Synaptic transmission 2

As an axon approaches a muscle it loses its myelin sheath and branches extensively to form several areas of contact with different muscle fibres. Each point of contact between the motor neurone and one muscle fibre is a special plate-like synapse called a neuromuscular junction or motor end plate. It lies in a shallow much infolded depression on the surface of the muscle fibre. Synaptic vesicles are clustered in groups opposite the infolded regions of the sarcolema. As with a neurone-to-neurone synapse, a neuromuscular junction has a small gap between the membrane of the muscle fibre called a synaptic cleft
A neuromuscular junction functions in a simialr way to a cholinergic synapse. Acetylecholine is always the neurotransmitter and, in vertebrate skeletal muscles, is always excitatory
The main events that take place when a nerve impulse passes along a motor nerve to a neuromuscular junction are; when an action potential reaches a neuromuscular junction, calcium ion channel proteins open and calcium ions diffuse into the synaptic cleft, the diffusion of calcium ions causes synaptic vesicles to move to the junction membrane an fuse with it, acetylcholine is released from the vesicles iinto the synaptic cleft, acetylcholine diffuses across the cleft and attaches onto receptor molecules on the sarcolema, receptor activation causes sodium ion channels to open in the membrane of the muscle fibre, an influx of sodium ions into the sarcoplasm leads to a localised depolarisation of the sarcolemma, the graded potential does not obery the all-or-nothing principle; its amplitude increases with the intensity of the stimulus until the stimulus reaches a threshold level,
1. At the threshold level of stimulation, enough actylcholine is released by the vesicles to generate an action potential across the muscle fibre, causing it to contract
Immediately after an action potential, actetylcholinesterase breaks down acetlycholine to ensure the muscle fibre is not over stimulated, and the sarcolema becomes repolarized. The acetylecholine is then resynthesised and stored in the synaptic vesicles
If a neuromuscular junction recieves a contimous stream of axon action potentials at high frequency, eventually transmission across the junction stops. This is because the neurotransmitter can not be resynetheised fast enough and it runs out. The synapse becomes fatigued.

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