Muscle fiber

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Transcript Muscle fiber

The Muscular System:
Skeletal Muscle Tissue
and Organization
There are three types of muscle tissue:
– Skeletal muscle
• Pulls on skeletal bones
• Voluntary contraction
– Cardiac muscle
• Pushes blood through arteries and veins
• Rhythmic contractions
– Smooth muscle
• Pushes fluids and solids along the digestive tract, for
• Involuntary contraction
Muscle tissues share four basic properties:
– Excitability
• The ability to respond to stimuli
– Contractility
• The ability to shorten and exert a pull or tension
– Extensibility
• The ability to continue to contract over a range of
resting lengths
– Elasticity
• The ability to recoil to its original length
Skeletal muscles perform the following functions
– Produce skeletal movement
• Pull on tendons to move the bones
– Maintain posture and body position
• Stabilize the joints to aid in posture
– Support soft tissue
• Support the weight of the visceral organs
– Regulate entering and exiting of material
• Voluntary control over swallowing, defecation, and
– Maintain body temperature
• Some of the energy used for contraction is converted to
• Gross anatomy
– Connective tissue of muscle
• Epimysium: dense collagen fiber tissue
that surrounds the entire muscle
• Perimysium: dense tissue that divides
the muscle into parallel compartments
of fascicles of ms. fibers
• Endomysium: dense tissue that
surrounds individual muscle fibers to
connect them together.
Capillary network
nerve fibers
satellite cells (stem cells that
repair damage)
Figure 9.1 Structural Organization of Skeletal Muscle
Muscle fascicle
Muscle fibers
Blood vessels
Muscle fiber
Blood vessels
and nerves
(bundle of cells)
Anatomy of Skeletal Muscles
• Connective Tissue of Muscle
• Epimysium, perimysium, and endomysium
converge to form tendons
• Tendons and Aponeuroses
• Tendons connect a muscle to a bone
• Aponeuroses connect a muscle to a muscle
Anatomy of Skeletal Muscles
– Nerves
• Nerves innervate the muscle
• There is a chemical communication between a nerve
and a muscle
• The nerve is “connected” to the muscle via the
motor end plate
– This is the neuromuscular
Anatomy of Skeletal Muscles
• blood vessels
– Blood vessels innervate the endomysium of the
– They then branch to form coiled networks to
accommodate flexion and extension of the muscle
Figure 9.2a Skeletal Muscle Innervation
LM  230
A neuromuscular synapse as seen on a
muscle fiber of this fascicle
Anatomy of Skeletal Muscles
• Microanatomy of skeletal muscle fibers
– Sarcolemma
• Membrane that surrounds the muscle cell
– Sarcoplasm
• The cytosol of the muscle cell
– Muscle fiber (same thing as a muscle cell)
• Can be 30–40 cm in length
• Multinucleated (each muscle cell has hundreds of
• Nuclei are located just deep to the sarcolemma
Figure 9.3ab The Formation and Structure of a Skeletal Muscle Fiber
Muscle fibers develop
through the fusion of
mesodermal cells
called myoblasts.
Development of a
skeletal muscle fiber
Myosatellite cell
muscle fiber
External appearance
and histological view
• Levels of Organization
– Skeletal muscles consist of - fascicles
– Muscle fascicles consist of - fibers
– Muscle fibers consist of- myofibrils
– Myofibrils consist of - sarcomeres
– Sarcomeres consist of- myofilaments
– Myofilaments are- actin and myosin
• Myofibrils and Myofilaments
– The sarcoplasm contains myofibrils
• Myofibrils are responsible for the contraction of muscles
• Myofibrils are attached to the sarcolemma at each end of the
muscle cell
• Surrounding each myofibril is the sarcoplasmic
– Myofibrils are segmented into sarcomeres
– Sarcomeres are made of myofilaments
• Actin
• Myosin
Figure 9.3b–d The Formation and Structure of a Skeletal Muscle Fiber
External appearance
and histological view
The external organization
of a muscle fiber
Terminal cisterna
Thin filament
Thick filament
Internal organization of a muscle fiber.
