48 nervous text

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

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 48Chapter 48

Nervous Systems


• Overview: 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


• Functional magnetic resonance imaging

– Is a technology that can reconstruct a three-dimensional map of brain activity

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


• Concept 48.1: 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 supporting cells

– That are essential for the structural integrity of the nervous system and for the normal functioning of neurons


• 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

Figure 48.8


• Concept 48.2: 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


• 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 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


• Concept 48.3: 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


• Concept 48.4: 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


• Concept 48.5: 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 system

Peripheralnervous 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 organsParasympathetic 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


• Concept 48.6: 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


• Concept 48.7: 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


PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 49Chapter 49

Sensory and Motor Mechanisms


• Overview: Sensing and Acting

• Bats use sonar to detect their prey

• Moths, a common prey for bats

– Can detect the bat’s sonar and attempt to flee

Figure 49.1


• Both of these organisms

– Have complex sensory systems that facilitate their survival

• The structures that make up these systems

– Have been transformed by evolution into diverse mechanisms that sense various stimuli and generate the appropriate physical movement


• Concept 49.1: Sensory receptors transduce stimulus energy and transmit signals to the central nervous system

• Sensations are action potentials

– That reach the brain via sensory neurons

• Once the brain is aware of sensations

– It interprets them, giving the perception of stimuli


• Sensations and perceptions

– Begin with sensory reception, the detection of stimuli by sensory receptors

• Exteroreceptors

– Detect stimuli coming from the outside of the body

• Interoreceptors

– Detect internal stimuli


Functions Performed by Sensory Receptors

• All stimuli represent forms of energy

• Sensation involves converting this energy

– Into a change in the membrane potential of sensory receptors


• Sensory receptors perform four functions in this process

– Sensory transduction, amplification, transmission, and integration


• Two types of sensory receptors exhibit these functions

– A stretch receptor in a crayfish

Figure 49.2a

(a)Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends, muscles and dendrites

stretch, producing a receptor potential in the stretch receptor. The receptor potential triggers action potentials in the axon of the stretch

receptor. A stronger stretch producesa larger receptor potential and higherrequency of action potentials.

Muscle

Dendrites

Stretchreceptor

Axon

Mem

bran

epo

tent

ial (

mV

)

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Action potentials

Receptor potential

Weakmuscle stretch

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Strongmuscle stretch


– A hair cell found in vertebrates

of action potentials in the sensory neuron. Bending in the other direction has the opposite effects. Thus, hair cells respond to the direction of motion as well as to its strength and speed.s

(b) Vertebrate hair cells have specialized cilia or microvilli (“hairs”) that bend when sur-rounding fluid moves. Each hair cell releases an excitatory neurotransmitter at a synapse

with a sensory neuron, which conducts action potentials to the CNS. Bending in one direction depolarizes the hair cell, causing it to release more neurotransmitter and increasing frequency

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Action potentials

No fluidmovement

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Receptor potential

Fluid moving inone direction

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Fluid moving in other direction

Mem

bran

epo

tent

ial (

mV

)

Mem

bran

epo

tent

ial (

mV

)

Mem

bran

epo

tent

ial (

mV

)

“Hairs” ofhair cell

Neuro-trans-mitter at synapse

Axon

Lessneuro-trans-mitter

Moreneuro-trans-mitter

Figure 49.2b


Sensory Transduction

• Sensory transduction is the conversion of stimulus energy

– Into a change in the membrane potential of a sensory receptor

• This change in the membrane potential

– Is known as a receptor potential


• Many sensory receptors are extremely sensitive

– With the ability to detect the smallest physical unit of stimulus possible


Amplification

• Amplification is the strengthening of stimulus energy

– By cells in sensory pathways


Transmission

• After energy in a stimulus has been transduced into a receptor potential

– Some sensory cells generate action potentials, which are transmitted to the CNS


