50

0

–50

–100

Time (msec)0 1 2 3 4 5

Threshold

Restingpotential Hyperpolarizations

Me

mb

ran

e p

ote

ntia

l (m

V)

Stimuli

(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization.

• Other stimuli trigger a depolarization– A reduction in the magnitude of the membrane

potential

Figure 48.12b

+50

0

–50

–100

Time (msec)0 1 2 3 4 5

Threshold

Restingpotential

Depolarizations

Me

mb

ran

e p

ote

ntia

l (m

V)

Stimuli

(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization.

• Hyperpolarization and depolarization are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus

Production of Action Potentials• In most neurons, depolarizations are graded

only up to a certain membrane voltage, called the threshold

• A stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response, called an action potential

Figure 48.12c

+50

0

–50

–100

Time (msec)0 1 2 3  4 5 6

Threshold

Restingpotential

Me

mb

ran

e p

ote

ntia

l (m

V)

Stronger depolarizing stimulus

Actionpotential

(c) Action potential triggered by a depolarization that reaches the threshold.

• An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane– Is the type of signal that carries information along

axons

• Both voltage-gated Na+ channels and voltage-gated K+ channels are involved in the production of an 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

• The generation of an action potential

–  –  –  –  –  –  –  –

+  +  +  +  +  +  +  + +  + +  ++  +

–  – –  – –  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

Na+ Na+

K+

Na+ Na+

K+

Na+ Na+

K+

Na+

K+

K+

Na+ Na+

5

1 Resting state

2 Depolarization

3 Rising phase of the action potential

4 Falling phase of the action potential

Undershoot

1

2

3

4

5 1

Sodiumchannel

Actionpotential

Resting potential

Time

Plasma membrane

Extracellular fluid ActivationgatesPotassium

channel

Inactivationgate

Threshold

Mem

bran

e po

tent

ial

(mV

)

+50

0

–50

–100

Threshold

Cytosol

Figure 48.13

Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respectto the outside.

The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative.

A stimulus opens theactivation gates on some Na+ channels. Na+

influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential.

The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained.

Both gates of the Na+ channelsare closed, but the activation gates on some K+ channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state.

Conduction of Action Potentials• An action potential can travel long distances

– By regenerating itself along the axon

Figure 48.14

– +– + + + + +

– +– + + + + +

+ –+ – + + + +

+ –+ – + + + +

+ –+ – – – – –+ –+ – – – – –

– – – –– – – –

– –– –

+ +

+ +

+ ++ + – – – –

+ ++ + – – – –

– –– – + + + +– –– – + + + +Na+

Na+

Na+

Actionpotential

Actionpotential

ActionpotentialK+

K+

K+

Axon

An action potential is generated as Na+ flows inward across the membrane at one location.

1

2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

3 The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

K+

• At the site where the action potential is generated, usually the axon hillock– An electrical current depolarizes the neighboring

region of the axon membrane

Conduction Speed• The speed of an action potential increases with

the diameter of an axon• In vertebrates, axons are myelinated, also

causing the speed of an action potential to increase

• 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 48.15

Neurons communicate with other cells at synapses•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

• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal

Figure 48.16

Postsynapticneuron

Synapticterminalof presynapticneurons

5 µ

m

• When an action potential reaches a terminal– The final result is the release of neurotransmitters

into the synaptic cleft

Figure 48.17

Presynapticcell

Postsynaptic cell

Synaptic vesiclescontainingneurotransmitter

Presynapticmembrane

Postsynaptic membrane

Voltage-gatedCa2+ channel

Synaptic cleft

Ligand-gatedion channels

Na+

K+

Ligand-gatedion 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 postsynaptic potentials (EPSPs)– Inhibitory postsynaptic potentials (IPSPs)

• After its release, the neurotransmitter diffuses out of the synaptic cleft– May be taken up by surrounding cells and degraded

by enzymes

Summation of Postsynaptic Potentials

• Unlike action potentials postsynaptic potentials are graded and do not regenerate themselves

• Since most neurons have many synapses on their dendrites and cell body– A single EPSP is usually too small to trigger an action

potential in a postsynaptic neuron

Figure 48.18a

E1 E1

Restingpotential

Threshold of axon ofpostsynaptic neuron

(a) Subthreshold, nosummation

Terminal branch of presynaptic neuron

Postsynaptic neuron E1

0

–70

Me

mb

ran

e p

ote

ntia

l (m

V)

• If two EPSPs are produced in rapid succession an effect called temporal summation occurs

