ACTION POTENTIAL

Dr. Ayisha QureshiProfessor & HoD

MBBS, MPhil

Objectives

By the end of the lecture, student should be able to:

• Define an Action Potential.

• Differentiate between a threshold, sub-threshold and supra-threshold stimulus.

• Draw and label the Action Potential of a nerve fiber.

• Name the various phases of the Action Potential of a nerve fiber.

• Explain the ionic basis of the various phases of the Action Potential of a nerve fiber.

DEFINITIONS:

Stimulus: A stimulus is an external force or event which when applied to

an excitable tissue produces a characteristic response.Subthreshold stimulus:A stimulus which is too weak to produce a response is called a

Subthreshold stimulus.Threshold stimulus:The minimum strength of stimulus that can produce excitation

is called a Threshold stimulus.Suprathreshold stimulus:Stimuli having strengths higher than threshold stimulus are

called Suprathreshold stimuli.

VOLTAGE-GATED CHANNELS

Channels and their activation

• Activation of Channels: Channel opening to allow ion flow is called channel activation.

For example, Na channels and K channels of axons are both activated by changes in voltage.

• Inactivation of Channels: The closing of channels to stop the flow of ions is called Inactivation.

This is similar to doors with an automatic open close mechanism. The door opens when you hit the button; then after a certain period of time, it closes itself, whether you are still standing in the doorway or not (Like the door of the lift).

AN INACTIVATED CHANNEL RETURNS TO ITS NORMAL RESTING STATE SHORTLY.

POINTS TO NOTE:

• Voltage-gated Sodium channels: are fast channels & have 2 gates:

- An inner Activation gate (closed in resting state)- An outer Inactivation gate (open in resting state)

(These are different from Sodium Leak channels.)

• Voltage-gated Potassium channels: are slow channels & have only ONE gate.

(These are different from Potassium Leak channels.)

• The Sodium-Potassium PUMP is present.

Important • When ion channels open, ions may move INTO or OUT of the cell.

• The direction of ion movement depends on the electrochemicalgradient of the ion.

• Potassium ions usually move out of the cell (called EFFLUX).

• Sodium, Chloride and Calcium usually move into the cell (called INFLUX).

WHAT HAPPENS WHEN IONS MOVE INTO & OUT OF THE CELL?

WHAT HAPPENS WHEN IONS MOVE INTO & OUT OF THE CELL?

The net flow of ions across the membrane depolarizes or

hyperpolarizes the cell, creating an electrical signal.

WHAT IS DEPOLARIZATION, REPOLARIZATION & HYPERPOLARIZATION?

• Polarization: Any time the value of the membrane potential is other than 0 mV, in either the positive or negative direction, the membrane is in a state of polarization.

• Depolarization: The membrane becomes less polarized; the inside becomes less negative or more positive than at resting potential, with the potential moving closer to 0 mV (e.g. a change from – 90 to – 80mv).

• Hyperpolarization: The membrane becomes more polarized; the inside becomes more negative than at resting potential, with the potential moving even farther from 0 mV (for instance, a change from -90 to -100 mV).

• Repolarization: The membrane returns to resting potential after having been depolarized or hyperpolarized.

REVIEW

• Would a cell with a resting membrane potential of – 70 mV depolarize or hyperpolarize in the following cases?

(a) Cell becomes more permeable to Ca2+.

(b) Cell becomes less permeable to K+.

• Would the cell membrane depolarizeor hyperpolarize if a small amount of Na+

leaked into the cell?

ACTION POTENTIAL

Action Potential

Definition:

An Action Potential is a self-propagating wave of electro-positivity that passes along the inner surface of the nerve fiber membrane from the cell body to the axon terminals.

- Duration: only 1 msec (same in all neurons). - Also called: A Spike (because of its spike-like appearance).

- Action potentials are long-distance signals because they do not diminish/ decrease in size as they travel from the site of initiation throughout the length of the cell membrane.

Phases of an Action Potential

An Action Potential has 3 phases:

Phase 1: Depolarization

Phase 2: Repolarization

Phase 3: Hyperpolarization

IONIC BASIS OF AN ACTION POTENTIAL:

1. DEPOLARIZATION: Sodium (Na) Influx

2. REPOLARIZATION: Potassium (K) Efflux

3. HYPERPOLARIZATION: Leakage of excess Potassium (K) ions through the slow closing K channels.

4. RETURN OF THE AP TO THE RMP FROM HYPERPOLARIZATION: Sodium-Potassium Pump

State of SODIUM channel gates:

• Resting state:- Inactivation gates: OPEN- Activation gates: CLOSED

• Depolarization: - Activation gates: OPEN- Inactivation gates: OPEN

• Peak: - Inactivation gates: CLOSED- Activation gates: OPEN

• Repolarization:- Inactivation gates: OPEN- Activation gates: CLOSED

Why does the depolarization not reach the Nernst potential of +66mv for sodium?

Why does the depolarization not reach the Nernst potential of +66mv for sodium?

There are 2 main reasons. At +35 mv:

• Sodium Influx stops because Inactivation gates of Sodium channels close although the activation gates are open & thus no sodium can enter.

• Potassium Efflux starts because slow Potassium channel gates open and potassium moves out.

DEFINITIONS:LATENT PERIOD:It is the time period between the application of a stimulus and

the start of the response (Action Potential)DEPOLARIZATION:During an action potential when the potential difference

between the inside of the membrane (-90mv) and outside (0mv) decreases it is called depolarization.

REPOLARIZATION:A return to the resting membrane potential from either

direction (i.e. de- or hyper-polarization) is called repolarization.

HYPERPOLARIZATION: During an action potential when the potential difference

between the inside of the membrane (-90mv) and the outside (0mv) increases it is called Hyperpolarization.