Download Sensory & Motor Mechanisms

Chapter 49
Sensory and Motor
Mechanisms
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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
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• 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
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• 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
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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 the
body
• 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
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• Sensory receptors perform four functions in this
process
– Sensory transduction, amplification,
transmission, and integration
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• Two types of sensory receptors exhibit these
functions
– A stretch receptor in a crayfish
Weak
muscle stretch
Muscle
Strong
muscle stretch
Stretch
receptor
Axon
Membrane
potential (mV)
Dendrites
–50 Receptor potential
–50
–70
–70
Action potentials
0
0
–70
–70
0 1 2 3 4 5 6 7
Time (sec)
(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
Figure 49.2a
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01 2 3 4 5 67
Time (sec)
receptor. A stronger stretch produces
a larger receptor potential and higher
requency of action potentials.
– A hair cell found in vertebrates
“Hairs” of
hair cell
Fluid moving in
one direction
No fluid
movement
Neurotransmitter at
synapse
More
neurotransmitter
Less
neurotransmitter
–50
–50
–70
Action potentials
0
–70
Membrane
potential (mV)
–50 Receptor potential
Membrane
potential (mV)
Membrane
potential (mV)
Axon
Fluid moving in
other direction
–70
0
(b) Vertebrate hair cells have specialized cilia
or microvilli (“hairs”) that bend when surrounding fluid moves. Each hair cell releases
an excitatory neurotransmitter at a synapse
0 1 2 3 4 5 6 7
Time (sec)
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
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0
–70
–70
01 2 3 4 5 6 7
Time (sec)
–70
01 2 3 4 5 6 7
Time (sec)
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 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
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• Many sensory receptors are extremely
sensitive
– With the ability to detect the smallest physical
unit of stimulus possible
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Amplification
• Amplification is the strengthening of stimulus
energy
– By cells in sensory pathways
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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
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• Sensory cells without axons
– Release neurotransmitters at synapses with
sensory neurons
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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 sensory
adaptation
– A decrease in responsiveness during
continued 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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Mechanoreceptors
• Mechanoreceptors sense physical deformation
– Caused by stimuli such as pressure, stretch,
motion, and sound
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• The mammalian sense of touch
– Relies on mechanoreceptors that are the
dendrites of sensory neurons
Cold
Light touch
Pain
Hair
Heat
Epidermis
Dermis
Figure 49.3
Nerve
Connective tissue
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Hair movement
Strong pressure
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
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• Two of the most sensitive and specific
chemoreceptors known
Figure 49.4
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0.1 mm
– Are present in the antennae of the male
silkworm moth
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 enough
to 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.
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• 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, apparently
sense Earth’s magnetic field and use the information, along with
other cues, for orientation.
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Thermoreceptors
• Thermoreceptors, which respond to heat or
cold
– Help regulate body temperature by signaling
both surface and body core temperature
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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
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• 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
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Sensing Gravity and Sound in Invertebrates
• Most invertebrates have sensory organs called
statocysts
– That contain mechanoreceptors and function in
their sense of equilibrium
Ciliated
receptor cells
Statolith
Figure 49.6
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Cilia
Sensory nerve fibers
• Many arthropods sense sounds with body hairs
that vibrate
– Or with localized “ears” consisting of a
tympanic membrane and receptor cells
Tympanic
membrane
Figure 49.7
1 mm
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Hearing and Equilibrium in Mammals
• In most terrestrial vertebrates
– The sensory organs for hearing and
equilibrium are closely associated in the ear
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• Exploring the structure of the human ear
1
2 The middle ear and inner ear
Overview of ear structure
Incus
Middle
ear
Inner ear
Outer ear
Skull
bones
Semicircular
canals
Stapes
Malleus
Auditory nerve,
to brain
Pinna
Tympanic
membrane
Auditory
canal
Hair cells
Cochlea
Eustachian
tube
Tectorial
membrane
Tympanic
membrane
Oval
window
Eustachian
tube
Round
window
Cochlear duct
Bone
Vestibular canal
Auditory nerve
Basilar
membrane
Figure 49.8
Axons of
sensory neurons
To auditory
nerve
4 The organ of Corti
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Tympanic canal
3 The cochlea
Organ of Corti
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
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• These vibrations create pressure waves in the
fluid in the cochlea
– That travel through the vestibular canal and
ultimately strike the round window
Cochlea
Stapes
Axons of
sensory
neurons
Oval
window
Vestibular
canal
Perilymph
Base
Figure 49.9
Round
window
Tympanic
Basilar
canal
membrane
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Apex
• 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
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• The cochlea can distinguish pitch
– Because the basilar membrane is not uniform
along its length
Cochlea
(uncoiled)
Apex
(wide and flexible)
Basilar
membrane
1 kHz
500 Hz
(low pitch)
2 kHz
4 kHz
8 kHz
16 kHz
(high pitch)
Figure 49.10
Base
(narrow and stiff)
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Frequency producing maximum
vibration
• 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
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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 in
the inner ear
– Function in balance and equilibrium
The semicircular canals, arranged in three
spatial planes, detect angular movements
of the head.
