I had long read (and long taught) about the importance of the stretch
reflex negative feedback control system in controlling motor activity.
That is until a herniated disk pressing on L4 and L5 eliminated the
reflexes in my right leg as well as cutaneous sense over a large area.
I lost a lot of muscle strength and found myself hesitating to put
full weight on my leg, as in climbing stairs, but that might have
related to reduction in motor activity, not loss of sensory.feedback.
I never noticed any problem in motor coordination or control.
My neurologist claimed that that was well known -- many people
function well without these reflexes and also wondered why the
stretch reflex was given such prominent attention in teaching about
motor control.
Of course, it does make a might pretty story, whether or not true!
tong po wrote in message <365AF50E.67F6F8F7 at gad.zooks.emy>...
>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.
>