Which type of ion channel mediates the depolarization of the muscle fiber membrane at the motor end plate?

The correct option is ligand-gated ion channels. At the motor end plate, which is the junction between a motor neuron and a skeletal muscle fiber, the neurotransmitter acetylcholine is released from the motor neuron and binds to specific receptors on the muscle fiber membrane. These receptors are part of ligand-gated ion channels.

When acetylcholine binds to these receptors, it causes a conformational change in the ligand-gated ion channels, allowing sodium ions (Na+) to flow into the muscle fiber. This influx of sodium ions depolarizes the muscle membrane, shifting the membrane potential in a positive direction. The depolarization is crucial for the initiation of action potentials in the muscle fiber, leading to muscle contraction.

While other types of ion channels exist, they do not play a direct role in mediating the depolarization at the motor end plate in this context. Voltage-gated ion channels primarily respond to changes in membrane potential and are involved in action potentials, while mechanically-gated channels respond to physical deformation or pressure, and leak channels are generally responsible for maintaining resting membrane potential rather than initiating depolarization.