Chapter 28

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

PowerPoint Lectures forBiology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon

Lectures by Chris Romero

Chapter 28Chapter 28

Nervous Systems


Can an Injured Spinal Cord Be Fixed?

• The spinal cord is the central communication conduit between the brain and the rest of the body

• Injuries to the spinal cord can produce paraplegia or quadriplegia

• The spinal cord cannot repair itself

• Researchers are working on ways to regenerate or replace damaged nerve cells

– Growth factor proteins

– Embryonic stem cells


NERVOUS SYSTEM STRUCTURE AND FUNCTION

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

• The nervous system obtains and processes sensory information and sends commands to effector cells

– Central nervous system (CNS): brain and spinal cord

– Peripheral nervous system (PNS): nerves that carry signals to and from CNS


• Nervous tissue

– Neuron: nerve cell specialized for carrying signals from one location in the body to another

– Nerve: bundle of neuron extensions wrapped in connective tissue

– Ganglia: clusters of neuron cell bodies; found in PNS


• A nervous system has three interconnected functions

– Sensory input: Sensory neurons conduct signals from sensory receptors to integration centers

– Integration: Interneurons interpret signals and formulate responses

– Motor output: Motor neurons conduct signals from integration centers to effector cells

• A reflex is an automatic response to stimuli

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Sensory receptor

Sensory input

Motor output

Integration

Effector cells

Brain and spinal cord

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

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Sensoryreceptor

Quadricepsmuscles

Flexormuscles

Sensory neuron

Ganglion

Motorneuron

Nerve

PNS

CNS

Spinalcord

Brain

Interneuron


28.2 Neurons are the functional units of nervous systems

• Neurons are cells specialized for carrying signals

– Cell body: contains most organelles

– Dendrites: highly branched extensions that carry signals from other neurons toward the cell body

– Axon: long extension that transmits signals to other cells


• Many axons are enclosed by an insulating myelin sheath

– Chain of Schwann cells

– Nodes of Ranvier: points where signals can be transmitted

– Speeds up signal transmission

• Supporting cells (glia) are essential for structural integrity and normal functioning of the nervous system


• The axon ends in a cluster of branches

– Each branch ends in a synaptic terminal

– A synapse is a site of communication between a synaptic terminal and another cell

LE 28-2LE 28-2

Signal direction Dendrites

Cell body

Nucleus

Axon

Signalpathway

Schwann cell

Nodes ofRanvier

Myelin sheath Synaptic terminals

Schwann cell

Nucleus

Node of RanvierLayers of myelinin sheath

SE

M 3

,60 0

Cell Body


NERVE SIGNALS AND THEIR TRANSMISSION

28.3 A neuron maintains a membrane potential across its membrane

• A resting neuron has potential energy

– Membrane potential: electrical charge difference across the neuron's plasma membrane

– Resting potential: voltage across the plasma membrane of a resting neuron


• The resting potential depends on differences in ionic composition inside and outside the cell

– More K+ than Na+ diffuses inward through membrane channels

– Sodium-potassium pumps actively transport Na+ out of cell and K+ in

– The ionic gradient produces a voltage across the membrane

• The basis of nervous system signals

Animation: Resting PotentialAnimation: Resting Potential Animation: Sodium Potassium PumpAnimation: Sodium Potassium Pump

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Plasmamembrane

Microelectrodeinside cell

Neuron

Axon

Microelectrodeoutside cell

Voltmeter

–70 mV

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Outside of cell

Inside of cell

Plasmamembrane

Na

channel

Na

Na

Na K

NaNa

Na

K

Na

Na

Na

K

Kchannel

K

Na

Na Na

K

Na

K

Na KK

K

K

Na-K

pump

K

Protein


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

• Electrical changes make up an action potential, a nerve signal that carries information along an axon

– Stimulus raises voltage from resting potential to threshold

– Action potential is triggered; membrane polarity reverses abruptly

– Membrane repolarizes; voltage drops

– Voltage undershoots and then returns to resting potential


• Cause of electrical changes of an action potential

– Movement of K+ and Na+ across the membrane

– Controlled by the opening and closing of voltage-gated channels

Animation: Action PotentialAnimation: Action Potential

LE 28-4-5LE 28-4-5

Actionpotential

50

0

50

100

Threshold

Resting potential

Time (msec)

Neuroninterior

Resting state: voltage-gated Na

and K channels closed; restingpotential is maintained.

