muscle by duggybrown


									        Muscle Physiology
Types of muscle

Contraction of skeletal muscle
Control of contractility and mechanical
Energetics of skeletal muscle

Comparison of cardiac and skeletal muscle

Comparison of smooth and skeletal muscle
               Skeletal Muscle

    Large, long, multinucleate cells. Multiunit control
      (ie each fibre can be activated independently
               Smooth Muscle

     Medium sized, small diameter fibres. Single nucleus.
 Multiunit: pilomotor. Mixed: blood vessel. Single Unit (i.e. all
fibres contract together): intestinal muscle
              Cardiac Muscle

Short diameter, medium length, branching cells. Single Unit
  Skeletal muscle; light microscope

Striations caused by ordered arangement of actin and myosin
  muscle: em


    Myosin   Z-disc
       Sarcoplasmic reticulum

    The sarcoplasmic reticulum acts as Ca2+ store
                 Triad Structure

An action potential from the outside of the cell travels down
The t-tubules. It is transmitted to a calcium release channel
on the SR. This opens, releasing Ca2+ into the cytoplasm
Actin, troponin and tropomyosin
           Muscle contraction

1) Myosin binds ATP and hydrolyses it to ADP and Pi
2) Myosin binds to actin
3) Myosin undergoes a conformational change in which the
actin is pulled forward
4) ADP is released, ATP binds and myosin dissociates from

Normally actin filaments are covered with troponin-
tropomyosin complexes, which prevents myosin binding. In
the presence of calcium (released from the sarcoplasmic
reticulum), the troponin allows the myosin to bind.
         Control of contractility (1)
 Recruit more motor units
              Motor Unit 1

                                   Muscle fibres

                Motor unit 2
             Control of contractility (2)
   Twitch v tetanus
          Single              Summation             Tetanus


        Action potential is much shorter than the contraction, so
        it is possible to re-excite the muscle fibre before the
        previous contraction has finished.
        Types of contraction
• Isometric contraction: muscle remains at
  the same length but produces tension
  (pushing, supporting).
• Isotonic contraction: muscle remains at the
  same tension but shortens (movement).
Force-length curve
• ATP is present at low concentrations and is hydrolysed
  to give ADP and Pi
• Large stores of creatinine phosphate (CP);
• Glycogen breakdown creates more ATP which can be
  used directly in contraction or to rephosphorylate
  creatinine. Glycogen breakdown is stimulated by
  increased [ADP]. It may be oxidative or anaerobic, in
  which case lactic acid is produced and the muscle goes
  into oxygen debt. When enough oxygen is available the
  lactate can be oxidised to give more ATP.
• Red (slow twitch) muscles have very high ATP
  generating capabilities and contain myoglobin; can
  produce sustained contractions. White (fast twitch)
  muscles produce powerful short contractions.
           Cardiac Muscle
Main differences from skeletal muscle
1) Source of Ca2+ for contraction
2) Single unit muscle
    Control of Contractile Force
• Calcium for contraction enters during the action
  potential which is as long as the contraction. The
  subsequent refractory period occurs when the
  muscle is at rest. Thus it is not possible to produce a
  tetanus (important for a pump!).
• Can get facilitation with increased frequency of
            Role of nerves
• Gap junctions ensure the spread of Ca2+
  from cell to cell so that the muscle
  behaves as a single unit. Contraction is
  initiated spontaneously. Nerves act to
  increase or decrease calcium entry.
               Smooth Muscle
Differences from skeletal muscle
1) Source of Ca2+ (extracellular)
2) Single unit muscle (sometimes)
3) Control of contraction
There is no troponin or tropomyosin. Myosin has extra
   subunits, myosin light chains (MLC), that prevent it from
   interacting with actin. Calcium activates an enzyme,
   myosin light chain kinase (MLCK) which phosphorylates
   the MLC. This allows myosin to interact with actin.
4) Structure
No striations.
The less regular oranisation
  of actin and myosin
  means that smooth
  muscle cells "round up"
  when contracting,
  producing a much greater
  degree of shortening.
  Due to the irregular
  arrangement of the actin
  and myosin, the muscle
  has a much broader
  working range over which
  the maximum tension can
  be developed.
• When a muscle is stretched, it initially has
  high tension. Over time, the cross bridges
  and filaments re-arrange to reduce this
  tension. Consequently smooth muscle can
  distend without exerting unacceptable
• There is a very low rate of cross-bridge
  cycling, so there is low ATP use.
  Consequently muscles are adapted for
  long, slow contractures.
       Force-Length curve

 Heart muscle fibres are usually overcompacted.
Stretching results in a greater force of contraction
          (Frank-Starling mechanism).

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