Each canal has at its base a
swelling called an ampulla,
containing a cluster of hair cells.
When the head changes its rate
of rotation, inertia prevents
endolymph in the semicircular
canals from moving with the head,
so the endolymph presses against
the cupula, bending the hairs.
Flow
of endolymph
Flow
of endolymph
Vestibular nerve
Cupula
Hairs
Hair
cell
Nerve
fibers
Vestibule
Utricle
Body movement
Saccule
Figure 49.11
The utricle and saccule tell the brain which
way is up and inform it of the body’s
position or linear acceleration.
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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.
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 both
sides of their body
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• The lateral line system contains
mechanoreceptors
– With hair cells that respond to water movement
Lateral
line
Lateral line canal
Scale
Epidermis Neuromast
Segmental muscles of body wall
Opening of lateral
line canal
Lateral nerve
Cupula
Sensory
hairs
Supporting cell
Figure 49.12
Nerve fiber
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Hair cell
• 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
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• The taste receptors of insects are located
within sensory hairs called sensilla
– Which are located on the feet and in
mouthparts
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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.
To brain
Chemoreceptors
Sensillum
Microelectrode
To voltage
recorder
CONCLUSION Any natural food probably stimulates multiple chemoreceptors. By
integrating sensations, the insect’s brain can apparently distinguish a very large
number of tastes.
Figure 49.13
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Pore at tip
Pipette containing
test substance
Number of action potentials
in first second of response
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.
Chemoreceptors
50
30
10
0
0.5 M
NaCl
0.5 M
Sucrose
Stimulus
Meat
Honey
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)
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• Transduction in taste receptors
– Occurs by several mechanisms
Taste pore
Sugar molecule
Taste bud
Sensory
receptor
cells
Sensory
neuron
Tongue
1
Sugar
A sugar molecule binds
to a receptor protein on
the sensory receptor cell.
G protein
Sugar
receptor
Adenylyl cyclase
2 Binding initiates a signal transduction pathway
involving cyclic AMP and protein kinase A.
ATP
cAMP
Protein
kinase A
3 Activated protein kinase A closes K+ channels in
the membrane.
SENSORY
K+
RECEPTOR
CELL
Synaptic
4 The decrease in the membrane’s permeability to
K+ depolarizes the membrane.
vesicle
—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.
Figure 49.14
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Sensory neuron
Smell in Humans
• Olfactory receptor cells
– Are neurons that line the upper portion of the
nasal cavity
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• When odorant molecules bind to specific
receptors
– A signal transduction pathway is triggered,
sending action potentials to the brain
Brain
Action potentials
Odorant
Olfactory bulb
Nasal cavity
Bone
Epithelial cell
Odorant
receptors
Chemoreceptor
Plasma
membrane
Figure 49.15
Odorant
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Cilia
Mucus
• 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
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Vision in Invertebrates
• Most invertebrates
– Have some sort of light-detecting organ
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• One of the simplest is the eye cup of
planarians
– Which provides information about light
intensity and direction but does not form
images
Light
Light shining from
the front is detected
Photoreceptor
Visual pigment
Ocellus
Figure 49.16
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Nerve to
brain
Screening
pigment
Light shining from
behind is blocked
by the screening pigment
• Two major types of image-forming eyes have
evolved in invertebrates
– The compound eye and the single-lens eye
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• Compound eyes are found in insects and
crustaceans
– And consist of up to several thousand light
detectors called ommatidia
2 mm
(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.
