The func~or~ o~ ~ e d ~ c e ~ d i n ~ r o ~ e~ ~he fifth nervo

• % D. Denw-Brown and N. Yanagisawa, B r a i n , 96 (i97a, 783-814

3 *7

in tlae m~caque mo~ke'g ~,~owing 8orsa~

E. J. Kirk and D. Denny-Brown, Z e o m p , N e u r o L , i39 (1970) 307--320

The message of Denny-Brown's recenc work on the macaque is that the area of sensory loss due to lesions of the posterior roots or horn ~ i~ ~hysiology and rmt anab. omy. It is not fixed, it is: functional, and it can be changed by time and strychnine.

From experiments ~n which posterior roots were cut so as m leave a region cf remaining sensibility, Denny=Brown and his colleagues have showrt thane there is a far greater . . . . amount . of overlap of ~s te r io r roots throughout the soma tha.n had previous1 y been:thought. ~e~atea Of'overlap is not normally manifeste~ because the input from a posteri0r~root is being constantly inNbited by ~he posterior horns of the0neignbouring~, segments; it can alsJ be fadHtated. This inhibition and facilitation d,pend on the input f romthe neighbou)ing roots: When strychnine is g~ver~, the Jn;fibition is tempor° arily stopped, and the cutaneous sensory loss of the neighbouring posterior roof a overlap is removed. The facilitation and inhibition are carried out by ~he substantia gelafinosa. They are spread locally by intrinsic fibres of the posterior horns and b y

Lissauer's tract, the laterat part of the tract being responsible for inNi>hion and the medial part for Ncilitation. Facilitation is less important t!aan inhibition° for cutting the whole tract has more or less the same effects as catting only t~e ]atera~ par0:

. . . . . . . . . . . :ior ~orn~. there ibres. This form sensory neurons ?sterior root; ~o ? or more path- primary senso~T e same point of

the spinal cord by intrinsic fibres of the grey matter of the posterior horns. 'Pain is more w~tnerable to togs Of spatialsummation than is temperature sensation, and this in turn more than touch'. In other words, the threshold of posterior born neurones is higher for ~ain tha.~ for thermal sensation, and for thermal than for tactile sensation.

l:he evidence ofspa*Nt s~mmat~on ir~ re .ard to ain leads Denw.--Brown ~"~ ~ts , ~ . . . . . . . . . . . . . . . , , g P . . . . . . . . . . . .

colleagues *o support Pain theories based on the bdief that pain is quami~ative rafl-~,,'r than a separate modaHty of sensation.