Action Potential Propagation

Action potentials are a means of sending a rapid, ultimately non-decremental signal from one part of a cell to another The last block considered the question of how Aps are initiated at one spot of excitable membrane This one will consider how they travel throughout an excitable cell. Subsequent lectures will show how a message that takes the form of an AP can jump from one cell to another.

What is a decremental signal?

Passive or decremental signals lose intensity exponentially as they spread within a cell

V x =V 0 (-x/  rm/ri ) Where V 0 is the membrane potential at the point where the signal is initiated, V x is the membrane potential at some distance x away along the axon, rm is the transmembrane resistance and ri is the axoplasmic resistance. The length constant describes the rate of voltage decrease with distance.

The time constant tau is a measure of how rapidly the membrane potential can change This equation is of the form used for any process of exponential change – it just says that the voltage at time t is equal to the initial voltage multiplied by e (the base of the natural logarithms) raised to the power of t/tau. Tau =RC Where R is the membrane resistance and C is the membrane capacitance

An AP spreads by sending a decremental signal ahead of itself, that activates voltage-sensitive Na + channels in adjacent membrane

The rate at which an action potential can spread into adjacent membrane is entirely dependent on how much adjacent membrane it can depolarize to threshold. This is determined almost entirely by geometry.

The speed of propagation of the AP is dramatically increased by increasing the diameter of the axon Effect on the length constant: The larger the diameter of the cell, the smaller the surface/volume ratio. Thus, as a cylindrical cell is made larger, core resistance decreases more rapidly than membrane resistance. As core resistance gets lower relative to membrane resistance, less of the initial current leaks out per unit distance.

Small-diameter axon Large diameter axon

This fact has led to the evolution of giant axons in some invertebrates and lower vertebrates Giant axons of the order of 1 mm diameter can achieve conduction velocities of the order of 20 m/sec. Such axons are found in the pathway that drives escape responses in squid.

A second strategy to increase conduction velocity of neurons involves decreasing the membrane resistance through electrical insulation. Such axons are said to be myelinated. Myelination has reached its highest evolution in vertebrates. The insulation is formed by glial cells that wrap processes around axons. Myelin-forming cells in the CNS are called oligodendrocytes - those in the peripheral nervous system are called Schwann cells. Myelination

The speed advantage is gained because each length of internode is crossed by a decremental current The process is not decremental in the macroscopic sense because an action potential regenerates the signal at each node. This mode of conduction is called saltatory (jumping) Myelinated axons can achieve conduction velocities as high as 120 m/sec. In vertebrates, myelination is characteristic of pathways that carry information about muscle length/joint position and rapidly changing stimuli delivered to the body surface.

Multiple Sclerosis, a demyelinating disease

Altered Conduction Patterns with MS

What causes multiple sclerosis? It is an autoimmune disease in which the immune system attacks oligodendrocytes or the myelin itself within the central nervous system (the brain and spinal cord). Conduction rate drops and sometimes fails. Treatment usually involves drugs that suppress the immune system.