TOUCH AND KINESTHESIS 



403 



while area 2 seems to receive only a collateral outflow 

 from it. What this important finding may imply lunc- 

 tionally is at present oiascure. 



MODALITY COMPONENTS OF MEDIAL LEMNISCAL SYSTEM 



is induced by general anesthesia, however light. In 

 any case, it is at present both convenient and necessary 

 to consider separately the activity in the lemniscal 

 system evoked by stimulation of the skin, touch- 

 pressure, and that provoked by stimulation of perios- 

 teum, bones and joints, deep sensibility. 



A second general property of the lemni-scal system 

 is that it handles, within a single topographical pat- 

 tern, activity evoked Ijy tactile as well as kinesthetic 

 and other mechanical stimuli acting upon deep tis- 

 sues. At each successive level of the system neurons 

 subserving various forms of mechanoreception are in- 

 termingled in a common topographical pattern. 

 Nevertheless, single unit studies indicate that the in- 

 dividual neurons at each level retain their modality 

 specificity. This rather surprising observation requires 

 an immediate comment. In work with an intact ani- 

 mal a given unit at a central station of the system 

 can be driven by stimuli delivered to the skin or to the 

 deep tissues. It is usually a simple matter to be certain 

 which of the two contains the effective receptors, and 

 this can be proved by direct surgical dissection of the 

 peripheral tissues. Some difficulty does arise when 

 the receptive fields lie in highly specialized regions at 

 the apices of the limbs, such as the claws of the cat. 

 Nevertheless, the lack of any evidence that stimuli to 

 the skin and to the deep tissues can excite the same 

 neuron is quite striking. Since all the findings are 

 derived from anesthetized preparations it is con- 

 ceivable, although we believe rather unlikely, that 

 his apparent lack of any clear excitatory interaction 



Touch-Pressure 



.\D.^PTIVE PROPERTIES OF RECEPTORS AND OF CENTRAL 



NEURONS. It has been known for a long time (2, 3, 6} 

 that the mechanoreceptors of the skin and the afferent 

 fibers to which they are connected are not uniform in 

 all their properties. One can classify these afferents 

 according to the following criteria : the adequate 

 stimulus required for each, the size and conduction 

 velocity of the fibers concerned, the rate of adaptation 

 to steady stimuli and the sizes of the peripheral recep- 

 tive fields (168, 280, 282). When working with the 

 intact anesthetized animal, however, it is useful to 

 classify the cutaneous mechanoreceptors as (i) those 

 which respond steadily to steady stimuli and (it) those 

 which adapt quickly to such stimuli (fig. 5). Neural 

 elements at each of the central relay stations of the 

 lemniscal system, which are driven by mechanical 

 stimulation of the skin, fall readiK' into one or the 

 other of these classes (2, 3, 6, 7, 181 ; Berman, A. L., 

 unpublished observations, and Mountcastle, V. B. & 

 J. E. Rose, unpublished observations). The type of 

 adaptation of a given unit is, so far as has been ob- 

 served, an unchanging functional property. In general, 

 afferents related to hairs are quickly adapting, while 



On 



Pressure 



Off 



II 



Moving one thread of hair 





J 





( «-\ 



^A. 



\" n 



FIG. 5. .-Xction potentials in single cutaneous nerve fibers of the cat, elicited by mechanical stimula- 

 tion of the skin. A : \ single fiber adapts rapidly to steady pressure applied to its receptive field, 

 shown in the inset drawing. A short train of impulses occurs at the onset and release of the pressure. 

 In the second record a fiber responds to movement of a single hair. B: The receptive field for this 

 particular fiber is punctiform. The fiber adapts slowly to a steady mechanical stimulus. Upper row of 

 dots apply to the first two, the lower row to the third record. Distances between the dots indicate 10 

 msec, intervals. [From Maruhashi et al. (168).] 



