gamma and alpha neurons in motor control

dag.stenberg at helsinki.nospam.fi dag.stenberg at helsinki.nospam.fi
Sun Dec 20 05:39:56 EST 1998


F. Frank LeFever <flefever at ix.netcom.com> wrote:
> In <367BB99B.807397A3 at uni-leipzig.de> Mark Elliott writes:
> >could someone explain why neurons involved in muscle coordination are
> >referred to as alpha and gamma neurons. Is it a property of their
> >firing frequency?

> My understanding is that these simply refer to the size (diameter) of
> the axons--alpha being big and gamma being small.

Correct: axons are classified into A, B and C groups (C thinnest and
unmyelinated), and the A group is subdivided into alpha, beta, gamma and
delta (delta is thinnest, but thicker than B).

> The gamma fibers innervate tiny muscles within the big muscle, within
> the "intrafusil cell", and- - - ...

The muscle spindle, or "fusus" (latin), contains a strain gauge-like
sensory organ telling the spinal cord how stretched it is, and also
little muscle fibers (intrafusal fibers), which are innervated by gamma
motoneurons, and increase the stretch on the sensory organ.
  (The "sensory organ" of the muscle spindle contains different
receptors, for fast and slow changes in stretch - fast adapting, and
slow adapting. Their afferent connections are also different axon types.
In addition, the gamma motoneurons are suibdivided into fast and
slow-acting - dynamic and static).

The stretch on the sensory organ increases by either
a) passive stretch of the muscle, like when a load is imposed on it
b) gamma motoneurons contract the intrafusal fibers, thus stretching the
sensory organ

The stretch on the sensory organ is diminshed by
a) active contraction of the muscle, if the muscle (and its muscle
spindles) then shortens. This is achieved by impulses of the alpha
motoneurons, which excite the working muscle fibers outside the spindles
(extrafusal fibers) 
b) diminished activity of the gamma motoneurons, thus relaxation of
intrafusal fibers

Stretch of the sensory organ is transmitted as impulses to the spinal
cord, where they excite the alpha motoneurons. This results in the
so-called "stretch reflex": passive stretch on the muscle will make it
contract, thus maintaining its previous length.
 
In voluntary contractions, alpha and gamma motoneurons usually work 
together (the "alpha-gamma linkage"). Suppose we would make a muscle
contract through activation of the alpha motoneurons. The muscle
including the spindles would shorten, and the sensory "strain gauge"
organ would send less impulses to the spinal cord, thus diminishing the
excitation of the alpha motoneurons. This would cancel out what the brain
wanted. We thus need to increase the stretch on the "strain gauge" in
proportion to the shortening of muscle we wished for. This is achieved
by activation of gamma motoneurons simultaneous with the activation of
the alpha motoneurons. 
  Likewise, if we wanted to let a muscle relax, the brain not only has
to decrease the excitation of alpha motoneurons, but that of gamma
motoneurons at the same time. The "strain gauge" will then be perfectly
happy to let the muscle relax in a lengthened state.

There are other aspects on alpha-gamma activation patterns as well, but
this is the most important one.

For Mark Elliott's information, ANY basic textbook in physiology or
neuroscience will contain these facts.

Dag Stenberg



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