The tract of Lissauer in relation to sensory transmission in the dorsal horn of spinal cord in the macaque monkey

Abstract

The classical method for study of the area of innervation of skin by one nerve root, isolated by section of three neighboring roots on either side, was used to study the contribution of neighboring segments of spinal cord to transmission at the first sensory synapse. The area from which response to pinscratch could be obtained in the conscious monkey was greatly increased if the spinal cord was sectioned just below the test segment, or hemisected above it, particularly if vascular infarction was avoided. The enlarged area was hypersensitive. The maximal area of skin innervated by any one nerve root was approximately twice as large as had been found by previous investigators. Each sensory point in the ventral parts of dermatomes must be innervated by at least five nerve roots. Work previously reported had shown that the area of test dermatome was reduced by sectioning neighboring roots medial to their ganglia after previous distal section of the same roots, or by section of more than three neighboring roots. These restrictions of area could be reversed by a subconvulsive dose of strychnine. The facilitatory and suppressive effects from neighboring nerve roots and spinal segments respectively were traced to the mechanism of Lissauer's tract, of which the medial division exerts a facilitatory effect and the lateral division a suppressor effect on transmission through the first synapse of the test root. These excitatory and inhibitory effects, thought to be derived from collaterals of nerve fibers of like and unlike function of all sizes entering the substantia gelatinosa of neighboring segments, must be transmitted by small cells with axons in the tract of Lissauer. The reversal of the inhibition by strychnine indicates a post‐synaptic mechanism at the first synapse. The prolongation of either tonic excitatory or tonic inhibitory influence for as long as five days after section of the corresponding input pathway represents a unique feature of the mass polysynaptic effects of the small interneurones making up the substantia gelatinosa, rather than the presence of a long‐lasting transmitter substance. There was evidence that both dorsal root ganglion cells and substantia gelatinosa continue to contribute to fringe effects after isolation from the periphery. The mechanism of the substantia gelatinosa could provide correlation for the type of sequential directional information such as is necessary for the spinal scratch reflex.