Introduction
An action potential (AP), also known as impulse, is a regenerating depolarization of membrane potential that propagates along an excitable tissue’s membrane.
Properties Of AP
Action potential:
- Is initiated by depolarization
- Obeys all or none principle (happens completely or not at all)
- Requires a minimum "threshold" voltage to be initiated
- Has constant amplitude and period (APs do not summate; information is coded by frequency, not by amplitude)
- Do not decay in strength
- Involves change in permeability of cell membrane to Na+ & K+ ions
- Relies on voltage-gated channels
- Is a very rapid process
- Is wave-like in nature
- Conduction of AP is an active process i.e. ATP is used
Ionic Basis/Events Of AP
The ionic basis of an action potential can be broken down into several distinct phases:
- Latency Period
- Depolarization
- Spike
- Repolarization
- Hyperpolarization
- Return to Resting Membrane Potential
Latency Period
This is an isopotential period in which the threshold stimulus does not generate action potential.
Depolarisation
- When the voltage generated by a stimulus reaches the threshold level
- Voltage-gated Na+ channels open up
- Na+ influx occurs
- Influx causes increasing positivity of the membrane potential {depolarization}
Spike
- A sharp rise (depolarization) and fall (repolarization) in the membrane potential produces a spike (or peak) potential
- It is the highest point of the action potential after which repolarization occurs
- When AP reaches the spike potential Na+ channels close
- Voltage-gated K+ channels open up
- K+ efflux occurs
- Efflux causes increasing negativity of the membrane potential {repolarization}
Repolarization
Hyperpolarization
- K+ channels close slowly leading to excess K+ efflux
- Membrane potential falls below the Resting Potential {hyperpolarization}
Return to Resting Membrane Potential
- Membrane potential becomes negative but ionic distribution has become unequal
- Na+/K+ pump restores Na+ and K+ conc. slowly by pumping 3 Na+ outward and 2 K+ inward
- Thus returning the membrane potential to the resting level
Graph Of AP
Conduction Of AP
Conduction of AP is the propagation of APs across the length of an axon. It is an active process and an irreversible one. There are two types of conduction of APs;
- Saltatory conduction: occurs in myelinated fibers and is faster than any other type of AP conduction because APs jump from one node of Ranvier to another
- Continuous conduction: takes place in unmyelinated fibers. AP is passed along to adjacent parts of the axon. Continuous conduction of AP is of two types
- Orthodromic conduction: AP is conducted in a forward direction
- Antidromic conduction: AP is conducted in a backward direction
Related Disorders
- Channelopathies: These are disorders associated with ion channels and result in the inability of affected cells or tissues to generate AP.
- Demyelination: A disorder in which neurons lose their myelin sheath. This affects the rate of conduction of AP.
- Myasthenia gravis: An autoimmune disorder that prevents the transmission of AP across a neuromuscular junction.
