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Chapter 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

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Chapter 49

Chapter 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



  • Sensations and perceptions and transmit signals to the central nervous system

    • 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
Functions Performed by Sensory Receptors and transmit signals to the central nervous system

  • All stimuli represent forms of energy

  • Sensation involves converting this energy

    • Into a change in the membrane potential of sensory receptors



Weak and transmit signals to the central nervous systemmuscle stretch

Strongmuscle stretch

Muscle

Dendrites

Receptor potential

–50

–50

–70

–70

Stretchreceptor

Membranepotential (mV)

Action potentials

0

0

Axon

–70

–70

4

5

1

2

6

7

0

0

1

2

3

4

5

6

7

3

Time (sec)

Time (sec)

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.

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

Figure 49.2a

  • Two types of sensory receptors exhibit these functions

    • A stretch receptor in a crayfish


Fluid moving in and transmit signals to the central nervous systemone direction

Fluid moving in other direction

No fluidmovement

“Hairs” ofhair cell

Neuro-trans-mitter at synapse

Moreneuro-trans-mitter

Lessneuro-trans-mitter

–50

–50

Axon

–50

Receptor potential

–70

–70

–70

Membranepotential (mV)

Membranepotential (mV)

Membranepotential (mV)

Action potentials

0

0

0

–70

–70

–70

5

6

7

1

2

3

4

0

0

0

1

2

3

4

5

6

7

1

2

3

4

5

6

7

Time (sec)

Time (sec)

Time (sec)

(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

Figure 49.2b

  • 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


Sensory transduction
Sensory Transduction and transmit signals to the central nervous system

  • 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



Amplification
Amplification and transmit signals to the central nervous system

  • Amplification is the strengthening of stimulus energy

    • By cells in sensory pathways


Transmission
Transmission and transmit signals to the central nervous system

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

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



Integration
Integration and transmit signals to the central nervous system

  • The integration of sensory information

    • Begins as soon as the information is received

    • Occurs at all levels of the nervous system


  • Some receptor potentials and transmit signals to the central nervous system

    • Are integrated through summation

  • Another type of integration is sensory adaptation

    • A decrease in responsiveness during continued stimulation


Types of sensory receptors
Types of Sensory Receptors and transmit signals to the central nervous system

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

    • Mechanoreceptors

    • Chemoreceptors

    • Electromagnetic receptors

    • Thermoreceptors

    • Pain receptors


Mechanoreceptors
Mechanoreceptors and transmit signals to the central nervous system

  • Mechanoreceptors sense physical deformation

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


Cold and transmit signals to the central nervous system

Light touch

Pain

Hair

Heat

Epidermis

Dermis

Hair movement

Nerve

Strong pressure

Connective tissue

  • The mammalian sense of touch

    • Relies on mechanoreceptors that are the dendrites of sensory neurons

Figure 49.3


Chemoreceptors
Chemoreceptors and transmit signals to the central nervous system

  • Chemoreceptors include

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

    • Specific receptors that respond to individual kinds of molecules


0.1 mm and transmit signals to the central nervous system

  • Two of the most sensitive and specific chemoreceptors known

    • Are present in the antennae of the male silkworm moth

Figure 49.4


Electromagnetic receptors
Electromagnetic Receptors and transmit signals to the central nervous system

  • Electromagnetic receptors detect various forms of electromagnetic energy

    • Such as visible light, electricity, and magnetism


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.


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 lines

  • Thermoreceptors, which respond to heat or cold

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


Pain receptors
Pain Receptors lines

  • 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



Sensing gravity and sound in invertebrates

Ciliated equilibrium detect settling particles or moving fluidreceptor cells

Cilia

Statolith

Sensory nerve fibers

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


Tympanic equilibrium detect settling particles or moving fluidmembrane

1 mm

  • Many arthropods sense sounds with body hairs that vibrate

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

Figure 49.7


Hearing and equilibrium in mammals
Hearing and Equilibrium in Mammals equilibrium detect settling particles or moving fluid

  • In most terrestrial vertebrates

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


1 equilibrium detect settling particles or moving fluid

2

Overview of ear structure

The middle ear and inner ear

Incus

Semicircularcanals

Skullbones

Stapes

Middleear

Outer ear

Inner ear

Malleus

Auditory nerve,to brain

Pinna

Tympanicmembrane

Cochlea

Eustachian tube

Auditory canal

Ovalwindow

Eustachian tube

Tympanicmembrane

Tectorialmembrane

Hair cells

Roundwindow

Cochlear duct

Bone

Vestibular canal

Auditory nerve

Axons of sensory neurons

Basilarmembrane

To auditorynerve

Tympanic canal

Organ of Corti

4

3

The organ of Corti

The cochlea

  • Exploring the structure of the human ear

Figure 49.8


Hearing
Hearing equilibrium detect settling particles or moving fluid

  • 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


Cochlea equilibrium detect settling particles or moving fluid

Stapes

Axons ofsensoryneurons

Oval window

Vestibularcanal

Perilymph

Apex

Base

Roundwindow

Tympaniccanal

Basilar membrane

  • 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


  • The pressure waves in the vestibular canal equilibrium detect settling particles or moving fluid