Mem

bra

ne

po

ten

tial

(mV

)

A stimulus opens some Na

channels; if threshold is reached,action potential is triggered.

Na

Na

Additional Na channels open,K channels are closed; interior ofcell becomes more positive.

Na

Na

K

Na channels close and

inactivate. K channels

open, and K rushesout; interior of cell morenegative than outside.

K

The K channels closerelatively slowly, causinga brief undershoot.

Neuroninterior

Return to resting state.


28.5 The action potential propagates itself along the neuron

• An action potential transmits a signal in a domino effect

1. Na+ channels open, Na+ rushes inward

2. K+ channels open, K+ diffuses outward; Na+ channels are closed and inactivated

3. Membrane returns to resting potential


• Action potentials are propagated only from cell body to synaptic cleft

– Cannot be generated where K+ is leaving axon and Na+ channels are inactivated

• Action potentials are all-or-none events

– Same events occur no matter how strong or weak the stimulus

– Intensity of stimulus determines frequency of action potentials

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Action potential

Na

K

Axon

Action potential

Axonsegment

K

Na

K

Na

Action potential

K


28.6 Neurons communicate at synapses

• The transmission of signals occurs at synapses

– Junction between synaptic terminal and another cell

• Electrical synapse

– Electrical current passes directly from one neuron to the next

– Receiving neuron stimulated quickly and at same frequency as sending neuron

– Found in human heart and digestive tract


• Chemical synapse

1. Action potential arrives in sending neuron

2. Vesicle containing neurotransmitter fuses with plasma membrane

3. Neurotransmitter is released into synaptic cleft

4. Neurotransmitter binds to receptor on receiving neuron

– Following events vary with different types of chemical synapses

LE 28-6LE 28-6

Sending neuron

Vesicles

Synapticterminal

Axon ofsendingneuron

Synapticcleft

Receivingneuron

Vesicle fuseswith plasmamembrane

Neurotransmitteris released intosynaptic cleft

Actionpotentialarrives

Synapse

Neurotrans-mitter bindsto receptor

ReceptorNeurotransmitter

Ion channelsNeurotransmittermolecules

Receivingneuron

Ions

Ion channel opens Ion channel closes

Neurotransmitter brokendown and releases


Animation: SynapseAnimation: Synapse


28.7 Chemical synapses make complex information processing possible

• A neuron may receive information from hundreds of other neurons via thousands of synaptic terminals

• Some neurotransmitters excite the receiving cell

• Other neurotransmitters inhibit the receiving cell's activity by decreasing its ability to develop action potentials

• If excitatory signals are strong enough to initiate an action potential, a neuron will transmit a signal

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Dendrites

Myelinsheath

Axon

Receivingcell body

Inhibitory Excitatory

Synapticterminals

SE

M 5

,500

Synaptic terminals


28.8 A variety of small molecules function as neurotransmitters

• Many small, nitrogen-containing molecules serve as neurotransmitters

– Acetylcholine

• Important in brain and at synapses between motor neurons and muscles

– Biogenic amines

• Important in central nervous system

• Seratonin, dopamine


• Amino acids

– Important in central nervous system

• Peptides

– Substance P, endorphins influence perception of pain

• Dissolved gases

– NO functions during sexual arousal


CONNECTION

28.9 Many drugs act at chemical synapses

• Many psychoactive drugs act at synapses and affect neurotransmitter action

– Caffeine

– Nicotine

– Alcohol

– Psychoactive prescription drugs

– Stimulants

– THC (marijuana)

– Opiates


AN OVERVIEW OF ANIMAL NERVOUS SYSTEMS

28.10 Nervous system organization usually correlates with body symmetry

• Sponges have no nervous system

• Radially symmetrical animals

– Nervous system arranged in a weblike system of neurons called a nerve net

– Though uncentralized, not simple


• Most bilaterally symmetrical animals

– Tendency to move through environment headfirst

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

– Centralization, presence of a central nervous system

LE 28-10aLE 28-10a

Nervenet

Hydra (cnidarian)

Neuron

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Transversenerve

Flatworm (planarian)

Nervecord

Brain

Eyespot

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Segmentalganglion

Leech (annelid)

