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 THE NERVOUS SYSTEM AND MOVEMENT

MUSCLE CONTRACTION

The motor end plate (neuromuscular junction)

  • An impulse arrives at the motor end plate from the axon of a motor neurone.

  • Acteyl choline is released from by the motor end plate into the synapse.

  • Acetyl choline diffuses across the synapse and binds with the receptor sites on the sarcolemma (the muscle cell surface membrane).

  • The sarcolemma depolarises and an action potential is created (from –90 mV to +40mV) once the threshold of the muscle cell is reached (all-or-nothing response).

  • Infoldings from the sarcolemma (T-tubules) connect to a system of membranes called the sarcoplasmic reticulum that cover the myofibrils.

  • Acetyl choline is broken down in the synaptic cleft by choline esterase enzyme. and the products are reabsorbed by the motor end plate.
     
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  • The depolarised sarcoplasmic reticulum becomes permeable to Ca2+
  • Ca2+ diffuses into the cytosol (cytoplasm of the muscle fibre)
  • Troponin/tropomyosin protein complex blocks actin filament and stops myosin head groups from binding to it.
  • Ca2+ lifts the blockage
  • Myosin is an actin-activated ATPase, when it binds with actin it hydrolyses ATP to ADP and inorganic Phosphate.
  • As ATP is hydrolysed the conformation (shape) of the myosin head changes. Hence ATP hydrolysis is coupled to movement at a molecular level.
  • This change pulls the myosin along the actin filament (the "power stroke")
  • As ADP is released from the myosin, myosin detaches itself from actin.
  • Mysosin picks up another ATP and the cycle is repeated. The muscle contracts.
  • This contraction continues as long as Ca2+ levels remain high in the cytosol.
  • Ca2+ is rapidly pumped back across the sarcoplasmic reticulum (this also requires ATP) and the muscle relaxes.

 

© Paul Billiet 2004