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Muscle Physiology

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Title: Muscle Physiology


1
Muscle Physiology
Chapter 8
2
There 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.

3
A 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

4
The 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.

5
Tropomyosin 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.

6
Skeletal 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.

7
By 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.

8
By 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.

9
The 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.

10
The 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.

11
Relaxation 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.

12
A 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.

13
A 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.

14
The 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.

15
The 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.

16
The 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.

17
Skeletal 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.

18
Bones, 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.

19
ATP 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.

20
These 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.

21
The 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.

22
Muscle 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.

23
Multiple 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.

24
Muscle 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.

25
Smooth 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.

26
Multiunit 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.

27
Single-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.

28
Single-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.

29
Smooth 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.

30
Cardiac 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.
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