Ventralnervecord

Brain

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Ganglia

Insect (arthropod)

Ventralnervecord

Brain

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Giantaxon

Squid (mollusc)

Brain


28.11 Vertebrate nervous systems are highly centralized and cephalized

• Peripheral nervous system (PNS) includes cranial and spinal nerves and ganglia

• Central nervous system (CNS) made up of spinal cord and brain

– Spinal cord

• Inside vertebral column

• Conveys information from brain

• Integrates simple responses


– Brain, the master control center

• Homeostatic centers keep body functioning smoothly

• Sensory centers integrate data from sense organs

• Can include centers of emotion and intellect

• Sends motor commands to muscles

LE 28-11aLE 28-11aCentral nervoussystem (CNS)

Brain

Spinal cord

Peripheralnervoussystem (PNS)

Cranialnerves

GangliaoutsideCNS

Spinalnerves


• Vast network of blood vessels services the CNS

• Blood-brain barrier maintains a stable chemical environment for the brain

• Ventricles in brain are continuous with central canal of spinal cord

– Filled with cerebrospinal fluid

– Protected by meninges (layers of connective tissue)

• Two distinct areas in CNS

– White matter: axons

– Gray matter: nerve cell bodies and dendrites

LE 28-11bLE 28-11b

Brain

Cerebrospinal fluid

Meninges

Ventricles

Central canalof spinal cord

Central canal

White matter

Gray matterDorsal rootganglion(part of PNS)

Spinal cord(cross section)

Spinal nerve(part of PNS)

Spinal cord


28.12 The peripheral nervous system of vertebrates is a functional hierarchy

• The PNS has two functional components

– Somatic nervous system

• Carries signals to and from skeletal muscles

• Responds mainly to external stimuli

– Autonomic nervous system

• Regulates internal environment

• Controls smooth and cardiac muscle, various organs; involuntary

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Peripheralnervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision


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

• The autonomic nervous system has two sets of neurons with opposing effects

– Parasympathetic division

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

– Sympathetic division

• Prepares the body for intense, energy-consuming activities


• The enteric division is regulated by the sympathetic and parasympathetic divisions

– Controls the digestive process

• Sympathetic and parasympathetic neurons emerge from different regions of the CNS

– Use different neurotransmitters

• Somatic and autonomic components of PNS cooperate to maintain homeostasis

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Parasympathetic division Sympathetic division

EyeBrain

Spinalcord

Constrictspupil Salivary

glands

Lung

Stimulatessalivaproduction

Heart

Constrictsbronchi

Slowsheart

Liver

Stomach

Dilatespupil

Inhibitssalivaproduction

Dilatesbronchi

Acceleratesheart

Adrenalgland

Pancreas

Intestines

Bladder

Stimulatesstomach,pancreas,and intestines

Stimulatesurination

Stimulatesepinephrineand norepi-nephrine release

Stimulatesglucose release

Inhibitsstomach,pancreas,and intestines

Genitalia

Promoteserection ofgenitals

Inhibitsurination

Promotes ejacu-lation and vaginalcontractions


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

• Early embryonic divisions of the vertebrate brain develop into different adult structures

– Forebrain, midbrain, hindbrain

• Evolution of complex behavior paralleled increases in forebrain integrative power

• During embryonic development, most profound changes occur in the forebrain

• The cerebrum, an outgrowth of the forebrain, controls homeostasis and integration

LE 28-14LE 28-14Embryonic

Brain Regions

Forebrain

Midbrain

Hindbrain

Midbrain

Hindbrain

Forebrain

Cerebralhemisphere

Diencephalon

Midbrain

Pons

Cerebellum

Medullaoblongata

Spinal cord

Medulla oblongata (part of brainstem)

Pons (part of brainstem), cerebellum

Midbrain (part of brainstem)

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

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

Embryo (one month old) Fetus (three months old)

Brain StructuresPresent in Adult


THE HUMAN BRAIN

28.15 The structure of a living supercomputer: The human brain

• The human brain is composed of around 100 billion neurons and more supporting cells

