Action Potential

Question 1

Describe the ionic basis of the action potential

Answer 1

At resting potential, ion channels are closed. In response to an electrical input, the gates of the ion channel open and sodium enters the cell causing it to become depolarized. The depolarization is what triggers action potential once it reaches threshold. In response to depolarization, the channel is inactivated and voltage gated K channels open and K flows out with a large driving force to hyperpolarize the cell for a short time (undershoot)

Question 2

Define the currents that are responsible for the different components of the action potential.

Answer 2

Depolarization from ionic currents causes the membrane potential to go towards zero. Once it reaches -55, an action potential is triggered. Capacitative current moves the signal down the axon. Ionic current from potassium then results in hyperpolarization and the undershoot.

Question 3

Define the absolute and relative refractory period.

Answer 3

Absolute is when sodium channels are inactivated and no AP is possible. Relative is when AP is possible but requires greater stimulus as the previous AP is slightly hyperpolarized.

Question 4

Compare continuous and saltatory conduction

Answer 4

Continuous—unmyelinated, signal is propagated down the axon at a continuous speed. Saltatory—myelinated. Signal jumps between nodes of ranvier because they travel so quickly through myelinated section of axon.

Question 5

Describe the clinical correlates if myelin loss

Answer 5

AP conduction is slowed or blocked, disrupting the pathways function as conduction is not quick enough. Failures of sensory input and motor command. Example: MS

Question 6

Compare temporal and spatial summation

Answer 6

Temporal: two excitatory signals close together which together can lead to an action potential
Spatial: when two signals synapse onto the same space to trigger an action potential (looks like one signal)

Question 7

Why do action potentials only go in one direction

Answer 7

Current flow from adjacent regions or nodes causes depolarization to activate voltage gated Na+ channels. This flow is one way due to the refractory period of Na+ channels