• Sensory cells without axons

– Release neurotransmitters at synapses with sensory neurons


Integration

• The integration of sensory information

– Begins as soon as the information is received

– Occurs at all levels of the nervous system


• Some receptor potentials

– Are integrated through summation

• Another type of integration is sensory adaptation

– A decrease in responsiveness during continued stimulation


Types of Sensory Receptors

• Based on the energy they transduce, sensory receptors fall into five categories

– Mechanoreceptors

– Chemoreceptors

– Electromagnetic receptors

– Thermoreceptors

– Pain receptors


Mechanoreceptors

• Mechanoreceptors sense physical deformation

– Caused by stimuli such as pressure, stretch, motion, and sound


• The mammalian sense of touch

– Relies on mechanoreceptors that are the dendrites of sensory neurons

Figure 49.3

Heat

Light touch Pain

Cold

Hair

Nerve Connective tissue Hair movement Strong pressure

Dermis

Epidermis


Chemoreceptors

• Chemoreceptors include

– General receptors that transmit information about the total solute concentration of a solution

– Specific receptors that respond to individual kinds of molecules


• Two of the most sensitive and specific chemoreceptors known

– Are present in the antennae of the male silkworm moth

Figure 49.4 0.1

mm


Electromagnetic Receptors

• Electromagnetic receptors detect various forms of electromagnetic energy

– Such as visible light, electricity, and magnetism


• Some snakes have very sensitive infrared receptors

– That detect body heat of prey against a colder background

Figure 49.5a

(a) This rattlesnake and other pit vipers have a pair of infrared receptors,one between each eye and nostril. The organs are sensitive enoughto detect the infrared radiation emitted by a warm mouse a meter away. The snake moves its head from side to side until the radiation is detected equally by the two receptors, indicating that the mouse is straight ahead.


• Many mammals appear to use the Earth’s magnetic field lines

– To orient themselves as they migrate

Figure 49.5b

(b) Some migrating animals, such as these beluga whales, apparentlysense Earth’s magnetic field and use the information, along with other cues, for orientation.


Thermoreceptors

• Thermoreceptors, which respond to heat or cold

– Help regulate body temperature by signaling both surface and body core temperature


Pain Receptors

• In humans, pain receptors, also called nociceptors

– Are a class of naked dendrites in the epidermis

– Respond to excess heat, pressure, or specific classes of chemicals released from damaged or inflamed tissues


• Concept 49.2: The mechanoreceptors involved with hearing and equilibrium detect settling particles or moving fluid

• Hearing and the perception of body equilibrium

– Are related in most animals


Sensing Gravity and Sound in Invertebrates

• Most invertebrates have sensory organs called statocysts

– That contain mechanoreceptors and function in their sense of equilibrium

Figure 49.6

Ciliatedreceptor cells

CiliaStatolith

Sensory nerve fibers


• Many arthropods sense sounds with body hairs that vibrate

– Or with localized “ears” consisting of a tympanic membrane and receptor cells

Figure 49.7

1 mm

Tympanicmembrane


Hearing and Equilibrium in Mammals

• In most terrestrial vertebrates

– The sensory organs for hearing and equilibrium are closely associated in the ear


• Exploring the structure of the human ear

Figure 49.8

Pinna

Auditory canal

Eustachian tube

Tympanicmembrane

Stapes

Incus

Malleus

Skullbones

Semicircularcanals

Auditory nerve,to brain

Cochlea

Tympanicmembrane

Ovalwindow

Eustachian tube

Roundwindow

Vestibular canal

Tympanic canal

Auditory nerve

BoneCochlear duct

Hair cells Tectorialmembrane

Basilarmembrane

To auditorynerve

Axons of sensory neurons

1 Overview of ear structure 2 The middle ear and inner ear

4 The organ of Corti 3 The cochleaOrgan of Corti

Outer earMiddle

ear Inner ear


Hearing

• Vibrating objects create percussion waves in the air

– That cause the tympanic membrane to vibrate

• The three bones of the middle ear

– Transmit the vibrations to the oval window on the cochlea


• These vibrations create pressure waves in the fluid in the cochlea

– That travel through the vestibular canal and ultimately strike the round window

