NMJ physiology

23 steps

What’s the 1st step in NMJ physiology

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

Whats the 2nd step in NMJ physiology

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

Whats the 3rd step in NMJ physiology

Ca causes synaptic vesicles with acetylcholine (ACh) to form

Whats the 4th step in NMJ physiology

Vesicles released ACh into the NMJ via exocytosis

Whats the 5th step in NMJ physiology

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

Whats the 6th step in NMJ physiology

Na-K ion channels of the sarcolemma open

Whats the 7th step in NMJ physiology

Na enters and K exists the cell

Whats the 8th step in NMJ physiology

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

Whats the 9th step in NMJ physiology

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

Whats the 10th step in NMJ physiology

Depolarization occurs (end plate potential spreads to adjacent pieces of the sarcolemma) and opens more Na channels that further depolarizes the sarcolemma and decrease voltage towards a threshold value that initiates 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

Whats the 11th step in NMJ physiology

Meanwhile, back at the beginning (at the NMJ) we begin repolarization where Na channels inactive 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

Whats the 12th step in NMJ physiology

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

Whats the 13th step in NMJ physiology

Sr Ca channels release Ca into the sarcoplasm

Whats the 14th step in NMJ physiology

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

Whats the 15th step in NMJ physiology

A cocked/ activated myosin head attaches its acting binding site to the myosin binding site of actin and forms the cross bridge

Whats the 16th step in NMJ physiology

ADP and P release from the myosin head

Whats the 17th step in NMJ physiology

Myosin head pivots and pulls actin towards the M line of the sarcomere (like a ratchet)

Whats the 18th step in NMJ physiology

ATP attaches to the myosin head, detaching it from actin (inactivation of mysosin head)

Whats the 19th step in NMJ physiology

ATP on the myosin head hydrolyzes to ADP and P

Whats the 20th step in NMJ physiology

Myosin head cocks/activates and is ready to attach again (repeats 15-20 until the muscle is fully contracted as in step 21)

Whats the 21st step in NMJ physiolody

Sarcomere shortens thus contracting the muscle fiber (this occurs in many sarcomeres so it shortens the “box cars” of sarcomeres) and movement of the muscle occurs

Whats the 22nd step in NMJ physiology

Ca is removed from troponin and is taken back up into the SR

Whats the 23rd step in NMJ physiology

Tropomyosin moves and re-covers the myosin binding sites on actin molecules and troponin locks it in place so it will no longer bind to the actin binding sites on myosin until another signal to contract is received