Title: Muscle Physiology
1Muscle Physiology
Chapter 8
2There are three kinds of muscle tissue
skeletal, cardiac, and smooth.
- These three kinds of muscle tissue compose about
50 percent of a humans body weight. - Skeletal muscle tissue is striated and subject to
voluntary control. - The skeletal muscles make up the muscular system.
- The skeletal muscles are innervated by the
somatic nervous system.
3A muscle fiber is a skeletal muscle cell. It is
large, elongated, and cylinder-shaped. The
fibers extend the entire length of a skeletal
muscle.
- A muscle fiber contains contractile elements
called myofibrils. A myofibril contains thick
filaments called myosin and thin filaments (e.g.,
actin). - Actin and myosin are arranged in units called
sarcomeres. A sarcomere is found between two Z
lines. The sarcomere is the functional unit of
the muscle. Its regions are - A band - myosin (thick) filaments stacked along
with parts of the actin (thin) filaments - H zone - middle of the A band where actin does
not reach - M line - extends vertically down the center of
the A band - I band - has part of actin that do not project
into A band
4The thick filaments of myosin have cross bridges.
The cross bridges can attach to actin binding
sites. The cross bridges also have myosin ATPase
activity.
- Actin is the main, thin structural protein in the
sarcomere. Each actin molecule has a binding
site that can attach with a myosin cross bridge.
- Actin and mysoin are contractile proteins.
5Tropomyosin and troponin are thin proteins. They
are regulatory proteins.
- Tropomyosin covers the actin binding sites,
preventing their union with myosin cross bridges. - Troponin has three binding sites one binds to
tropomyosin, one to actin, and one to Ca ions. - When calcium combines with troponin,
tropomyosin slips away from its blocking position
between actin and myosin. - With this change actin and myosin can interact
and muscle contraction can occur.
6Skeletal muscle contraction is a molecular
phenomenon.
- The myosin cross bridges can bind to the actin,
pulling these thin filaments toward the center of
the sarcomere. This is the sliding filament
mechanism. - The width of the A band remains unchanged.
- The H zone is shortened horizontally.
- The I band decreases in width as the actin
overlaps more with the myosin. - Neither the thick nor thin filaments change in
length. They change their position with one
another. - The actin slides closer together between the
thick filaments.
7By the power stroke the myosin cross bridges pull
the thin actin filaments inward. The cross
bridges bind to the actin and bend inward.
- A single power stroke pulls the actin inward only
a small percentage of the total shortening
distance. Complete shortening occurs by repeated
cycles of the power stroke. - This interaction can occur when troponin and
tropomyosin are pulled out of the way by the
release of calcium. - The link between myosin and actin is broken at
the end of one cross bridge cycle. A cross
bridge returns to its original position and can
return to the next actin molecule position,
pulling the actin filament further.
8By excitation-coupling a series of events link
muscle excitation to muscle contraction.
- Calcium is the link for this process.
- A somatic efferent sends action potentials to
muscle fibers. - This neuron releases acetylcholine at the
neuromuscular junction. - This produces an action potential over the entire
muscle cell membrane. - This action potential passes along the membrane
of the T tubule in the central part of the muscle
fiber. - This action potential meets the membranes of the
adjacent sarcoplasmic reticulum (SR) deep inside
the muscle cell. - A permeability change is produced in the
membranes of the SR, releasing calcium from the
SR.
9The release of calcium ions from the SR into the
cytosol allows it to bind with troponin.
- By this event, tropomyosin is pulled off the
binding sites of actin, allowing the myosin cross
bridges to bind to actin and slide this protein. - Myosin cross bridges had been previously
energized by splitting ATP into ADP plus P. The
cross bridges have binding sites for this change.
This energy places the myosin cross bridges
into a cocked position. - The contact between myosin and actin pulls the
trigger, allowing myosin to pull the actin
toward the center of the sarcomere (power
stroke). This shortens the sarcomere. - P is released from the cross bridges during the
power stroke. ADP is released with power stroke
completion.
10The addition of a new ATP to myosin cross bridges
detaches them from actin. The bridges return to
their original conformation.
- The cross bridges return to their original shape
for a repeat of the power stroke cycle. - By the continued, repetition of the cycle, the
rowing of the myosin cross bridges slide the
actin toward the center of the sarcomere for
muscle contraction. - However, for this repetition, calcium ions must
be available.
11Relaxation of a skeletal muscle depends on the
reuptake of calcium ions, from the cytosol into
the SR.
- With the absence of calcium, troponin and
tropomyosin can resume their blocking role. - However, calcium can be released again from the
SR into the cytosol if a somatic efferent neuron
signals the muscle cell with another action
potential.
12A whole muscle is a group of muscle fibers.