Cornea
Crystalline
cone
Rhabdom
Axons
Figure 49.17a–b
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Photoreceptor
Ommatidium
Lens
• Single-lens eyes
– Are found in some jellies, polychaetes, spiders,
and many molluscs
– Work on a camera-like principle
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The Vertebrate Visual System
• The eyes of vertebrates are camera-like
– But they evolved independently and differ from
the single-lens eyes of invertebrates
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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 surface
of 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
Sclera
Choroid
Retina
Ciliary body
Fovea (center
of visual field)
Suspensory
ligament
Cornea
Iris
Optic
nerve
Pupil
Aqueous
humor
Lens
Vitreous humor
Central artery and
vein of the retina
Figure 49.18
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Optic disk
(blind spot)
• Humans and other mammals
– Focus light by changing the shape of the lens
Front view of lens
and ciliary muscle
Lens (rounder)
Ciliary muscles contract, pulling
border of choroid toward lens
Choroid
Suspensory ligaments relax
Retina
Ciliary
muscle
Lens becomes thicker and rounder,
focusing on near objects
Suspensory
ligaments
(a) Near vision (accommodation)
Ciliary muscles relax, and border of
choroid moves away from lens
Suspensory ligaments
pull against lens
Lens becomes flatter, focusing on
distant objects
Figure 49.19a–b
(b) Distance vision
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Lens (flatter)
• 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
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Sensory Transduction in the Eye
• Each rod or cone in the vertebrate retina
– Contains visual pigments that consist of a lightabsorbing molecule called retinal bonded to a
protein called opsin
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• Rods contain the pigment rhodopsin
– Which changes shape when it absorbs light
Rod
Outer
segment
H
H2 C
H
CH3
C
CH3
C
H
H2 C
Disks
C
C
C
C
CH3 H
H3 C
H
C
C
C
C
H
O
C
H
C
C
H
H
CH3
cis isomer
Inside
of disk
Cell body
Enzymes
Light
Synaptic
terminal
H
H
H2 C
CH3
CH3
C
H
H
H
H2 C
C
C
C
C
Cytosol
Rhodopsin
Retinal
C
C
C
CH3 H
C
CH3
C
CH3
H
C
C
CH3
C
O
H
trans isomer
Opsin
Figure 49.20a, b
(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.
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(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.
Processing Visual Information
• The processing of visual information
– Begins in the retina itself
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• Absorption of light by retinal
– Triggers a signal transduction pathway
Light
EXTRACELLULAR
FLUID
INSIDE OF DISK
Active rhodopsin
PDE
CYTOSOL
Plasma
membrane
Membrane
potential (mV)
0
Dark Light
Inactive rhodopsin
Transducin
cGMP
Disk membrane
– 40
GMP
Na+
1 Light
isomerizes
retinal, which
activates
rhodopsin.
2 Active
rhodopsin
in turn
activates a G
protein called
transducin.
3 Transducin
activates the
enzyme
phosphodiesterae
(PDE).
Figure 49.21
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4 Activated PDE
detaches cyclic
guanosine
monophosphate
(cGMP) from
Na+ channels in
the plasma
membrane by
hydrolyzing
cGMP to GMP.
– 70
– Hyperpolarization
Time
Na+
5 The Na+ channels
close when cGMP
detaches. The
membrane’s
permeability to
Na+ decreases,
and the rod
hyperpolarizes.
