Thursday, March 29, 2012

Neuromuscular Junction

Neuromuscular junction is the junction of an axon terminal with the motor end plate, which in turn is the region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon. The axon terminals of a motor neuron contain vesicles similar to the vesicles found at synaptic junctions between two neurons. The events occurring at the neuromuscular junction are as follow: when an action potential in a motor neuron arrives at the axon terminal, it depolarizes the plasma membrane, opening voltage-sensitive calcium channels and allowing calcium ions to diffuse into the axon terminal from the extracellular fluid. This calcium binds to proteins that enable the membranes of acetylcholine-containing vesicles to fuse with the neuronal plasma membrane, thereby releasing acetylcholine into the extracellular cleft separating the axon terminal and the motor end plate.

Acetylcholine (ACh) diffuses from the axon terminal to the motor end plate where it binds to receptors (of the nicotinic type). The binding of ACh opens an ion channel in each receptor protein. Both sodium and potassium ions can pass through these channels. Because of the differences in electrochemical gradients across the plasma membrane, more sodium moves in than potassium out, producing a local depolarization of the motor end plate known as an end-plate potential (EPP). Thus, an EPP is analogous to an EPSP (excitatory postsynaptic potential) at a neuron–neuron synapse. The magnitude of a single EPP is, however, much larger than that of an EPSP because neurotransmitter is released over a larger surface area, binding to many more receptors and opening many more ion channels.

Neuromuscular junction: the motor axon terminals are embedded in grooves in muscle fiber's surface