49 Senses Text

8/14/2019 49 Senses Text

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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 49

Sensory and Motor

Mechanisms


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Overview: Sensing and Acting

Bats use sonar to detect their prey

Moths, a common prey for bats

Can detect the bats sonar and attempt to flee

Figure 49.1


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Both of these organisms

Have complex sensory systems that facilitatetheir survival

The structures that make up these systems

Have been transformed by evolution intodiverse mechanisms that sense various stimuliand generate the appropriate physicalmovement


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Concept 49.1: Sensory receptors transduce

stimulus energy and transmit signals to thecentral 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


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Sensations and perceptions

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

Exteroreceptors

Detect stimuli coming from the outside of thebody

Interoreceptors Detect internal stimuli


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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


7/113Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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 dendritesembedded in abdominal muscles. When theabdomen bends, muscles and dendrites

stretch, producing a receptor potential in thestretch receptor. The receptor potential triggersaction potentials in the axon of the stretch

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

Muscle

Dendrites

Stretchreceptor

Axon M e m

b r a n e

p o

t e n

t i a l ( m

V )

50

70

0

70

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

Action potentials

Receptor potential

Weak

muscle stretch

50

70

0

70

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

Strong

muscle stretch



A hair cell found in vertebrates

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

(b) Vertebrate hair cells have specialized ciliaor microvilli (hairs) that bend when sur-rounding fluid moves. Each hair cell releasesan excitatory neurotransmitter at a synapse

with a sensory neuron, which conducts actionpotentials to the CNS. Bending in one directiondepolarizes the hair cell, causing it to releasemore 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 inother direction

M e m

b r a n e

p o

t e n

t i a l ( m

V )

M e m

b r a n e

p o

t e n

t i a l ( m

V )

M e m

b r a n e

p o

t e n

t i a l ( m

V )

Hairs of hair cell

Neuro-trans-mitter atsynapse

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 withsensory neurons



Integration

The integration of sensory information

Begins as soon as the information is received

Occurs at all levels of the nervous system


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Some receptor potentials

Are integrated through summation

Another type of integration is sensoryadaptation

A decrease in responsiveness duringcontinued stimulation


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Types of Sensory Receptors

Based on the energy they transduce, sensory

receptors fall into five categories Mechanoreceptors

Chemoreceptors

Electromagnetic receptors

Thermoreceptors

Pain receptors


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The mammalian sense of touch

Relies on mechanoreceptors that are thedendrites of sensory neurons

Figure 49.3

Heat

Light touch Pain

Cold

Hair

Nerve Connective tissue Hair movement Strong pressure

Dermis

Epidermis


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Chemoreceptors

Chemoreceptors include

General receptors that transmit informationabout the total solute concentration of asolution

Specific receptors that respond to individualkinds of molecules


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Two of the most sensitive and specific

chemoreceptors known Are present in the antennae of the male

silkworm moth

Figure 49.4 0 . 1

m m


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Electromagnetic Receptors

Electromagnetic receptors detect various forms

of electromagnetic energy Such as visible light, electricity, and

magnetism


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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 detectedequally by the two receptors, indicating that the mouse is straight ahead.


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Many mammals appear to use the Earths

magnetic field lines To orient themselves as they migrate

Figure 49.5b

(b) Some migrating animals, such as these beluga whales, apparentlysense Earths magnetic field and use the information, along withother cues, for orientation.


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Thermoreceptors

Thermoreceptors, which respond to heat or cold

Help regulate body temperature by signalingboth surface and body core temperature


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Pain Receptors

In humans, pain receptors, also callednociceptors

Are a class of naked dendrites in the epidermis

Respond to excess heat, pressure, or specificclasses of chemicals released from damagedor inflamed tissues


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Concept 49.2: The mechanoreceptors involvedwith hearing and equilibrium detect settlingparticles or moving fluid

Hearing and the perception of body equilibrium

Are related in most animals


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Sensing Gravity and Sound in Invertebrates

Most invertebrates have sensory organs calledstatocysts

That contain mechanoreceptors and function intheir sense of equilibrium

Figure 49.6

Ciliatedreceptor cells

CiliaStatolith

Sensory nerve fibers


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Many arthropods sense sounds with body hairsthat vibrate

