MBBS FIRST YEAR

1. ACTION POTENTIAL & ITS IONIC
BASIS
Presented by : KAIF Q.

2. ACTION POTENTIAL
Definition:
An Action Potential is a brief sequential change of membrane potential after
application of a threshold stimulus.
• It can be recorded by a CATHODE RAY OSCILLOSCOPE

3. • We know that the inside of the nerve membrane is
negative with respect to the outside [POLARISED
STATE]
(RMP=—90 mv)
• When an effective stimulus(threshold or
suprathreshold) is applied, the electrical charge on the
membrane is reversed ie at the active part of the nerve
fibre the outside becomes negative as compared to
the corresponding region in the interior. This is called
DEPOLARIZATION and forms the Action Potential.

4. PHASES OF AN ACTION
POTENTIAL:
Phase 1: Depolarization
Phase 2: Repolarization
Phase 3: Hyperpolarization

5. ACTION POTENTIAL

6. DEPOLARISATION
LATENT PERIOD:
It is the time period between the application of a
stimulus and the start of the response (Action Potential)
FIRING POTENTIAL: The sharp change observed in the MP
which rapidly goes upto 0 and ultimately crosses the ‘0’
line to become positive.
SPIKE POTENTIAL: The pointed peak in the graph ,
attained due to depolarisation and rapid repolarisation

7. 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

13. WHY DOES THE
DEPOLARIZATION NOT
REACH THE NERNST
POTENTIAL OF +65MV 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.

16. • REPOLARIZATION: A return to the resting membrane potential from either
direction (i.e. de- or hyperpolarization) is called repolarization.
• HYPERPOLARIZATION: When during the transit changes in the action potential,
the Potential difference between the inside of the membrane (-90mv) and the
outside (0mv) increases it is called Hyperpolarization.

17. REPOLARISATION

21. AFTER REPOLARISATION