Muscle contraction can be understood as the consequence of a process of transmission of action potentials from one neuron to another. A chemical called acetylcholine is the neurotransmitter released from the presynaptic neuron. As the postsynaptic cells on the muscle cell membrane receive the acetylcholine, the channels for the cations sodium and potassium are opened. These cations produce a net depolarization of the cell membrane and this electrical signal travels along the muscle fibers. Through the movement of calcium ions, the muscle action potential is taken into actual muscle contraction with the interaction of two types of proteins, actin and myosin.
Different steps of activities are involved in muscle contraction. The sequence of contraction process is originated in the central nervous system. It can be either a voluntary activity from the brain or a reflex activity from the spinal cord. An action potential passes outward in a ventral root of the spinal cord as a motor neuron in the ventral horn is stimulated. The axon branch to supply numerous muscle fibers or motor units and the action potential is carried to a motor end plate on each muscle fiber. Then the action potential releases quanta of acetylcholine into the synaptic clefts on the surface of the muscle fiber. Acetylcholine which is the neurotransmitter initiates an action potential which passes in both directions along the surface of the muscle fiber. Successively vesicles containing the neurotransmitter acetylcholine is bonded with the plasma membrane. Then acetylcholine is released into the extracellular space between the axon terminal and the motor end plate of the skeletal muscle fiber. The action potential spreads inside the muscle fiber at the opening of each transverse tubule onto the muscle fiber surface.
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