Figure 48.18b

E1 E1

Actionpotential

(b) Temporal summation

E1

Axonhillock

• In spatial summation EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

Figure 48.18c

E1 + E2

Actionpotential

(c) Spatial summation

E1

E2

• Through summation an IPSP can counter the effect of an EPSP

Figure 48.18d

E1 E1 + II

(d) Spatial summationof EPSP and IPSP

E1

I

Indirect Synaptic Transmission• In indirect synaptic transmission

– A neurotransmitter binds to a receptor that is not part of an ion channel

• This binding activates a signal transduction pathway– Involving a second messenger in the postsynaptic

cell, producing a slowly developing but long-lasting effect

Neurotransmitters• The same neurotransmitter can produce

different effects in different types of cells

• Major neurotransmitters

Table 48.1

Acetylcholine• Acetylcholine

– Is one of the most common neurotransmitters in both vertebrates and invertebrates

– Can be inhibitory or excitatory

Biogenic Amines• Biogenic amines

– Include epinephrine, norepinephrine, dopamine, and serotonin

– Are active in the CNS and PNS

Amino Acids and Peptides• Various amino acids and peptides are active in

the brain

Gases• Gases such as nitric oxide and carbon monoxide

are local regulators in the PNS

The vertebrate nervous system is regionally specialized•In all vertebrates, the nervous system

– Shows a high degree of cephalization and distinct CNS and PNS components

Figure 48.19

Central nervoussystem (CNS)

Peripheral nervoussystem (PNS)

Brain

Spinal cordCranialnerves

GangliaoutsideCNSSpinalnerves

• 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

• 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

Gray matter

Whitematter

Ventricles

Figure 48.20

The Peripheral Nervous System

• The PNS transmits information to and from the CNS– And plays a large role in regulating a vertebrate’s

movement and internal environment

• The cranial nerves originate in the brain and terminate mostly in organs of the head and upper body

• The spinal nerves originate in the spinal cord and extend to parts of the body below the head

• The PNS can be divided into two functional components– The somatic nervous system and the autonomic

nervous systemPeripheral

nervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision

Figure 48.21

• The somatic nervous system carries signals to skeletal muscles

• The autonomic nervous system regulates the internal environment, in an involuntary manner– Is divided into the sympathetic, parasympathetic,

and enteric divisions

• The sympathetic and parasympathetic divisions– Have antagonistic effects on target organs

Parasympathetic division Sympathetic division

Action on target organs: Action on target organs:

Location ofpreganglionic neurons:brainstem and sacralsegments of spinal cord

Neurotransmitterreleased bypreganglionic neurons:acetylcholine

Location ofpostganglionic neurons:in ganglia close to orwithin target organs

Neurotransmitterreleased bypostganglionic neurons:acetylcholine

Constricts pupilof eye

Stimulates salivarygland secretion

Constrictsbronchi in lungs

Slows heart

Stimulates activityof stomach and

intestines

Stimulates activityof pancreas

Stimulatesgallbladder

Promotes emptyingof bladder

Promotes erectionof genitalia

Cervical

Thoracic

Lumbar

Synapse

Sympatheticganglia

Dilates pupilof eye

Inhibits salivary gland secretion

Relaxes bronchiin lungs

Accelerates heart

Inhibits activity of stomach and intestines

Inhibits activityof pancreas

Stimulates glucoserelease from liver;inhibits gallbladder

Stimulatesadrenal medulla

Inhibits emptyingof bladder

Promotes ejaculation and vaginal contractionsSacral

Location ofpreganglionic neurons:thoracic and lumbarsegments of spinal cord

Neurotransmitterreleased bypreganglionic neurons:acetylcholine

Location ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near spinal cord

Neurotransmitterreleased bypostganglionic neurons:norepinephrine

Figure 48.22

• The sympathetic division– Correlates with the “fight-or-flight” response

• The parasympathetic division– Promotes a return to self-maintenance functions

• The enteric division– Controls the activity of the digestive tract, pancreas,

and gallbladder

Embryonic Development of the Brain

• In all vertebrates the brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain

Figure 48.23a

Forebrain

Midbrain

Hindbrain

Midbrain Hindbrain

Forebrain

(a) Embryo at one month

Embryonic brain regions

• By the fifth week of human embryonic development– Five brain regions have formed from the three

embryonic regions

Figure 48.23b

Telencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

(b) Embryo at five weeks

MesencephalonMetencephalon

Myelencephalon

Spinal cord

Diencephalon

Telencephalon

Embryonic brain regions

• As a human brain develops further the most profound change occurs in the forebrain, which gives rise to the cerebrum

