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Author Topic: What is the process of neuromuscular transmission at a neuromuscular junction?  (Read 6009 times)

Offline portugal

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Would someone be able to tell me the physiological events occuring at the axon and muscle fibre between the delivery of the stimulus and start of recorded response via electromyography?
« Last Edit: 04/05/2009 00:32:24 by chris »


 

Offline chris

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Hi there. Put simply, action potentials (waves of electrical activity) travel down the motor nerve fibre to its junction with the muscle - referred to as the neuromuscular junction (NMJ). Here the nerve ending is separated from the muscle surface by a small gap about 20nm wide (1/50,000'th of a millimetre), which is known as the synaptic cleft.

The arrival of the action potential triggers the nerve terminal to release a spurt of the transmitter chemical acetyl choline (ACh), which is stored in the nerve ending in tiny packets called vesicles. The ACh travels across the synaptic cleft where it binds to an array of acetyl choline receptors (AChRs) which are concentrated on the membrane of the muscle cell.

When the ACh binds to the receptors it triggers the opening of fast ion channels which allow sodium and calcium to flow into the muscle cells. This depolarises (excites) the muscle cell triggering other "voltage-gated" channels to activate, allowing more sodium and calcium to enter. These channels are also found on a specialised structure called the sarcoplasmic reticulum, which contains a large reservoir of stored calcium inside the cell.

The result is a flood of calcium into the cytoplasm of the muscle cell; calcium binds to a molecule called troponin C, which activates myosin, the major motor protein that causes muscles to contract.

The process is continuously being reversed by repolarisation of the muscle cell and re-sequestering of calcium back into the sarcoplasmic reticulum. If the the nerve supply is shut off the muscle stops contracting.

The flow of ions into and out of the muscle produces small but detectable electrical currents around the muscle, and even over the skin above the muscle. Sticking electrodes onto the skin, or even into the muscle allows the recording of these currents, which are displayed using an oscilloscope. The trace that you see is the EMG - electromyograph. It will show initally a depolarisation of the muscle cell (the cell becomes positive as large amounts of positively charged sodium and calcium ions flow in), followed by re- and hyper-polarisation (the cell will become negative as positively-charged potassium ions flow out to reset the membrane potential).

I hope that helps.

Chris
« Last Edit: 05/08/2007 12:28:08 by chris »
 

Offline pinky

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can anyone please tell me how to prepare 1mg% acetone solution whose specific gravity is 0.792
 

Offline Chemistry4me

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A 1mg% solution? ?????? Should that not be 1%?
 

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