Excitation-Contraction Coupling

Question 1

Step 1

Answer 1

An action potential travels down the motor neuron and arrives at the axon terminal at the Neuromuscular Junction.

Question 2

Step 2

Answer 2

This causes the voltage gated Ca2+ channels to open allowing Ca2+ to diffuse into the axon terminal, moving down its electrochemical gradient.

Question 3

Step 3

Answer 3

The entrance of Ca2+ activates the synaptic vesicles to release acetylcholine via exocytosis.

Question 4

Step 4

Answer 4

At the motor end plate, ACh diffuses across the synaptic cleft and binds to ACh receptors at the sarcolemma.

Question 5

Step 5

Answer 5

The binding of ACh causes chemically gated Na+ and K+ channels to open. Due to the greater concentration gradient, there is an influx of Na+. Because the K+ gated channels open slowly, the K+ can not leave the muscle until they are fully open. Causing depolarization of the membrane potential of the sarcolemma at the motor end plate.

Question 6

Step 6

Answer 6

A depolarization wave in response to the change occurs, the Na+ voltage-gated channel’s adjacent patch to the motor end plate open to allow more Na+ to enter the muscle.

Question 7

Step 7

Answer 7

The initial depolarization wave creates an action potential.

Question 8

Step 8

Answer 8

The action potential propagates along the sarcolemma from the voltage-gated Na+ channels down the T tubules to the sarcoplasmic reticulum in which voltage-gated channels open to release Ca+.

Question 9

Step 9

Answer 9

The Ca+ binds to troponin and removes the blocking action of tropomyosin, allowing the myosin heads of the thick filament to attach to the actin active site of the thin filament, forming cross bridges. Giving us the Sliding Filament Model of Contraction.

Question 10

Step 10

Answer 10

In the Sliding Filament Model of Contraction, the myosin head is raised and binds to the actin site hydrolyzing ATP. ADP and Pi are released and the myosin head pivots and bends pulling the actin filament towards the M line.

Question 11

Step 11

Answer 11

ATP then binds to the myosin and the link between the myosin and the actin weakens, detaching the myosin head in the process.

Question 12

Step 12

Answer 12

As the newly attached ATP is hydrolyzed to ADP and Pi, the myosin head returns to its cocked position, ready to reattach. This process continues until the muscle is fully contracted.

Question 13

Step 13

Answer 13

At the same time, at the motor end plate, the enzyme acetylcholinesterase is released and binds to the ACh to break it down into choline and acidic acid causing the influx of Na+ and efflux of K+ to stop and the chemical-gated channels to close.

Question 14

Step 14

Answer 14

The sodium potassium pumps brings the membrane potential back to resting potential.

Question 15

Step 15

Answer 15

This reversal of charge, back to negative, causes the Na+ voltage-gated channels to close and by this time the K+ voltage-gated channels have finally opened and K+ is leaving the cell.

Question 16

Step 16.

Answer 16

This allows for a repolarization wave, which propagates down the T-tubules and closes all voltage-gated channels.

Question 17

Step 17

Answer 17

Along the sarcolemma many Na+ and K+ pumps are helping to re-establish the resting membrane potential which travels down the muscle fiber and eventually closes the calcium gated channels. Which in turn causes the proper site for myosin and actin to not line up and breaks the bond which causes the muscle to go into the relaxed state.