Download Motor Systems

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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  Both bats and moths 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  Concept: Sensory receptors transduce stimulus
energy and transmit signals to the central
nervous system
•  Sensations and perceptions begin with sensory
reception, the detection of stimuli by sensory
receptors
•  Sensations are action potentials that reach the
brain via sensory neurons
•  Exteroreceptors
•  Once the brain is aware of sensations it
interprets them, giving the perception of stimuli.
–  Detect stimuli coming from the outside of the
body
•  Interoreceptors
–  Detect internal stimuli
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1
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 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 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
•  Integration is the evaluation and coordination of stimuli to
produce a response. It occurs at all levels of the nervous
system.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hair cell found in vertebrates
Fluid moving in
one direction
No fluid
movement
“Hairs” of
hair cell
Neurotransmitter at
synapse
Types of Sensory Receptors
More
neurotransmitter
Less
neurotransmitter
–70
Action potentials
0
Membrane
potential (mV)
Membrane
potential (mV)
–50
–70
0
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
–  Chemoreceptors
–70
–  Electromagnetic receptors
0
–70
–70
0 1 2 3 4 5 6 7
Time (sec)
(b) Vertebrate
–70
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
•  Based on the energy they transduce, sensory
receptors fall into five categories
–  Mechanoreceptors:
–50
Receptor potential
Membrane
potential (mV)
–50
Axon
Fluid moving in
other direction
0 1 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
–  Thermoreceptors
–  Pain receptors
Figure 49.2b
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2
Mechanoreceptors
Mechanoreceptors
•  Mechanoreceptors sense physical deformation
•  The mammalian sense of touch relies on
mechanoreceptors that are the dendrites of
sensory neurons
–  Caused by stimuli such as pressure, stretch,
motion, and sound
Cold
Light touch
Pain
Hair
Heat
Epidermis
Dermis
Figure 49.3
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nerve Connective tissueHair movementStrong pressure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Chemoreceptors
•  Chemoreceptors include
–  General receptors that transmit information
about the total solute concentration of a
solution
•  Two of the most sensitive and specific
chemoreceptors known are present in the
antennae of the male silkworm moth. The males
use their antennae to detect two components in
pheromones released by females.
Figure 49.4
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
0.1 mm
–  Specific receptors that respond to individual
kinds of molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3
Electromagnetic Receptors
•  Electromagnetic receptors detect various forms
of electromagnetic energy such as visible light,
heat, electricity, and magnetism
•  Some snakes have very sensitive infrared
receptors that detect body heat of prey against
a colder background
(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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Thermoreceptors
•  Many mammals and birds appear to use the
Earth’s magnetic field lines to orient
themselves as they migrate
•  Thermoreceptors, which respond to heat or
cold help regulate body temperature by
signaling both surface and body core
temperature
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
4
Pain Receptors
Hearing, balance and the ear.
•  In humans, pain receptors, also called
nociceptors are a class of naked dendrites in
the epidermis
•  The mechanoreceptors involved with hearing
and equilibrium detect settling particles or
moving fluid
•  They respond to excess heat, pressure, or
specific classes of chemicals released from
damaged or inflamed tissues
•  Hearing and the perception of body equilibrium
are related in most animals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hearing and Equilibrium in Mammals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure of the human ear
•  In most terrestrial vertebrates the sensory organs for
hearing and equilibrium are closely associated in the
ear.
1 Overview of ear structure
2 The middle ear and inner ear
Incus
Semicircular
Skull
bones canals for balance
Stapes
Middle
ear Inner ear Malleus
Outer ear
Auditory nerve,
to brain
•  The ear is divided into three areas:
Pinna
–  The outer ear which contains the auditory canal.
