Nerves Action Potentials and Conductance

Question 1

Explain the concept of threshold in terms of the underlying ion channel activity

Answer 1

An action potential is typically triggered by depolarization, which opens voltage gated Na+ channels A degree of depolarization, termed threshold, will result in generation of an action potential 50% of the time If an action potential is generated it is an all or none phenomenon

Question 2

Describe how changes in resting membrane potential affect the action potential.

Answer 2

Modest depolarization brings the membrane closer threshold and can make it easier for an action potential to be achieved. More extreme depolarization, like staying over threshold, can make it impossible to generate another action potential because it is the threshold voltage that opens VG Na+ channels. Hyperpolarization brings the membrane further away from threshold can make it harder for an action potential to occur (relative refractory period)

Question 3

Describe the refractory period and how it is produced mechanistically.

Answer 3

Period of time where it is harder to generate another action potential, or where you absolutely cannot generate one. Absolute: no action potential can be generated regardless of how much stimulus is applied. Na+ inactivation gates reset based on time, no amount of voltage can change this. Relative: an action potential can be generated but more stimulus is required to due so (this is due to the fact that the membrane potential is hyperpolarized because of increased K+ conductance)

Question 4

Explain the propagation of an action potential down an axon and describe how the refractory period contributes to this.

Answer 4

EPSP’s and IPSP’s sum at the axon hillock. If sum of charges is less than threshold, VG Na+ channels open and charge begins to flow down the axon. At nodes of Ranvier, the flowing charge opens more V.G. Na+ channels. The area behind, which has just undergone an action potential, is in its refractory period. Initially Na+ channels are inactivated and you cannot generate another action potential (absolute refractory period). After that, the membrane potential is briefly hyperpolarized. So you can get an action potential, but the stimulus has to be great enough to overcome the extra polarization (relative refractory period). So great stimuli = more frequent Action Potentials.

Question 5

How would extremely elevated extracellular K+ affect action potential generation?

Answer 5

Cannot generate another action potential, because you would be above threshold and crossing threshold is what would be required to open the voltage gated Na+ channels again.

Question 6

How would low extracellular K+ affect action potential generation?

Answer 6

More potassium would leave the cell through leak channels down the concentration gradient. So, membrane would be hyperpolarized, and you would be further away from threshold (i.e. you would need a great stimulus to generate an A.P.)

Question 7

What would be the result of a toxin blocking V.G. K+ channels?

Answer 7

You would stay above threshold and continue to dump neurotransmitter into the synapse. Muscle contractions/spasms.

Question 8

What would be the result of a toxin blocking V.G. Na+ channels?

Answer 8

Could not generate an action potential. Paralysis. Probably death.

Question 9

Draw out and label the parts of a depolarization vs time graph for a typical action potential.

Answer 9