Or with localized ears consisting of atympanic membrane and receptor cells

Figure 49.7

1 mm

Tympanicmembrane


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Hearing and Equilibrium in Mammals

In most terrestrial vertebrates

The sensory organs for hearing andequilibrium are closely associated in the ear


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Exploring the structure of the human ear

Figure 49.8

Pinna

Auditorycanal

Eustachiantube

Tympanicmembrane

Stapes

Incus

Malleus

Skullbones

Semicircular canals

Auditory nerve,to brain

Cochlea

Tympanicmembrane

Ovalwindow

Eustachiantube

Roundwindow

Vestibular canal

Tympanic canal

Auditory nerve

BoneCochlear duct

Hair cells Tectorialmembrane

Basilar membrane

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 ear Middle

ear Inner ear


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Hearing

Vibrating objects create percussion waves inthe air

That cause the tympanic membrane to vibrate

The three bones of the middle ear

Transmit the vibrations to the oval window onthe cochlea


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The cochlea can distinguish pitch

Because the basilar membrane is not uniformalong its length

Cochlea(uncoiled)

Basilar membrane

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 maximumvibration

Figure 49.10


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Each region of the basilar membrane vibratesmost vigorously

At a particular frequency and leads toexcitation of a specific auditory area of thecerebral cortex

E ilib i


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Equilibrium

Several of the organs of the inner ear

Detect body position and balance


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The utricle, saccule, and semicircular canals inthe inner ear

Function in balance and equilibrium

Figure 49.11

The semicircular canals, arranged in threespatial planes, detect angular movementsof the head.

Body movement

Nervefibers

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

When the head changes its rateof rotation, inertia preventsendolymph 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 whichway is up and inform it of the bodysposition or linear acceleration.

The hairs of the hair cellsproject into a gelatinous capcalled the cupula.

Bending of the hairs increases thefrequency of action potentials insensory neurons in directproportion to the amount of

rotational acceleration.

Vestibule

Utricle

Saccule

Vestibular nerve

Flowof endolymph

Flowof endolymph

CupulaHairs

Hair cell

H i g d E ilib i i Oth V t b t


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Hearing and Equilibrium in Other Vertebrates

Like other vertebrates, fishes and amphibians

Also have inner ears located near the brain


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Most fishes and aquatic amphibians

Also have a lateral line system along bothsides of their body


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The lateral line system containsmechanoreceptors

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

Scale EpidermisLateral line canal

Neuromast

Opening of lateralline canal

Lateralline


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Concept 49.3: The senses of taste and smellare closely related in most animals

The perceptions of gustation (taste) andolfaction (smell)

Are both dependent on chemoreceptors thatdetect specific chemicals in the environment


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The taste receptors of insects are locatedwithin sensory hairs called sensilla

Which are located on the feet and inmouthparts


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Figure 49.13

EXPERIMENT Insects taste using gustatory sensilla (hairs) on their feet andmouthparts. Each sensillum contains four chemoreceptors with dendrites thatextend to a pore at the tip of the sensillum. To study the sensitivity of eachchemoreceptor, researchers immobilized a blowfly ( Phormia regina ) by attachingit to a rod with wax. They then inserted the tip of a microelectrode into onesensillum to record action potentials in the chemoreceptors, while they used apipette to touch the pore with various test substances.

N u m

b e r o

f a c t

i o n p o

t e n

t i a l s

i n f i r s t s e c o n d

o f r e s p o n s e

CONCLUSION Any natural food probably stimulates multiple chemoreceptors. Byintegrating sensations, the insects brain can apparently distinguish a very largenumber of tastes.

