Question

What initiates the contraction of the ventricular myocyte. List the steps starting with the action potential, those that cause crossbridge formation, and the process of relaxation (i.e. what happens to cytosolic Ca)?

Answer 1

The initiation of a ventricular myocyte contraction begins with an action potential that leads to the release of acetylcholine, which causes Ca²⁺ to be released from the sarcoplasmic reticulum, leading to cross-bridge formation between actin and myosin, muscle contraction, and eventually relaxation when calcium is reuptaken.

Step-by-step explanation:

The contraction of a ventricular myocyte is initiated by a sequence of events starting with an action potential. Here are the steps involved:

1. Acetylcholine (ACh) is released from the axon terminal of a motor neuron and binds to receptors on the sarcolemma.
2. An action potential is generated and travels down the T-tubule.
3. Calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum in response to the change in voltage.
4. Ca²⁺ binds to troponin causing the actin-binding sites to be exposed as tropomyosin shifts away, allowing myosin heads to form cross-bridges with actin.
5. The power stroke occurs as ADP and Pi are released from the myosin head, causing it to pivot and pull the actin filament inward, thus shortening the sarcomere and resulting in muscle contraction.
6. ATP binds to the myosin head, leading to the dissociation of the actin-myosin complex.
7. Hydrolysis of ATP provides energy for the myosin head to return to its original position, ready for a new cycle.
8. For relaxation to occur, Ca²⁺ must be removed from the cytosol. The sarcoplasmic reticulum reuptakes Ca²⁺ through active transport, which is ATP-dependent, leading to the dissociation of Ca²⁺ from troponin and subsequent relaxation of the muscle fiber.

The entire process is known as excitation-contraction coupling and is crucial for muscle fiber contraction and relaxation.