• 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
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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
Dark Responses
Rhodopsin inactive
Rhodopsin active
Na+ channels open
Na+ channels closed
Rod depolarized
Rod hyperpolarized
Glutamate
released
Figure 49.22
Light Responses
Bipolar cell either
depolarized or
hyperpolarized,
depending on
glutamate receptors
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No glutamate
released
Bipolar cell either
hyperpolarized or
depolarized,
depending on
glutamate receptors
• Three other types of neurons contribute to
information processing in the retina
– Ganglion cells, horizontal cells, and amacrine
cells
Retina
Optic nerve
To
brain
Retina
Photoreceptors
Neurons
Cone Rod
Amacrine
cell
Figure 49.23
Optic
nerve
fibers
Ganglion
cell
Horizontal
cell
Bipolar
cell
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Pigmented
epithelium
• 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
Left
visual
field
Right
visual
field
Left
eye
Right
eye
Optic nerve
Optic chiasm
Lateral
geniculate
nucleus
Figure 49.24
Primary
visual cortex
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• 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
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• 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
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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
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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
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• Annelids use their hydrostatic skeleton for
peristalsis
– A type of movement on land produced by
rhythmic waves of muscle contractions
(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.)
Longitudinal
muscle relaxed
(extended)
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.
Figure 49.25a–c
(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.
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Circular
muscle
contracted
Circular
muscle
relaxed
Longitudinal
muscle
contracted
Head
Exoskeletons
• An exoskeleton is a hard encasement
– Deposited on the surface of an animal
• Exoskeletons
– Are found in most molluscs and arthropods
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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
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• 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
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• The human skeleton
key
Axial skeleton
Appendicular
skeleton
Skull
Examples
of joints
Head of
humerus
Scapula
1
Shoulder
girdle
Clavicle
Scapula
Sternum
Rib
Humerus
2
Vertebra
3
Radius
Ulna
Pelvic
girdle
1 Ball-and-socket joints, where the humerus contacts
the shoulder girdle and where the femur contacts the
pelvic girdle, enable us to rotate our arms and
legs and move them in several planes.
Humerus
Carpals
Phalanges
Ulna
Metacarpals
Femur
Patella
2 Hinge joints, such as between the humerus and
the head of the ulna, restrict movement to a single
plane.
Tibia
Fibula
Ulna
Figure 49.26
Tarsals
Metatarsals
Phalanges
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Radius
3 Pivot joints allow us to rotate our forearm at the
elbow and to move our head from side to side.
Physical Support on Land
• In addition to the skeleton
– Muscles and tendons help support large land
vertebrates
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• Concept 49.6: Muscles move skeletal parts by
contracting
• The action of a muscle
– Is always to contract
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• Skeletal muscles are attached to the skeleton
in antagonistic pairs
– With each member of the pair working against
each other
Human
Grasshopper
Extensor
muscle
relaxes
Biceps
contracts
Triceps
relaxes
Flexor
muscle
contracts
Forearm
flexes
Extensor
muscle
contracts
Biceps
relaxes
Forearm
extends
Figure 49.27
Triceps
contracts
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Tibia
flexes
Tibia
extends
Flexor
muscle
relaxes
Vertebrate Skeletal Muscle
• Vertebrate skeletal muscle
– Is characterized by a hierarchy of smaller and
smaller units
Muscle
Bundle of
muscle fibers
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Z line
Light
band
Dark band
Sarcomere
0.5 m
TEM
I band
A band
I band
M line
Thick
filaments
(myosin)
Figure 49.28
Thin
filaments
(actin)
Z line
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H zone
Sarcomere
Z line
• 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
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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 myosin
molecules
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• Skeletal muscle is also called striated muscle
– Because the regular arrangement of the
myofilaments creates a pattern of light and
dark 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
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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
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• As a result of this sliding
– The I band and the H zone shrink
0.5 m
(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 and
thin filaments slide past each other, reducing the width of the
I bands and H zone and shortening the sarcomere.
Figure 49.29a–c
(c) Fully contracted muscle fiber. In a fully contracted muscle
fiber, 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.
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Z
H
A
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
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• Myosin-actin interactions underlying muscle
fiber contraction
Thick filament
1 Starting here, the myosin head is
bound to ATP and is in its lowenergy confinguration.
Thin filaments
5 Binding of a new molecule of ATP releases the
myosin head from actin,
and a new cycle begins.
Thin filament
Myosin head (lowenergy configuration)
ATP
ATP
Thick
filament
Thin filament moves
toward center of sarcomere.
