chapter 49 sensory and motor mechanisms. copyright © 2005 pearson education, inc. publishing as...

Chapter 49Chapter 49

Sensory and Motor Mechanisms

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

• 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

chapter 49 sensory and motor mechanisms. copyright © 2005 pearson education, inc. publishing as...

Documents