    • 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


Cochlea equilibrium detect settling particles or moving fluid(uncoiled)

Apex(wide and flexible)

Basilarmembrane

500 Hz(low pitch)

1 kHz

2 kHz

4 kHz

8 kHz

16 kHz(high pitch)

Frequency producing maximum vibration

Base(narrow and stiff)

  • The cochlea can distinguish pitch

    • Because the basilar membrane is not uniform along its length

Figure 49.10



Equilibrium
Equilibrium equilibrium detect settling particles or moving fluid

  • Several of the organs of the inner ear

    • Detect body position and balance


Each canal has at its base a equilibrium detect settling particles or moving fluidswelling called an ampulla, containing a cluster of hair cells.

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

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.

Flowof endolymph

Flowof endolymph

Vestibular nerve

Cupula

Hairs

Haircell

Nervefibers

Vestibule

Utricle

Body movement

Saccule

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

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

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

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

    • Function in balance and equilibrium

Figure 49.11


Hearing and equilibrium in other vertebrates
Hearing and Equilibrium in Other Vertebrates equilibrium detect settling particles or moving fluid

  • Like other vertebrates, fishes and amphibians

    • Also have inner ears located near the brain



Lateral equilibrium detect settling particles or moving fluidline

Opening of lateralline canal

Lateral line canal

Scale

Epidermis

Neuromast

Lateral nerve

Segmental muscles of body wall

Cupula

Sensoryhairs

Supporting cell

Hair cell

Nerve fiber

  • The lateral line system contains mechanoreceptors

    • With hair cells that respond to water movement

Figure 49.12




EXPERIMENT hairs called sensilla 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.

To brain

Chemo-receptors

Sensillum

Microelectrode

To voltagerecorder

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.

Pore at tip

Pipette containingtest substance

Chemoreceptors

50

Number of action potentials

in first second of response

30

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

10

0

Honey

0.5 MSucrose

0.5 MNaCl

Meat

Figure 49.13

Stimulus


Taste in humans
Taste in Humans hairs called sensilla

  • 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 hairs called sensilla

Sugar molecule

Taste bud

Sensoryreceptorcells

Sensoryneuron

Tongue

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

Sugar

Adenylyl cyclase

G protein

Sugarreceptor

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

ATP

cAMP

Proteinkinase A

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

SENSORYRECEPTORCELL

K+

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

Synapticvesicle

—Ca2+

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

Neurotransmitter

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

Sensory neuron

  • Transduction in taste receptors

    • Occurs by several mechanisms

Figure 49.14


Smell in humans
Smell in Humans hairs called sensilla

  • Olfactory receptor cells

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


Brain hairs called sensilla

Action potentials

Odorant

Olfactory bulb

Nasal cavity

Bone

Epithelial cell

Odorantreceptors

Chemoreceptor

Plasmamembrane

Cilia

Figure 49.15

Odorant

Mucus

  • When odorant molecules bind to specific receptors

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



Vision in invertebrates
Vision in Invertebrates the animal kingdom

  • Most invertebrates

    • Have some sort of light-detecting organ


Light the animal kingdom

Light shining from the front is detected

Photoreceptor

Nerve to brain

Visual pigment

Screening pigment

Ocellus

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



(a) invertebrates The faceted eyes on the head of a fly,

photographed with a stereomicroscope.

2 mm

(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.

Cornea

Lens

Crystallinecone

Rhabdom

Photoreceptor

Axons

Ommatidium

  • Compound eyes are found in insects and crustaceans

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

Figure 49.17a–b


  • Single-lens eyes invertebrates

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

    • Work on a camera-like principle


The vertebrate visual system
The Vertebrate Visual System invertebrates

  • The eyes of vertebrates are camera-like

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


Structure of the eye
Structure of the Eye invertebrates

  • 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



Sclera invertebrates

Choroid

Retina

Ciliary body

Fovea (centerof visual field)

Suspensoryligament

Cornea

Iris

Opticnerve

Pupil

Aqueoushumor

Lens

Vitreous humor

Central artery and

vein of the retina

Optic disk(blind spot)

  • The structure of the vertebrate eye

Figure 49.18


Front view of lens invertebratesand ciliary muscle

Ciliary muscles contract, pullingborder of choroid toward lens

Lens (rounder)