To brain

Chemo-receptors

Pore at tip

Pipette containingtest substance

To voltagerecorder

Sensillum

Microelectrode

50

30

10

00.5 M NaCl

Meat 0.5 M Sucrose

Honey

Stimulus

Chemoreceptors

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

Taste in Humans


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Taste in Humans

The receptor cells for taste in humans

Are modified epithelial cells organized intotaste buds

Five taste perceptions involve several signaltransduction mechanisms

Sweet, sour, salty, bitter, and umami (elicitedby glutamate)


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Taste pore Sugar molecule

Sensoryreceptor cells

Sensoryneuron

Taste bud

Tongue

G protein Adenylyl cyclase

Ca 2+

ATP

cAMP

Proteinkinase A

Sugar

Sugar receptor

SENSORYRECEPTORCELL Synaptic

vesicle

K+

Neurotransmitter

Sensory neuron

Transduction in taste receptors

Occurs by several mechanisms

Figure 49.14

4 The decrease in the membranes permeability toK+ depolarizes the membrane.

5 Depolarization opens voltage-gated calcium ion(Ca 2+) channels, and Ca 2+ diffuses into the receptor cell.

6 The increased Ca 2+ concentration causessynaptic vesicles to release neurotransmitter.

3 Activated protein kinase A closes K + channels inthe membrane.

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

1 A sugar molecule binds

to a receptor protein onthe sensory receptor cell.

Smell in Humans


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Smell in Humans

Olfactory receptor cells

Are neurons that line the upper portion of thenasal cavity


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When odorant molecules bind to specificreceptors

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


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Concept 49.4: Similar mechanisms underlievision throughout the animal kingdom

Many types of light detectors

Have evolved in the animal kingdom and maybe homologous

Vision in Invertebrates


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Vision in Invertebrates

Most invertebrates

Have some sort of light-detecting organ


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Light

Light shining fromthe front is detected

Photoreceptor

Visual pigment

Ocellus

Nerve tobrain

Screeningpigment

Light shining frombehind is blockedby the screening pigment

One of the simplest is the eye cup of planarians

Which provides information about lightintensity and direction but does not formimages

Figure 49.16


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Two major types of image-forming eyes haveevolved in invertebrates

The compound eye and the single-lens eye


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Compound eyes are found in insects andcrustaceans

And consist of up to several thousand lightdetectors called ommatidia

Figure 49.17ab

Cornea

Crystallinecone

Rhabdom

Photoreceptor Axons

Ommatidium

Lens

2 m m

(a) The faceted eyes on thehead of a fly,

photographed witha stereomicroscope.

(b) The cornea and crystalline cone of each ommatidium function asa lens that focuses light on therhabdom, a stack of pigmented

plates inside a circle of photoreceptors. The rhabdomtraps light and guides it tophotoreceptors. The imageformed by a compound eye is amosaic of dots produced by differentintensities of light entering themany ommatidia from different angles.


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Structure of the Eye


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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 surfaceof the sclera


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The iris, which regulates the pupil

The retina, which contains photoreceptors The lens, which focuses light on the retina


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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 (center of visual field)

Retina

ChoroidSclera


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The human retina contains two types of photoreceptors

Rods are sensitive to light but do notdistinguish colors

Cones distinguish colors but are not assensitive

Sensory Transduction in the Eye


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y y

Each rod or cone in the vertebrate retina

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


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Rods contain the pigment rhodopsin

Which changes shape when it absorbs light

Figure 49.20a, b

Rod

Outer segment

Cell body

Synapticterminal

Disks

Insideof disk

(a) Rods contain the visual pigment rhodopsin, which is embedded ina stack of membranous disks in the rods outer segment.Rhodopsin consists of the light-absorbing molecule retinalbonded to opsin, a protein. Opsin has seven helices that spanthe disk membrane.

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

Retinal

OpsinRhodopsin

Cytosol

HC

CH2C

CH2C C

HCH 3

CH 3H

CC

CH 3 H CH 3

CC

CC

CC

C

H

H

H

H

OH

H3C

HC

CH2C

CH

2C C

HCH 3

CH 3H

CC

CH 3 H CH 3

C C C C

HH

CH 3

H

C C CHO

CH 3

trans isomer

cis isomer

EnzymesLight

Processing Visual Information


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g

The processing of visual information

Begins in the retina itself


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In the dark, both rods and cones

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


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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 either depolarized or hyperpolarized,depending onglutamate receptors

Light Responses

Rhodopsin active

Na + channels closed

Rod hyperpolarized

No glutamatereleased

Bipolar cell either hyperpolarized or depolarized,depending onglutamate receptors


