Question

Compare and contrast the excitation-contraction coupling mechanisms of skeletal muscle versus contractile cardiac muscle.

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Answer 1

Compare and contrast the excitation-contraction coupling mechanisms of skeletal muscle versus contractile cardiac muscle.

Excitation-Contraction Coupling ECC in cardiac muscle depends on the phenomenon called calcium-induced calcium release (CICR) in which the influx of calcium ions into the cell which further triggers the ion influx. The receptors of cardiomyocyte that bind to calcium ions which opens the channel during depolarization causes release of more calcium ion into the cell. In skeletal muscle, the initial depolarization is triggered by the influx of sodium ions. However, the influx of calcium ions sustains the depolarization of cadiac muscle which lasts longer. The CICR induced plateu phase makes the cell slightly positive before it becomes more negative as it repolarizes due to potassium ion influx whereas, the repolarization is rapid in skeletal muscles.

Pathway of Cardiac Muscle Contraction in cardiac muscle:

• Occurs via the sliding filament model of contraction.
• Myosin filaments slide along actin filaments to shorten or lengthen the muscle fiber for contraction and relaxation.

It can be described in five steps:

1. An action potential originated from the sinoatrial (SA) and atrioventricular (AV) nodes,
2. Conducted to contractile cardiomyocytes through gap junctions.
3. Calcium channels in the T-tubules caues influx of calcium ions into the cardiomyocyte.
4. Calcium binds to troponin-C triggers actin- myosin bindingand initiates contraction.
5. Actin filaments pulled toward the center of the sarcomere by the myosin head causes msucle contraction.

In skeletal muscle:

• Release of acetylcholine (ACh), a neurotransmitter by motor neurons.
• Binding to receptor in  the motor end plate.
• Action potential travels down the motor neuron’s axon.
• Permeability of the synaptic terminal membrane and an influx of calcium.
• Release of Ach in synaptic cleft.
• As a neurotransmitter binds, these ion channels open, and Na+ ions cross the membrane into the muscle cell.
• Causes voltage difference between the inside and outside of the cell, which is called depolarization.
• The depolarization then spreads along the sarcolemma and down the T tubules, creating an action potential.
• The action potential triggers the sarcoplasmic reticulum to release of Ca2+, which activate troponin and stimulate muscle contraction.