Problems with myelin theory

Hugo Witters hugo.witters at tornado.be
Fri Feb 13 08:55:56 EST 1998


Jeremy Leipzig <jeremyl at mindspring.com> wrote:

>Several neuroscience textbooks cite the ability of myelin to increase
>the capacitance of the neural membrane as one of its principal means of
>increasing the velocity of propagation of neural impulses.  Increasing
>the capacitance of the membrane (by effectively making it thicker) means
>less charge is needed to create the same potential difference across
>that membrane.
 
Capacitance is inversely proportional to the thickness of the
dielectricum. The thicker the myelin, the smaller the capacitance.

>I am confused by this claim for the following reasons:
>    1) There is no myelin in the Nodes of Ranvier where the
>voltage-gated ion channels are located.

Myelin acts as an insulator. No ions move through an insulator. There
is only a charge displacement within the atoms of the insulator when
an electric field is applied. Hence the lack of myelin at the nodes
where ions flow.

>    2) Increasing the capacitance of a membrane would increase the
>electric potential across that membrane but it would not increase the
>electric field.  The same way, for instance, walking up a flight of
>stairs increases your potential energy while your weight remains
>conserved.

The simple relationship between electric potential and electric field
is that the field is equal to the potential divided by the distance
across which the potential is applied, assuming a homogeneous
dielectricum. When applying a pulse such as an action potential of 100
mVpp across the insulating myelin, the lower the capacitance is, the
smaller the amount of charge absorbed by the the myelin at the leading
edge of the pulse, resulting in a faster transmission of this edge
across the axon.

>    Hence, the force exerted on ions would be the same regardless of the
>presence of myelin.  As I remember, voltage-gated ion channels are
>governed by the force exerted on charged amino acid helices that form
>pores through which ions can travel.  These channels would only respond
>to a change in the electric field, not merely electric potential.

The basic function of the myelin is to force the local electric
current accompanying the action potential at one node to the next site
of initiation, which because of the insulation provided by the myelin,
is the next node of Ranvier. This saltatory conduction is much faster
than with a smoothly moving action potential in a similar sized axon
without myelin.

Cheers, 
	Hugo





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