General Physiology of Muscle Contraction

Muscle contraction

The initiation and execution of muscle contraction occur in the following sequential steps.

  1. An action potential travels along a motor nerve to its endings on muscle fibers.
muscle contraction- action potential move along nerve fibre to neuromuscular junction
Action potentials move along nerve fibre to neuromuscular junction // Ganong's review of medical physiology
muscle contraction
. A motor unit consists of a motor neuron and the group of skeletal muscle fibers it innervates. A single motor axon may branch to innervate several muscle fibers that function together as a group. Although each muscle fiber is innervated by a single motor neuron, an entire muscle may receive input from hundreds of different motor neurons. // Guyton and Hall textbook of medical physiology

2. At each ending, the nerve secretes a small amount of the neurotransmitter acetylcholine.

3. The acetylcholine acts on a local area of the muscle fiber membrane to open “acetylcholinegated” cation channels through protein molecules floating in the membrane.

4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This action causes a local depolarization that in turn leads to opening of voltage-gated sodium channels, which initiates an action potential at the membrane.

5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes.

6. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.

 

Calcium ions released in muscle contraction
Calcium ions released
Sarcoplasmic reticulum in the extracellular spaces between the myofibrils, s
Sarcoplasmic reticulum in the extracellular spaces between the myofibrils, showing a longitudinal system paralleling the myofibrils. Also shown in cross section are T tubules (arrows) that lead to the exterior of the fiber membrane and are important for conducting the electrical signal into the center of the muscle fiber. (From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process.

 

The “walk-along” mechanism for contraction of the muscle.
The “walk-along” mechanism for contraction of the muscle. // Guyton and Hall Textbook of Medical Physiology

8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump and remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease.

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  1. Leticia April 5, 2022 Reply

    Very good write up. Highly educative

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