"Intrinsic Organization of a Motoneuron Pool in the Adult Rat"

Abstract

<p>It is becoming increasingly clear from physiological and histochemical observations that many skeletal muscles are not used in an all-or-none fashion, but rather that some parts of the muscle are more active in one movement than another. Such a functional compartmentalization" could have an appropriate anatomical correlate; for example, within the pool of motoneurons supplying an individual muscle, subsets of motoneurons could be spatially grouped according to the location of their peripheral muscle fields. For a number of technical reasons, this possibility is difficult to investigate in the typical skeletal muscle. The cutaneous trunci muscle (CTM) of the rat, however, is ideally suited for such an investigation. This vast thin sheet of muscle is inserted into the deep surface of back and flank skin and is reflexly activated by nociceptive information from the overlying skin. A punctate stimulus evokes a localized contraction of the CTM in the immediate vicinity of the stimulus: the reflex activation of this muscle is therefore organized behaviorally into functional compartments. The simplest expectation would be that the peripheral nerves that drive the CTM reflex are segmentally organized. While this is true for the sensory nerves, I find that it is not the case for the motor ones. I have used electrophysiological and histochemical techniques to examine the pattern of motor innervation of the CTM and retrograde tracers to study its motoneuron pool. Interestingly, all the CTM motoneurons are located in the cervical spinal cord, several segments rostral to even the most rostral sensory input that activates the CTM. My findings indicate that there is a spatial organization within the CTM motoneuron pool; this organization corresponds to the pattern of motor nerve innervation of the muscle, and moreover, seems appropriate for the compartmentalized nature of the reflex activation of the muscle by cutaneous sensory nerves.</p>