Action Potential Overview Part 2, Lecture notes of Biology

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BISC 421: Neural signal transduction

Dr. Dion Dickman

USC

Mon/Wed/Fri

11:00 am – 11:50 am

Sal 101

 In the resting state, most Na + channels are closed. ◦ Non-gated K+^ channels are open. ◦ Negative-inside.

 If Na +^ channels open, the resulting influx of Na+ overwhelms the efflux of K+^.

◦ Net inward movement of (+)- charges.

◦ Now the cytosolic face of the membrane has a net (+)-charge.  Depolarized.  Moved towards E (^) Na.

 At peak, gated- K+^ channels open and causes an efflux of (+)-charge.

 Na +^ channels begin to close due to their intrinsic properties.

 Axon repolarizes.

Three channel types:

**1. Non-gated K +

2. Voltage-gated Na +
3. Voltage-gated K +**

Coordinated opening and closing of Na+/K+ channels

The refractory period is a period of time

after the initiation of one action potential

when it is impossible to initiate a second

action potential no matter how much the

cell is depolarized.

The refractory period

Regenerative nature of the Action Potential

• V (^) x = voltage at any distance c along membrane.
• Vo = voltage change at point of current injection.
• e = base of natural log.
  • r (^) i = resistance of the intracellular cytoplasm.
  • r (^) o = resistance of the extracellular fluid.
  • r (^) m = resistance across the membrane.

The length constant ( λ ) is an indication of how far a potential will spread along an axon in response to a subthreshold stimulus

The larger the length constant,

the more passive charge flow

The length constant

Capacitative current

• Propagation will be faster if:
  • λ is large so that charge spreads far.
  • The time constant ( τ ) for membrane charging is small.
• So you can increase velocity by:
  • Increasing the diameter of the axon
  • Increasing r (^) m – this can be done by myelination****.
• Large diameter neurons conduct faster than small diameter axons.

What parameters can change to conduct AP faster?

• What if myelin were to cover the whole axon?
  • How would Na +^ get in and K +^ get out?
• Nodes of Ranvier

Myelinated neurons conduct faster (Insulator!)

Myelin increases AP conduction

Multiple Sclerosis: Axonal demyelination