The rôle of the dorsal roots in muscle tonus.

It is hard to see why cutting the sensory fibers for a muscle should abolish its tonus since powerful tonic contraction can be induced from the vestibular labyrinth, from the sensory nerves of the opposite limb and from other sources. It would seem as if afferent impulses from these sources should be able to maintain at least a moderate degree of tonus in the absence of those coming from the muscle itself.

For this reason Frank 1 has suggested that the dorsal roots are important in this connection not so much because they carry sensory fibers from the muscles as because they contain efferent fibers through which tonic impulses are conveyed to the muscles. Such special tonic fibers might be thought of as causing a jellying or increase in viscosity of the contracted muscle thus delaying its relaxation. Frank failed to subject his hypothesis to a thorough experimental test and partly for this reason, and partly because it is in conflict with Bell's law, his theory has received but scant consideration.

The results of our experiments can be made to agree with neither of these theories regarding the origin of tonus. If the long dorsal roots of the sacral and lower lumbar nerves are cut close to the spinal cord and at a distance of from 1 to 11/2 inches from the spinal ganglia, the muscles of the corresponding hind limb become atonic for only about 24 hours. After this tonus gradually returns, the extensor muscles usually become markeclly tonic and the limb is held in a hyperextended position. There develops considerable resistance to passive flexion. This stiffness usually reaches its maximum about one week after the operation and then gradually subsides during the next two weeks.