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Nerve fibers and neurotrophins
Mixed nerves
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Nerves are made up of multiple axons, and all those axons have varying thresholds
When a subthreshold stimulus is applied, only the axons with the lowest threshold will fire; an electrode in that nerve
will only record a small potential change
The response to stimulus increases in proportion to the electrical stimulus- it’s a reflection of the fact that more and
more axons are brought to firing threshold
A stimulus that is required to excite ALL the axons is the MAXIMAL STIMULUS – any greater stimulus wont get a
greater response (because of the “all or none” law)
Latency
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When you apply a stimulus to a nerve, there is a LATENT period- the nerve takes a little while to respond with an
action potential.
If you know the latent period and the distance between the stimulating and the recording electrode, you can measure
AXONAL CONDUCTION VELOCITY
Classification of nerve fibers
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General rule: THE THICKEST FIBERS CONDUCT THE FASTEST
Erlanger and Gasser worked out a way to classify nerve fibers according to diameters, electrical characteristics, and
function (this got them the 1944 Nobel prize for physiology)
o They ended up with A, B and C fibers, with the A fibers being subdivided into alpha, beta, gamma and delta
IN ORDER OF THICKNESS:
A-Alpha
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proprioception and somatic motor fibers
fastest conduction velocity
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touch and pressure sensation
A-Beta
A and B fibers are myelinated
A- Gamma
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motor fibers to muscle spindles
A-Delta
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pain, cold and light touch fibers
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preganglionic autonomic fibers
B-fibers
C-fibers: dorsal root
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pain, temperature and some mechano-receptors
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postganglionic sympathetic fibers: slowest of all
C fibers are unmyelinated
C-fibers: sympathetic
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THE VARIOUS CLASSES OF FIBERS VARY ALSO IN THEIR RESPONSE TO LOCAL ANAESTHETICS AND TO HYPOXIA
C fibers get affected by local anaesthetic LONG BEFORE A-delta touch fibers; so when you stitch somebody, they may
feel no pain, but they may complain of a weird pulling and dragging sensation.
A-fibers are more rapidly affected by pressure, and pressure on a thick nerve may depress all motor touch and
pressure fibers while leaving pain fibers intact
Briefly about Neurotrophins
SUSCEPTIBILITIES:
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A –fibers are most susceptible to pressure
B-fibers are most susceptible to hypoxia
C-fibers are most susceptible to local anaesthetics
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C-fibers are least susceptible to hypoxia and pressure
A-fibers are least susceptible to local anaesthetics
References: Ganong Review of Medical physiology, 23rd ed, chapter 4
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Some of these growth hormones rae produced by the
innervated structures (eg. muscle) and others by the
astrocytes
These growth factors are absorbed into the nerve ending,
and transported to the neuron body by retrograde
transport
Schwann cells can also generate neurotrophins, causing a
severed axon to sprout in the direction of the distal stump
Sadly, CNS myelin is an inhibitor of nerve growth and
in the CNS axonal regeneration is rare. There just
aren’t any growth-promoting chemicals.