Neurophysiology: Action Potentials

Exam 3 Chapter 11.6

Action Potentials (APs)

do not die out over distance

only occur in the axon (graded, threshold required)

“nerve impulse”

all or none: stronger stimulus does not produce larger AP

Depolarization of AP

Voltage-gated Na+ channels

fast to open and to close

Repolarization of AP

Voltage-gated K+ Channels

slow to open and close

Hyperpolarization of AP

Slow closing K+ Gates

Action Potential graph

6 steps from RMP to return to RMP

If all APs are the same, how does the integrator know that one stimulus is different from another?

Frequency!

stronger stimulus will produce MORE APs rather than larger ones

more NT = More APs

absolute refractory period

1-2 msec before another AP can be initiated

time to reset NA+ channel gates 

keeps APs from overlapping

Relative Refractory Period

Stronger than normal stimulus required to reach threshold

K+ Channels still open

7 Characteristics of APs

1. APs are produced when a graded potential reaches threshold

2. all-or-none

3. Depolarization is a result of increased membrane permeability to NA+ and movement of NA+ into the cell. activation gates of the Voltage-gated Na+ channels open

4. Repolarization is a result of decreased membrane permeability to Na+ and increased membrane permeability to K+, which stops Na+ movement into the cell and increases K+ movement out of the cell. the inactivation gates of the v-g Na+ channels close, and the v-g K+ channels open.

5. During the absolute refractory period, no AP is produced by stimulus, no matter how strong. during the relative refractory period, a stronger-than-threshold stimulus can produce an AP

6. APs are propagated and, for a given axon/muscle fiber, the magnitude of the AP is constant

7. Stimulus strength determines the frequency of APs

Graded potential vs Action Potential

GP and AP

GP and AP cont.