Answer
An action potential from a somatic motor neuron cannot cross the synaptic cleft between its synaptic knob and the sarcolemma of the motor end plate of the opposing muscle fiber. Instead, it induces the release of acetylcholine from synaptic vesicles. The released ACH pours into the synaptic cleft and binds to acetylcholine receptors in the sarcolemma of the muscle fiber end plate.
Work Step by Step
The ACH acts as a ligand ; it opens ligand gated sodium ion channels in the sarcolemma and allow s Na+ ions to pour into the sarcoplasm. . Sodium ions pour in briefly , then K+ ions pour out . The result is a rapid depolarization and repolarization of the muscle cell . These events generate an endplate potential which if adequate generates an action pottential (AP) which is prppagated in all dirctions . The AP causes the sarcoplasmic reticulum to release calcium (Ca++) which regulates the positions of myosin and actin; this enables the binding of myosin heads to actin active sites ( using ATP generated energy) and thus effecting contraction.
After ACH has stimulated muscle contraction, it is broken down by acetylcholinesterase which is released from the basement membrane and the sarcolemma . This ends the activity of the neurotransmitter , and the muscle fiber is able to relax. The molecular fragments of ACH are reabsorbed by the synaptic knob of the somatic axon terminals for further use.