- A muscle is covered by a sheath of connective
tissue. It divides the muscle internally into
bundles. Each individual muscle fiber is
enveloped by a layer of connective tissue. - A tendon attaches a muscle to a bone.
- A single action potential is a muscle fiber
produces a twitch. - Gradations of whole muscle tension depend on the
number of muscle fibers contracting and the
tension developed by each contracting fiber.
13A motor unit is one motor neuron and the muscle
fibers it innervates.
- Whole muscle tension depends on the size of the
muscle, the extend of motor unit recruitment,
and the size of each motor unit. - The number of muscle fibers varies among
different motor units. - Muscles performing refined, delicate movements
have few muscle fibers per motor unit. - Muscles performing coarse, controlled movements
have a large number of fibers per motor unit. - The asynchronous recruitment of motor units
delays or prevents muscle fatigue.
14The tension developed by a muscle depends on the
frequency of stimulation.
- Repetitive stimulation of a muscle increases its
tension by twitch summation. - Contractile responses (twitches) can add together
by two actions potentials signaling a muscle
fiber closely together in time. - If a muscle fiber is stimulated very rapidly, it
cannot relax between stimuli. The twitches merge
into a smooth, sustained, maximal contraction
called tetanus. - Twitch summation results from sustained elevation
of calcium in the cytosol.
15The tension of a tetanic contraction also depends
on the length of the fiber at the onset of
contraction.
- The optimal length is the resting muscle length.
It offers the maximal opportunity for
cross-bridge interaction. - At lengths other than the optimal length, not all
cross bridges are able to interact for muscle
shortening. - Whole muscle tension also depends on he extent of
fatigue and the thickness of the fiber. - Muscle tension is transmitted to bone as the
contractile component tightens the series-elastic
component. - The origin is the fixed end of attachment of a
muscle to a bone. The insertion is the movable
end of attachment. If - If muscle tension overcomes a load, it pulls the
insertion toward the origin.
16The two primary types of contraction are isotonic
and isometric.
- By isometric contraction the muscle tension
developed is less than its opposing load. The
muscle cannot shorten and lift the object with
that load. - By isotonic contraction the muscle tension
developed is greater than its opposing load. The
muscle usually shortens and lifts an object. The
muscle maintains a constant tension throughout
the period of shortening. - The velocity of muscle shortening is inversely
proportional to the magnitude of the load.
17Skeletal muscles can perform work.
- Work is calculated by multiplying the magnitude
of the load times the distance the load is moved
(Force times distance). - Much of the energy applied is converted into
heat. - About 25 percent is realized work.
- About 75 percent is converted to heat.
18Bones, muscles, and joints interact to form lever
systems.
- A lever is a rigid structure capable of moving
around a pivot. - The pivot is the fulcrum.
- The power arm is the part of the lever between
the fulcrum and the point where an upward force
is applied. - The load arm is the part of the lever between the
fulcrum and the downward force from the load. - Often the velocity and distance of muscle
shortening is increased to increase the speed and
range of motion of the body part moved by muscle
contraction. The muscle must exert more force
than the opposing load for this increased speed
and range.
19ATP is generated three ways for the muscle
contraction.
- Creatine phosphate plus ADP is converted
enzymatically to creatine plus ATP. This is the
first source of ATP for the first minute or less
of exercise. - Oxidative phosphorylation generates large amounts
of ATP in the mitochondria if oxygen is available
for the muscle cell. This supports aerobic
exercise. Some types of muscle fibers have
myoglobin which can transfer oxygen into muscle
cells. - Glycolysis makes a small amount of ATP in the
absence of oxygen. A net of 2ATPs is formed per
glucose molecule. This process uses large
quantities of stored glycogen and produces lactic
acid. The accumulation of this acid produces
muscle soreness. - Glycolysis supports anaerobic exercise. One
glucose molecule is converted into two molecules
of pyruvic acid. - It the absence of oxygen for the muscle cell,
pyruvic acid does not enter oxidative
phosphorylation. It is converted to pyruvic acid.
20These are two types of fatigue.
- Muscle fatigue occurs when an exercising muscle
can no longer respond to the same degree of
stimulation with the same degree of contractile
activity. - Factors for this include an increase in inorganic
phosphate, accumulation of lactic acid, and the
depletion of energy reserves. Increased oxygen
consumption is needed to recover from exercise
(paying off an oxygen debt). - Central fatigue occurs when the CNS can no longer
activate motor neurons supplying working muscles. - It is often psychological and is related to
biochemical changes at the synapses in the brain.
21The three types of skeletal muscle fibers are
- slow oxidative (type I) fibers
- fast-oxidative (type IIa) fibers
- fast-glycolytic (type IIb) fibers
- Most humans have a mixture of all three types of
fibers. - They are classified by the pathway used for ATP
synthesis (oxidative vs. glycolytic) and the
rapidity by which they split ATP and contract
(fast vs. slow). - Oxidative fibers are red with a high
concentration of myoglobin.