Figure 49.9

Cochlea

Stapes

Oval window

Apex

Axons ofsensoryneurons

Roundwindow Basilar

membrane

Tympaniccanal

Base

Vestibularcanal Perilymph


• The pressure waves in the vestibular canal

– Cause the basilar membrane to vibrate up and down causing its hair cells to bend

• The bending of the hair cells depolarizes their membranes

– Sending action potentials that travel via the auditory nerve to the brain


• The cochlea can distinguish pitch

– Because the basilar membrane is not uniform along its length

Cochlea(uncoiled)

Basilarmembrane

Apex(wide and flexible)

Base(narrow and stiff)

500 Hz(low pitch)1 kHz

2 kHz

4 kHz

8 kHz

16 kHz(high pitch)

Frequency producing maximum vibration

Figure 49.10


• Each region of the basilar membrane vibrates most vigorously

– At a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex


Equilibrium

• Several of the organs of the inner ear

– Detect body position and balance


• The utricle, saccule, and semicircular canals in the inner ear

– Function in balance and equilibrium

Figure 49.11

The semicircular canals, arranged in three spatial planes, detect angular movements of the head.

Body movement

Nervefibers

Each canal has at its base a swelling called an ampulla, containing a cluster of hair cells.

When the head changes its rateof rotation, inertia prevents endolymph in the semicircular canals from moving with the head, so the endolymph presses against the cupula, bending the hairs.

The utricle and saccule tell the brain which way is up and inform it of the body’s position or linear acceleration.

The hairs of the hair cells project into a gelatinous cap called the cupula.

Bending of the hairs increases the frequency of action potentials in sensory neurons in direct proportion to the amount of rotational acceleration.

Vestibule

Utricle

Saccule

Vestibular nerve

Flowof endolymph

Flowof endolymph

CupulaHairs

Haircell


Hearing and Equilibrium in Other Vertebrates

• Like other vertebrates, fishes and amphibians

– Also have inner ears located near the brain


• Most fishes and aquatic amphibians

– Also have a lateral line system along both sides of their body


• The lateral line system contains mechanoreceptors

– With hair cells that respond to water movement

Figure 49.12 Nerve fiber

Supporting cell

Cupula

Sensoryhairs

Hair cell

Segmental muscles of body wall Lateral nerve

ScaleEpidermis

Lateral line canal

Neuromast

Opening of lateralline canal

Lateralline


• Concept 49.3: The senses of taste and smell are closely related in most animals

• The perceptions of gustation (taste) and olfaction (smell)

– Are both dependent on chemoreceptors that detect specific chemicals in the environment


• The taste receptors of insects are located within sensory hairs called sensilla

– Which are located on the feet and in mouthparts


Figure 49.13

EXPERIMENT Insects taste using gustatory sensilla (hairs) on their feet and mouthparts. Each sensillum contains four chemoreceptors with dendrites that extend to a pore at the tip of the sensillum. To study the sensitivity of each chemoreceptor, researchers immobilized a blowfly (Phormia regina) by attaching it to a rod with wax. They then inserted the tip of a microelectrode into one sensillum to record action potentials in the chemoreceptors, while they used a pipette to touch the pore with various test substances.

Num

ber

of

act

ion

pot

en

tials

in f

irst

seco

nd

of r

esp

ons

e

CONCLUSION Any natural food probably stimulates multiple chemoreceptors. By integrating sensations, the insect’s brain can apparently distinguish a very large number of tastes.

To brain

Chemo-receptors

Pore at tip

Pipette containingtest substance

To voltagerecorder

Sensillum

Microelectrode

50

30

10

00.5 MNaCl

Meat 0.5 MSucrose

Honey

Stimulus

Chemoreceptors

RESULTS Each chemoreceptor is especially sensitive to a particular class of substance, but this specificity is relative; each cell can respond to some extent to a broad range of different chemical stimuli.