Note the relationships among myofibrils,
sarcoplasmic reticulum, mitochondria,
triads, and thick and thin filaments.
Triad Sarcoplasmic T tubules
• Sarcomere Organization
– Myosin (thick filament) Anisotropic -dark
– Actin (thin filament) isotropic-light
– Both are arranged in repeating units called
– All the myofilaments are arranged parallel to the
long axis of the cell
Anatomy of Skeletal Muscles
• Sarcomere Organization
– Sarcomere
Main functioning unit of muscle fibers
Approximately 10,000 per myofibril
Consists of overlapping actin and myosin
This overlapping creates the striations that give the
skeletal muscle its identifiable characteristic
Muscle Contraction
• A contracting muscle shortens in length
• Contraction is caused by interactions between thick and thin
filaments within the sarcomere
• Muscle contraction requires the presence of ATP
• When a muscle contracts, actin filaments slide toward each
– This sliding action is called the sliding filament theory
• Sarcomere Organization
– Each sarcomere consists of:
Z line (Z disc)
I band
A band (overlapping A bands create striations)
H band
M line
Figure 9.4b Sarcomere Structure
A band
I band
H band
Zone of overlap
M line
Z line
I band
A band
H band
Z line
A corresponding view of a sarcomere in a myofibril in
the gastrocnemius muscle of the calf and a diagram
showing the various components of this sarcomere
Zone of overlap
M line
Z line
TEM  64,000
The sliding filament theory
• Upon contraction:
• The H band and I
band get smaller
• The zone of
overlap gets larger
• The Z lines move
closer together
• The width of the A
band remains
throughout the
A relaxed sarcomere showing location
of the A band, Z lines, and I band
I band
A band
Z line
H band
Z line
During a contraction, the A band stays the same
width, but the Z lines move closer together and
the I band gets smaller. When the ends of a
myofibril are free to move, the sarcomeres
shorten simultaneously and the ends of the
myofibril are pulled toward its center.
• The neuromuscular junction is formed by an enlarged
nerve terminal that rests in the invaginations of the
Neuromuscular Junction
• Motor neurons are specialized nerve cells that propagate
action potentials to skeletal muscle fibers.
• each axon branch projects to one muscle fiber and forms a
neuromuscular junction (synapse),
• each muscle fiber receives a branch of an axon
• each axon innervates more than one muscle fiber.
• Motor unit- a motor neuron and all the muscle fibers it
• motor neurons reside in the spinal cord - their axons
extend to the muscle.
• axons divide into multiple axonal terminals & attach to
multiple muscle fibers
• presynaptic terminal -An enlarged nerve terminal
• synaptic cleft -the space between pre and post synaptic
• Postsynaptic terminal – accepts the neurotransmitter
• motor endplate -the muscle cell membrane in the area of
the junction or the postsynaptic terminal
• synaptic vesicles -spherical sacs in the presynaptic terminal
containing acetylcholine a neurotransmitter
• neurotransmitter - substance released from a presynaptic
terminal that diffuses across the synaptic cleft and
stimulates (or inhibits) the production of an action
potential in the postsynaptic terminal.
• Actin--Twisted filament consisting of G actin molecules
– Each G actin molecule has an active site (binding site)
– Myosin heads binds to active sites & forms cross-bridges
Z line Titin
The attachment
of thin filaments
to the Z line
H band
G actin
F actin
The detailed structure of a thin filament showing
the organization of G actin, troponin, and
Z line
M line
– Tropomyosin: A protein that covers the binding sites
(when the muscle is relaxed)
– Troponin: Holds tropomyosin in position
• Myosin filaments consist of an elongated tail and a globular
head ( forms cross-bridges)
H band
Z line
M line
The structure of
thick filaments
M line
Myosin tail
– Myosin is a stationary molecule. It is held in place by:
• Protein forming the M line
• A core of titin connecting to the Z lines
A nerve impulse is sent from the central nervous system
The action potential reaches the presynaptic terminal
causes calcium (Ca2*) channels in the axon's cell membrane to open
Ca ions diffuse into the cell
Ca ions cause synaptic vesicles to secrete ACh by exocytosis from
the presynaptic terminal into the synaptic cleft.
