132401
Description
Mind Map by harry_bygraves, updated more than 1 year ago
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Created by harry_bygraves
about 11 years ago
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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,
- 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
- 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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