Taste in Humans

• The receptor cells for taste in humans

– Are modified epithelial cells organized into taste buds

• Five taste perceptions involve several signal transduction mechanisms

– Sweet, sour, salty, bitter, and umami (elicited by glutamate)


Taste pore Sugar molecule

Sensoryreceptorcells

Sensoryneuron

Taste bud

Tongue

G protein Adenylyl cyclase

—Ca2+

ATP

cAMP

Proteinkinase A

Sugar

Sugarreceptor

SENSORYRECEPTORCELL Synaptic

vesicle

K+

Neurotransmitter

Sensory neuron

• Transduction in taste receptors

– Occurs by several mechanisms

Figure 49.14

4 The decrease in the membrane’s permeability to K+ depolarizes the membrane.

5 Depolarization opens voltage-gated calcium ion (Ca2+) channels, and Ca2+ diffuses into the receptor cell.

6 The increased Ca2+ concentration causes synaptic vesicles to release neurotransmitter.

3 Activated protein kinase A closes K+ channels in the membrane.

2 Binding initiates a signal transduction pathway involving cyclic AMP and protein kinase A.

1 A sugar molecule binds to a receptor protein on the sensory receptor cell.


Smell in Humans

• Olfactory receptor cells

– Are neurons that line the upper portion of the nasal cavity


• When odorant molecules bind to specific receptors

– A signal transduction pathway is triggered, sending action potentials to the brain

Brain

Nasal cavity

Odorant

Odorantreceptors

Plasmamembrane

Odorant

Cilia

Chemoreceptor

Epithelial cell

Bone

Olfactory bulb

Action potentials

MucusFigure 49.15


• Concept 49.4: Similar mechanisms underlie vision throughout the animal kingdom

• Many types of light detectors

– Have evolved in the animal kingdom and may be homologous


Vision in Invertebrates

• Most invertebrates

– Have some sort of light-detecting organ


Light

Light shining from the front is detected

Photoreceptor

Visual pigment

Ocellus

Nerve to brain

Screening pigment

Light shining from behind is blockedby the screening pigment

• One of the simplest is the eye cup of planarians

– Which provides information about light intensity and direction but does not form images

Figure 49.16


• Two major types of image-forming eyes have evolved in invertebrates

– The compound eye and the single-lens eye


• Compound eyes are found in insects and crustaceans

– And consist of up to several thousand light detectors called ommatidia

Figure 49.17a–b

Cornea

Crystallinecone

Rhabdom

PhotoreceptorAxons

Ommatidium

Lens

2 m

m

(a) The faceted eyes on the head of a fly,photographed with a stereomicroscope.

(b) The cornea and crystalline cone of each ommatidium function as a lens that focuses light on the rhabdom, a stack of pigmented plates inside a circle of photoreceptors. The rhabdom traps light and guides it to photoreceptors. The image formed by a compound eye is a mosaic of dots produced by different intensities of light entering the many ommatidia from different angles.


• Single-lens eyes

– Are found in some jellies, polychaetes, spiders, and many molluscs

– Work on a camera-like principle


The Vertebrate Visual System

• The eyes of vertebrates are camera-like

– But they evolved independently and differ from the single-lens eyes of invertebrates


Structure of the Eye

• The main parts of the vertebrate eye are

– The sclera, which includes the cornea

– The choroid, a pigmented layer

– The conjunctiva, that covers the outer surface of the sclera


– The iris, which regulates the pupil

– The retina, which contains photoreceptors

– The lens, which focuses light on the retina


• The structure of the vertebrate eye

Figure 49.18

Ciliary body

Iris

Suspensoryligament

Cornea

Pupil

Aqueoushumor

Lens

Vitreous humor

Optic disk(blind spot)

Central artery andvein of the retina

Opticnerve

Fovea (centerof visual field)