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Three other types of neurons contribute toinformation processing in the retina

Ganglion cells, horizontal cells, and amacrinecells

Figure 49.23Opticnervefibers

Ganglioncell

Bipolar cell

Horizontalcell

Amacrinecell

Pigmentedepithelium

NeuronsCone Rod

Photoreceptors

Retina

Retina

Optic nerve

Tobrain


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Signals from rods and cones

Travel from bipolar cells to ganglion cells

The axons of ganglion cells are part of the opticnerve

That transmit information to the brain

Figure 49.24

Leftvisualfield

Rightvisualfield

Lefteye

Righteye

Optic nerve

Optic chiasm

Lateralgeniculatenucleus

Primaryvisual cortex


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Most ganglion cell axons lead to the lateralgeniculate nuclei of the thalamus

Which relays information to the primary visualcortex

Several integrating centers in the cerebralcortex

Are active in creating visual perceptions


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Concept 49.5: Animal skeletons function insupport, protection, and movement

The various types of animal movements

All result from muscles working against some

type of skeleton


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Hydrostatic Skeletons


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A hydrostatic skeleton

Consists of fluid held under pressure in aclosed body compartment

This is the main type of skeleton

In most cnidarians, flatworms, nematodes, andannelids


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Annelids use their hydrostatic skeleton for peristalsis

A type of movement on land produced byrhythmic waves of muscle contractions

Figure 49.25ac

(a) Body segments at the head and just in frontof the rear are short and thick (longitudinalmuscles contracted; circular musclesrelaxed) and anchored to the ground bybristles. The other segments are thin andelongated (circular muscles contracted;longitudinal muscles relaxed.)

(b) The head has moved forward because

circular muscles in the head segments havecontracted. Segments behind the head andat the rear are now thick and anchored, thuspreventing the worm from slipping backward.

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

Longitudinalmuscle relaxed(extended)

Circular musclecontracted

Circular musclerelaxed

Longitudinalmusclecontracted

HeadBristles

Exoskeletons


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An exoskeleton is a hard encasement

Deposited on the surface of an animal

Exoskeletons

Are found in most molluscs and arthropods

Endoskeletons


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An endoskeleton consists of hard supportingelements

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

Endoskeletons Are found in sponges, echinoderms, and

chordates


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The mammalian skeleton is built from morethan 200 bones

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


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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 skeletonAppendicular skeleton

Skull

Shoulder girdle

Clavicle

Scapula

Sternum

RibHumerus

Vertebra

Radius

UlnaPelvicgirdle

Carpals

Phalanges

Metacarpals

Femur Patella

Tibia

Fibula

TarsalsMetatarsalsPhalanges

1

Examplesof joints

2

3

Head of humerus

Scapula

Humerus

Ulna

UlnaRadius

Physical Support on Land


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In addition to the skeleton

Muscles and tendons help support large landvertebrates


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Skeletal muscles are attached to the skeletonin antagonistic pairs

With each member of the pair working againsteach other

Figure 49.27

Human Grasshopper

Bicepscontracts

Tricepsrelaxes

Forearmflexes

Bicepsrelaxes

Tricepscontracts

Forearmextends

Extensor musclerelaxes

Flexor musclecontracts

Tibiaflexes

Extensor musclecontracts

Flexor musclerelaxes

Tibiaextends

Vertebrate Skeletal Muscle


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Vertebrate skeletal muscle

Is characterized by a hierarchy of smaller andsmaller units

Figure 49.28

Muscle

Bundle of muscle 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


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A skeletal muscle consists of a bundle of longfibers

Running parallel to the length of the muscle

A muscle fiber

Is itself a bundle of smaller myofibrils arrangedlongitudinally


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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 myosinmolecules


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Skeletal muscle is also called striated muscle

Because the regular arrangement of themyofilaments creates a pattern of light anddark bands


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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


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According to the sliding-filament model of muscle contraction

The filaments slide past each other longitudinally, producing more overlapbetween the thin and thick filaments


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As a result of this sliding

The I band and the H zone shrink

Figure 49.29ac

(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands

and 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 of the thick filaments contact the Z lines.

