What is the outcome of ACh binding to its receptors on the muscle cell?

When acetylcholine (ACh) binds to its receptors on the muscle cell, it initiates a sequence of events that ultimately lead to muscle contraction. The primary outcome of this binding is a change in ion permeability across the muscle cell membrane. Specifically, the binding of ACh to its receptors opens chemically-gated ion channels, allowing sodium ions to flow into the muscle cell.

This influx of sodium ions decreases the membrane potential, causing depolarization of the muscle cell. Depolarization is a crucial step in the initiation of an action potential, which then propagates along the muscle cell membrane and into the transverse tubules. This electrical signal triggers further events leading to muscle contraction, such as the release of calcium ions from the sarcoplasmic reticulum.

Understanding this process is vital because it highlights the role of ACh not just as a neurotransmitter, but as a key initiator of the electrical changes that lead to muscle contraction. The other options do not accurately represent the immediate physiological events occurring after ACh binding—while muscle contraction eventually follows, it does not start immediately. Ion channels closing would counteract the depolarization, and myofibrils shortening is a result of contraction but is not the direct outcome of ACh binding