Chapter 48/49

Nervous Systems

The human brain contains an estimated 100 billion nerve cells, or neurons Each neuron may communicate with thousands of

other neurons Complex information processing network is at work

Different neurons do different things

Functional magnetic resonance imaging (fMRI)can reconstruct a three-dimensional map of brain activity

The results of brain imaging and other research methods revealed 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 They have neurons arranged in nerve nets

Nerve net

(a) Hydra (cnidarian)

Sea Stars

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

Nervering

Radialnerve

(b) Sea star (echinoderm)

Flatworms In relatively simple cephalized animals, such

as flatworms a central nervous system (CNS) is evident

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

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

Anteriornerve ring

Longitudinalnerve cords

Ganglia

Brain

Ganglia

(f) Chiton (mollusc) (g) Squid (mollusc)

Molluscs

Nervous systems in molluscs correlate with the animals’ lifestyles

Sessile molluscs have simple systems More complex molluscs have more

sophisticated systems

Vertebrates

The central nervous system consists of a brain and dorsal spinal cord

The PNS connects to the CNS via nerves

Brain

Spinalcord(dorsalnervecord)

Sensoryganglion

(h) Salamander (chordate)

Information Processing

Nervous systems process information in three stages: Sensory input, integration, and motor output

Sensor

Effector

Motor output

Integration

Sensory input

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Sensory neurons transmit information from sensors (receptors) 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

Cells of the Nervous System

Neuron Structure Cell body Dendrites Axons Myelin Sheath

Dendrites

Cell body

Nucleus

Axon hillock

AxonSignal direction

Synapse

Myelin sheath

Synapticterminals

Presynaptic cell Postsynaptic cell

Schwann cells Synaptic Terminals Synapse Neurotransmitters

Types of neurons

Sensory Neurons Interneurons Motor Neurons

Supporting Cells (Glia)

Glia are supporting cells They are essential for the structural integrity of

the nervous system and for the normal functioning of 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 Resting Membrane Potential: membrane potential

of a neuron that is not transmitting signals The inside of a cell is negative relative to the

outside

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

Gated Ion Channels

Gated ion channels open or close In response to membrane stretch or the binding of

a specific ligand In response to a change in the membrane

potential

Action Potential

Action potentials are the signals conducted by nerve fibers

If a cell has gated ion channels Its membrane potential may change in response to

stimuli that open or close those channels

Hyperpolarization

Stimuli may trigger an increase in the magnitude of the membrane potential

+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.

Depolarization

stimuli may trigger a reduction in the magnitude of the membrane potential

+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

+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.

Action Potential

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

Step 1…Sodium gates

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

Step 2…Potassium gates

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

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

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.

– +– + + + + +

– +– + + + + +

+ –+ – + + + +

+ –+ – + + + +

+ –+ – – – – –

+ –+ – – – – –

– – – –

– – – –

– –

– –

+ +

+ +

+ ++ + – – – –

+ ++ + – – – –

– –– – + + + +

– –– – + + + +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+

Conduction of Action Potentials

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

Saltatory Conduction

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)

++ +

++ +

++ +

++

– –

– –

– –

–––

–

–

–

Synapse Electrical synapse

Electrical current flows directly from one cell to another via a gap junction

Chemical synapse A presynaptic neuron

releases chemical neurotransmitters, which are stored in the synaptic terminal

Postsynapticneuron

Synapticterminalof presynapticneurons

5 µ

m

Neurotransmitter release

When an action potential reaches a terminal the final result is the release of neurotransmitters into the synaptic cleft

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

Synaptic Transmission

Neurotransmitters bind to ligand-gated ion channels

Binding causes the ion channels to open, generating a postsynaptic potential and generation of action potential

Fate of neurotransmitter in the cleft Diffuses out of the synaptic cleft Taken up by surrounding cells and degraded by

enzymes Degraded in the cleft

Neurotransmitters

Different neurons may release different neurotransmitters

The same neurotransmitter can produce different effects in different types of cells

Neurotransmitters

Neurotransmitters

Acetylcholine Is one of the most common neurotransmitters in both

vertebrates and invertebrates Can be inhibitory or excitatory

Biogenic amines Include epinephrine, norepinephrine, dopamine, and

serotonin Are active in the CNS and PNS

Amino acids and peptides Are active in the brain

Gases such as nitric oxide and carbon monoxide Are local regulators in the PNS

Vertebrate Nervous System

Regionally specialized In all vertebrates, the

nervous system shows a high degree of cephalization and distinct CNS and PNS components

