Z-line Z-lineActin Myosin

Na+

Na+ Na+Na+

Na+ Na+

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“Polarized” Neuromuscular Junction

Neuromuscular Junction at “Resting Potential”

K+

K+

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1. A “signal” is sent from the brain down the axon of a motor neuron.

“signal” direction

Action Potential!!!

**Please note – Aside #1

• In this case a “signal” is a depolarization wave which travels away from the brain and spinal cord and towards the neuromuscular junction.

The question to ask at this point is, what is a depolarization wave?

To explain this we must go back for a moment and look at resting potential and polarization.

To maintain resting potential, the sodium-potassium pump (Na+/K+) pumps out 3 sodium ions (Na+) for each 2 potassium ions (K+) pumped into the neuron. This results in more potassium ions inside and more sodium ions on the outside.

Outside neuron

Unequal pumping (3 Na+ out to 2 K+ in) results in more positive charge on the outside compared to the inside. The membrane is polarized.

Some K+ channels are open so K+ tends to leak out. This adds to negative charge inside. The charge difference prevents further leakage.

When the neuron cell body is stimulated, an electrical impulse is sent down the axon. This impulse is simply the movement of “charged particles” (ions) across a semi-permeable membrane. Think of it as “Sodium in/Sodium out” in its simplest form.

“Sodium in” = depolarization

“Sodium out” = repolarization

Na+ Na+

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2. The signal hits the terminal bud (button) of the axon.

3. The vesicles break open releasing the neurotransmitter, acetylcholine, in to the synaptic cleft.

4. The acetylcholine crosses the synaptic cleft and binds to receptor sites on the sarcolemma.

*Please Note – Aside #2

• The “lock & key” mechanism of membrane binding.

• Please refer to the “board” and draw along.

5. The sarcolemma undergoes a permeability shift from Na+ impermeable to Na+ permeable.

6. Na+ ions rush in to the muscle fiber through channels in the sarcolemma, but they “overshoot” equilibrium.

7. Cholinesterase, an enzyme that “destroys” acetylcholine, deactivates and removes the acetylcholine from the receptor sites on the sarcolemma.

8. The permeability of the sarcolemma shifts back to Na+ impermeable thus trapping the Na+ ions inside the muscle fiber.

9. The neuromuscular junction is “depolarized” with respect to Na+.

Steps 1 through 9, documented as “Action Potential” are actually the steps of depolarization. The depolarization of the neuromuscular junction also means that the sarcolemma of the muscle fiber has been depolarized, with respect to Sodium. The second part of action potential is “repolarization” and the steps for this are as follows:

1. K+ ions diffuse out of the muscle fiber to help balance the positive charge “imbalance” caused by the Na+ being trapped inside the fiber.

2. Na+ ions are “pumped” out of the muscle fiber and K+ ions are “pumped” back in by the active transport mechanism known as the Na+/K+ pump in a ratio of 3 Na+ out to 2 K+ in.

Inside Muscle Fiber

Outside muscle fiber

3. Once the original balance of Na+ and K+ is restored, the NMJ is said to be repolarized.

**Please note - This means the Na+ is mostly back in the synaptic cleft, outside the muscle fiber, and the K+ is on both sides, but more is inside the muscle fiber.

Inside muscle fiber

Action Potential

• Depolarization followed by Repolarization of the sarcolemma of the muscle.

• 1. The neuromuscular junction, and thus the sarcolemma, goes from polarized to depolarized to repolarized, which is the same as polarized again.

• 2. The neuromuscular junction goes from resting potential through action potential back to resting potential.

The trick here is that depolarization of the N.M.J. is what stimulates the muscle to go through contraction, while repolarization stimulates relaxation.

Depolarization of NMJ Contraction

Repolarization of NMJ Relaxation

However, the timing of these events is as follows:

Time in milliseconds0 x

Depolarization

of NMJ Repolarization

of NMJ

Contraction

of sarcomere

Relaxation

of sarcomere

Another way of looking at the timing is as follows:

Time in milliseconds0 x

Depolarization

of NMJ

Repolarization

of NMJ

Contraction

of sarcomere

Relaxation

of sarcomere

Our Goals !!(a reminder!)

• 1. To define and track the sequence of “electrical” events which happen in a phenomenon called “Action Potential”.

• 2. To define and track the sequence of interactions between myofilaments that occur during contraction and relaxation of muscle tissue.

• 3. To determine the connections between numbers one and two above.