Figure 48.23c

Brain structures present in adult

Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

(c) Adult

Cerebral hemisphereDiencephalon:

Hypothalamus

ThalamusPineal gland(part of epithalamus)

Brainstem:

Midbrain

Pons

Medullaoblongata

Cerebellum

Central canal

Spinal cord

Pituitarygland

The Brainstem• The brainstem consists of three parts

– The medulla oblongata, the pons, and the midbrain

• The medulla oblongata– Contains centers that control several visceral

functions

• The pons– Also participates in visceral functions

• The midbrain– Contains centers for the receipt and integration of

several types of sensory information

Arousal and Sleep• A diffuse network of neurons called the reticular

formation– Is present in the core of the brainstem

Figure 48.24

Eye

Reticular formation

Input from touch, pain, and temperature receptors

Input from ears

• A part of the reticular formation, the reticular activating system (RAS)– Regulates sleep and arousal

The Cerebellum• The cerebellum

– Is important for coordination and error checking during motor, perceptual, and cognitive functions

• The cerebellum is also involved in learning and remembering motor skills

The Diencephalon• The embryonic diencephalon develops into

three adult brain regions– The epithalamus, thalamus, and hypothalamus

• The epithalamus includes the pineal gland and the choroid plexus

• The thalamus is the main input center for sensory information going to the cerebrum and the main output center for motor information leaving the cerebrum

• The hypothalamus regulates – Homeostasis– Basic survival behaviors such as feeding, fighting,

fleeing, and reproducing

Circadian Rhythms• The hypothalamus also regulates circadian

rhythms– Such as the sleep/wake cycle

• Animals usually have a biological clock– Which is a pair of suprachiasmatic nuclei (SCN)

found in the hypothalamus

• Biological clocks usually require external cues to remain synchronized with environmental cycles

Figure 48.25

In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and endsat dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captivesquirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness.The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating andwhen it was still.

EXPERIMENT

Light Dark Light

20

15

10

5

1

(a) 12 hr light-12 hr dark cycle (b) Constant darkness

12 16 20 24 4 8 12 12 16 20 24 4 8 12

Time of day (hr) Time of day (hr)

When the squirrelswere exposed to a regular light/darkcycle, their wheel-turning activity (indicated by the dark bars) occurredat roughly the same time every day.However, when they were kept inconstant darkness, their activity phasebegan about 21 minutes later each day.

RESULTS

The northern flying squirrel’s internal clock can run in constant darkness, but it does so onits own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.

CONCLUSION

Dark

Day

s of

exp

erim

ent

The Cerebrum• The cerebrum develops from the embryonic

telencephalon

• The cerebrum has right and left cerebral hemispheres– That each consist of cerebral cortex overlying white

matter and basal nuclei

Left cerebralhemisphere

Corpuscallosum

Neocortex

Right cerebralhemisphere

Basalnuclei

Figure 48.26

• The basal nuclei are important centers for planning and learning movement sequences

• In mammals the cerebral cortex has a convoluted surface called the neocortex

• 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

• A thick band of axons, the corpus callosum provides communication between the right and left cerebral cortices

The cerebral cortex controls voluntary movement and cognitive functions•Each side of the cerebral cortex has four lobes

– Frontal, parietal, temporal, and occipital

Frontal lobe

Temporal lobe Occipital lobe

Parietal lobe

Frontalassociationarea

Speech

Smell

Hearing

Auditoryassociationarea

Vision

Visualassociationarea

Somatosensoryassociationarea

Reading

Speech

TasteS

omat

osen

sory

cor

tex

Mot

or c

orte

x

Figure 48.27

• Each of the lobes contains primary sensory areas and association areas

Information Processing in the Cerebral Cortex

• Specific types of sensory input– Enter the primary sensory areas

• Adjacent association areas– Process particular features in the sensory input and

integrate information from different sensory areas

• 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

TongueJawLips

Face

Eye

Brow

Neck

Thumb

Fingers

HandW

ristForearmE

lbowS

houlderT

runk

Hip

Knee

Primarymotor cortex Abdominal

organs

Pharynx

Tongue

TeethGumsJaw

Lips

Face

Nose

Eye

Fingers

HandForearm

Elbow

Upper arm

Trunk

Hip

Leg

Thumb

Neck

Head

Genitalia

Primarysomatosensory cortex

Toes

Parietal lobeFrontal lobe

Lateralization of Cortical Function• During brain development, in a process called

lateralization, competing functions segregate and displace each other in the cortex of the left and right cerebral hemispheres

