Action Potential Conduction

Introduction of Action Potential Conduction

The action potential is a transient depolarization of the membrane potential of excitable cells. Its main functions are to transmit and encode information and initiate multiple cellular events. Relative to extracellular space, the inside of unstimulated cells is at a negative potential, that is, the resting potential. Through stimulating processes, the action potential is generated in a section of the axon. A small electrical current will flow between the regions of different polarity and this current will depolarize the resting region of the cell membrane. A sufficiently large current in the local circuit will depolarize the quiescent region of the membrane to the threshold and initiate an action potential in this region.

Fig.1 The local circuit hypothesis underlying action potential conduction in an unmyelinated axon. (Fry, 2010)

Factors Affecting Action Potentials Conduction Velocity

Action potentials conduction along with excitable cells can be affected by multiple factors. For example, the speed of C-fibres from somatosensory receptors is as low as 0.5 m/s, whilst type IA afferent nerve fibers from muscle spindles propagate up to 120 m/s. And the cardiac impulse propagates even more slowly at 0.05 cm/s in tissues such as the atrioventricular node.

• Fiber diameter

Fiber diameter is the basic factor to determine propagation velocity. In general, the larger the diameter is, the greater the velocity will be.

• Myelination

For vertebrates, myelination is a key evolutionary to increase action potential conduction speed. Some studies have shown that a 10 μm myelinated fiber conducts an action potential at about 50 m/s which is almost twice as that in an invertebrate unmyelinated axon of about 500 μm diameter.

• Temperature, pressure, hypoxia

An increase in temperature will lead to an increase in ion channel dynamics and ultimately an increase in action potentials conduction speed. In contrast, the conduction speed would be slow when the body temperatures are lower than normal. However, conduction block may occur at temperatures > 40℃ and the generation of action potentials may stop, because K⁺ opening is more enhanced than Na⁺ channels. In addition, physical pressure applied to nerves and hypoxia also slow conduction speed.

• Ion channel number

The conduction velocity can be influenced by the number of ion channels contributing to the depolarizing phase of the action potential. A decreased ion channel number reduces the magnitude of local circuits and thus reduces conduction velocity.

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Reference

1. Fry, C.; Jabr, R. The action potential and nervous conduction. Surgery (Oxford). 2010, 28(2): 49-54.