Question

explain how the opening of the AChR found in skeletal muscle is able to lead to depolarization when the ion channel allows both na+ and k+ to flow through the ion channel

Answer 1

Each motor neuron axon terminal divides into a cluster of synaptic end bulbs .suspended in the cytosol within each endbulb are hundreds of membrane-enclosed sacs called synaptic vesicles. Inside each synaptic vescicles are thousands of molecules of acetylcholine. The muscle fiber part of the neuromuscular junction is the sarcolemma opposite the synaptic end bulbs This region of sarcolemma is called motor endplate. Within each motor endplate are 30 to 40 million acetylcholine receptors.which are integral transmembrane proteins that bind specifically to ACh.

A Nerve impulse elicits a muscle action potential in the following way.

1)RELEASE OF ACETYLCHOLINE:

The arrival of a nerve impulse at synaptic end bulbs causes many synaptic vesicles to undergo exocytosis . During exocytosis synaptic vesicles fuse with motor neurons plasma membrane which liberates Ach into the synaptic cleft. The ACh then diffuses across the synaptic cleft between the motor neuron and the motor endplate.

2)ACTIVATION OF ACH RECEPTORS:

Binding of two molecules of ACh to the receptor opens the ion channel part of the ACh receptor. Once the channel is open small cations most importantly sodium ions can flow across the membrane.

3)PRODUCTION OF MUSCLE ACTION POTENTIAL:

The inflow of sodium ions(down its electrochemical gradient)makes the inside of muscle fiber more positively charged. This change in membrane potential is signal that triggers a muscle action potential . Each nerve impulse normally elicits one muscle action potential. The muscle action potential then propagates along the sarcolemma and into the T tubule system.

4)TERMINATION OF ACH ACTIVITY:

The effect of Ach binding lasts only briefly because Ach is rapidly broken by an enzyme called acetylcholine esterase(AchE). This enzyme is attached to collagen fibers in extracellular matrix of the synaptic cleft.AChE breaks down Ach into acetyl and choline , products that cannot activate the Ach receptor.

If another nerve impulse releases more Ach steps 2 and 3 repeats.When action potential ceases in the motor neuron , Ach release stops and AchE rapidly breaks down the Ach already present in the synaptic cleft. This ends the production of muscle action potential and calcium ions release channels in the sarcoplasmic reticulum membrane close.