chapter 28 nervous system

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

Chapter 28

Nervous System


Can an Injured Spinal Cord Be Fixed?

• Injuries to the spinal cord

– Disrupt communication between the central nervous system (brain and spinal cord) and the rest of the body

Spinalcord


• The late actor Christopher Reeve

– Suffered a spinal cord injury during an equestrian competition

– Was an influential advocate for spinal cord research


NERVOUS SYSTEM STRUCTURE AND FUNCTION

28.1 Nervous systems receive sensory input, interpret it, and send out appropriate commands

• Nervous systems

– Are the most intricately organized data-processing systems on Earth


• The nervous system obtains and processes sensory information

– And sends commands to effector cells, such as muscles, that carry out appropriate responses

Sensory input

Sensory receptor

Effector cells

Motor output

Integration

Peripheral nervous system (PNS)

Brain and spinal cord

Central nervoussystem (CNS)Figure 28.1A


• Sensory neurons

– Conduct signals from sensory receptors to the central nervous system (CNS)

• The CNS

– Consists of the brain and spinal cord


• Interneurons in the CNS

– Integrate information and send it to motor neurons

• Motor neurons

– Convey signals to effector cells


Quadricepsmuscles

Flexormuscles

Brain

Spinalcord

Nerve

PNS

Ganglion

CNS

• Automatic responses called reflexes

– Provide an example of nervous system function

Interneuron

4

Sensory neuron2

Motorneuron 3

1 Sensory receptor

Figure 28.1B


• Located outside the CNS, the peripheral nervous system (PNS)

– Consists of nerves (bundles of fibers of sensory and motor neurons) and ganglia (clusters of cell bodies of the neurons)


28.2 Neurons are the functional units of nervous systems

• Neurons

– Are cells specialized for carrying signals


• A neuron consists of

– A cell body

– Two types of extensions (fibers) that conduct signals, dendrites and axons

Schwann cell

Signal direction Dendrites

Cell body

Nucleus

Axon

Schwann cell

Signalpathway

Myelin sheath

Nodes ofRanvier

Node of RanvierLayers of myelinin sheath

Nucleus

Synaptic terminals

Cell body

SE

M 3

,600

Figure 28.2


• Many axons are enclosed by cellular insulation called the myelin sheath

– Which speeds up signal transmission


NERVE SIGNALS AND THEIR TRANSMISSION

28.3 A neuron maintains a membrane potential across its membrane

• At rest, a neuron’s plasma membrane

– Has an electrical voltage called the resting potentialVoltmeter

Plasmamembrane Microelectrode

outside cell– 70 mV

Axon

Neuron

Microelectrodeinside cell

+ + + + + + + +

– – – – – – – –

– – – – – – – –

+ + + + + + + +

Figure 28.3A


• The resting potential

– Is caused by the membrane’s ability to maintain a positive charge on its outer surface opposing a negative charge on its inner surface

Outside of cell Na+

Na+Na+

K+

Na+

Na+

Na+K+ Na+ Na+

Na+Na+

Na+Na+

K+

Protein

Na+

Plasmamembrane

K+ channel

K+

K+

K+K+K+

K+

K+

K+

Na+ - K+

pump

Na+

Na+

K+

Na+

channel

Inside of cellFigure 28.3B


28.4 A nerve signal begins as a change in the membrane potential

• A stimulus alters the permeability of a portion of the membrane

– Allowing ions to pass through and changing the membrane’s voltage


• A nerve signal, called an action potential

– Is a change in the membrane voltage from the resting potential to a maximum level and back to the resting potential

3

2

1

4

5

1

Na+

+ + + + + + +

– – + + – – –

– – + + – – –

+ + + + + + +

Additional Na+ channels open, K+ channels are closed; interior of cell becomes more positive.

A stimulus opens some Na+ channels; if threshold is reached, action potential is triggered.

Resting state: voltage-gated Na+ and K+ channels closed; resting potential is maintained.

Na+ channels close and inactivate. K+ channels open, and K+ rushes out; interior of cell more negative than outside.

The K+ channels closerelatively slowly, causinga brief undershoot.

Return to resting state.

Na+

+ + + + + + +

– – + – – – –

– – + – – – –

+ + + + + + +

+ + + + + + + +

– – – – – – – –

– – – – – – – –

+ + + + + + + +

+ + + + + + +

– – – – – – –

– – – – – – –

+ + + + + + +

+ + + + + + + +

– – – – – – – –

– – – – – – – –

+ + + + + + + +

Neuroninterior Neuron

interior

Actionpotential

Resting potential

Threshold

+50

0

– 50

– 100Time (msec)

