The neuromuscular junction (NMJ) is a specialized synapse between a motor neuron and a muscle fiber. It plays a crucial role in converting electrical signals from the nervous system into mechanical muscle contractions. Here are the key physiological events that occur at the NMJ:
- Action Potential Arrival: The process begins when an action potential travels down the motor neuron and reaches the axon terminal at the NMJ.
- Calcium Influx: The arrival of the action potential triggers the opening of voltage-gated calcium channels, allowing calcium ions (Ca²⁺) to flow into the axon terminal.
- Vesicle Fusion: The influx of calcium ions causes synaptic vesicles, which contain the neurotransmitter acetylcholine (ACh), to fuse with the presynaptic membrane and release ACh into the synaptic cleft.
- Acetylcholine Binding: ACh diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the postsynaptic membrane of the muscle fiber.
- Ion Channel Opening: The binding of ACh to nAChRs causes these ligand-gated ion channels to open, allowing sodium ions (Na⁺) to enter the muscle fiber and potassium ions (K⁺) to exit.
- End-Plate Potential (EPP): The movement of ions generates an end-plate potential, which is a local depolarization of the muscle fiber membrane. If the EPP reaches the threshold, it triggers an action potential in the muscle fiber.
- Muscle Contraction: The action potential propagates along the muscle fiber, leading to the release of calcium from the sarcoplasmic reticulum and initiating the contraction of the muscle.
- Acetylcholine Breakdown: After ACh has done its job, it is rapidly broken down by the enzyme acetylcholinesterase (AChE) into choline and acetate, which are then recycled.
These events ensure that the nervous system can effectively control muscle activity, allowing for precise and coordinated movements.