6. The acetylcholine molecules then diffuse across the cleft
7. ACh binds to receptor molecules on the membrane of the
postsynaptic terminal.
8. Receptors cause the sarcolemma to become temporarily
permeable to sodium ions which rush into the muscle
9. This gives the cell interior an excess of positive ions, which
upsets and changes the electrical conditions of the
sarcolemma and causes an action potential.
10. the action potential travels over the entire surface of the
sarcolemma, conducting the electrical impulse from
one end of the cell to the other
excitation contraction coupling =The mechanism by which action
potential production causes contraction of a muscle fiber
11. action potential Na is propagated along sarcolemma & penetrates Ttubules.
12. T tubules carry the action potentials into the muscle fiber's interior.
13. when action potentials reach the area of the sarcoplasmic reticulum
membranes increase their permeability to Ca+ ions.
14. Ca+ ions rapidly diffuse out from the sarcoplasmic reticulum
15. Ca2+ ions bind to troponin of the actin myofilaments
16. Ca+ causes the tropomyosin to move deeper into the groove
between the two F-actin molecules
17. this exposes the active sites on the actin
18. exposed active sites bind to the heads of the myosin molecules to
form cross bridges
19. hinged areas of the myosin move… causing the actin to slide past the
20. Thus causing the sarcomere to shorten …(contraction)
• When the heads of the myosin
molecules bind to actin, a series of
events resulting in contraction
which proceeds very rapidly.
• The myosin heads bend at their
hinged area, forcing the actin to
slide over the surface of the
• After movement, each myosin
head releases from the actin and
returns to its original position.
• It can then form another cross
Energy Requirements for Contraction
one ATP energy molecule is required for each cycle of crossbridge formation, cross-bridge movement, and cross-bridge
After a cross bridge has formed and movement has
occurred, ATP binds to the head of the myosin molecule
allowing its release from the actin
The ATP is broken down by ATPase in the head of the
myosin and energy is stored in the head of the myosin
The cross bridge is then released and the myosin head is
restored to its original position (Figure 10-13, A). When the
myosin molecule binds to actin to form another cross
bridge, much of the stored energy is used for cross bridge
formation and movement (Figure 10-13, B and C). Before
the cross bridge can be released for another cycle, once
again, an ATP molecule must bind to the head of the myosin
Movement of the myosin molecule while the cross bridge is
attached is a power stroke, whereas return of the myosin
head to its original position after cross-bridge release is a
recovery stroke. Many cycles of power and recovery strokes
occur during each muscle contraction. While muscle is
relaxed, energy stored in the heads of the myosin molecules
is held in reserve until the next contraction. When calcium
is released from the sarcoplasmic reticulum in response to
an action potential, the cycle of cross-bridge formation and
release, which results in contraction, begins (Table 10-2).
Other events of skeletal muscle contraction
• Before contraction ATP energy is stored in the
head of the myosin
• ATP binds to the head of the myosin and is
broken down to ADP
• energy is needed to release actin from myosin
• energy causes the hinged area of myosin to
return to its original position.
• The remainder of the energy is stored in the
head of the myosin
• As long as actin-active sites are available, the
process continues resulting in further
• If no additional action potentials are produced
in the skeletal muscle fibers Ca ions are taken
up by the sarcoplasmic reticulum,
• Ca ions unbind from troponin & the troponintropomyosin complex covers the actin-active
• Then relaxation occurs.
• ACh is rapidly broken down to acetic acid and choline by
• Acetylcholine’s rapid degradation in the neuromuscular junction
ensures that one presynaptic action potential yields only one
postsynaptic action potential.
• Choline molecules are actively reabsorbed by the presynaptic
terminal and then combined with the acetic acid produced within the
cell to form acetylcholine.
• Recycling choline molecules requires less energy and is more rapid
than completely synthesizing new acetylcholine molecules each time
they are released from the presynaptic terminal.