Retina

ChoroidSclera


• Humans and other mammals

– Focus light by changing the shape of the lens

Figure 49.19a–b

Lens (flatter)

Lens (rounder)

Ciliarymuscle

Suspensoryligaments

Choroid

Retina

Front view of lensand ciliary muscleCiliary muscles contract, pulling

border of choroid toward lens

Suspensory ligaments relax

Lens becomes thicker and rounder, focusing on near objects

(a) Near vision (accommodation)

(b) Distance vision

Ciliary muscles relax, and border of choroid moves away from lens

Suspensory ligaments pull against lens

Lens becomes flatter, focusing on distant objects


• The human retina contains two types of photoreceptors

– Rods are sensitive to light but do not distinguish colors

– Cones distinguish colors but are not as sensitive


Sensory Transduction in the Eye

• Each rod or cone in the vertebrate retina

– Contains visual pigments that consist of a light-absorbing molecule called retinal bonded to a protein called opsin


• Rods contain the pigment rhodopsin

– Which changes shape when it absorbs light

Figure 49.20a, b

Rod

Outersegment

Cell body

Synapticterminal

Disks

Insideof disk

(a) Rods contain the visual pigment rhodopsin, which is embedded in a stack of membranous disks in the rod’s outer segment. Rhodopsin consists of the light-absorbing molecule retinal bonded to opsin, a protein. Opsin has seven helices that span the disk membrane.

(b) Retinal exists as two isomers. Absorption of light converts the cis isomer to the trans isomer, which causes opsin to change its conformation (shape). After a few minutes, retinal detaches from opsin. In the dark, enzymes convert retinal back to its cis form, which recombines with opsin to form rhodopsin.

Retinal

OpsinRhodopsin

Cytosol

HC

CH2C

CH2C C

HCH3

CH3

H

CC

CH3 H CH3

CC

CC

CC

C

H

H

H

H

OH

H3C

HC

CH2C

CH2C C

HCH3

CH3

H

CC

CH3 H CH3

CC

CC

HH

CH3

H

CC

CH

O

CH3

trans isomer

cis isomer

EnzymesLight


Processing Visual Information

• The processing of visual information

– Begins in the retina itself


• Absorption of light by retinal

– Triggers a signal transduction pathway

Figure 49.21

EXTRACELLULARFLUID

Membranepotential (mV)

0

– 40

– 70

Dark Light

– Hyper- polarization

Time

Na+

Na+

cGMP

CYTOSOL

GMP

Plasmamembrane

INSIDE OF DISK

PDEActive rhodopsin

Light

Inactive rhodopsin Transducin Disk membrane

2 Active rhodopsin in turn activates a G protein called transducin.

3 Transducinactivates theenzyme phos-phodiesterae(PDE).

4 Activated PDE detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP.

5 The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases, and the rod hyperpolarizes.

1 Light isomerizes retinal, which activates rhodopsin.


• In the dark, both rods and cones

– Release the neurotransmitter glutamate into the synapses with neurons called bipolar cells, which are either hyperpolarized or depolarized


• In the light, rods and cones hyperpolarize

– Shutting off their release of glutamate

• The bipolar cells

– Are then either depolarized or hyperpolarized

Figure 49.22

Dark Responses

Rhodopsin inactive

Na+ channels open

Rod depolarized

Glutamatereleased

Bipolar cell eitherdepolarized orhyperpolarized,depending onglutamate receptors

Light Responses

Rhodopsin active

Na+ channels closed

Rod hyperpolarized

No glutamatereleased

Bipolar cell eitherhyperpolarized ordepolarized, depending on glutamate receptors


• Three other types of neurons contribute to information processing in the retina

