Showing papers in "American Journal of Physiology in 1939"

Conduction velocity and diameter of nerve fibers

Abstract: Previous studies of the relationship between the size of nerve fibers and . the velocity of conduction of impulses have resulted in the bringing forward of three formulations: 1. v = kD (Gasser and Erlanger, 1927; Gasser, 1934) 2. v = kD2 (Blair and Erlanger, 1933) 3. v = kD0s5 (Pumphrey and Young, 1938) At the time at which the present studies were started only the first two proposals had been made, and it was hoped that by the utilization of an experimental method different from those used for the measurements that constitute the basis of the original derivation, new evidence would be acquired which would throw the balance toward one or the other of the two formulas. The method employed depends upon the generally accepted assumption that in any given nerve the largest fiber conducts at the highest velocity. The procedure in accord with the method is to select nerves with different maximal velocities, measure the velocities, and then prepare histological sections of the nerve in which measurements of the diameter of the largest fiber can be made. A similar method was used by Pumphrey and Young for the small fibers in their nerves. The velocities in the large axons were determined individually and the diameters were measured directly in the fresh state. Squid nerves were used exclusively in their experiments, and this fact must be kept in mind when their data are compared with the data to be presented later, all of which were obtained from medullated vertebrate nerve. METHOD. In order to obtain valid velocity readings it is essential that the fibers be in the best possible condition. Precaution is especially necessary when small fibers are concerned, as they are more susceptible to injury than large fibers, and if injured they acquire a velocity rating that is too low. The method is a favorable one from the standpoint of the conservation of fibers, as dissection is limited to freeing the nerves of adherent fascia, and it is not necessary to use small strands. Excised nerves of the cat were mounted on silver-silver chloride electrodes in a moist

Axon diameters in relation to the spike dimensions and the conduction velocity in mammalian a fibers

Abstract: The problem of the relationship between the diameters of nerve fibers and the velocity at which the fibers conduct impulses cannot be considered solved until it is possible, within the range of a homogeneous group of fibers, to predict correctly the form of the action potential on the basis of the histological picture. Of all the methods as yet proposed, the re-\\ construction method is the most sensitive. As the method was first employed (11) it appeared to work satisfactorily on the basis of direct pro-portionality between conduction velocity and the diameter of the fibers. But the full range of the fiber velocities was then unknown and later attempts to apply the procedure to the complete series failed to give an acceptable result, or involved an assumption the validity of which can no longer be maintained (10). Recent developments have been made with other methods. Blair and Erlanger observed that in frog nerves the size of single-fiber spikes varies directly as the velocity of conduction, and they argued that if the size of the spike is proportional to the cross-section of the fiber, the velocity must vary as the square of the diameter. And Zotterman came to the same conclusion after confirming their results on mammalian fibers. Reconstructions , however, in accord with the power relationship fail to match the recorded potentials (Erlanger, 1937,' fig. 14). A different conclusion was reached by Hursh (1939a), who compared the maximal velocities in a series of mammalian nerves with the respective sizes of the largest fibers. All the points relating the two properties fell about a straight line. A still different relationship was found by Pumphrey and Young for squid fibers. The diameters of the large axons in the fresh state were measured and compared with the individually determined velocities; those of the small axons were measured after fixa-tion and compared with the maximal velocity for the group. The result was a distribution of points best connected by a curve describing the velocity as varying with the 0.6 power of the diameter. The fact that the relationship of velocity and fiber diameter has been 393