22Muscle fibers can adapt to the different demands
placed on them.
- Regular endurance exercise (e.g., long-distance
jogging) promotes improved oxidative capacity in
oxidative fibers. - The fibers use oxygen more efficiently.
- High-intensity resistance training promotes
hypertrophy of fast glycolytic fibers. The
fibers increase diameter. Testosterone promotes
protein synthesis for this increase. - The extent of training determines the
interconversion of the two types of fast-twitch
fibers. Whether a fiber is fast- twitch or
slow-twitch depends on its nerve supply. - Fast-twitch and slow-twitch fibers are
interconvertible. For example weight training
can convert fast -oxidative fibers to
fast-glycolytic fibers. - Skeletal muscles atrophy when not used. There is
disuse atrophy and denervation atrophy. - Skeletal muscles have a capacity for limited
repair.
23Multiple neural inputs influence motor unit
output. There are three inputs of control for
motor neuron output.
- (1) spinal reflex pathways that arise from
afferent neurons - (2) corticospinal (pyramidal) motor system that
arise from the primary motor cortex It is
involved mainly with the intricate movements of
the hands. - (3) pathways of the multineuronal
(extrapyramidal) motor system that originate from
the brain stem It is involved mainly with
postural adjustments and involuntary movements of
the trunk and limbs.
24Muscle receptors provide afferent information
needed to control skeletal muscle activity.
- This input can communicate changes in muscle
length (monitored by muscle spindles) and muscle
tension (monitored by Golgi tendon organs).
Golgi tendon organs are located in the tendons of
muscles. - A stretch reflex is triggered when a whole muscle
is passively stretched. - Muscle spindles are stretched. This triggers the
reflex contraction of that muscle. This response
resists passive changes in muscle length. - The classic example of the stretch reflex is the
patellar-tendon or knee jerk reflex.
25Smooth muscle composes the internal, contractile
organs except the heart. The heart is composed
of cardiac muscle.
- Smooth muscle cells are small and unstriated.
- These cells have actin and myosin. Their
arrangement is not organized compared to
skeletal muscle cells. Therefore, smooth muscle
cells are not striated. - Smooth muscle cells contract when calcium ions
enter the cells from the ECF. Calcium is also
released from intracellular stores. - This release activates a series of biochemical
reactions leading to myosin cross bridge
movement. - Myosin cross bridges are phosphorylated and bind
to actin.
26Multiunit smooth muscle is neurogenic.
- It has properties partway between skeletal muscle
and single-unit smooth muscle. - Smooth muscle is supplied by the involuntary
autonomic nervous system. - Multi-unit smooth muscle is found in the walls of
large vessels, the large airways of the lungs.
the ciliary muscle, the iris of the eye, and
the base of hair follicles. - Single-unit smooth muscle cells form functional
syncytia. - A syncyticium is a group of interconnected cells.
When an action potential develops in one cell,
it quickly spreads to other cells. - Therefore, the cells in a syncyticium contract as
a single, coordinated unit.
27Single-unit smooth muscle is myogenic.
- It is self-excitable. It does not require
nervous stimulation for contraction. It can
develop pacemaker potentials or slow-wave
potentials. - Automatic shifts in ion concentrations in the ECF
and ICF cause spontaneous depolarizations to
threshold potential. - By myogenic activity the smooth muscle develops
nerve-independent contractile activity. It is
initiated by the muscle itself.
28Single-unit smooth muscle can produce gradations
of contraction.
- It differs from the mechanism for producing the
gradations of skeletal muscle contraction. - It depends on the level of calcium ions in the
cytosol. - Many single-unit smooth muscle cells have enough
calcium in the cytosol to maintain tone (low
level of tension). This occurs in the absence of
action potentials. - Signaling by the ANS and hormones alter the
strength of self-induced, smooth muscle
contractions. - Other factors, such as local metabolites and
certain drugs, alter the contraction of smooth
muscle. - All of these influences alter the level of
calcium ions in the cells cytosol.
29Smooth muscle can develop tension when it is
stretched significantly. It inherently relaxes
when stretched.
- Its contraction is slow and energy-efficient.
- Single-unit smooth muscle can exist at a many
lengths without a change in tension. It is
well-suited for forming the walls of distensible,
hollow organs.
30Cardiac muscle has properties of skeletal and
smooth muscle.
- It is found in the walls of the heart.
- It is highly organized and striated. These are
similarities to skeletal muscle tissue. - It can generate action potentials which spread
throughout the walls of the heart. This is
similar to single-unit smooth muscle.