– Ganglion cells, horizontal cells, and amacrine cells

Figure 49.23Opticnervefibers

Ganglioncell

Bipolarcell

Horizontalcell

Amacrinecell

Pigmentedepithelium

NeuronsCone Rod

Photoreceptors

Retina

Retina

Optic nerve

Tobrain


• Signals from rods and cones

– Travel from bipolar cells to ganglion cells

• The axons of ganglion cells are part of the optic nerve

– That transmit information to the brain

Figure 49.24

Leftvisualfield

Rightvisualfield

Lefteye

Righteye

Optic nerve

Optic chiasm

Lateralgeniculatenucleus

Primaryvisual cortex


• Most ganglion cell axons lead to the lateral geniculate nuclei of the thalamus

– Which relays information to the primary visual cortex

• Several integrating centers in the cerebral cortex

– Are active in creating visual perceptions


• Concept 49.5: Animal skeletons function in support, protection, and movement

• The various types of animal movements

– All result from muscles working against some type of skeleton


Types of Skeletons

• The three main functions of a skeleton are

– Support, protection, and movement

• The three main types of skeletons are

– Hydrostatic skeletons, exoskeletons, and endoskeletons


Hydrostatic Skeletons

• A hydrostatic skeleton

– Consists of fluid held under pressure in a closed body compartment

• This is the main type of skeleton

– In most cnidarians, flatworms, nematodes, and annelids


• Annelids use their hydrostatic skeleton for peristalsis

– A type of movement on land produced by rhythmic waves of muscle contractions

Figure 49.25a–c

(a) Body segments at the head and just in front of the rear are short and thick (longitudinal muscles contracted; circular muscles relaxed) and anchored to the ground by bristles. The other segments are thin and elongated (circular muscles contracted; longitudinal muscles relaxed.)

(b) The head has moved forward because circular muscles in the head segments have contracted. Segments behind the head and at the rear are now thick and anchored, thus preventing the worm from slipping backward.

(c) The head segments are thick again and anchored in their new positions. The rear segments have released their hold on the ground and have been pulled forward.

Longitudinalmuscle relaxed(extended)

Circularmusclecontracted

Circularmusclerelaxed

Longitudinalmusclecontracted

HeadBristles


Exoskeletons

• An exoskeleton is a hard encasement

– Deposited on the surface of an animal

• Exoskeletons

– Are found in most molluscs and arthropods


Endoskeletons

• An endoskeleton consists of hard supporting elements

– Such as bones, buried within the soft tissue of an animal

• Endoskeletons

– Are found in sponges, echinoderms, and chordates


• The mammalian skeleton is built from more than 200 bones

– Some fused together and others connected at joints by ligaments that allow freedom of movement


• The human skeleton

Figure 49.26

1 Ball-and-socket joints, where the humerus contactsthe shoulder girdle and where the femur contacts thepelvic girdle, enable us to rotate our arms andlegs and move them in several planes.

2 Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.

3 Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.

keyAxial skeletonAppendicularskeleton

Skull

Shouldergirdle

Clavicle

Scapula

Sternum

RibHumerus

Vertebra

RadiusUlnaPelvicgirdle

Carpals

Phalanges

Metacarpals

Femur

Patella

Tibia

Fibula

TarsalsMetatarsalsPhalanges

1

Examplesof joints

2

3

Head ofhumerus

Scapula

Humerus

Ulna

UlnaRadius


Physical Support on Land

• In addition to the skeleton

– Muscles and tendons help support large land vertebrates


• Concept 49.6: Muscles move skeletal parts by contracting

• The action of a muscle

– Is always to contract


• Skeletal muscles are attached to the skeleton in antagonistic pairs

– With each member of the pair working against each other

Figure 49.27

Human Grasshopper

Bicepscontracts

Tricepsrelaxes

Forearmflexes

Bicepsrelaxes

Tricepscontracts

Forearmextends

Extensormusclerelaxes

Flexormusclecontracts

Tibiaflexes

Extensormusclecontracts

Flexormusclerelaxes

Tibiaextends


Vertebrate Skeletal Muscle

• Vertebrate skeletal muscle

– Is characterized by a hierarchy of smaller and smaller units

Figure 49.28

Muscle

Bundle ofmuscle fibers

Single muscle fiber(cell)