Central nervoussystem (CNS)

Peripheral nervoussystem (PNS)

Brain

Spinal cordCranialnerves

GangliaoutsideCNSSpinalnerves

Central Nervous System

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

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

PNS components

The somatic nervous system The autonomic nervous system

Peripheralnervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision

Somatic and Autonomic

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

Sympathetic and parasympathetic divisions They 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

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

The Brainstem The brainstem consists of three parts: the medulla

oblongata, the pons, and the midbrain

Brainstem

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

All three areas are center of reflex actions

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

Eye

Reticular formation

Input from touch, pain, and temperature receptors

Input from ears

The Cerebellum Important for

coordination and error checking during motor, perceptual, and cognitive functions

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

The Cerebrum

Cerebral Hemispheres The cerebrum has right and left cerebral

hemispheres that each consist of cerebral cortex overlying white matter and basal nuclei

Left cerebralhemisphere

Corpuscallosum

Cortex

Right cerebralhemisphere

Basalnuclei

Basal Nuclei

The basal nuclei Are important centers for planning and learning

movement sequences

Cerebral Cortex

In mammals The cerebral cortex has a convoluted surface

called the neocortex In humans it’s the largest and most complex

part of the brain: 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

Cerebral Cortex

The cerebral cortex controls voluntary movement and cognitive functions

Frontal, parietal, temporal, and occipitalFrontal 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

Information Processing in the Cerebral Cortex

Each of the lobes contains primary sensory areas and association areas

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

Hearingwords

Seeingwords

Speakingwords

Generatingwords

Max

MinPET scans showing brain activity levels

Language and Speech

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

Broca’s area damage: comprehend speech but have impaired ability to mechanically form speech

Wernicke’s area damage: no language comprehension, can’t understand either spoken or written language. Speak in rapid nonsensical manner

Emotions The limbic system is a ring of structures

around the brainstem

HypothalamusThalamus

Prefrontal cortex

Olfactorybulb

Amygdala Hippocampus

Limbic system

This limbic system includes three parts of the cerebral cortex: The amygdala, hippocampus, and olfactory bulb. Also some parts of the thalamus and hypothalamus

These structures interact with the neocortex to mediate primary emotions and attach emotional “feelings” to survival-related functions (reproduction, aggression, feeding)

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

CNS injuries and diseases

Unlike the PNS, the mammalian CNS Cannot repair itself when damaged or diseased

Research on nerve cell development and stem cells Hot area of research May one day make it possible for physicians to

repair or replace damaged neurons

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

10

m

Diseases and Disorders of the Nervous System

Mental illnesses and neurological disorders take a huge toll on society patient’s loss of a productive life high cost of long-term health care The direct costs of mental health services in the United States in 1996 totaled

$69.0 billion. This figure represents 7.3 percent of total health spending. An additional $17.7 billion was spent on Alzheimer’s disease The indirect costs of

mental illness were estimated in 1990 at $78.6 billion

Causes (genetic & environmental) are not often clear at present

Treatments that exist are not great and usually amount to control of symptoms and not cure for the underlying abnormality

Schizophrenia

About 1% of the world’s population suffers from schizophrenia

There are several forms each characterized by an inability to distinguish reality

Symptoms Hallucinations, delusions, blunted emotions, and

many other symptoms Treatments have focused on brain pathways

that use dopamine as a neurotransmitter

Depression

Two broad forms of depressive illness: 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 (amyloid protein) in the brain

Senile plaque Neurofibrillary tangle 20 m

Parkinson’s Disease

Parkinson’s disease is a motor disorder Caused by the death of dopamine-secreting

neurons in midbrain nucleus (substantia nigra) Characterized by difficulty in initiating movements,

slowness of movement, and rigidity

Multiple sclerosis

Caused by immune destruction of myelin sheaths

Leads to improper nerve impulse conduction and associated loss of function

Nerve toxins

Tetanus Blocks inhibitory synapses leading to spastic

paralysis Botulism

Prevents release of Acetylcholine leading to flaccid paralysis

Curare Prevents binding of Acetylcholine to receptors

leading to flaccid paralysis