Tympanic
membrane
–  The middle ear which includes the tympanic
membrane and the three ear bones that transmit
vibrations to the inner ear
Hair cells
Cochlea
Eustachian
tube
Auditory
canal
Tectorial
membrane
Tympanic
membrane
Oval
window
Cochlear duct
–  The inner ear which contains the cochlea where
vibrations are converted to nerve impulses and
sent to the brain as well as the organs of
balance: utricle, saccule and semicircular canals.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Round
window
Eustachian
tube
Bone
Vestibular canal
Basilar
membrane
Figure 49.8
Axons of
sensory neurons
Auditory nerve
To auditory
nerve
4 The organ of Corti
Tympanic canal
3 The cochlea
Organ of Corti
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
5
Hearing
Hearing
•  Vibrating objects create percussion waves in
the air that cause the tympanic membrane to
vibrate
•  These vibrations create pressure waves in the
fluid in the cochlea that travel through the
vestibular canal and ultimately strike the round
window
•  The three bones of the middle ear transmit the
vibrations to the oval window on the cochlea
Cochlea
Stapes
Oval
window
Axons of
sensory
neurons
Vestibular
Perilymph
canal
Apex
Base
Figure 49.9
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Round
window
Tympanic
Basilar
canal
membrane
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hearing
Hearing
•  The pressure waves in the vestibular canal
cause the basilar membrane in the organ of
Corti within the cochlea to vibrate up and down
causing its hair cells to bend
•  The cochlea can distinguish pitch because the basilar
membrane is not uniform along its length. 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.
Cochlea
(uncoiled)
•  The bending of the hair cells depolarizes their
membranes sending action potentials that
travel via the auditory nerve to the brain where
they are interpreted as sounds.
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Frequency producing maximum
vibration
Base
(narrow and stiff)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
6
Equilibrium
•  Several of the organs of the inner ear detect
body position and balance
The semicircular canals, arranged
in three spatial planes, detect
angular movements of the head.
•  The utricle and saccule tell the brain which way
is up.
•  Semicircular canals in the inner ear detect
angular movements of the head and function in
balance and equilibrium
Each canal has at its base
a swelling called an
ampulla, containing a
cluster of hair cells.
Flow
of endolymph
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
Vestibular nerve
Cupula
Hairs
Hair
cell
Nerve
fibers
Vestibule
Utricle
Body movement
Saccule
The utricle and saccule tell the
brain which way is up and inform it
of the body’s position or linear
acceleration.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The hairs of the hair cells
project into a gelatinous
cap called the cupula.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Bending of the hairs
increases the frequency of
action potentials in sensory
neurons in direct proportion
to the amount of rotational
acceleration.
Odor detection in humans
Odor detection in humans
•  Olfactory receptor cells
•  When odorant molecules bind to specific
receptors a signal transduction pathway is
triggered, sending action potentials to the brain
–  Are neurons that line the upper portion of the
nasal cavity
Brain
Action potentials
Odorant
Olfactory bulb
Nasal cavity
Bone
Epithelial cell
Odorant
receptors
Chemoreceptor
Plasma
membrane
Figure 49.15
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Odorant
Cilia
Mucus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
7
Vision
Structure of the Vertebrate Eye
•  Many types of light detectors have evolved in
the animal kingdom ranging from simple eye
cups of planarians (flatworms) to the
compound eyes of insects and crustaceans to
the camera-like eyes of vertebrates and
cephalopods (octopuses and relatives).
•  The main parts of the vertebrate eye are
–  The sclera, a layer of tough connective tissue,
which includes the transparent cornea which
allows light into the eye and acts as a fixed
lens.
–  The choroid, a pigmented layer inside the
sclera.
–  The conjunctiva, that covers the outer surface
of the sclera (except the cornea) and keeps it
moist.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure of the Vertebrate Eye
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The structure of the vertebrate eye
–  The iris, which regulates the size of the pupil
the hole in the middle of the iris that lets light
in.
Sclera
Choroid
Retina
Ciliary body
Fovea (center
of visual field)
Suspensory
ligament
–  The retina, which contains photoreceptors that
detect light and convert it to nervous impulses.
Cornea
Iris
–  The lens, which focuses light on the retina
Optic
nerve
Pupil
Aqueous
humor
Lens
Vitreous humor
Central artery and
vein of the retina
Figure 49.18
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Optic disk
(blind spot)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
8
Humans and other mammals focus light by changing the shape of the lens
Ciliary muscles contract, pulling
border of choroid toward lens
Suspensory ligaments
relax
Choroid
Front view of lens
and ciliary muscle
Lens (rounder)
Retina
Ciliary
muscle
Lens becomes thicker and
rounder, focusing on near
objects
(a) Near vision
(accommodation)
Ciliary muscles relax, and
border of choroid moves away
from lens
Suspensory
ligaments
•  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 to light.