0.5 m

Z HA

Sarcomere


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The sliding of filaments is based on

The interaction between the actin and myosinmolecules 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


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Myosin-actin interactions underlying musclefiber 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 movestoward center of sarcomere.

Thickfilament

ActinCross-bridgebinding site

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

2 The myosin head hydrolyzesATP to ADP and inorganicphosphate ( I ) and is in itshigh-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 themyosin head from actin,

and a new cycle begins.

The Role of Calcium and Regulatory Proteins


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A skeletal muscle fiber contracts

Only when stimulated by a motor neuron


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When a muscle is at rest

The myosin-binding sites on the thin filamentare blocked by the regulatory proteintropomyosin

Figure 49.31a

ActinTropomyosin Ca 2+-binding sites

Troponin complex

(a) Myosin-binding sites blocked


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For a muscle fiber to contract

The myosin-binding sites must be uncovered This occurs when calcium ions (Ca 2+)

Bind to another set of regulatory proteins, thetroponin complex

Figure 49.31b

Ca 2+

Myosin-binding site

(b) Myosin-binding sites exposed


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The stimulus leading to the contraction of askeletal muscle fiber

Is an action potential in a motor neuron thatmakes a synapse with the muscle fiber

Figure 49.32

Motor neuron axon

Mitochondrion

Synapticterminal

T tubule

Sarcoplasmicreticulum

Myofibril

Plasma membraneof muscle fiber

Sarcomere

Ca 2+ releasedfrom sarcoplasmicreticulum


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The synaptic terminal of the motor neuron

Releases the neurotransmitter acetylcholine,depolarizing the muscle and causing it toproduce an action potential


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ACh

Synapticterminalof motor

neuronSynaptic cleft T TUBULE

PLASMA MEMBRANE

SR

ADP

CYTOSOL

Ca 2+

Ca 2+

P 2

Cytosolic Ca 2+ isremoved by activetransport intoSR after actionpotential 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 fibers plasma membrane,triggering an action potential in muscle fiber.

1

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

2

Action potentialtriggers Ca 2+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


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Contraction of a whole muscle is graded

Which means that we can voluntarily alter theextent and strength of its contraction


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There are two basic mechanisms by which thenervous system produces graded contractionsof whole muscles

By varying the number of fibers that contract

By varying the rate at which muscle fibers arestimulated


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In a vertebrate skeletal muscle

Each branched muscle fiber is innervated byonly one motor neuron

Each motor neuron

May synapse with multiple muscle fibers

Figure 49.34

Spinal cord

Nerve

Motor neuroncell body

Motor unit 1

Motor unit 2

Motor neuronaxon

Muscle

Tendon

Synaptic terminals

Muscle fibers


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A motor unit

Consists of a single motor neuron and all themuscle fibers it controls

Recruitment of multiple motor neurons

Results in stronger contractions


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A twitch

Results from a single action potential in amotor neuron

More rapidly delivered action potentials

Produce a graded contraction by summation

Figure 49.35

Actionpotential Pair of

actionpotentials

Series of actionpotentials at

high frequency

Time

T e n s i o n

Singletwitch

Summation of two twitches

Tetanus


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Tetanus is a state of smooth and sustainedcontraction

Produced when motor neurons deliver a volleyof action potentials

Types of Muscle Fibers


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Skeletal muscle fibers are classified as slowoxidative, fast oxidative, and fast glycolytic

Based on their contraction speed and major pathway for producing ATP


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Types of skeletal muscles


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C 49 7 L i i


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Concept 49.7: Locomotion requires energy toovercome 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

O i f i i


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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

W lki i h i li l d


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Walking, running, hopping, or crawling on land

Requires an animal to support itself and moveagainst gravity

Di d i f li l d


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Diverse adaptations for traveling on land

Have evolved in various vertebrates

Figure 49.36

Flying

Fli ht i th t i d l h lift


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Flight requires that wings develop enough lift

To overcome the downward force of gravity


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A i l th t i li d f i i g


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Animals that are specialized for swimming

Expend less energy per meter traveled thanequivalently sized animals specialized for flying or running

49 Senses Text

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