Cutaneous receptive field and morphological properties of hamstring flexor alpha-motoneurones in the rat.

Abstract

Intracellular recordings were made from 20 antidromically identified posterior biceps femoris/semitendinosus (p.b.s.t.) hamstring flexor .alpha.-motoneuron in the decerebrate-spinal rat. The hamstring motoneurons had either low or no spontaneous background activity. In 19 of the 20 cells high-frequency phasic responses could be elicited by stimulation of the ipsilateral hind paw with firm pressure or pinch. There was no response to light touch or brush. Contralateral cutaneous mechanoreceptive fields with higher threshold and weaker responses were present in 70% of the motoneurons. Noxious heating of the ipsilateral hind paw produced excitatory responses in 6 of 8 cells tested and 2 of these cells also responded to heating of the contralateral hind paw. Stimulation of the ipsilateral sural nerve at graded strengths that successively activated A.beta., A.delta. and C afferents produced excitatory post-synaptic potentials (e.p.s.p.s) at progressively longer latencies in the motoneurons. The C-fiber induced e.p.s.p. lasted up to 200 ms. Horseradish peroxidase was injected into 10 motoneurons and in 7 cases full reconstructions of dendritic field, cell body and axon could be made. In agreement with previous reports from studies in the cat, the dendritic fields of rat motoneurons are very extensive in the rostrocaudal, mediolateral and dorsoventral planes. The general pattern of dendritic branching for each motoneuron in this functionally homogeneous population was uniformly organized. Three major spatial orientations were always present: a rostrocaudally restricted series of dendrites emerging from the cell body and directed dorsolaterally towards the dorsolateral funiculus with branches in the lateral dorsal horn, a laterally and a ventromedially directed series of branches arranged obliquely in the ventral horn, both of which were distributed rostrocaudally for equal distances from the cell body. Many of these dendritic branches terminated within the lateral and ventral white columns. Although the sizes of the rat flexor motoneurons'' somas (51 .+-. 4.9 .mu.m, SE, n = 10) were similar to those of cat lumbosacral .alpha.-motoneurons, the tip-to-tip rostrocaudal extent of their dendritic fields (1130 .+-. 34 .mu.m, SE, n = 7) was 1/2 that reported in the cat. Results are discussed in terms of the organization of the cutaneous flexor withdrawal reflex in the rat.