Friday, March 23, 2012

Saltatory Conduction

Action potential propagation along a myelinated nerve axon is often said to occur by “saltatory conduction.” The 4th edition of Intermediate Physics for Medicine and Biology follows this traditional explanation.
“We have so far been discussing fibers without the thick myelin sheath. Unmyelinated fibers constitute about two-thirds of the fibers in the human body . . . Myelinated fibers are relatively large, with outer radii of 0.5 – 10 μm. They are wrapped with many layers of myelin between the nodes of Ranvier . . . In the myelinated region the conduction of the nerve impulse can be modeled by electrotonus because the conductance of the myelin sheath is independent of voltage. At each node a regenerative Hodgkin-Huxley-type (HH-type) conductance change restores the shape of the pulse. Such conduction is called saltatory conduction because saltare is the Latin verb ‘to jump.’ ”
I have never liked the physical picture of an action potential jumping from one node to the next. The problem with this idea is that the action potential is in fact distributed over many nodes simultaneously as it propagates along the axon. Consider an action potential with a rise time of about half a millisecond. Let the radius of the axon be 5 microns. Table 6.2 in Intermediate Physics for Medicine and Biology indicates that the speed of propagation for this axon is 85 m/s, which implies that the upstroke of the action potential is spread over (0.5 ms) x (85 mm/ms) = 42.5 mm. But the distance between nodes for this fiber (again, from Table 6.2) is 1.7 mm. Therefore, the action potential upstroke is distributed over 25 nodes! The action potential is not rising at one node and then jumping to the next, but it propagates in a nearly continuous way along the myelinated axon. I grant that in other cases, when the speed is slower or the rise time is briefer, you can observe behavior that begins to look saltatory (e.g., Huxley and Stampfli, 1949, J. Physiol, 108:315-339), but even then the action potential upstroke is distributed over many nodes (see their Fig. 13).

If saltatory conduction is not the best description of propagation along a myelinated axon, then what is responsible for the speedup compared to unmyelinated axons? Primarily, the action potential propagates faster because of a reduction of the membrane capacitance. Along the myelinated section of the membrane, the capacitance is low because of the many layers of myelin (N capacitors C in series result in a total capacitance of C/N). At a node of Ranvier, the capacitance per unit area of the membrane is normal, but the area of the nodal membrane is small. Adding these two contributions together leads to a very small average, or effective, capacitance, which allows the membrane potential to increase very quickly, resulting in fast propagation.

In summary, I don’t find the idea of an action potential jumping from node to node to be the most useful image of propagation along a myelinated axon. Instead, I prefer to think of propagation as being nearly continuous, with the reduced effective capacitance increasing the speed. This is not the typical explanation found in physiology books, but I believe it is closer to the truth. Rather than using the term salutatory conduction, I suggest we use curretory conduction, for the Latin verb currere, “to run.”

3 comments:

  1. Very Nice! Just one example of how you have helped me 'see.'

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  2. I'm a week behind. Thanks for the post on a topic that is particularly important to me. I'll enjoy considering two this week.

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  3. Thanks for link to '49 Huxley of which I am beginning a careful reading.

    His summary point #6,

    "Conduction was blocked reversibly by increasing the external resistance between two adjacent nodes. It is concluded that the action potential at each node excites the next node by current flowing forward in the axis cylinder and back in the fluid outside the myelin sheath."

    supports my contention that low intensity, pulsed, focused ultrasound can be employed to reversibly, noninvasively block nerve conduction.

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