Figure 9.10bc The Neuromuscular Synapse
Glial cell
Synaptic terminal
One portion of a
neuromuscular synapse
Arriving action
Synaptic cleft
Sarcolemma of
motor end plate
Detailed view of a terminal,
synaptic cleft, and motor end
plate. See also Figure 9.2.
• Anything that affects the production, release, and degradation of acetylcholine
or its ability to bind to its receptor molecule will also affect the transmission of
action potentials across the neuromuscular junction. For example, some
insecticides contain organophosphates that bind to and inhibit the function of
acetylcholinesterase. As a result, acetylcholine is not degraded and
accumulates in the synaptic cleft where it acts as a constant stimulus to the
muscle fiber. Insects exposed to the insecticide die. partly because their
muscles contract and cannot relaxa condition called spastic paralysis. Other
organic poisons such as curare bind to the acetylcholine receptors, preventing
acetylcholine from binding to them. Curare does not allow activation of the
receptors; therefore the muscle is not capable of contracting in response to
nervous stimulation—a condition called flaccid paralysis. Myasthenia graVIs
{mi'as-the'ne-ah grS'vis) results from the production of antibodies that bind to
acetylcholine receptors, eventually causing the destruction of the receptor and
thus reducing the number of receptors. As a consequence, muscles exhibit a
degree of flaccid paralysis or are extremely weak. A class of drugs that'
includes neostigmine partially blocks the action of acetylcholinesterase and
sometimes is used to treat myasthenia gravis. The drugs cause acetylcholine
levels to increase in the synaptic cleft and combine more effectively with the
remaining acetylcholine receptor sites.
Muscle Twitch
• A muscle twitch is contraction of a whole muscle in response to a stimulus
that causes an action potential in one or more muscle fibers.
• lag, or latent phase =The time period between application of the stimulus
to the motor neuron and the beginning of contraction
• contraction phase =the time during which contraction occurs
• relaxation phase =the time during which relaxation occurs
• The action potential is an electrochemical event, but contraction is a
mechanical event.
• =
• all-or-none law of skeletal muscle contraction = an isolated skeletal muscle fiber
either contracts maximally or does not contract at all.
• subthreshold stimulus does not produce an action potential, and no muscle
• threshold stimulus = an action potential that results in contraction of the muscle
• submaximal stimuli = activates additional motor units until all of the motor units
are activated by
• maximal stimulus = contracts all motor units
• supramaximal stimulus = an action potential of the same magnitude as the
threshold stimulus and therefore produces an identical contraction.
• =
• multiple motor unit summation = As the
stimulus strength increases between threshold
and maximum values, motor units are recruited,
and the force of contraction produced by the
muscle increases in a graded fashion.
• A whole muscle contracts with either a small
force or a large force, depending on the number
of motor units recruited, but each motor unit
responds to an action potential either maximally
or not at all.
• =
Stimulus Frequency and Muscle Contraction
• incomplete tetanus = muscle fibers partially relax between
• complete tetanus = action potentials occur so rapidly there is no
muscle relaxation between the action potentials.
• multiple wave summation = tension produced by a muscle increases
as the stimulus frequency increases.
• Treppe - a second contraction produces a greater tension than the
first, and the third produces greater tension than the second. After
only few stimuli, the tension produced by all the contractions is
• =
• isometric contractions = the length of the muscle does
not change, but the amount of tension does increase
• isotonic contractions = the amount of tension is
constant during contraction, but the length of the
muscle changes
• Concentric contractions – an isotonic contraction that
is big enough to overcome the opposing resistance and
the muscle shortens
• Eccentric contractions – an isotonic contraction that
maintains tension while the muscle increases in
length.(lowering a weight)
• Most muscle contractions are a combination of
isometric and isotonic contractions
• =
• Muscle tone = constant tension by muscles of
the body for long periods of time.
Muscle tone is responsible for keeping the
back and legs straight, the head held in an
upright position, and the abdomen from
Length versus Tension
• Active tension – force applied to an object
when a muscle contracts
• Passive tension – the tension applied to a load
when a muscle is stretched but not stimulated
• The decreased capacity to do work following a
period of activity
• Psychologic fatigue – person perceives that
more muscle work is not possible.(most
common type)
• Muscular fatigue – depletion of ATP
• Synaptic fatigue – acetylcholine synthesis cant
keep up with ms. use
Muscle disorders are caused by disruption of normal innervation,
degeneration and replacement of muscle cells, injury, lack of use, or disease.