Lens (flatter)
Suspensory
ligaments pull
against lens
Lens becomes flatter, focusing
on distant objects
Figure 49.19a–b
(b) Distance vision
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sensory Transduction in the Eye
•  Each rod or cone in the vertebrate retina
contains visual pigments that consist of a lightabsorbing pigment called retinal bonded to a
protein called opsin.
•  Opsins vary in structure from one type of
photoreceptor to another
•  Rods contain the visual pigment rhodopsin,
which changes shape when it absorbs light.
•  Cones contain one or other of three different
photopsins and are either red, green or blue
cones.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  Rods are used under low light conditions and
cones in bright light for color vision.
•  Absorption spectra of the different types of
cones overlap and the differential stimulation of
the three types of cones allows the brain to
produce the perception of color.
•  For example, if both red and green cones are
stimulated we may see orange or yellow
depending on which cones are more strongly
stimulated.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
9
Processing Visual Information
•  The processing of visual information begins in
the retina itself
Light
PDE
•  When a cone or rod is struck it absorbs light
and its retinal changes shape.
•  This change in shape triggers a signal
transduction pathway that generates an action
potential and nerve impulses that travel to the
brain.
CYTOSOL
Inactive rhodopsin
Transducin
Na+
2 Active
rhodopsin
in turn
activates a
G protein
called
transducin.
3 Transducin
activates
the
enzyme
phosphodiestera
e(PDE).
0
Dark Light
– 40
GMP
1 Light
isomerizes
retinal,
which
activates
rhodopsin.
Membrane
potential (mV)
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  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
Left
Right
brain
visual
visual
field
Optic nerve
Plasma
membrane
cGMP
Disk membrane
Figure 49.21
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
EXTRACELLULAR
FLUID
INSIDE OF DISK
Active rhodopsin
Left
eye
field
•  Several integrating centers in the cerebral
cortex are active in creating visual perceptions.
•  You should be aware that the “reality” you
perceive is a a simulation generated by your
brain that builds a mental model of the world.
Right
eye
Optic chiasm
Lateral
geniculate
nucleus
Primary
visual cortex
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
10
Animal Skeletons
Functions of the skeleton
•  Animal skeletons function in support,
protection, and movement
•  The three main functions of a skeleton are
–  Support,
•  The various types of animal movements all
result from muscles working against some type
of skeleton
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
–  Protection,
–  and Movement
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Human skeleton
•  The mammalian skeleton is an endoskeleton of
bones buried within the soft tissue. It is built
from more than 200 bones
•  Some bones are fused together and others are
connected at joints by ligaments that allow
freedom of movement
key
Axial skeleton
Appendicular
skeleton
Skull
Examples
of joints
Head of
humerus
Scapula
1
Shoulder
girdle
Sternum
Rib
Humerus
Vertebra
Radius
Ulna
Pelvic
girdle
Carpals
Clavicle
Scapula
2
3
Phalanges
Metacarpals
Femur
Patella
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
Ulna
2 Hinge joints, such as between the humerus and
the head of the ulna, restrict movement to a single
plane.
Tibia
Fibula
Ulna
Tarsals
Metatarsals
Phalanges
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Radius
3 Pivot joints allow us to rotate our forearm at the
elbow and to move our head from side to side.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
11
Human
•  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 e.g. triceps and
biceps in the upper arm.
Grasshopper
Extensor
muscle
relaxes
Biceps
contracts
Tibia
flexes
Flexor
muscle
contracts
Triceps
relaxes
Forearm
flexes
Extensor
muscle
contracts
Biceps
relaxes
Tibia
extends
Forearm
extends
Triceps
contracts
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Flexor
muscle
relaxes
Figure 49.27
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Muscle
•  Vertebrate skeletal muscle is characterized by
a hierarchy of smaller and smaller units.
Bundle of
muscle fibers
•  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.
Z line
Sarcomere
TEM
I band
•  Myofibrils are composed of myofilaments.
Figure 49.28
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Light
band Dark band
Thick
filaments
(myosin)
Thin
filaments
(actin)
Z line
A band
M line
H zone
Sarcomere
0.5 µm
I band
Z line
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
12
•  The myofibrils are composed of 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  Each repeating unit is a sarcomere bordered
by Z lines. The Z lines are aligned vertically
and are attached to the thin filaments (made of
actin).
