Unit 2: Lecture 12: Neurophysiology Pt.3

Action Potential

An electrical signal generated by neurons

occurs along the length of the axon

the ion channels that produce action potentials are mainly voltage-gated channels

Step 1 of Action Potential

cell starts at -70 mv= RMP

local potential at axon hillock increases until it rises to threshold=-60 mv

Step 2 of Action Potential

Neurons produce AP

voltage-gated Na+ channels open more and more

More Na+ enters the cell

K+ channels open very slowly when threshold is reached

rapid depolarization

Step 3 of Action Potential

Rush of Na+ ions enter to depolarize

Voltage peaks at approx +35 mv

K+ voltage gated channels are becoming fully opened

Step 4 of Action Potential

K+ leaves the cell repolarizing the membrane

causes a shift back to negative inside and positive on the outside of the cell

K+ channels remain open a little longer than the Na+ channels

More K+ ions leave the cell than Na+ ions causing hyperpolarization

Step 5 of Action Potential

Na+/K+ ATPase pump helps to restore cell to RMP

removes excess Na+

Gain the loss of K+

Absolute Refractory Period

Impossible to make another AP on a membrane segment

Threshold at +35 mv

All or None Principle

threshold met= AP will occur

threshold not met = No AP

Irreversible

Relative Refractory Period

Difficult to make an AP membrane segment

Can generate another AP but requires STRONG stimulus

needs to reach threshold with K+ leaving but greater force of Na+ entering

Sodium Voltage Gated Channels

RMP

Threshold

Peak

Repolarize

activation gate= outside of cell

inactivation gate= inside of cell

RMP voltage gate

-70 mv

Activation gate closed

inactivation gate opened

Threshold voltage gate

-60 mv

activation gate opened

inactivation gate opened

Peak voltage gate

+35 mv

activation gate opened

inactivation gate closed

Repolarize voltage gate

-50 mv

activation gate reclosed

Inactivation gate reopens

Propagation

Continuous

Saltatory

Continuous Propagation

unmyelinated axons

AP spreads to the next segment

Saltatory Propagation

myelinated axons

Salt=Skip

Does not have to propagate every single segment of the axon
This allows for much faster APs

Axon Diameter

Large Diameter= faster propagation

Smaller diameter= slower propagation

Type A= largest + myelinated

Type B= medium+ myelinated

Type C= Smallest + unmyelinated