Forces and energy changes in the leg during walking

Abstract: The study of locomotion yields information not only concerning the mechanics of progression but also concerning the part that muscles play in this intricate activity. Early attempts at a quantitative analysis of human locomotion were made by the Webers, Marey and Otto Fischer, followed in recent years by Bernstein (1927, 1935) and by Fenn (1929, 1930). The present paper is concerned with a detailed analysis of the dynamics of the human leg in walking, providing data concerning muscle function. In his fundamental investigations of the kinematics of walking, Fischer (1901) was hindered by the fact that he could not determine the point of application of the force exerted by the ground on the foot. The path of this point during the time the foot is on the ground was determined by Elftman and Manter (1934) from cinematic records of pressure distribution in the foot, obtained by a method described by Elftman (1934). For the purposes of the present research, however, a new apparatus has been devised (Elftman, 1938), which gives not only the point of application of this force, but also the magnitude of the force in three components. In addition to solving Fischer's difficulty, this obviates the necessity of dealing with the entire body when only one portion, such as the leg, is of immediate interest. The point of application of this force, as it passes forward during the course of the step, is shown in figure 1. The force itself is plotted in two components, one in the plane of progression, the other lateral, in the horizontal plane. Only the component in the plane of progression will be considered in the present discussion. It is apparent from the diagram that it has two maxima and that it is at first directed upward and backward against the foot, later upward and forward. This reaction of the platform is plotted in figure 6 in two components, one vertical and the other horizontal, in the plane of progression. In addition to this knowledge concerning the external force exerted by the platform, it is necessary to know the disposition of the leg in space. This is shown in figure 2 for the left leg during the double step under con-339 340 HERBERT ELFTMAN sideration. The original information was obtained from cinematic records taken at the rate of 92 exposures per second as the subject walked behind a rectangular grid. The timing was …

The range and variability of the blood flow in the human fingers and the vasomotor regulation of body temperature

Abstract: The physical regulation of body temperature depends mainly upon the physiological control of the peripheral circulation, when the environmental temperature is below that where ment of the peripheral blood flow sweating the effec plays a major role. tive thermal conduc By adjusttivity of the tissues may be altered over a five or six-fold range of values, as shown by Burton and Bazett (1936) in water baths and by Winslow, Herrington and Gagge (1937) in atmospheric environments. The regulation of the average flow to the requirements of heat eliminacourse, brought tion is, of peripheral blood vessels. about by the nervous control of This control is mediated chiefly the bY tone of the the sympathetic system, though a measure of “metabolic” regulation of flow may exist in a denervated limb (Freeman, 1935). An inhibitory action mediroots ated by fibres from the dorsal Bozler (1936) but it is not yet has been demonstrated in the frog by clear how much this must be considered in the mammal. A series of researches in this laboratory on the activity of the sympathetic system has shown state of continual that under fluctuation the influence of afferent stimuli it is in a of tone. The activity of the sympathetic centres may be of a rhythmic and intermittent character which bears no obvious relation to any corresponding intermittence of afferent influences (Bronk, 1936). The activity of the cardiac sympathetic nerves (Bronk et al., 1936) shows the interaction of several rhythms of various origins. The setting of the average peripheral blood flow to the level required by the demands of temperature regulation must then be made by the modifica.g sympathetic tone which itself has a complicated tion of pattern an of underlyin activity. Another complicating factor is that the regulation of peripheral flow to serve the function of elimination of body heat must proceed simultaneously with other types of regulation of blood flow. Among these are the supply of oxygen in accordance with the needs of local metabolic activity, and the

The location and function of the chemoreceptors of the aorta

Abstract: Although Heymans and Heymans (1927) demonstrated tIhe presence of chemically sensitive nerve receptors in the region of the aortic arch several years before Heymans et al. (1931) discovered similar receptors in the carotid sinus area, the former have received little attention since that time while the latter have been investigated repeatedly. Consequently the localization and physiological significance of the carotid receptors are now fairly well understood, but even the existence of the aortic receptors has been denied (Dautrebande and Wegria, 1937; Beyne, Gautrelet and Halpern, 1933), and their localization and significance are bot.h uncertain. A number of investigators (Selladurai and Wright, 1932; Schmidt, 1932; Jongbloed, 1936; Gesell and Moyer, 1937; Lambert and Gellhorn, 1938) have confirmed the presence of extracarotid chemoreceptors by showing that, even after complete carotid denervation, anoxemia still produces some increase in respiration which disappears when the vagodepressor nerves are cut. However no attempts at precise physiological localization have been reported since the work of Heymans and Heymans (1927). Anatomical studies made by Penitschka (1931), Palme (1934), Muratori (1934), Seto (1935), Nonidez (1935, 1937), and Boyd (1937) have demonstrated the presence about the aortic arch of cell groups which are similar in appearance to the chemoreceptors of the carotid bodies-a structural relationship which has led to the suggestion that these cells represent the chemically sensitive areas of the aortic region. Until now this suggestion lacked physiological confirmation. The objects of the present experiments were to ascertain the anatomical

