what are the events leading to the shortening of a muscle . what causes its relaxation ?
Mechanism of Muscle Contraction − Sliding Filament Theory
Muscle contraction is initiated by the signal sent by the CNS.
Neural signal, on reaching the neuromuscular junction, releases a neurotransmitter (acetylcholine) that generates action potential in the sarcolemma.
Action potential spreads through the muscle fibre and causes release of calcium ions in the sarcoplasm.
These calcium ions bind with troponin subunits. Hence, the masking of the active sites of myosin is removed.
Exposed active sites on actin now binds with the myosin head to form a cross bridge.
Cross bridge formation pulls the attached actin filament towards the centre of A band. The Z line attached to the actin is also pulled in, leading to muscle contraction.
Myosin releases ADP + Pi, and relaxes. A new ATP binds and the cross bridge is broken. ATP is again hydrolysed by the myosin head, and cycle of cross bridge formation and breakage is repeated, causing further sliding.
Process continues till calcium ions are pumped back and actin filaments are masked again. The Z lines return to their original position and the muscle relaxes.
The mechanism for muscle contraction can be summarised as:
(1) The sequence of events leading to contraction is initiated somewhere in the central nervous system, either as voluntary activity from the brain or as reflex activity from the spinal cord.
(2) A motor neuron in the ventral horn of the spinal cord is activated, and an action potential passes outward in a ventral root of the spinal cord.
(3) The axon branches to supply a number of muscle fibers called a motor unit, and the action potential is conveyed to a motor end plate on each muscle fiber.
(4) At the motor end plate, the action potential causes the release of packets or quanta ofacetylcholine into the synaptic clefts on the surface of the muscle fiber.
(5) Acetylcholine causes the electrical resting potential under the motor end plate to change, and this then initiates an action potential which passes in both directions along the surface of the muscle fiber.
(6) At the opening of each transverse tubule onto the muscle fiber surface, the action potential spreads inside the muscle fiber.
(7) At each point where a transverse tubule touches part of the sarcoplasmic reticulum, it causes the sarcoplasmic reticulum to release Ca++ ions.
(8) The calcium ions result in movement of troponin and tropomyosin on their thin filaments, and this enables the myosin molecule heads to “grab and swivel” their way along the thin filament. This is the driving force of muscle contraction.