• Hindbrain

– Pons and medulla oblongata

• Conduct information to and from other brain areas

• Control involuntary activities

• Help coordinate whole-body movement


– Cerebellum

• Coordinates movement of limbs

• Is responsible for learned motor responses

• Midbrain

– Integrates auditory information

– Coordinates visual reflexes

– Relays sensory data to higher brain centers

• Midbrain and hindbrain make up the brain stem


• Forebrain

– Thalamus

• Relays information to and from cerebral cortex

– Hypothalamus

• Regulates homeostasis

• Controls hormonal output of pituitary gland

• Serves as biological clock, regulating circadian rhythms


– Cerebrum

• Two cerebral hemispheres connected by corpus callosum

• Performs sophisticated integration

• Plays major role in memory, learning, speech, emotions

• Formulates complex behavioral responses

– Basal ganglia important in motor coordination

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Forebrain

Cerebrum

Thalamus

Hypothalamus

Pituitary gland

Midbrain

Hindbrain

Pons

Medullaoblongata

Cerebellum

Cerebralcortex

Spinalcord

LE 28-15bLE 28-15bLeft cerebralhemisphere

Right cerebralhemisphere

Basalganglia

Corpuscallosum


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

• The cerebral cortex occupies more than 80% of total brain mass

• The most distinctively human traits are produced in the cerebral cortex

• Right and left hemispheres are connected through the corpus callosum

– Lateralization specializes the two sides for different functions

– Each side has four lobes with different functional areas

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Frontal lobe Parietal lobe

Frontalassociationarea

Speech

Smell

Hearing

Taste

Speech

Reading

Somatosensoryassociationarea

Mo

tor

cort

exS

om

atos

enso

ryco

rtex

Visualassociationarea

Vision

Occipital lobeTemporal lobe

Auditoryassociationarea


CONNECTION

28.17 Injuries and brain operations provide insight into brain function

• Injuries have revealed how healthy brains operate

– Example: Phineas Gage

• PET scans, MRIs, and neurosurgery have enhanced understanding of brain function

– Example: hemispherectomy


28.18 Several parts of the brain regulate sleep and arousal

• Hypothalamus, medulla oblongata, and pons help regulate our sleep/wake cycles

• Sensory information sent from reticular formation to cortex makes us alert and aware

• EEG measures electrical activity in the brain during arousal and sleep

– In REM sleep, brain waves are similar to the awake state; most dreams occur

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Eye

Reticular formation

Input from touch, pain,and temperature receptors

Input fromears

Data to cerebralcortex

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Awake but quiet (alpha waves)

Awake during intense mental activity (beta waves)

Asleep

Delta waves Delta wavesREM sleep


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

• The limbic system is a group of integrating centers in the cerebral cortex, thalamus, and hypothalamus

– Amygdala

• Lays down emotional memories

• Acts as a memory filter

– Hippocampus

• Involved in formation and recall of memories

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Hypothalamus

ThalamusCerebrum

HippocampusAmygdalaOlfactorybulb

Smell

Prefrontalcortex


• Memory is the ability to store and retrieve information derived from experience

– Short-term memory: lasts only a few minutes

– Long-term memory: lasts weeks or longer

– Information can be transferred from short-term to long-term memory

– Factual memories are different from skill memories

• Information processing by the brain involves complex interplay of integrating centers


28.20 Changes in brain physiology can produce neurological disorders

• Schizophrenia is a severe mental disturbance

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

– Causes unknown, but research is looking for gene for predisposition

– Treated by drugs that affect the neurotransmitter dopamine


• Two broad forms of depressive illness have been identified

– Major depression leaves a person unable to live a normal life

– Bipolar disorder is characterized by extreme mood swings

– Most common treatment is selective serotonin reuptake inhibitors (SSRIs)

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140

120

100

80

60

40

20

01995 1996 1997 1998 1999 2000 2001 2002 2003

Year

Pre

scr

ipti

on

s (

mill

ion

s)


• Alzheimer's disease (AD) is a form of mental deterioration

– Characterized by confusion, memory loss, and many other symptoms

– Progressive; usually age related

– Neurofibrillary tangles and senile plaques destroy neurons in brain

– Currently no cure

LE 28-20bLE 28-20bSenile plaque Neurofibrillary tangle


• Parkinson's disease is a motor disorder

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

– Progressive; increases with age

– Symptoms result from death of neurons in midbrain

– Results from combination of environmental and genetic factors

– At present, no cure available

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