I need help on the following:
Imagine that a motor unit, with its nerve intact, is placed in a solution that is calcium-free (but has the proper ionic strength – don’t worry how). Further imagine that the ends of the muscle fibers in the motor unit are held such that length is constant. If the nerve is stimulated and fires an action potential will the motor unit contract?
To determine whether the motor unit will contract in a calcium-free solution, we need to understand the role of calcium in muscle contraction.
Muscle contraction occurs when the nerve that innervates the muscle releases an action potential, which propagates along the nerve's axon. This action potential triggers the release of calcium ions from storage sites within the muscle fiber, specifically from a structure called the sarcoplasmic reticulum.
The released calcium ions bind to a protein called troponin, which is part of the muscle's contractile apparatus. This interaction causes a conformational change in the troponin-tropomyosin complex, enabling the myosin heads to bind to actin filaments. This binding initiates the sliding of actin and myosin filaments, resulting in muscle contraction.
In a calcium-free solution, however, there is no available calcium to bind to troponin. Without this binding, the troponin-tropomyosin complex remains in its blocked state, preventing the myosin heads from binding to actin. Consequently, muscle contraction cannot occur.
Therefore, if the nerve fires an action potential, but the motor unit is in a calcium-free solution, the motor unit will not contract.