K+

K+

Mem

bra

ne

pote

ntia

l(m

V) 3

1

2

4

51

Na+

Na+

Figure 28.4


28.5 The action potential propagates itself along the neuron

• Action potentials

– Are self-propagated in a one-way chain reaction along a neuron

– Are all-or-none events


Axon

Action potential

Action potential

Action potential

Axonsegment

Na

K

K

Na

K

K

Na

• Propagation of the action potential along an axon

1

2

3

Figure 28.5


• The frequency of action potentials, but not their strength

– Changes with the strength of the stimulus


28.6 Neurons communicate at synapses

• The transmission of signals between neurons

– Or between neurons and effector cells occurs at junctions called synapses


• Electrical signals

– Pass between cells at electrical synapses


• At chemical synapses

– The sending cell secretes a chemical signal, a neurotransmitter

• The neurotransmitter

– Crosses the synaptic cleft and binds to a receptor on the surface of the receiving cell


Sending neuron

Axon ofsendingneuron

Receivingneuron

Synapticcleft

Receivingneuron

Synapticterminal

Neurotransmittermolecules

Vesicles

Ion channels

Neurotransmitter brokendown and released

Ions

NeurotransmitterReceptor

Synapse

• Neuron communication

Vesicle fuseswith plasmamembrane

23

Neurotransmitteris released intosynaptic cleft

4Neurotransmitterbinds to receptor

1Actionpotentialarrives

5 Ion channel opens 6 Ion channel closesFigure 28.6


28.7 Chemical synapses make complex information processing possible

• A neuron may receive information

– From hundreds of other neurons via thousands of synaptic terminals

Dendrites

Myelinsheath

Axon

Synaptic terminals

Inhibitory

Receivingcell body

Excitatory

Synapticterminals

SE

M 5

,500

Figure 28.7


• Some neurotransmitters

– Excite the receiving cell

• Other types of neurotransmitters

– Inhibit the receiving cell’s activity by decreasing its ability to develop action potentials


• The summation of excitation and inhibition

– Determines whether or not a neuron will transmit a nerve signal


28.8 A variety of small molecules function as neurotransmitters

• Many small, nitrogen-containing molecules

– Serve as neurotransmitters


CONNECTION

28.9 Many drugs act at chemical synapses

• Many psychoactive drugs

– Act at synapses and affect neurotransmitter action

Figure 28.9


AN OVERVIEW OF ANIMAL NERVOUS SYSTEMS

28.10 Nervous system organization usually correlates with body symmetry

• Radially symmetrical animals

– Have a nervous system arranged in a weblike system of neurons called a nerve net

Nervenet

Neuron

A Hydra (cnidarian)Figure 28.10A


• Most bilaterally symmetrical animals exhibit

– Cephalization, the concentration of the nervous system in the head region

– Centralization, the presence of a central nervous system

B Flatworm (planarian)

Eyespot

Brain

Nervecord

Tranversenerve

E Squid (mollusc)

Brain

Giantaxon

Brain

Ventralnervecord

Segmentalganglion

C Leech (annelid)

Brain

Ventralnervecord

Ganglia

D Insect (arthropod)

Figure 28.10B–E


28.11 Vertebrate nervous systems are highly centralized and cephalized

Central nervoussystem (CNS)

Brain

Spinal cord

Peripheralnervoussystem (PNS)

Cranialnerves

Gangliaoutside CNS

Spinalnerves

Figure 28.11A


• The brain and spinal cord

– Contain fluid-filled spaces

BrainCerebrospinal fluid

Meninges

White matter

Gray matter Dorsal rootganglion(part of PNS)

Spinal nerve(part of PNS)

Central canal

Spinal cord(cross section)

Ventricles

Central canalof spinal cord

Spinal cord

Figure 28.11B


• Cranial and spinal nerves

– Make up the peripheral nervous system


28.12 The peripheral nervous system of vertebrates is a functional hierarchy

• The PNS can be divided into two functional components

– The somatic nervous system and the autonomic nervous system

Peripheralnervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision

Figure 28.12


• The somatic nervous system

– Carries signals to and from skeletal muscles, mainly in response to external stimuli

• The autonomic nervous system

– Regulates the internal environment by controlling smooth and cardiac muscles and the organs of various body systems


28.13 Opposing actions of sympathetic and parasympathetic neurons regulate the internal environment

• The parasympathetic division of the autonomic nervous system

– Primes the body for activities that gain and conserve energy for the body

• The sympathetic division of the autonomic nervous system

– Prepares the body for intense, energy-consuming activities


• The autonomic nervous system

Parasympathetic division Sympathetic division

Eye

Salivaryglands

Lung

Liver

Heart Adrenalgland

Pancreas

Stomach

Intestines

Bladder

Genitalia

Spinalcord

Brain

Constrictspupil

Stimulatessalivaproduction

Constrictsbronchi

Slowsheart

Stimulatesstomach,pancreas,and intestines

Stimulatesurination

Promoteserection ofgenitals

Promotesejaculation and vaginalcontractions

Inhibitsurination

Inhibitsstomach,pancreas,and intestines

Stimulatesglucose release

Stimulatesepinephrineand norepinep-hrine release

Acceleratesheart

Dilates bronchi

Inhibitssalivaproduction

Dilatespupil

Figure 28.13


28.14 The vertebrate brain develops from three anterior bulges of the neural tube

• The vertebrate brain

– Develops from the forebrain, midbrain, and hindbrain

EmbryonicBrain Regions

Brain StructuresPresent in Adult

Forebrain

Midbrain

Hindbrain

Midbrain Hindbrain

Forebrain

Embryo (one month old)

