4o8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



12 3 4 5 



Number of spikes per response 



FIG. 1 1 . Curves illustrating the distributions of response 

 populations around their modal values for several neurons of 

 the thalamic ventrobasal nuclear complex of the cat. Number of 

 impulses per response to a brief peripheral stimulus graphed 

 against the percentage of responses with the stated number of 

 impulses. Although values on the abscissa are always integers, 

 the points belonging to the same distribution are connected by 

 lines to aid the eye. The number which follows the letters 

 TM identifies the experiment, the number in parentheses 

 identifies the neuron studied. .V indicates the number of 

 responses upon which each graph is based. Locus of peripheral 

 stimulus constant for each unit. In each case the neuron is 

 activated by electrical stimulation of the contralateral skin. 

 Locus of stimulation: TM 34(1), second toe on hindfoot; 

 TM 32(2), lower abdomen; TM 32(5), upper thigh; TM 32(6), 

 ankle; TM 27(2), wrist; TM 28(2), wrist. Note that most 

 responses in each series do not differ from the modal value by 

 more than one impulse. [From Rose & Mountcastle (208).] 



transmits at a higher frequency than that predicted 

 by the recovery cycle studies. 



c) Mode reduction. The records of figure 12 show 

 finally that, when responding at higher frequencies, 

 the cell discharges but a single impulse to each stimu- 

 lus which is effective in contrast to the repetitive re- 

 sponse to the first stimulus of the train. The repet- 

 itive response ' singles up' as a rule when the fre- 

 quency of the stimuli increases beyond 10 to 15 per 

 sec. 



The equilibration type of response occurs in about 

 60 per cent of the neurons studied at thalamic and 

 cortical levels. The remaining neurons display a 

 different sort of lieha\ior. While following the stimulus 



rate up to values which differ greatly for different 

 units, they respond to the presentation of still faster 

 trains with but an initial response and are thereafter 

 silent during the train, or discharge randomly at the 

 spontaneous rate (see fig. 13). It is not clear at present 

 whether the ' equilibration' and the ' cut-off' types of 

 response can be obtained for the same unit by suitable 

 manipulation of the stimulus. Most of the units ob- 

 served which show the ' cut-off' response pattern follow 

 only a low rate of stimulation. On the other hand, it 

 seems to be true, at least occasionally, that a typical 

 equilibration type of response becomes a cut-off type 

 when the stimulus rate is made very high (200 to 300 

 per sec). 



AFFERENT INHIBITION. It has been shown recently 

 (181, 198) that the afferent volleys evoked by periph- 

 eral stimuli while excitatory for some cells of the sys- 

 tem will tend to inhibit others (fig. 14). All inhibitory 

 phenomena are very sensitive to anesthetic agents and 

 are probably at least partially abolished even under 

 very light general anesthesia. Nevertheless, the inhibi- 

 tion of both the spontaneous and the evoked activity 

 of central neurons has been observed for a consider- 

 able number of cells (198). The peripheral inhibitory 

 receptive field for a given neuron (in the postcentral 

 cortex) may surround or lie adjacent to its excitatory 

 field. It is an interesting observation that a cell 

 excited, for example by movement of a joint, may be 

 inhibited by skin stimulation, although a purely 

 excitatory intermodality interaction has not thus far 

 been demonstrated for units driven from the skin and 

 from deep receptors. In the cortex, pairs of cells which 

 are in one case excited and in the other inhibited from 

 the same receptive field have been observed at a single 

 electrode position. They must therefore lie very close 

 to one another. This suggests, of course, that afferent 

 inhibition may play an important role in reducing 

 the discharge zone of cells activated by a local periph- 

 eral stimulus. It need hardly be added that restricted, 

 sharply focused discharge zones may be instrumental 

 in recognizing a single localized peripheral event and 

 in more complex discriminations as well. 



suMM.\RY. The single unit studies at several stations of 

 the somatic afferent system have produced a consider- 

 able mass of data concerning the relation of a dis- 

 charge of a single cell to the quantitative parameters 

 of the peripheral stimuli. For a population of cells the 

 data allow a reconstruction of the distribution ot 

 activity set up by a single brief peripheral stimulus 

 occurring in that population. However, no informa- 



