närvilihasühendus

Närvilihasühendus, also known as the neuromuscular junction, is the specialized synapse where a motor neuron transmits a signal to a muscle fiber. This communication is crucial for initiating muscle contraction and thus for all voluntary movements. The process begins when an electrical impulse, the action potential, travels down the motor neuron to its axon terminal. At the axon terminal, the arrival of the action potential triggers the release of neurotransmitters, primarily acetylcholine, into the synaptic cleft, the small gap between the neuron and the muscle fiber. These acetylcholine molecules then diffuse across the synaptic cleft and bind to specific receptors on the muscle fiber membrane, known as nicotinic acetylcholine receptors. This binding causes a change in the membrane permeability, allowing ions to flow into the muscle fiber, leading to depolarization and the generation of an action potential in the muscle fiber itself. This muscle action potential then propagates along the sarcolemma and into the t-tubules, ultimately leading to the release of calcium ions from the sarcoplasmic reticulum. The increased intracellular calcium concentration is the direct trigger for the interaction of actin and myosin filaments, resulting in muscle contraction. Acetylcholine in the synaptic cleft is rapidly broken down by the enzyme acetylcholinesterase, ensuring that the signal is transient and allowing the muscle fiber to relax once the nerve impulse ceases.