Choroid

Retina

Suspensory ligaments relax

Ciliarymuscle

Lens becomes thicker and rounder, focusing on near objects

Suspensoryligaments

(a) Near vision (accommodation)

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

Lens (flatter)

Suspensory ligaments pull against lens

Lens becomes flatter, focusing on distant objects

(b) Distance vision

  • Humans and other mammals

    • Focus light by changing the shape of the lens

Figure 49.19a–b



Sensory transduction in the eye
Sensory Transduction in the Eye invertebrates

  • 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


Rod invertebrates

Outersegment

H

H

O

C

H

CH3

C

C

H3C

H

CH3

C

H2C

C

H

H

H2C

C

C

C

C

Disks

C

C

C

C

H

H

H

CH3

CH3

cis isomer

Insideof disk

Cell body

Enzymes

Light

Synapticterminal

H

H

CH3

C

CH3

H

H

H2C

C

H

H

H2C

C

C

C

C

C

O

C

C

C

C

C

C

H

CH3

CH3

H

CH3

CH3

Cytosol

trans isomer

Retinal

Rhodopsin

Opsin

(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.

(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.

Figure 49.20a, b

  • Rods contain the pigment rhodopsin

    • Which changes shape when it absorbs light


Processing visual information
Processing Visual Information invertebrates

  • The processing of visual information

    • Begins in the retina itself


Light invertebrates

EXTRACELLULARFLUID

INSIDE OF DISK

Active rhodopsin

PDE

Membranepotential (mV)

Plasmamembrane

CYTOSOL

0

Dark Light

cGMP

Inactive rhodopsin

Transducin

Disk membrane

– 40

GMP

– Hyper- polarization

Na+

– 70

3Transducinactivates theenzyme phos-phodiesterae(PDE).

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

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

1Light isomerizes retinal, which activates rhodopsin.

Time

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

Na+

  • Absorption of light by retinal

    • Triggers a signal transduction pathway

Figure 49.21


  • In the dark, both rods and cones invertebrates

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


Dark Responses invertebrates

Light Responses

Rhodopsin active

Rhodopsin inactive

Na+ channels closed

Na+ channels open

Rod hyperpolarized

Rod depolarized

Glutamate

released

No glutamate

released

Bipolar cell either

hyperpolarized or

depolarized, depending on glutamate receptors

Bipolar cell either

depolarized or

hyperpolarized,

depending on

glutamate receptors

  • 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


Retina invertebrates

Optic nerve

Tobrain

Retina

Photoreceptors

Neurons

Cone

Rod

Amacrinecell

Horizontalcell

Opticnervefibers

Pigmentedepithelium

Ganglioncell

Bipolarcell

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

    • Ganglion cells, horizontal cells, and amacrine cells

Figure 49.23


Left invertebrates

visual

field

Right

visual

field

Left

eye

Right

eye

Optic nerve

Optic chiasm

Lateralgeniculatenucleus

Primaryvisual cortex

  • 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




Types of skeletons
Types of Skeletons protection, and movement

  • 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
Hydrostatic Skeletons protection, and movement

  • 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


Circular protection, and movement

muscle

contracted

Longitudinal

muscle relaxed

(extended)

Circular

muscle

relaxed

Longitudinal

muscle

contracted

(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.)

Head

Bristles

(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.

  • Annelids use their hydrostatic skeleton for peristalsis

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

Figure 49.25a–c


Exoskeletons
Exoskeletons protection, and movement

  • An exoskeleton is a hard encasement

    • Deposited on the surface of an animal

  • Exoskeletons

    • Are found in most molluscs and arthropods


Endoskeletons
Endoskeletons protection, and movement

  • 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



Head of protection, and movementhumerus

key

Examplesof joints

Axial skeleton

Skull

Appendicularskeleton

Scapula

1

Clavicle

Shouldergirdle

Scapula

Sternum

1Ball-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.

Rib

2

Humerus

3

Vertebra

Radius

Ulna

Humerus

Pelvicgirdle

Carpals

Ulna

Phalanges

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

Metacarpals

Femur

Patella

Tibia

Fibula

Ulna

Radius

Tarsals

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

Metatarsals

Phalanges

  • The human skeleton

Figure 49.26


Physical support on land
Physical Support on Land protection, and movement

  • In addition to the skeleton

    • Muscles and tendons help support large land vertebrates



Human protection, and movement

Grasshopper

Extensormusclerelaxes

Bicepscontracts

Tibiaflexes

Flexormusclecontracts

Tricepsrelaxes

Forearmflexes

Extensormusclecontracts

Tibiaextends

Bicepsrelaxes

Forearmextends

Flexormusclerelaxes

Triceps

contracts

  • Skeletal muscles are attached to the skeleton in antagonistic pairs

    • With each member of the pair working against each other

Figure 49.27


Vertebrate skeletal muscle

Muscle protection, and movement

Bundle ofmuscle fibers

Nuclei

Single muscle fiber

(cell)

Plasma membrane

Myofibril

Z line

Lightband

Dark band

Sarcomere

TEM

0.5 m

A band

I band

I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zone

Z line

Z line

Sarcomere

Vertebrate Skeletal Muscle

  • Vertebrate skeletal muscle

    • Is characterized by a hierarchy of smaller and smaller units

Figure 49.28






The sliding filament model of muscle contraction
The Sliding-Filament Model of Muscle Contraction protection, and movement

  • 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


0.5 m protection, and movement

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

Z

H

A

Sarcomere

(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.

