Point 6 of 10 — Lesson 2: Movement in Plants and Muscle Contraction Mechanics in HumansQuestion bank: 3 questions (3 with pictures)
06 How the nerve impulse reaches the muscle
Resting state. The outer surface of the muscle-fibre membrane is positive and the inner surface negative (the fibre is polarised), because of different concentrations of ions inside and outside (more Na⁺ outside, more K⁺ inside). This gives a potential difference across the membrane.
Stimulus. The stimulus for contraction is the motor impulse coming from the brain or spinal cord along a motor nerve that is connected to the muscle fibre by a synapse (neuromuscular junction).
Transmitter release.The nerve ending containssynaptic vesicles filled with the chemical transmitter acetylcholine. When the impulse arrives at the ending, calcium channels open and Ca²⁺ ions enter the nerve ending; this causes the vesicles to fuse with the membrane and release acetylcholine into the synaptic cleft between the nerve ending and the muscle-fibre membrane.
Depolarisation. Acetylcholine changes the permeability of the muscle-fibre membrane, so sodium ions enter and the membrane becomes depolarised (positive inside, negative outside). Depolarisation spreads along the fibre and triggers contraction.
Repolarisation. After a fraction of a second, the enzyme cholinesterase, present in large amounts at the neuromuscular junctions, breaks down acetylcholine into choline and acetic acid. The stimulation stops, the membrane returns to its resting state (repolarisation), and the fibre is ready for a new stimulus.
Why is cholinesterase important? If acetylcholine were not destroyed, the membrane would stay depolarised, the muscle would be stimulated continuously, and could not relax or respond again to a new impulse (spasm).
Some poisons (organophosphate insecticides, nerve gases) inhibit cholinesterase and cause convulsions for this reason.
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