1. Arrival at the Synapse:
* The nerve impulse (action potential) reaches the axon terminal of the motor neuron, which is the end of the neuron that connects to the muscle cell.
2. Neurotransmitter Release:
* The arrival of the action potential triggers the release of a neurotransmitter called acetylcholine (ACh) from synaptic vesicles within the axon terminal.
* ACh is stored in these vesicles and is released into the synaptic cleft, the small gap between the neuron and the muscle cell.
3. Binding to Receptors:
* ACh diffuses across the synaptic cleft and binds to specific receptors on the muscle cell's membrane, called acetylcholine receptors.
4. Muscle Cell Depolarization:
* The binding of ACh to its receptors causes a change in the permeability of the muscle cell membrane, allowing sodium ions (Na+) to flow into the cell.
* This influx of Na+ depolarizes the muscle cell membrane, initiating a muscle action potential.
5. Muscle Contraction:
* The muscle action potential travels along the muscle cell membrane and into the interior of the muscle fiber via structures called T-tubules.
* This triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum, a network of internal membranes within the muscle cell.
* Ca2+ binds to proteins called troponin and tropomyosin, which are associated with the thin filaments (actin) in the muscle fiber.
* This binding causes a conformational change in these proteins, exposing binding sites on the actin filaments for myosin heads to attach.
* The myosin heads then bind to actin, forming cross-bridges. The myosin heads use energy from ATP to pull the actin filaments towards the center of the sarcomere (the basic unit of muscle contraction), causing the muscle to shorten and contract.
6. Termination of Signal:
* The signal is terminated by the enzyme acetylcholinesterase, which breaks down ACh in the synaptic cleft. This prevents continued muscle stimulation.
* Calcium ions are actively pumped back into the sarcoplasmic reticulum, allowing the muscle to relax.
In short, the nerve impulse triggers a series of events that lead to the release of calcium, which then initiates the muscle contraction cycle.