Cerebrum (cerebral hemispheres; includescerebral cortex, white matter, basal ganglia)

Diencephalon (thalamus, hypothalamus,posterior pituitary, pineal gland)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

Cerebralhemisphere Diencephalon

Midbrain

PonsCerebellum

Medullaoblongata

Spinal cord

Fetus (three months old)Figure 28.14


• The size and complexity of the cerebrum in birds and mammals

– Correlates with their sophisticated behavior


THE HUMAN BRAIN

28.15 The structure of a living supercomputer: The human brain

• The human brain

– Is more powerful than the most sophisticated computer


• The human brain is composed of three main parts

– The forebrain, the midbrain, and the hindbrain

Forebrain

Midbrain

Hindbrain

Cerebrum

Thalamus

Hypothalamus

Pituitary gland

Pons

Medullaoblongata

Cerebellum

Spinalcord

Cerebralcortex

Figure 28.15A


• Major structures of the human brain

Table 28.15


• The midbrain and subdivisions of the hindbrain, together with the thalamus and hypothalamus

– Function mainly in conducting information to and from higher brain centers

– Regulate homeostatic functions, keep track of body position, and sort sensory information


• The forebrain’s cerebrum

– Is the largest and most complex part of the brain


• Most of the cerebrum’s integrative power

– Resides in the cerebral cortex of the two cerebral hemispheres

Left cerebralhemisphere

Right cerebralhemisphere

Corpuscallosum

BasalgangliaFigure 28.15B


28.16 The cerebral cortex is a mosaic of specialized, interactive regions

• Specialized integrative regions of the cerebral cortex include

– The somatosensory cortex and centers for vision, hearing, taste, and smell

Frontal lobe Parietal lobe

Occipital lobeTemporal lobe

Frontalassociationarea M

otor

cor

tex

Som

atos

enso

ry c

orte

x

Somatosensory associationarea

Speech

TasteReading

Hearing

Smell

Auditory associationarea

Visual associationarea

Vision

Speech

Figure 28.16


• The motor cortex

– Directs responses

• Association areas

– Concerned with higher mental activities such as reasoning and language, make up most of the cerebrum

• The right and left cerebral hemispheres

– Tend to specialize in different mental tasks


CONNECTION

28.17 Injuries and brain operations provide insight into brain function

• Brain injuries and surgeries

– Have been used to study brain function

Figure 28.17A, B


28.18 Several parts of the brain regulate sleep and arousal

• Sleep and arousal involve

– Activity by the hypothalamus, medulla oblongata, pons, and neurons of the reticular formation


• The reticular formation

– Receives data from sensory receptors and sends useful data to the cerebral cortex

Eye

Reticular formation

Input from touch, pain,and temperature receptors

Input fromears

Data to cerebral cortex

Figure 28.18A


• An EEG

– Measures brain waves during sleep and arousal

Awake but quiet (alpha waves)

Awake during intense mental activity (beta waves)

Asleep

Delta waves REM sleep Delta wavesFigure 28.18B, C


28.19 The limbic system is involved in emotions, memory, and learning

• The limbic system

– Is a functional group of integrating centers in the cerebral cortex, thalamus, and hypothalamus

– Is involved in emotions, memory, and learning

Smell

Olfactorybulb Amygdala Hippocampus

Cerebrum

Thalamus

Hypothalamus

Prefrontalcortex

Figure 28.19


CONNECTION

20.20 Changes in brain physiology can produce neurological disorders


Schizophrenia

• Schizophrenia is a severe mental disturbance

– Characterized by psychotic episodes in which patients lose the ability to distinguish reality


Depression

• Two broad forms of depressive illness have been identified

– Major depression and bipolar disorder


• Selective serotonin reuptake inhibitors (SSRIs)

– Are prescribed to treat depression

140

120

100

80

60

40

20

01995 1996 1997 1998 1999 2000 2001 2002 2003

Year

Pre

scrip

tions

(m

illio

ns)

Figure 28.20A


Alzheimer’s Disease

• Alzheimer’s disease (AD)

– Is characterized by confusion, memory loss, and a variety of other symptoms

Senile plaque Neurofibrillary tangle

LM

25

0

Figure 28.20B


Parkinson’s Disease

• Parkinson’s disease is a motor disorder

– Characterized by difficulty in initiating movements, slowness of movement, and rigidity

Figure 28.20C

chapter 28 nervous system

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