Geoff G. answered • 05/13/21
Extensive Experience Editing and Proofreading Formal Papers
There are two major roles for calcium in muscle contractions although one is more direct.
A normal skeletal muscle contraction requires stimulation from a motor neuron. The neuron releases neurotransmitter at the neuromuscular junction (NMJ), and when the neurotransmitter binds to the cell membrane of the muscle, sodium ions rush in which initiates the electrical signal which eventually leads to contraction in the muscle. The release of the neurotransmitter is stimulated by calcium entering the neuron near the NMJ. Voltage-gated calcium channels open when the action potential reaches the end of the neuron, and the entrance of calcium allows the neurotransmitter to be released.
Now let's go back to the electrical signal in the muscle cell. When we left off, it was initiated at the plasma membrane. Now it travels along the transverse (or T-) tubules where it eventually contacts the sarcoplasmic reticulum which is where calcium is stored in the muscle cell. The release of calcium is necessary for muscle contraction to occur - but, why?
The answer relates to the sliding filament mechanism. The actual contraction of a muscle is the result of certain protein filaments in the muscle to binding to other proteins and pulling on them. This binding cannot occur without calcium. The sites on the thin filament (actin) where the thick filament (myosin) attaches are blocked by a protein called tropomyosin. Tropomyosin in turn is attached to another protein called troponin. Calcium binds to troponin causing it to rotate and pull tropomyosin out of the way, allowing myosin to bind to actin and therefore allowing contraction to occur.
In summary calcium is necessary to initiate the electrical signal in the muscle cell and to allow the necessary protein binding for contraction to occur. Review the events occurring at the NMJ and the sliding filament mechanism and cross-bridge cycling.
