What is an action potential?
An action potential is rapid depolarization followed by repolarization of the membrane after a threshold/supra-threshold stimulus is applied to an excitable membrane (nerve & muscle).
Different phases
- Stimulus artifact
- Latent period
- Firing level/Threshold
- Depolarization
- Repolarization
- Hyperpolarization
Electrical signals are critical to the function of the nervous system the human.
Different kinds of stimuli may be utilized for excitation such as electrical, mechanical, chemical, thermal, sound or light, etc.
Latent period:
It is a time it takes the impulse to travel along the axon from the site of stimulation to the recording electrode.
Threshold Potential (firing level):
It refers to the “magnitude” of depolarization of the excitable membrane at which the membrane undergoes spontaneous changes in permeability to different ions which leads to the generation of an action potential.
Differences:
| Graded Potential | Action potential |
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Graded potentials die out over short distances. However, when their magnitude is sufficient, they lead to the generation of action potentials.
The following graded potentials:
- Generator/receptor potential- cutaneous sensory receptors,
- Endplate potential- skeletal muscle.
- Post synaptic potential- the synapse.
- Pacemaker potentials- the heart.
- Slow-wave potentials- GI smooth muscle.
Ionic basis of action potential:
Action potentials take place as a result of the opening and subsequent closing of two specific types of channels.
Na+ channels that are closed at resting membrane potential (RMP) open rapidly upon depolarization allowing Na+ entry that further depolarizes the membrane and potentiates additional Na+ channel opening positive feedback.
Action potential generated by sequential changes in membrane permeability to ions (Na+ & K+).
- Resting (polarized) state:
- At the resting state; the inside of the membrane is negative & the outside is positive as K+ permeability is greater than Na+ permeability.
- Depolarization:
- Depolarization state, Membrane becomes very permeable to Na+ ions, allowing tremendous numbers of positively charged Na+ ions to diffuse to the interior of axon via Voltage-gated Na+ channels.
- Repolarization:
- It starts with K+ efflux due to the opening of voltage-gated K+ channels & decreases in further Na+ influx. It re-establishes the normal negative resting membrane potential (RMP).
- Hyperpolarization:
- It occurs due to late closure of voltage-gated K+ channels, more K+ efflux {potential is even more negative than resting membrane potential (RMP)}.
- The Na+-K+ pump gradually restores the concentration gradients of ions disrupted by action potentials.
Duration of action potential:
Unlike the variable or alter the duration of a graded potential, the duration of an action potential is always the same in a given excitable cell.
- In a neuron, an action potential lasts for only 1 Millisecond (0.001 sec).
- It lasts longer in muscle, with the duration depending on muscle type (200 to 300 Millisecond in cardiac muscle cell).
Physiological effects of action potentials:
These include
- Transmission of impulses along nerve fibers.
- Release of neurotransmitter or chemical transmitters in synapses.
- Muscle contraction occurred.
- Activation or inhibition of glandular secretion.
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