  • As a result of this sliding

    • The I band and the H zone shrink

Figure 49.29a–c


  • The sliding of filaments is based on protection, and movement

    • 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


Thick filament protection, and movement

Thin filaments

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

5 Binding of a new mole-

cule of ATP releases the

myosin head from actin,

and a new cycle begins.

Thin filament

Myosin head (low-energy configuration)

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

ATP

2

ATP

Cross-bridge binding site

Thick filament

P

Actin

Thin filament moves toward center of sarcomere.

Myosin head (high-energy configuration)

ADP

Myosin head (low-energy configuration)

P i

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

3

ADP

+

Cross-bridge

ADP

P i

P i

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

4

P

  • Myosin-actin interactions underlying muscle fiber contraction

Figure 49.30


The role of calcium and regulatory proteins
The Role of Calcium and Regulatory Proteins protection, and movement

  • A skeletal muscle fiber contracts

    • Only when stimulated by a motor neuron


Tropomyosin protection, and movement

Ca2+-binding sites

Actin

Troponin complex

(a) Myosin-binding sites blocked

  • When a muscle is at rest

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

Figure 49.31a


Ca protection, and movement2+

Myosin-binding site

(b) Myosin-binding sites exposed

  • 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


Motor protection, and movementneuron axon

Mitochondrion

Synapticterminal

T tubule

Sarcoplasmicreticulum

Ca2+ releasedfrom sarcoplasmicreticulum

Myofibril

Sarcomere

Plasma membraneof muscle fiber

  • 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



  • Action potentials travel to the interior of the muscle fiber protection, and movement

    • 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


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.

Synapticterminalof motorneuron

1

PLASMA MEMBRANE

Synaptic cleft

T TUBULE

Action potential is propa-

gated along plasma

membrane and down

T tubules.

2

ACh

SR

4

Action potential

triggers Ca2+

release from sarco-

plasmic reticulum

(SR).

3

Ca2

Calcium ions bind to troponin;

troponin changes shape,

removing blocking action

of tropomyosin; myosin-binding

sites exposed.

Tropomyosin blockage of myosin-

binding sites is restored; contraction

ends, and muscle fiber relaxes.

7

Ca2

CYTOSOL

Cytosolic Ca2+ is

removed by active

transport into

SR after action

potential ends.

6

ADP

P2

Myosin cross-bridges alternately attach

to actin and detach, pulling actin

filaments toward center of sarcomere;

ATP powers sliding of filaments.

5

  • Review of contraction in a skeletal muscle fiber

Figure 49.33


Neural control of muscle tension
Neural Control of Muscle Tension across synaptic

  • Contraction of a whole muscle is graded

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



Motor produces graded contractions of whole musclesunit 1

Motorunit 2

Spinal cord

Synaptic terminals

Nerve

Motor neuroncell body

Motor neuronaxon

Muscle

Muscle fibers

Tendon

  • 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


  • A motor unit produces graded contractions of whole muscles

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

  • Recruitment of multiple motor neurons

    • Results in stronger contractions


Tetanus produces graded contractions of whole muscles

Tension

Summation of two twitches

Singletwitch

Time

Actionpotential

Pair ofactionpotentials

Series of action potentials at high frequency

  • 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



Types of muscle fibers
Types of Muscle Fibers produces graded contractions of whole muscles

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

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



Other types of muscle
Other Types of Muscle produces graded contractions of whole muscles

  • 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




Swimming
Swimming friction and gravity

  • 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
Locomotion on Land friction and gravity

  • Walking, running, hopping, or crawling on land

    • Requires an animal to support itself and move against gravity


Figure 49.36


Flying
Flying friction and gravity

  • Flight requires that wings develop enough lift

    • To overcome the downward force of gravity


EXPERIMENT friction and gravity

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.

RESULTS

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.

Flying

Running

102

10

Energy cost (J/Kg/m)

1

Swimming

10–1

10–3

1

103

106

Body mass(g)

Comparing Costs of Locomotion

  • The energy cost of locomotion

    • Depends on the mode of locomotion and the environment

CONCLUSION

For 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.

CONCLUSION

Figure 49.37



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