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From Guyton and Hall Textbook of Medical Physiology Fourth, South Asia edition, writer, 5 -mark short answer on Describe Neuromuscular Transmission

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Neuromuscular Transmission

Neuromuscular transmission is the process by which an action potential in an alpha motor neuron produces an action potential and contraction in a skeletal muscle fiber. It occurs at the neuromuscular junction (motor end plate).
Structure
  • The motor nerve terminal lies in a depression of the muscle membrane called the synaptic gutter.
  • It is separated from the muscle membrane by a synaptic cleft, about 20-30 nm wide.
  • The postsynaptic membrane has deep subneural folds containing nicotinic acetylcholine receptors and voltage-gated Na⁺ channels.
  • The nerve terminal contains numerous synaptic vesicles filled with acetylcholine (ACh). Acetylcholinesterase is present in the cleft.
Steps of transmission
  1. A nerve action potential reaches the presynaptic motor nerve terminal.
  2. Depolarization opens voltage-gated Ca²⁺ channels, causing Ca²⁺ influx.
  3. Ca²⁺ triggers fusion of ACh vesicles with the presynaptic membrane and release of ACh by exocytosis. About 125 vesicles may be released with one nerve impulse.
  4. ACh diffuses across the cleft and binds to nicotinic ACh receptors on the motor end plate.
  5. These ligand-gated channels open, allowing mainly Na⁺ influx, producing a local depolarization called the end-plate potential (EPP).
  6. The EPP opens nearby voltage-gated Na⁺ channels, generating a muscle action potential. This spreads along the sarcolemma and T-tubules, initiating excitation-contraction coupling and muscle contraction.
  7. ACh is rapidly hydrolyzed by acetylcholinesterase into acetate and choline. Choline is taken back into the nerve terminal for synthesis of new ACh.
Safety factor: Normally, the EPP is much larger than the threshold required to trigger a muscle action potential. Therefore, each nerve impulse usually produces one muscle action potential and contraction.
Source: Guyton and Hall Textbook of Medical Physiology, Chapter 7, “Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling,” pp. 104-107.
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