• Exercise causes muscular hypertrophy.
• disuse of muscle results in muscular atrophy.
Extreme disuse of muscle results in muscular atrophy in which there
is a permanent loss of skeletal muscle fibers and the replacement of
those fibers by connective tissue.
• Immobility caused by damage to the nervous system or by old age
may lead to permanent and severe muscular atrophy.
• Denervation
• When motor neurons innervating skeletal muscle fibers are severed,
the result is flaccid paralysis. If the muscle is reinnervated, muscle
function is restored, and atrophy is stopped. However, if skeletal
muscle is permanently denervated, it atrophies and exhibits
permanent flaccid paralysis. Muscles that have been denervated
sometimes are stimulated electrically to prevent severe atrophy. The
strategy is to slow the process of atrophy while motor neurons
slowly grow toward the muscles and eventually reinnervate them.
Neither cardiac muscle nor smooth muscle atrophies in response to
Muscular Dystrophy
Muscular dystrophy refers to a group of diseases called myopa-thies that destroy skeletal muscle tissue. Usually the diseases are inherited
and are characterized by degeneration of muscle cells, leading to atrophy and eventual replacement by fatty tissue. Duch-enne muscular
dystrophy affects only males, and by early adolescence the individual is confined to a wheelchair. As the muscles atrophy, they shorten,
causing conditions such as immobility of the joints and postural abnormalities such a scoliosis. Facioscapulohu-moral (fa'sT-o-skap'u-Iohu'mor-al) muscular dystrophy is generally less severe, and it affects both sexes later in life. The muscles of the face and shoulder girdle are
primarily involved. Both types of muscular dystrophy are inherited and progressive, and no drugs prevent the progression of the disease.
Therapy primarily involves exercises. Braces and corrective surgery sometimes help correct
abnormal posture caused by the advanced disease.
Fibrosis is the replacement of damaged cardiac muscle or skeletal muscle by connective tissue.
Fibrosis, or scarring, is associated with severe trauma to skeletal muscle and with heart attack
(myocardial infarction) in cardiac muscle.
Fibrositis is an inflammation of fibrous connective tissue, resulting in stiffness, pain, or soreness.
It is not progressive, nor does it lead to tissue destruction. Fibrositis may be caused by repeated
muscular strain or prolonged muscular tension.
Painful, spastic contractions of muscles (cramps) are usually due to an irritation within a muscle
that causes a reflex contraction (see Chapter 13). Local inflammation resulting from a buildup of
lactic acid and fibrositis causes reflex contraction of muscle fibers surrounding the irritated
Motor Units and Muscle Control
• Motor Units (motor neurons controlling muscle
– Precise control
• A motor neuron controlling two or three muscle
• Example: the control over the eye muscles
– Less precise control
• A motor neuron controlling perhaps 2000 muscle fibers
• Example: the control over the leg muscles
Figure 9.12 The Arrangement of Motor Units in a Skeletal Muscle
of motor
Motor unit 1
Motor unit 2
Motor unit 3
Muscle fibers
Motor Units and Muscle Control
• Muscle Tension
– Muscle tension depends on:
• The frequency of stimulation
• The number of motor units involved
Motor Units and Muscle Control
• Muscle Tone
– The tension of a muscle when it is
– Stabilizes the position of bones and joints
• Muscle Spindles
– These are specialized muscle cells that are
monitored by sensory nerves
Motor Units and Muscle Control
• Muscle Hypertrophy
– Exercise causes:
An increase in the number of mitochondria
An increase in the activity of muscle spindles
An increase in the concentration of glycolytic enzymes
An increase in the glycogen reserves
An increase in the number of myofibrils
The net effect is an enlargement of the muscle
Motor Units and Muscle Control
• Muscle Atrophy
– Discontinued use of a muscle
– Disuse causes:
• A decrease in muscle size
• A decrease in muscle tone
– Physical therapy helps to reduce the effects
of atrophy
Types of Skeletal Muscle Fibers
• Three major types of skeletal muscle
– Fast fibers (white fibers)
• Associated with eye muscles
– Intermediate fibers (pink fibers)
– Slow fibers (red fibers)
• Associated with leg muscles
Figure 9.13a Types of Skeletal Muscle Fibers
Slow fibers
Smaller diameter,
darker color due to
myoglobin; fatigue
LM  170
Fast fibers
Larger diameter,
paler color;
easily fatigued
LM  170
Note the difference in the size of
slow muscle fibers (above) and
fast muscle fibers (below).
