Lecture 4- Action Potential

Voltage-gated ion channels

open/close by membrane potential changes

Why is the membrane potential at rest relative negative?

Although Na+ wants to come inside, there are only a few Na+ channels open. There are many K+ channels open, and K+ leaves the cell. This is why resting potential is negative

Action Potential

The electrical signal used for neural communication

What is action potential

It is the active response to a neuron when it is depolarizing ( becoming more positive)

Why are voltage gated channels important to action potential

because every part of action potential is dependent on these voltage gates

What are the parts of action potential

1. threshold

2. upstroke

3. downstroke

4. afterhyperpolarization

5. relative refractory period

What is action potential caused by? (threshold)

Ions flowing through the voltage gated channels. There is an early inward current of sodium (na+) and late outward current of potassium (k+)

Explain threshold

at equalibrium Na+ influx is balanced by k+ eflux. At threshold there is more na+ ions entering. This triggers the voltage gated channels to open, and as more voltage gated channels (self-regenrative) are opening the causes an increase in na+ influx (ALL OR NOTHING AT THIS POINT)

Upstroke

Strong na+ influx, weak K+ efflux (due to k+ channels not being open yet)

Downstroke

strong K+ efflux (potassium channels are open – strong driving force), weak Na+ influx (sodium channels have inactivated – weak driving force) Neuron repolarizes

Afterhyperpolarization

K+ is slow to open and slow to close. This is why the cell becomes even more negative (hyperpolarizes) despite the cell reaching its original starting state (POTASSIUM EFLUX)

Inactivation

Dependent on the voltage of the surroundings

Deactivation

passive recovery to recover from depolarization

Give example of inactivation

The Na+ channels are inactivated by a ligand because the inside of the cell becomes too positive at one point, so a ligand comes to attach to the ion channel.

Give example of deactivation

They open when depolarized and stay open until neuron gets back into resting potential

Absolute Refractory Period

Occurs when sodium channels are inactivated, during this time an action potential is impossible to occur again

Relative Refractory Period

follows after the Absolute Refractory Period. time after an Action

Potential when enough Na channels have recovered from inactivation to trigger an action potential, but K efflux is still active so the cell is hyperpolarized

Can action potential trigger during relative refractory period

YES! more stimulus is required through to trigger action potential

why are refractory periods important to neural activity

because it ensures that action potential only goes downstream (in ab. refractory period AP cannot be triggered)

Important points

-ions that flow during action potential is relatively small compared to the total ions surrounded inside and outside the neuro

-little effect on the total number of ions

-Resting ion concentrations are rapidly re‑established by the movement of ions

Properties of Action Potentials

Initiated at the axon hillock

• Threshold potential

• All or none

• Non-decremental

• Refractory periods

• Very rapid

Why is myelin important for action potential

speeds up

Where are voltage gated channels mainly located

Voltage gated channels are restricted mainly to breaks in the myelin

called Nodes of Ranvier

Mutliple Scelrois

When myelin is damaged, cannot properly send the action potential signals.APs conducted more slowly or

unreliably. APs may fail to be conducted to the

axon terminal