McKeen Cattell

Sensory discharges in single cutaneous nerve fibres

SEVERAL investigations have dealt with the sensory impulses produced in the cutaneous nerves of the frog by stimulating the skin [Adrian, 1926; Bronk, 1929; Matthews, 1929], but until recently there was no certain method of restricting the discharge to a single nerve fibre. Sharp localization of the stimulus goes some way towards this, but there is so much overlapping in the areas of distribution of the sensory fibres that we can rarely be sure of confining the excitation to one of them. Section of all but one of the fibres might be tried, but owing to the arrangement of the connective tissue in the nerves of the frog it would be exceedingly difficult to carry out. This lack of a convenient method of observing the discharge in single fibres has made it impossible in the past to investigate the cutaneous receptors with anything like the accuracy obtainable in the case of the muscle receptors, but a method has now been found for the end-organs responding to touch, and in this paper we give some preliminary results obtained by its use. The method was discovered by chance in an investigation of the impulses in sympathetic fibres [Adrian, 1930 a]. In the spinal frog if one of the dorsal cutaneous nerves is cut where it enters the skin and suspended on electrodes it is found that sympathetic discharges take place when various parts of the body surface are rubbed or pinched. The impulses are easily recognizable by their slow time relations, and they cease when either the spinal cord or the sympathetic ganglion chain is destroyed3. But in some animals stimulation of the body surface gives 1 Cornell University Medical College. 2 Parker Fellow, Harvard University. 3 In many preparations a light touch on almost any part of the body surface gives an immediate reflex discharge in the sympathetic fibres. The impulses cease when the stimulus is withdrawn, but with stronger stimulation (pinching) there is often a long after-discharge. Pinching the leg of a spinal frog produces a considerable secretion from the mucous glands of the skin and presumably the sympathetic reflexes are chiefly concerned with skin secretion.

Response of tactile receptors to intermittent stimulation

THE preceding paper [Adrian, Cattell and Hoagland] describes a preparation of the dorsal cutaneous nerves of the frog which enables us to record the action potentials in single nerve fibres from receptors in the skin. One of the methods of stimulation employed-an interrupted air blast-makes it possible to study the response to a rapid sequence of stimuli and the present paper deals with certain aspects of this response. To some extent the investigation is complementary to those of Bronk [1929] and of Matthews [1931 a] on the stretch receptors in muscle, for like theirs, it is chiefly concerned with the gradual failure of the response under continued stimulation, but the nature of the stimulation differs considerably from that used in previous investigations of the sensory discharge. The muscle receptors become very slowly adapted to a steady stimulus and most of the work on them deals with the response to continued stretch, its relation to the intensity of the stimulus and its gradual decline with time. The receptors in the frogs skin behave much more like an ordinary nerve fibre, they become adapted almost at once to a steady stimulus, and we are here concerned with the failure of the response to rapidly repeated stimuli. We have attempted to determine some of the factors which contribute to this failure, and it will be shown that both the stimulus and the resulting activity may be held responsible. The method of experiment is identical with that described in the preceding paper. Leads were placed on one of the dorsal cutaneous nerves of the frog cut near the skin, to record the antidromic impulses which are often produced in a single fibre by stimulation of some cutaneous area. The action potentials were amplified and then recorded with a loud speaker and a Matthews oscillograph used with a rotating mirror and camera. Stimulation of any desired frequency, duration or

Heat versus tension in relation to the problem of plurisegmental innervation

As a criterion of the energy output accompanying muscular activity, it is usual to employ either the heat production or the development of tension, two quantities which under most conditions bear a constant relationship to each other. For the estimation of the number of active fibres in a muscle mass, either the determination of the heat production or tension should give a satisfactory answer, provided our methods permit of the measurement of a constant proportion of the energy produced. Both procedures have been employed in an attempt to gain evidence on the problem of plurisegmental innervation, and more specifically to determine the proportion of fibres activated by a given nerve root. But Katz [1925] has found that the two methods give contradictory answers to this question. In a series of sixteen experiments on the gastrocnemius muscle of the frog, he found that the sum of the heats obtained on separate stimulation of the eighth and ninth roots of the sciatic nerve was on the average only 1 p.c. greater than the heat obtained by stimulating both components together, while the measurement of tension in twelve experiments gave an average superiority of 41 p.c. for the separate stimulation of the two roots. Katz believes that the heat measurements serve as a reliable guide, and that the discrepancy is due to the impossibility of recording all of the tension developed by the individual fibres when the whole of the gastrocnemius muscle contracts. Because of our interest in the problem of plurisegmental innervation, we have thought it worth while to repeat Katzs experiments, and to attempt to throw further light on the validity of heat and tension measurements in relation to the problem of plurisegmental innervation. The experiments have all been carried out on the isolated sciatic-gastrocnemius preparations from the frog (Rana pipiens). For the measurement of the initial heat production and tension we have used the same thermopile, galvanometer system, and tension lever described in the preceding paper [1930 a], and the methods of recording have been the same. Special double electrodes were fixed at the end of two grooves cut in a hard rubber block, along which each of the two nerve roots was laid. A projection of hard rubber between the two grooves served to minimize the tendency of current spread from one root to the other.

The delayed anaerobic heat production of stimulated muscle

THE production of a considerable quantity of heat following a short tetanic contraction under strictly anaerobic conditions has been constantly observed, and during the past ten years has been the subject of a number of investigations from these laboratories [Hartree and Hill, 1922, 1923; Furusawa and Hartree, 1926; Hartree and Hill, 1928; Hartree, 1929; Blaschko, 1930]. While variable in magnitude the delayed anaerobic heat has been an easily demonstrable phenomenon which could not be explained by some peculiarity of the technique. It appeared to represent a true physiological activity on the part of the muscle, although its significance remained questionable. With the confirmation of the findings of Embden and his associates [1926, 1927] through the recent investigations of Lehnartz [1931], of Lundsgaard [1931], and of Meyerhof and Schulz [1931], showing that in a short tetanus a large fraction of the total lactic acid production occurs after the contraction is over, the delayed anaerobic heat production receives a definite place in the chain of events associated with muscular activity. It becomes, therefore, of special interest to inquire further into the conditions which influence the magnitude and time course of the delayed anaerobic heat, which recent refinements in technique have made possible to an accuracy hitherto unobtainable. The present study has been taken up from this point of view with particular reference to the delayed heat following a series of twitches, and the changes occurring with fatigue.