TOUCH AND KINESTHESIS 



389 



formation of skin without injury. Since most of the 

 experimental data pertain to animals, it is usually not 

 practical to distinguish between stimuli which cause 

 sensations of touch from those of light pressure. 



KINESTHETIC STIMULI. Stimuli pressing upon or dis- 

 placing without injury the connective tissue under- 

 neath the skin, periosteum, bones, sheaths of tendons 

 or muscle fascia, and capsules of the joints lead to sen- 

 sations often referred to as deep sensibility. We shall 

 be concerned in particular with those stimuli which 

 cause displacement or compression of joint capsules 

 and shall refer to them as kinesthetic. Under physio- 

 logical conditions, of course, it is the contraction of 

 the muscle which acts as a major kinesthetic stimulus. 

 This fact, however, has no bearing on the assertion 

 which is justified on page 410 that stretch receptors in 

 the muscle itself play no direct role in the arousal of 

 kinesthetic sensations. 



Electrophysiological Methods Jor Study of Somatic 

 Afferent Systems 



When a light tactile stimulus is delivered to a small 

 area on the body .surface, it evokes a burst of impulses 

 in afferent nerve fibers. This volley is relayed through 

 afferent pathways and synaptic regions of the spinal 

 cord, brain stem and thalamus, and invades the 

 sensory areas of the cerebral cortex. An electrode 

 placed at .some point in the system samples the elec- 

 trical signs of this ev^oked activity and provides a tool 

 for study of its whereabouts and nature and of the 

 patterning of the central reflection of the body form. 

 The variables of the experiment are the form of stimu- 

 lation used, the state of excitability of the neurons 

 and the method of recording. Each deserves a com- 

 ment. 



METHODS OF STIMULATION. The somatic afferent system 

 presents difficulties for experimental study for only 

 exceptionally has it so far been possible to deliver 

 quantitatively precise stimuli such as those available 

 for activation of the auditory and visual systems. 

 Various mechanical devices for quick displacement of 

 hairs or skin or for rotation of joints are commonly 

 used, but few of them provide a wide range of action. 

 This had led many investigators to resort to electrical 

 stimulation of nerve trunks to achieve an exact 

 temporal positioning and pattern of the stimuli. It 

 was often believed that a dependable correlation exists 

 between certain groups of fibers, separable by stimu- 

 lus strength, and certain modalities of sensations. 



However, this correlation is far from exact, and it can 

 hardly be doubted that the use of massive nerve \ol- 

 leys has led occasionally to conclusions of question- 

 able physiological significance. The large electrical 

 fields created around massive volleys traversing the 

 central nervous system are likely also to lead to 

 ephaptic excitation of neurons of perhaps completely 

 unrelated function. Electrical stimuli deliv^ered within 

 or across the skin allow a topographical positioning 

 of the stimulus, but since the volley e\'oked is a svn- 

 chronous one it cannot, of course, be easily compared 

 with that produced Ijy a natural stimulus. A new ad- 

 vance in stimulation technique is badly needed. 



EXCITABILITY STATES OF CENTRAL NEURONS. The re- 

 markable safety factor of synaptic transmission at 

 relays of at least some corticopetal tactile and kines- 

 thetic pathways renders them in certain aspects rela- 

 tively immune to the depressing effects of anesthetic 

 agents. By contrast the activity evoked by tactile and 

 kinesthetic stimuli in systems which do not conduct 

 towards the cortex and therefore are not likely to con- 

 tribute directly to conscious perception are extra- 

 ordinarily susceptible to these depressing effects. This 

 differential is of great advantage for it allows a de- 

 tailed mapping of the place and patterns of the 

 central projection of the sensory surface. However, 

 the temporal capacity of the corticopetal systems for 

 transmission as well as certain other functional char- 

 acteristics are severely depressed by barbiturates. 



The desire to retain a high level of excitability in 

 an anesthetized animal has led many investigators to 

 use chloralose as an anesthetic agent, frequently com- 

 bined with a neuromuscular blocking drug. While 

 connections revealed under these conditions un- 

 doubtedly exist, the abnormal excitability of the 

 brain calls for particular caution in evaluating the 

 findings obtained. Under these conditions the trans- 

 mission capacity revealed is as seriously abnormal in 

 one direction as it is in the other under deep bar- 

 biturate narcosis. Recently a combination of light 

 barbiturate narcosis with neuromuscular blocking 

 agents has allowed a somewhat closer approach to the 

 normal state. An important advance has been made 

 by Bremer (30, 31) by introducing the encephale isole 

 preparation, although the high cervical transection 

 makes such a preparation suitable for study of only 

 certain sensory somatic problems. On the other hand, 

 many investigators resort to the use of unanesthetized 

 animals held motionless by neuromuscular blocking 

 drugs. The latter method, apart from the hesitations 

 one may ha\e in using such preparations, hardly 