Figure 49.30
+
Cross-bridge
binding site
Actin
ADP
Myosin head (lowenergy configuration)
ADP
2 The myosin head hydrolyzes
ATP to ADP and inorganic
phosphate ( P I ) and is in its
high-energy configuration.
Pi
ADP
Pi
4 Releasing ADP and ( P i), myosin
relaxes to its low-energy configuration,
sliding the thin filament.
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Pi
Cross-bridge
Myosin head (highenergy configuration)
13 The myosin head binds to
actin, forming a crossbridge.
The Role of Calcium and Regulatory Proteins
• 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 filament
are blocked by the regulatory protein
tropomyosin
Tropomyosin
Actin
Figure 49.31a
Ca2+-binding sites
(a) Myosin-binding sites blocked
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Troponin complex
• 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
Ca2+
Myosinbinding site
Figure 49.31b
(b) Myosin-binding sites exposed
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• 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
Motor
neuron axon
Mitochondrion
Synaptic
terminal
T tubule
Sarcoplasmic
reticulum
Ca2+ released
from sarcoplasmic
reticulum
Myofibril
Figure 49.32
Plasma membrane
of muscle fiber
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Sarcomere
• The synaptic terminal of the motor neuron
– Releases the neurotransmitter acetylcholine,
depolarizing the muscle and causing it to
produce an action potential
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• 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
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• Review of contraction in a skeletal muscle fiber
Synaptic
terminal
of motor
neuron
1 Acetylcholine (ACh) released by synaptic terminal diffuses across synaptic
cleft and binds to receptor proteins on muscle fiber’s plasma membrane,
triggering an action potential in muscle fiber.
Synaptic cleft
ACh
2 Action potential is propagated along plasma
membrane and down
T tubules.
SR
3 Action potential
triggers Ca2+
release from sarcoplasmic reticulum
(SR).
Ca2
7 Tropomyosin blockage of myosinbinding sites is restored; contraction
ends, and muscle fiber relaxes.
Ca2
CYTOSOL
ADP
P2
PLASMA MEMBRANE
T TUBULE
4 Calcium ions bind to troponin;
troponin changes shape,
removing blocking action
of tropomyosin; myosin-binding
sites exposed.
2+
6 Cytosolic Ca is
removed by active
transport into
SR after action
potential ends.
Figure 49.33
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5 Myosin cross-bridges alternately attach
to actin and detach, pulling actin
filaments toward center of sarcomere;
ATP powers sliding of filaments.
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
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• 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
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• 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
Motor
unit 1
Spinal cord
Motor
unit 2
Synaptic terminals
Nerve
Motor neuron
cell body
Motor neuron
axon
Muscle
Muscle fibers
Figure 49.34
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Tendon
• 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
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• A twitch
– Results from a single action potential in a
motor neuron
• More rapidly delivered action potentials
– Produce a graded contraction by summation
Tension
Tetanus
Summation of
two twitches
Single
twitch
Action
potential
Time
Pair of
action
potentials
Figure 49.35
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Series of action
potentials at
high frequency
• Tetanus is a state of smooth and sustained
contraction
– Produced when motor neurons deliver a volley
of action potentials
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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
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• Types of skeletal muscles
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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
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• 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
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• 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
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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
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Locomotion on Land
• Walking, running, hopping, or crawling on land
– Requires an animal to support itself and move
against gravity
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• Diverse adaptations for traveling on land
– Have evolved in various vertebrates
Figure 49.36
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Flying
• Flight requires that wings develop enough lift
– To overcome the downward force of gravity
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Comparing Costs of Locomotion
•The energy cost of locomotion
–Depends on the mode of locomotion and the
environment
EXPERIMENT
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.
CONCLUSION
Flying
Energy cost (J/Kg/m)
For animals of a given
body
mass, swimming is the most energyCONCLUSION
efficient and running the least energyefficient mode of locomotion. In any mode,
a small animal expends more energy per
kilogram of body mass than a large animal.
102
Running
10
1
Swimming
10–1
10–3
Figure 49.37
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1
103
Body mass(g)
106
• Animals that are specialized for swimming
– Expend less energy per meter traveled than
equivalently sized animals specialized for
flying or running
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