388 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



INTRODUCTION 



WE SHALL DEAL IN THIS CHAPTER With neural events 

 which occur in the nervous system in response to me- 

 chanical stimulation of the skin and of some tissues 

 beneath it. We shall focus our attention on those 

 events which presumably provide the substrate for 

 tactile and kinesthetic sensations. 



In the past it was possible to study such sensations 

 only by relating the introspective report of a human 

 observer to the experimental manipulations at the 

 periphery. In this way a large body of psychophysical 

 data was gathered. The events which take place in 

 the nervous system between the time an appropriate 

 stimulus is applied to the receptive zone and the time 

 sensation is reported were not accessible to systematic 

 experimentation, and virtually all the knowledge 

 about them was derived from clinical neurological 

 and neurosurgical observ-ations. 



Even though an introspective report is still the only 

 way Ijy which sensations can be studied directly, a 

 considerable body of electrophysiological data, relat- 

 ing the tactile and kinesthetic stimuli to the response 

 in the central nervous system, has been collected in 

 the past three decades. Perhaps because such data do 

 not permit, at present, an interpretation of sensations 

 in terms of neural events, their impact on theory has 

 been quite modest. Even in recent accounts no need 

 is felt to deal in any detail with the neural impulses 

 farther centrally than the peripheral nerve. Since it is 

 reasonable to believe that a detailed knowledge of 

 central neural events is a prerequisite for any sound 

 approach to the problem of sensations, we shall deal 

 with the material pertaining to our subject in a some- 

 what unorthodox manner. Thus, we shall not review 

 the formidable body of psychophysical data since such 

 data are readily available in all texts of experimental 

 psychology. Likewise, we shall deal with the mor- 

 phology of the skin receptors only to the extent which 

 is necessary for our purposes. We refer the interested 

 reader to a recent review (268) which contains an ex- 

 tensive list of literature on this subject. On the other 

 hand, we shall consider the electrophysiological data 

 pertaining to the receptors and the peripheral afferent 

 fibers and shall deal with the morphology and elec- 

 trical activity of the central pathways and synaptic 

 regions which appear relevant for tactile and kines- 

 thetic sensations. 



We propose to consider these sensations together 

 since the available evidence indicates that the afferent 

 impulses evoked by stimulation of skin and joints are 

 handled in the central ncr\ous svstcin in a similar 



fashion and in the same synaptic regions. By kines- 

 thetic sensations we understand the appreciation of 

 movement and position of the joints. We shall use the 

 term ' kinesthetic' which is current in the literature of 

 experimental psychology instead of the term * proprio- 

 ceptive' which was introduced by Sherrington and 

 which is almost universally used in physiological texts. 

 For once it appears that a Sherringtonian concept 

 tended to obscure rather than clarify the issues. It was 

 already established by Goldscheider (99) that ap- 

 preciation of movement of the limbs derives essen- 

 tially from stimulation of the joints rather than 

 muscles. In harmony with these findings there is, in 

 our opinion, no evidence for and strong evidence 

 against the notion that impulses provoked by stretch 

 receptors in muscles provide information for percep- 

 tion of mo\ement or position of the joints. Thus, it ap- 

 pears that classical proprioceptors may not contribute 

 at all to the arousal of 'proprioceptive' sensations. 

 Hence, the more neutral term of kinesthesis has been 

 adopted. 



Definitions 



Since it is desirable to utilize electrophysiological 

 data from animal experimentations in describing 

 events leading to tactile and kinesthetic sensations, it 

 is useful to consider such sensations in terms of the 

 stimuli which provoke them and to assume that in 

 mammals, other than man, comparable sensations 

 arise when similar stimuli are applied. Three diffi- 

 culties arise in this connection. First, the stimuli 

 cannot usually \x related to the receptors themselves, 

 as would be desirable, but must be related to the tissues 

 containing them; second, for the time being neural 

 events cannot be related in any simple way to sensa- 

 tions; and third, not every activity in the central 

 nerv'ous system evoked by tactile and kinesthetic 

 stimuli necessarily has a bearing on the arousal of 

 sensations. 



From the electrophysiological point of view then, 

 one can speak in a strict sense only of the electrical 

 signs of neural activity aroused by tactile or kines- 

 thetic stimuli. We shall speak, however, specifically of 

 tactile or kinesthetic activity if the stimuli evoke 

 responses in the direct corticopetal pathways and ap- 

 propriate synaptic regions since it seems fair to infer 

 that at least this activity must be instrumental for the 

 arousal of the appropriate sensations. 



TACTILE STIMULI. We shall consider as tactile stimuli 

 all those which cause displacement of hairs or de- 