•  The thin filaments project into the sarcomere.
•  The thick filaments are aligned in the center of
the sarcomere and align with and partially
overlap with the thin filaments.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Muscle
•  At rest the thick and thin fibers only partially
overlap and this produces a pattern of bands in
the sarcomere that are identified by letters.
Bundle of
muscle fibers
•  The A-band is the area in the middle of the
sarcomere where the thick filaments are
located and appears dark.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Z line
Sarcomere
•  The I-band is the area at either end of the
sarcomere where only thin filaments are found.
This appears light.
•  The H-zone in the middle is where only thick
filaments occur. It appears less dark than the
rest of the A-band.
Nuclei
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Light
band Dark band
TEM
I band
Figure 49.28
Thick
filaments
(myosin)
Thin
filaments
(actin)
Z line
A band
M line
H zone
Sarcomere
0.5 µm
I band
Z line
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
13
I band
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.
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bands
and H zone are relatively wide.
•  As a result of this sliding the I band and the H
zone shrink.
0.5 µm
Z
H
A
Sarcomere
(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.
(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.
Figure 49.29a–c
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Myosin-actin interactions underlying muscle fiber contraction
•  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
Thick filament
1 Starting here, the myosin
head is bound to ATP and
is in its low-energy
confinguration.
Thin filaments
Thin filament
5 Binding of a new molecule of ATP releases the
myosin head from actin,
and a new cycle begins.
ATP
ATP
Thin filament moves
toward center of sarcomere.
Thick
filament
Cross-bridge
binding site
Actin
Pi
ADP
Pi
4 Releasing ADP
and ( Pi), myosin
relaxes to its low-energy configuration,
sliding the thin filament.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
myosin head hydrolyzes
ATP to ADP and inorganic
phosphate ( PI ) and is in its
high-energy configuration.
ADP
Myosin head (lowenergy configuration)
ADP + P
i
2 The
Myosin head (lowenergy configuration)
Cross-bridge
Myosin head (highenergy configuration)
13 The myosin head binds to
actin, forming a crossbridge.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
14
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
Tropomyosin
Actin
Figure 49.31a
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Ca2+-binding sites
Troponin complex
(a) Myosin-binding sites blocked
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  For a muscle fiber to contract the myosinbinding sites must be uncovered
•  This occurs when calcium ions (Ca2+) bind to
another set of regulatory proteins, the troponin
complex
•  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
Ca2+
T tubule
Myosinbinding site
Sarcoplasmic
reticulum
Figure 49.31b
(b) Myosin-binding sites exposed
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 49.32
Myofibril
Plasma membrane
of muscle fiber
Ca2+ released
from sarcoplasmic
reticulum
Sarcomere
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
15
Neural Control of Muscle Tension
•  The synaptic terminal of the motor neuron
releases the neurotransmitter acetylcholine,
depolarizing the muscle and causing it to
produce an action potential
•  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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  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
•  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
Motor Motor
unit 1 unit 2 Synaptic terminals
Spinal cord
Nerve
Motor neuron
cell body
Motor neuron
axon
Muscle
Muscle fibers
Figure 49.34
Tendon
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
16
•  A twitch results from a single action potential in
a motor neuron
•  More rapidly delivered action potentials
produce a graded contraction by summation
•  Tetanus is a state of smooth and sustained
contraction produced when motor neurons
deliver a volley of action potentials.
Tension
Tetanus
Summation of
two twitches
Single
twitch
Action
potential
Time
Pair of
action
potentials
Series of action
potentials at
high frequency
Figure 49.35
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cardiac muscle
Smooth muscle
•  Cardiac muscle, found only in the heart
•  In smooth muscle, found mainly in the walls of
hollow organs (e.g. the digestive tract) the
contractions are relatively slow and may be
initiated by the muscles themselves
–  Consists of striated cells that are electrically
connected together. As a result an action
potential in one part of the heart spreads to all
cardiac cells and the heat contracts.
–  Cardiac muscle can generate action potentials
without neural input because the heart has its
own pacemaker cells that cause rhythmic
depolarizarions.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•  In addition, contractions may be caused by
stimulation from neurons in the autonomic
nervous system
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
17