Figure 9.13b Types of Skeletal Muscle Fibers
Slow fibers
Smaller diameter,
darker color due to
myoglobin; fatigue
Fast fibers
Larger diameter,
paler color;
easily fatigued
LM  783
The relatively slender slow
muscle fiber (R) has more
mitochondria (M) and a more
extensive capillary supply (cap)
than the fast muscle fiber (W).
• Fast-twitch muscle fibers
• break down ATP more rapidly than slow-twitch muscle fibers
• cross bridges that form, release, and reform more rapidly than those in
slow-twitch muscles
• less well-developed blood supply than slow-twitch muscles
• very little myoglobin
• fewer and smaller mitochondria.
• large deposits of glycogen and are well adapted to perform
• anaerobic metabolism
• contract rapidly for a shorter time and fatigue relatively quickly.
• Training causes fast-twitch muscles to improve their ability to carry out
aerobic metabolism. Trained fast-twitch muscles are called fatigueresistant fast-twitch muscles.
• =
Types of Skeletal Muscle Fibers
• Features of fast fibers:
– Large in diameter
– Large glycogen reserves
– Relatively few mitochondria
– Muscles contract using anaerobic metabolism
– Fatigue easily
– Can contract in 0.01 second or less after stimulation
– Produce powerful contractions
Slow Twitch muscle
• Type 1 oxidative
• Slow-twitch muscle fibers contract more slowly
• smaller in diameter, have a
• better developed blood supply, have
• more mitochondria, and are
• more fatigue resistant than fast-twitch muscle fibers.
• Aerobic metabolism is the primary source
• large amounts of myoglobin
• =
Types of Skeletal Muscle Fibers
• Features of slow fibers:
– Half the diameter of fast fibers
– Take three times longer to contract after stimulation
– Can contract for extended periods of time
– Contain abundant myoglobin (creates the red color)
– Muscles contract using aerobic metabolism
– Have a large network of capillaries
Types of Skeletal Muscle Fibers
• Features of intermediate fibers:
– Similar to fast fibers
• Have low myoglobin content
• Have high glycolytic enzyme concentration
• Contract using anaerobic metabolism
– Similar to slow fibers
• Have lots of mitochondria
• Have a greater capillary supply
• Resist fatigue
Table 9.1 Properties of Skeletal Muscle Fiber Types
Types of Skeletal Muscle Fibers
• Distribution of fast, slow, and intermediate
– Fast fibers
• High density associated with eye and hand muscles
• Sprinters have a high concentration of fast fibers
• Repeated intense workouts increase the fast fibers
Types of Skeletal Muscle Fibers
• Distribution of fast, slow, and intermediate
– Slow and intermediate fibers
None are associated with the eyes or hands
Found in high density in the back and leg muscles
Marathon runners have a high amount
Training for long distance running increases the
proportion of intermediate fibers
Aging and the Muscular System
• Changes occur in muscles as we age
– Skeletal muscle fibers become smaller in diameter
– There is a decrease in the number of myofibrils
– Contain less glycogen reserves
– Contain less myoglobin
• All of the above results in a decrease in strength and
• Muscles fatigue rapidly
Aging and the Muscular System
• Changes occur in muscles as we age (continued)
– There is a decrease in myosatellite cells
– There is an increase in fibrous connective tissue
• Results in fibrosis
• The ability to recover from muscular injuries decreases