Muscle rigidity in cats with severed spinal cords

tong po tp at gad.zooks.emy
Tue Nov 24 13:03:59 EST 1998


eladyt at my-dejanews.com wrote:

> Hello,
>
>   I have been told that when the spinal cord of a cat is cut, the
> muscles
> controlled by the cut nerves become rigid. This suggests that motor
> commands
> to muscle "tell" the muscle how much to relax, and not how much to
> contract.
>
>    Could anyone point me to a reference about this phenomena (in cats
> or
> possibly in humans) ?
>
>

Rigidity is a cardinal feature of  an "upper motor neuron syndrome",
where the corticospinal tract has been disrupted above the level of the
spinal motoneurons (which project to the muscles).  Disrupting the
motoneuron-to-muscle pathway results in "lower motor neuron syndrome"
which is characterized by flaccidity rather than rigidity (as well as
other things).  Briefly, the physiology underlying rigidity/spasticity
is that once the descending control over the spinal motoneurons and
interneurons is eliminated, the stretch reflex (via 1a afferents) acts
unopposed, thus any slight stretch of a muscle produces strong
contractions of that muscle, leading to rigidity.  It's not so much that
motor commands "tell muscles to relax rather than contract", but that
they are involved in mediating/gating the entire spinal motor apparatus
- not just direct excitation of motoneurons but also gating the stretch
reflex, etc. via their projections to interneurons.  A long-standing
(albeit controversial and, most likely, wrong) theory of motor control
is the "equilibrium point hypothesis", which suggests that descending
motor commands simply set the gain of the stretch reflex of appropriate
muscles to specific values such that the limb (or whatever) moves
passively to the appropriate "equilibrium position".    Anyway, any
basic systems Neuroscience (e.g. Kandel, Schwartz and Jessel) or
Physiology text will describe this in greater detail.




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