Plasma membrane

Myofibril

Lightband Dark band

Z line

Sarcomere

TEM 0.5 mI band A band I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zoneSarcomere

Z lineZ line

Nuclei


• A skeletal muscle consists of a bundle of long fibers

– Running parallel to the length of the muscle

• A muscle fiber

– Is itself a bundle of smaller myofibrils arranged longitudinally


• The myofibrils are composed to two kinds of myofilaments

– Thin filaments, consisting of two strands of actin and one strand of regulatory protein

– Thick filaments, staggered arrays of myosin molecules


• Skeletal muscle is also called striated muscle

– Because the regular arrangement of the myofilaments creates a pattern of light and dark bands


• Each repeating unit is a sarcomere

– Bordered by Z lines

• The areas that contain the myofilments

– Are the I band, A band, and H zone


The Sliding-Filament Model of Muscle Contraction

• According to the sliding-filament model of muscle contraction

– The filaments slide past each other longitudinally, producing more overlap between the thin and thick filaments


• As a result of this sliding

– The I band and the H zone shrink

Figure 49.29a–c

(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.

(b) Contracting muscle fiber. During contraction, the thick andthin filaments slide past each other, reducing the width of theI bands and H zone and shortening the sarcomere.

(c) Fully contracted muscle fiber. In a fully contracted musclefiber, the sarcomere is shorter still. The thin filaments overlap,eliminating the H zone. The I bands disappear as the ends ofthe thick filaments contact the Z lines.

0.5 m

Z HA

Sarcomere


• The sliding of filaments is based on

– The interaction between the actin and myosin molecules of the thick and thin filaments

• The “head” of a myosin molecule binds to an actin filament

– Forming a cross-bridge and pulling the thin filament toward the center of the sarcomere


• Myosin-actin interactions underlying muscle fiber contraction

Figure 49.30

Thick filament

Thin filaments

Thin filament

ATPATP

ADPADP

ADP

P i P i

P i

Cross-bridge

Myosin head (low-energy configuration)

Myosin head (high-energy configuration)

+

Myosin head (low-energy configuration)

Thin filament moves toward center of sarcomere.

Thick filament

ActinCross-bridge binding site

1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.

2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.

P

1 The myosin head binds toactin, forming a cross-bridge.

3

4 Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.

P

5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.


The Role of Calcium and Regulatory Proteins

• A skeletal muscle fiber contracts

– Only when stimulated by a motor neuron


• When a muscle is at rest

– The myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin

Figure 49.31a

ActinTropomyosin Ca2+-binding sites

Troponin complex

(a) Myosin-binding sites blocked


• For a muscle fiber to contract

– The myosin-binding sites must be uncovered

• This occurs when calcium ions (Ca2+)

– Bind to another set of regulatory proteins, the troponin complex

Figure 49.31b

Ca2+

Myosin-binding site

(b) Myosin-binding sites exposed


• The stimulus leading to the contraction of a skeletal muscle fiber

– Is an action potential in a motor neuron that makes a synapse with the muscle fiber

Figure 49.32

Motorneuron axon

Mitochondrion

Synapticterminal

T tubule

Sarcoplasmicreticulum

Myofibril

Plasma membraneof muscle fiber

Sarcomere

Ca2+ releasedfrom sarcoplasmicreticulum


• The synaptic terminal of the motor neuron

– Releases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potential


• Action potentials travel to the interior of the muscle fiber

– Along infoldings of the plasma membrane called transverse (T) tubules

• The action potential along the T tubules

– Causes the sarcoplasmic reticulum to release Ca2+

• The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments

– Exposing the myosin-binding sites and allowing the cross-bridge cycle to proceed


ACh

Synapticterminalof motorneuron

Synaptic cleft T TUBULEPLASMA MEMBRANE

SR

ADP

CYTOSOL

Ca2

Ca2

P2

Cytosolic Ca2+ is removed by active transport into SR after action potential ends.

