TOUCH AND KINESTHESIS 



393 



call a receptor potential since this potential — in con- 

 trast to the local response of the nerve — is not affected 

 by near absence of sodium. The receptor potential 

 can summate and its amplitude depends on stimulus 

 strength. It is set up in less than 0.2 msec, and is the 

 earliest electrical event which is known to occur. 

 While the mechanism of its generation is at the mo- 

 ment quite obscure, its occurrence provides a final 

 indication — if such be needed — that the Pacinian 

 corpuscle must be regarded as a full-fledged receptor. 

 An electronmicroscopical description of its fairly 

 complex structure has been given recently by Pease & 

 Quilliam (195). The mechanism of excitation of 

 Pacinian corpuscles is discussed by Gray in Chapter 

 I\' of this work. 



Peripheral Cutaneous Nerve Fibers 



IMPULSES IN PERIPHER.AL NERVE FIBERS. ThuS far WC 



have discussed only to what extent discharges in single 

 units reflect some properties of the receptors which 

 initiate the impulses. We shall now turn to electro- 

 physiological evidence derived from studies of cutane- 

 ous nerves. Since all experimental data are derived 

 either from studies of single units or from studies of 

 the electroneurogram, it will be useful to recall some 

 properties of the peripheral fibers. 



As is well known, a cutaneous nerve consists of 

 fibers of different sizes. Fibers of different diameters 

 do not, however, occur with equal frequency and 

 the size-frequency distribution curve for any cutane- 

 ous nerve shows characteristically several peaks. It is 

 customary to classify all fibers into A, B and C groups 

 according to certain characteristics of their action 

 potentials which are different for each of them. How- 

 ever, it is unnecessary to relate here these character- 

 istics since in the cutaneous nerves only A and C 

 fibers are known and the myelinated and unmye- 

 linated fibers form the A and C groups, respectively. 



It is known that the velocity of conduction in A 

 fibers varies with their diameters. If an as.sumption is 

 made that the amplitude of the action potential as 

 recorded across the membrane is about the same for 

 any A fiber but that the height of the externally re- 

 corded potential varies as the square of the diameter 

 of the fiber, it is possible to estimate the size of the 

 fiber from the externally recorded amplitude of the 

 discharge in a single unit preparation. Making these 

 assumptions for the entire nerve the shape of the com- 

 pound action potential can be reconstructed with 

 great accuracy if the fiber composition of the nerve is 

 known. Conversely, it is justifiable to infer from the 



different elevations of the electroneurogram the 

 presence of fiber groups of specified diameters in a 

 given nerve. Usually, Greek letters are used to denote 

 the different elevations, each successiv-e letter referring 

 to a group of fibers of smaller diameter (fig. i). Some 

 confusion resulted occasionally in the past with the 

 use of this notation, for different nerves do in fact 

 differ in their fiber compositon and thus an elevation 

 denoted by the same letter in different nerves may 

 indicate at least somewhat different fiber groups. For 

 the corresponding nerves in the same species the dis- 

 tribution of fibers is, according to Gasser (88), quite 

 constant. 



IMPULSES EVOKED IN FIBERS OF DIFFERENT SIZE BY 



T.-iiCTiLE STIMULI. All workers agree that tactile stimuli 

 activate the largest fibers in the cutaneous nerves and 

 those who distinguish gentle contact (touch) from 

 sustained displacement (pressure) invariably state 

 that it is a gentle contact which activates the largest 

 fibers. Maruhashi et al. (168) report that the diam- 

 eters of the fibers activated hy touch vary between 8 

 to 14 /J in the cat and 8 to 15 |i in the frog. They 

 further found that movements of hairs activate fibers 

 in the range of 6 to 1 2 /i, while diameters of fibers 



FIG. 1. Compound action potential of the saphenous nerve 

 of the cat recorded at a distance of 54 mm from the locus of 

 stimulation. Several elevations (denoted by Greek letters) are 

 recorded because, in the nerve, fibers of different sizes are 

 giouped around several peaks. Since the saphenous nerve lacks 

 the largest afferents arising from the muscle stretch receptors 

 no a elevation is indicated even though the sizes in the a and 

 (3 groups overlap. The /3 and 7 elevations as denoted here are 

 sometimes referred to as a and peaks. All elevations pertain to 

 \ fibers. The elevation due to C fibers is not shown. Time 

 line: 5,000 cps. 



