It is a theory that suggests that muscle movement is not directly controlled by the central nervous system but rather that the central nervous system (CNS) supplies the final point of a movement as lengths for the tensor and extensor muscles. This idea suggests that if the muscles were to encounter extra force that the muscles and the peripheral nervous system would manage this themselves without extra information flow with the CNS.
Thumb Tensor Experiment.
The most illustrative experiment was performed by Rothwell et al. In 1982. It uses the main muscle in the thumb in two subjects. The first subject is a normal man with full feeling in his hand and the second is a man with a rare neurological condition that results in him effectively having no sensor nerves and so is unable to get any feedback about the load on his limbs. The subjects are unable to see their thumbs and asked simply to move them from one angle to another. There were two sets of conditions for this experiment were as follows:
- In the first case the thumbs are simply allowed to move freely with a small constant load. This forms the control for the experiment.
- In the next case the viscosity of the load is increased fivefold. So it feels like they are moving their thumb through thick treacle (I am unsure if they actually used treacle but see no reason why they shouldn't).
In the first experiment the position/time graphs are very similar but the treacle experiment showed a slight overcompensation i.e. The thumb overshot the final position. In the normal man this overshoot is corrected very quickly but in the numb man the final position is slightly beyond the required 20 degree movement.
The most interesting result from this experiment is that the electro-magnetogram (EMG) from the thumb tensor in the numb (deafferented) man is the same in both situations. That is the CNS gives the thumb the same information in both cases even though the force generated by the muscles is very different in the two cases.
Monkey Torture Experiment.
This experiment is very much the same as the one above. It involves monkeys that have been trained to point to the light that is lit in front of them at one of a set positions by rewarding them with food. Some of the monkeys are then taken away through the RSPCA pickets to have their nerves severed so that they are in the same condition as the numb man in the above experiment. The experiments are then repeated. The conditions are similar to the thumb experiments except that the force applied to the limb is not constant but instead a small push is given either towards or away from the target. Again the results from both sets of monkeys are very similar and show that the disturbances make very little difference to the final position with or without nervous feedback.
From the above it can be seen that the CNS can be seen to only specify the final position and not "how to get there". This can be further developed by considering the two muscles involved in a movement the flexor and the extensor. When a joint is stationary these two are in equilibrium both in terms of length and force (to balance any external force) and since length and force (which can be seen as tension in the muscle) are intertwined concepts in muscles it is not unreasonable to combine the three values into a new graph of length/tension versus joint angle with the values for the two antagonistic muscles on parallel axes. The lines on the graph are the length/tension curves and so if a muscle is tensed it will increase the gradient of the line thus shifting the equilibrium between the two muscles; Thus is produced the figure below where the intersections of the lines represent points of equilibrium. Taking the two marked intersections as examples it can be seen that a change in the relative length/tensions of the two muscles produces a change in the angle of the joint.
length/Tension v length/Tension
| ___/| |
| Muscle Tension causes ___/ | |
| Joint to snap into ___/ | |
| position _<_/ | |
| ___/ ^ No Movement: Tension
| ___/ | | increases in
| ___/ | | both muscles
| 2 /_____________<______________|1 |
| /\___ No Resistance ___/\ |
| / \___ ___/ \ |
| / \___ ___/ \ |
| / _\___/ \ |
| / ___/ \___ \ |
| / ____/ \____ \ |
| / ____/ \____ \ |
| /___/ \___ \ |
Flexor Muscle Joint Angle Extensor Muscle
The major hole in this theory can be seen by asking the question, "Why
doesn't my arm always move at maximum speed?"; If only the final point on the tension/angle graph is specified then how does my muscle ever move slowly without me consciously controlling its movement? The answer to this question is provided by a further monkey torture experiment. In this the long suffering monkeys have their arms stopped from moving altogether for a time after the light has been lit. They are therefore trying to move their arms and not being able to. Their arms are then released and they shoot over towards the target. This is explained theoretically by adding a virtual length/tension to the graph which will move at the normal arm movement speed.
It is this virtual length/tension that controls the muscle tension not the actual length of the muscle. This virtual length/tension will continue to move in the initial direction of the desired movement until the equilibrium point is reached by the real muscle. This can lead to disastrous overshooting of the target force and explains my propensity to spray the contents of stiff drawers all over the room. In the graph above the virtual length/tension point goes straight from 1 to 2 but because the muscles cannot do this the tension is increased. When the muscles are allowed to move the tension is released and the joint goes to its desired point very quickly probably overshooting due to momentum.
E. Bizzi, N. Accornero, W. Chaoole, N. Hogan; "Arm Trajectory Formation in Monkeys", Experimental Brain Research, 1982.
John C. Rothwell, "Control of Voluntary Human Movement", Chappman and Hall, 1993.
Node Your Homework!
Also, all the flippancy in this paper is due to the fact that it was originally delivered as a talk and so humourous memory aids were a consideration.