PHYSIOLOGY OF NERVE AND MUSCLE CELLS lecture 10

CONTRACTILE RESPONSES
It is important to distinguish between the electrical and mechanical events in muscle. Although one response does not normally occur without the other, their physiologic basis and characteristics are different. Muscle fiber membrane depolarization normally starts at the motor end-plate, the specialized structure under the motor nerve ending; the action potential is transmitted along the muscle fiber and initiates the contractile response.
The Muscle Twitch
•A single action potential causes a brief contraction followed by relaxation. This response is called a muscle twitch. The twitch starts about 2 ms after the start of depolarization of the membrane, before repolarization is complete.
Sequence of events in contraction and relaxation of skeletal muscle.
• Steps in contraction
•(1) Discharge of motor neuron.
•(2) Release of transmitter (acetylcholine) at motor end-plate.
•(3) Binding of acetylcholine to nicotinic acetylcholine receptors.
•(4) Increased Na+ and K+ conductance in end-plate membrane.
•(5) Generation of end-plate potential.
•(6) Generation of action potential in muscle fibers.
•(7) Inward spread of depolarization along T tubules.
•(8) Release of Ca2+ from terminal cisterns of sarcoplasmic reticulum and diffusion to thick and thin filaments.
•(9) Binding of Ca2+ to troponin C, uncovering myosin-binding sites on actin.
•(10) Formation of cross-linkages between actin and myosin and sliding of thin on thick filaments, producing shortening.
Steps in relaxation
(1) Ca2+ pumped back into sarcoplasmic reticulum.
(2) Release of Ca2+ from troponin.
(3) Cessation of interaction between actin and myosin.