NMJ Physiology

What is the first step in muscle contraction?

Action potential signal (AP) arrives at the axon terminal of the motor neuron attached to the muscle fiber through the nueromusclar junction (NMJ)

What happens after the AP reaches the axon terminal?

Voltage gated Ca channels open and Ca floods into the motor neuron

What does Ca²⁺ cause inside the motor neuron?

Ca causes synaptic vesicles with acetylcholine (ACh) to form

How is ACh released into NMJ?

Vesicles release ACh into the NMJ via exocytosis

What happens after ACh is released?

ACh diffuses across the synaptic clef of the NMJ and bind to receptors on the junctional folds on the muscle fiber’s sarcolemma

What opens when ACh binds to its receptors?

Na-K ion channels of the sarcolemma open

What happens with Na⁺ and K⁺ ions?

Na enters and K exits the cell

What is the results of more Na⁺ entering than K⁺ leaving?

More Na comes in than K going out producing an end plate potential

What happens to ACh after it binds?

ACh is taken back up, degraded by acetylcholinesterase or is diffused away from the NMJ

What causes an AP to propagate along the sarcolemma?

Depolarization occurs (end plate potential spreads to adjacent pieces of the sarcolemma) and opens more Na channels that further depolarizes the sarcolemma and decreases voltage towards a threshold value that initates an action potential (AP) if exceeded and propagates (or moves the AP) using Na channels in adjacent pieces of the sarcolemma along the way to the T tubules

What happens during repolarization at the NMJ?

We begin repolarization where Na channels inactivate and close and K channels open (allows only K to go out of the muscle fiber) along with the Na-K pump to restore the resting membrane potential (RMP). Note: the muscle fiber cannot be triggered to contract again during this time! It is called the refractory period.

What happens when the AP reaches the T-tubule?

Now the AP has arrive at the T tubule and T tubule proteins change shape and stimulate the sarcoplasmic reticulum (SR) to open SR Ca channels via a protein in the SR membrane

What does the SR do in response to the T-tubule signal?

SR Ca channels release Cs into the sarcoplasm

What does Ca bind to in the sarcoplasm?

Ca binds to troponin which causes tropomyosin to move and expose the binding sites on actin molecules