• 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

Hearingwords

Seeingwords

Speakingwords

Generatingwords

Max

Min

• Portions of the frontal lobe, Broca’s area and Wernicke’s area are essential for the generation and understanding of language

Emotions• The limbic system

– Is a ring of structures around the brainstem

Figure 48.30

HypothalamusThalamus

Prefrontal cortex

Olfactorybulb

Amygdala Hippocampus

• This limbic system includes three parts of the cerebral cortex– The amygdala, hippocampus, and olfactory bulb

• These structures interact with the neocortex to mediate primary emotions – And 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

Cellular Mechanisms of Learning• Experiments on invertebrates have revealed the

cellular basis of some types of learning

Figure 48.31a, b

(a) Touching the siphon triggers a reflex thatcauses the gill to withdraw. If the tail isshocked just before the siphon is touched,the withdrawal reflex is stronger. Thisstrengthening of the reflex is a simple formof learning called sensitization.

(b) Sensitization involves interneurons thatmake synapses on the synaptic terminals ofthe siphon sensory neurons. When the tailis shocked, the interneurons releaseserotonin, which activates a signaltransduction pathway that closes K+

channels in the synaptic terminals ofthe siphon sensory neurons. As a result,action potentials in the siphon sensoryneurons produce a prolongeddepolarization of the terminals. That allowsmore Ca2+ to diffuse into the terminals, which causes the terminals to release more of their excitatory neurotransmitter onto the gill motor neurons. In response, the motor neuronsgenerate action potentials at a higher frequency,producing a more forceful gill withdrawal.

Siphon

Mantle

Gill

Tail

Head

Gill withdrawal pathway

Touchingthe siphon

Shockingthe tail Tail sensory

neuron

Interneuron

Sensitization pathway

Siphon sensoryneuron

Gill motorneuron

Gill

• In the vertebrate brain, a form of learning called long-term potentiation (LTP) involves an increase in the strength of synaptic transmission

Figure 48.32

PRESYNAPTIC NEURON

NO

Glutamate

NMDAreceptor

Signal transduction pathways

NO

Ca2+

AMPA receptor

POSTSYNAPTIC NEURON

Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.

5

Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).

6

The presynapticneuron releases glutamate.1

Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.

2

Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.

3

Ca2+ diffuses into thepostsynaptic neuron.

4

NO diffuses into thepresynaptic neuron, causing it to release more glutamate.

7

P

Consciousness• Modern brain-imaging techniques suggest that

consciousness may be an emergent property of the brain that is based on activity in many areas of the cortex

CNS injuries and diseases are the focus of much research•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

Nerve Cell Development• Signal molecules direct an axon’s growth by

binding to receptors on the plasma membrane of the growth cone

• This receptor binding triggers a signal transduction pathway which may cause an axon to grow toward or away from the source of the signal

Figure 48.33a, b

Midline ofspinal cord

Developing axonof interneuron

Growthcone

Netrin-1receptor

Netrin-1

Floorplate

Celladhesionmolecules

SlitreceptorSlit

Developing axon of motor neuron

Netrin-1receptor

Slitreceptor

Slit

Netrin-1

1 Growth toward the floor plate.Cells in the floor plate of thespinal cord release Netrin-1, whichdiffuses away from the floor plateand binds to receptors on thegrowth cone of a developinginterneuron axon. Binding stimulatesaxon growth toward the floor plate.

2 Growth across the mid-line.Once the axon reaches thefloor plate, cell adhesion moleculeson the axon bind to complementarymolecules on floor plate cells,directing the growth of the axonacross the midline.

3 No turning back. Now the axon synthesizes receptors that bind to Slit,a repulsion protein re-leased by floor plate cells.This prevents the axonfrom growing back acrossthe midline.

Netrin-1 and Slit, produced by cellsof the floor plate, bind to receptorson the axons of motor neurons. Inthis case, both proteins act to repelthe axon, directing the motor neuronto grow away from the spinal cord.

(a) Growth of an interneuron axon toward and across the midline of the spinal cord(diagrammed here in cross section)

(b) Growth of a motor neuron axon awayfrom the midline of the spinal cord

• The genes and basic events involved in axon guidance are similar in invertebrates and vertebrates

• Knowledge of these events may be applied one day to stimulate axonal regrowth following CNS damage

Neural Stem Cells• The adult human brain contains stem cells that

can differentiate into mature neurons

Figure 48.34

10

m

• The induction of stem cell differentiation and the transplantation of cultured stem cells are potential methods for replacing neurons lost to trauma or disease

Diseases and Disorders of the Nervous System

• 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

Figure 48.35

Senile plaque Neurofibrillary tangle20 m

• 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