6

• Review of contraction in a skeletal muscle fiber

Figure 49.33

Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.

1

Action potential is propa-gated along plasmamembrane and downT tubules.

2

Action potentialtriggers Ca2+

release from sarco-plasmic reticulum(SR).

3

Myosin cross-bridges alternately attachto actin and detach, pulling actinfilaments toward center of sarcomere;ATP powers sliding of filaments.

5

Calcium ions bind to troponin;troponin changes shape,removing blocking actionof tropomyosin; myosin-bindingsites exposed.

4

Tropomyosin blockage of myosin-binding sites is restored; contractionends, and muscle fiber relaxes.

7


Neural Control of Muscle Tension

• Contraction of a whole muscle is graded

– Which means that we can voluntarily alter the extent and strength of its contraction


• There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles

– By varying the number of fibers that contract

– By varying the rate at which muscle fibers are stimulated


• In a vertebrate skeletal muscle

– Each branched muscle fiber is innervated by only one motor neuron

• Each motor neuron

– May synapse with multiple muscle fibers

Figure 49.34

Spinal cord

Nerve

Motor neuroncell body

Motorunit 1

Motorunit 2

Motor neuronaxon

Muscle

Tendon

Synaptic terminals

Muscle fibers


• A motor unit

– Consists of a single motor neuron and all the muscle fibers it controls

• Recruitment of multiple motor neurons

– Results in stronger contractions


• A twitch

– Results from a single action potential in a motor neuron

• More rapidly delivered action potentials

– Produce a graded contraction by summation

Figure 49.35

Actionpotential Pair of

actionpotentials

Series of action potentials at

high frequency

Time

Ten

sion

Singletwitch

Summation of two twitches

Tetanus


• Tetanus is a state of smooth and sustained contraction

– Produced when motor neurons deliver a volley of action potentials


Types of Muscle Fibers

• Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic

– Based on their contraction speed and major pathway for producing ATP


• Types of skeletal muscles


Other Types of Muscle

• Cardiac muscle, found only in the heart

– Consists of striated cells that are electrically connected by intercalated discs

– Can generate action potentials without neural input


• In smooth muscle, found mainly in the walls of hollow organs

– The contractions are relatively slow and may be initiated by the muscles themselves

• In addition, contractions may be caused by

– Stimulation from neurons in the autonomic nervous system


• Concept 49.7: Locomotion requires energy to overcome friction and gravity

• Movement is a hallmark of all animals

– And usually necessary for finding food or evading predators

• Locomotion

– Is active travel from place to place


Swimming

• Overcoming friction

– Is a major problem for swimmers

• Overcoming gravity is less of a problem for swimmers

– Than for animals that move on land or fly


Locomotion on Land

• Walking, running, hopping, or crawling on land

– Requires an animal to support itself and move against gravity


• Diverse adaptations for traveling on land

– Have evolved in various vertebrates

Figure 49.36


Flying

• Flight requires that wings develop enough lift

– To overcome the downward force of gravity


CONCLUSIONFor animals of a given

body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.

FlyingRunning

Swimming

10–3 103 1061

10–1

10

102

1

Body mass(g)

En

erg

y co

st (

J/K

g/m

)CONCLUSION

This graph compares the energy cost, in joules per kilogram of body mass per meter traveled, for animals specialized for running, flying, and swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.

RESULTS

Physiologists typically determine an animal’s rate of energy use during locomotion by measuring its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that collects the air the bird exhales as it flies.

EXPERIMENT

•The energy cost of locomotion

–Depends on the mode of locomotion and the environment

Figure 49.37

Comparing Costs of Locomotion


• Animals that are specialized for swimming

– Expend less energy per meter traveled than equivalently sized animals specialized for flying or running

48 nervous text

Documents

cns figure

circuits of neurons

axon figure

clusters of neurons

neurons organelles

themotor neurons

nerve cells

cell body figure