Localization of the medullary respiratory centers in the cat

Abstract: Beginning with the work of Longet (1847) there has accumulated an impressive mass of evidence which indicates that the indispensable mechanism for the neural control of respiration is localized in the reticular formation of the caudal half of the medulla (Brown-Sequard, 1860; Gad and Marinesco, 1893; Bechterew, 1885; Kohnstamm, 1900; etc.). Although there have appeared sporadic assertions that isolated structures play a predominant role in respiratory control, e.g., the ala cinerea (Schiff, 1870; 1871), the tractus solitarius (Gierke, 1873), the central nuclei of the medullary raphe (Mislawsky, 1885), recent reviews of the subject by Finley (1931) and Cordier and Heymans (1935) adequately demonstrate the preponderant importance of the medullary reticular formation. A functional subdivision of the respiratory center into inspiratory and expiratory portions has been attempted by Marckwald and Kronecker (1880), Lewandowsky (1896), Mislawsky (1885) and Gad and Marinesco (1893). Lumsden (1923) claims to have distinguished 4 centers, a pneumotaxic, an apneustic, an expiratory and a gasping center. Henderson and Sweet (1929) maintain that only 1 respiratory center exists. Attempts at more definite localization within the reticular formation by the transection, partial section, isolated destruction and crude stimulation methods of the early investigators have succeeded only in delimiting the cephalic limits of the respiratory center as somewhere caudal to the entrance of the eighth nerves. Gesell, Bricker and Magee (1936) in a study of amplified potentials showing a respiratory rhythm tapped from the brain stem place the cephalic limit of the respiratory center within the reticular formation at the level of entrance of the upper vagal rootlets and the cephalic end of the inferior olive. The caudal limit they place 2 mm. below the obex at the level of the decussation of the pyramids. In view of the indefinite localization of the medullary respiratory center and the disagreement as to its functional subdivision it seemed to us that a careful study of the respiratory responses to stimulation of the brain stem

The electro-saltatory transmission of the nerve impulse and the effect of narcosis upon the nerve fiber

Abstract: Nervous transmission is generally believed to be effected through excitation of each section of the nerve by the activity of adjacent parts. Most investigators assume that the restimulating agent which excites the resting region may be electrical in nature. This view of electrical transmission has been considerably strengthened by the recent work of Hodgkin (1938). In the present paper I have attempted, by means of micro-technique, to obtain conclusive experimental evidence for the theory of electrical transmission, and to establish a more complete conception of nervous transmission. The problem of “decrement” in the narcotized region of nerve is also discussed with special reference to the new experimental data. METHODS. In all experiments, isolated single nerve-fibers (sciaticgastrocnemius preparation) of the Japanese toad were used. The procedures of isolation and experimentation were essentially similar to those described in previous papers (Tasaki, 1939). For multipolar stimulation, the circuit reported previously was employed, with a slight modification in the position of the current reversers. For determining the least interval between two stimuli necessary to give a summated muscular contraction, break induction shocks supplied by two inductoria were used; the secondaries were connected in series and the strength of the shock was controlled by precision resistances in the primary circuits. In a single-fiber preparation, a summated contraction is distinctly larger than an ordinary single twitch and can easily be distinguished by visual observation. The least interval for muscular sum-

The electrolytes of muscle and liver in potassium-depleted rats.

Abstract: The gross effects of withholding potassium from the diet of young rats has been the subject of a number of investigations (Osborne and Mendel, 1918; Miller, 1923; Leulier and Vanhems, 1934; Schrader, Prickett and Salmon, 1937; Grijns, 1938; Heppel and Schmidt, 1938). In general it was found that the deficiency results in failure to continue growth followed by death after a variable number of weeks. No reports are available in the literature dealing with chemical studies of the tissues of potassiumdepleted animals. The present investigation was designed to determine the effect of potassium deprivation on the concentration of the principal electrolytes in serum, muscle and liver tissue. In normal muscles the concentration of potassium is much greater than . that of sodium. It is generally held that potassium is concentrated inthe intracellular phase while sodium is largely restricted to the extracellular phase (see Fenn, 1936, and Manery and Hastings 1939). The most striking result of the work reported here lies in the fact that the muscles of animals deprived of potassium take up sodium instead of potassium. This occurs to such an extent that in some instances the muscles of the experimental animals are found to be richer in sodium than in potassium. We have here a clear-cut example of a condition in which sodium must occur largely as an intracellular cation.