To memorize or not to memorize nerve fiber diameters and conduction velocities
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E.S.Prakash
Dec 23, 2010, 4:33:20 AM12/23/10
to Medical Physiology at AUFOM
A Year 1 Student wrote Is it necessary for us to remember the diameter
and conduction velocity values of various types of nerve fibers
[Erlanger - Gasser classification]?
Reply: No, I wouldn't worry about memorizing a list of those values
except perhaps note that the action potential in the largest diameter
fibres (A alpha) is conducted at a speed of approximately 100 meters
per second (360 km/hour - faster than the fastest car). The diameter
of type A fibres is typically about 10-20 times higher than
unmyelinated Type C fibres (whose diameter is 1 micron). And the
average conduction speed in type C fibres is 1 meter per second.
You will need to note that:
Conduction speed is proportional to fiber diameter and fibres are
classified in descending order of fiber diameter.
Additionally, myelination increases nerve fiber diameter, and both of
these factors combined increase conduction velocity considerably i.e.,
it is about 50-100 times greater in Type A fibres compared to Type C
fibres.
I would advise trying to know one function subserved by each type of
fibre - the larger diameter of somatosensory and somatic motor neurons
explains their susceptibility to conduction blockade by pressure. In
contrast, unmyelinated type C fibres are most sensitive to blockade by
local anesthetics which block Na ion channels on nerve fiber
membranes.
Sympathetic postganglionic neurons are the slowest to conduct whereas
reflex responses mediated by A beta afferent neurons and A alpha
efferent motor neurons are several times faster. You could reflect
upon the survival value of faster somatic reflex responses.
May be we could ask why all nerve fibres did not get myelinated in
evolution - this might happen sometime in the future, but much more
space would be required to accommodate those huge nerves if we
"wanted" everything to be fast, and we would have to be much larger
E.S.Prakash
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and conduction velocity values of various types of nerve fibers
[Erlanger - Gasser classification]?
Reply: No, I wouldn't worry about memorizing a list of those values
except perhaps note that the action potential in the largest diameter
fibres (A alpha) is conducted at a speed of approximately 100 meters
per second (360 km/hour - faster than the fastest car). The diameter
of type A fibres is typically about 10-20 times higher than
unmyelinated Type C fibres (whose diameter is 1 micron). And the
average conduction speed in type C fibres is 1 meter per second.
You will need to note that:
Conduction speed is proportional to fiber diameter and fibres are
classified in descending order of fiber diameter.
Additionally, myelination increases nerve fiber diameter, and both of
these factors combined increase conduction velocity considerably i.e.,
it is about 50-100 times greater in Type A fibres compared to Type C
fibres.
I would advise trying to know one function subserved by each type of
fibre - the larger diameter of somatosensory and somatic motor neurons
explains their susceptibility to conduction blockade by pressure. In
contrast, unmyelinated type C fibres are most sensitive to blockade by
local anesthetics which block Na ion channels on nerve fiber
membranes.
Sympathetic postganglionic neurons are the slowest to conduct whereas
reflex responses mediated by A beta afferent neurons and A alpha
efferent motor neurons are several times faster. You could reflect
upon the survival value of faster somatic reflex responses.
May be we could ask why all nerve fibres did not get myelinated in
evolution - this might happen sometime in the future, but much more
space would be required to accommodate those huge nerves if we
"wanted" everything to be fast, and we would have to be much larger
E.S.Prakash
*****************************
E.S.Prakash
Dec 23, 2010, 4:57:22 AM12/23/10
to Medical Physiology at AUFOM, MBBS Batch 16
An important point I failed to include in my post below is the functional consequence of demyelination of myelinated motor or sensory nerve fibres. In this YouTube Clip [http://www.youtube.com/watch?v=TNPQFWk8fqs] please see Dan Powell, an art professor who lives with multiple sclerosis, speak about living with this disease.
Prakash
*************************
Prakash
*************************
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