Contains reports on four research projects.Bell Telephone Laboratories, IncorporatedThe Teagle Foundation, IncorporatedNational Science Foundatio

Duchane, E. M.

Gesteland, R. C.

Johnson, A. R.

Lettvin, Jerome Y.

McCulloch, Warren S.

Pitts, Walter H.

Wall, P. D.

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XI. NEUROPHYSIOLOGYW. S. McCulloch R. C. Gesteland W. H. PittsE. M. Duchane A. R. Johnson P. D. WallJ. Y. LettvinA. STUDIES ON THE NODE OF RANVIER AND SYNTHETIC NEUROLOGYOur findings within the past half year have been disquieting. When we found out howto record from single nodes of Ranvier in the cat, the procedure looked not only easy(medical students at a demonstration mastered it in five minutes) but profitable, and weforesaw many interesting data. But the results that we did get make our general goalseven more distant. It is idle to dwell on technique that is simply an elaboration ofstandard methods, and so we proceed directly to our findings.First, nerve fibers differ among themselves even more widely than is commonlythought. For example, some fibers that arise from stretch receptors show a very longnegative after-potential that cannot be accounted for simply on the basis of the meas-ured membrane potential. Other fibers show short negative after-potentials, or shortpositive ones, or mixtures, and the patterns seem more connected with modality thanwith the level of membrane potential, nor are they easily converted one to the other bypolarizing currents. Ordinarily such differences in response are attributed to, orcorrelated with, size of fibers and velocity of conduction. But even here the experimentson nerve bundles have hidden rather marked variations. For example, it is possible toshow that a motor nerve fiber with the same conduction velocity and the same membranepotential as a sensory nerve fiber not only has a different after-potential but is verymuch more resistant to, for example, topical anesthesia. While the large and fastdischarge seems the most obvious and important event in a peripheral nerve fiber, moreand more evidence suggests that secular processes play an important part in centraltransmission - as in post-tetanic potentiation. If it turns out either that the diameter ofa fiber determines the nature of the after-potential or that different species of fibersgive rise to entirely different central processes under similar anatomy (which we thinkmore likely), whichever is the case we have a mess.Second, while we were studying the node, we measured, among other impedances,that of the axoplasm during activity. To our astonishment it appeared that axoplasmdoes not seem to be a linear ohmic fluid but is markedly nonlinear, and its resistivityin the region of the node changes, not during the rising phase of the spike (like mem-brane), but later, during the falling phase, and returns to normal some time after thespike. At first we thought that this was an artefact and we began trying to trace it downThis work was supported in part by Bell Telephone Laboratories, Incorporated; inpart by The Teagle Foundation, Incorporated; and in part by National ScienceFoundation.162(XI. NEUROPHYSIOLOGY)when Dr. Strickholm, from Minnesota, came by with exactly similar results on musclewhere the sources of trouble we were attacking were obviated. At the same timeJohn Segal in Dr. Jenerick's laboratory, M. I. T. , showed the myoplasm to be probablyseverely nonlinear in dc measure of resistance.Finally, with Prof. M. Minsky, we built some artificial neurons for the artificialintelligence project at Lincoln Laboratory. The output of each neuron was a fixed fre-quency of pulses at rest - and could be set to any arbitrary frequency. There were twoinputs per neurun (high impedance points); one raised the rate of firing on receivingpulses from another neuron, the other lowered the rate - excitation and inhibition. Eachneuron could excite or inhibit itself with output connected to input. It was very amusingto discover that whereas one could easily synthesize arbitrary patterns of firing in threeor four interconnected neurons, one could not, from looking at the patterns, state howthe devices were interconnected. Of course, we all knew this from a priori considera-tions - rather well digested by now - but it was quite startling to be confronted with thepoint in actual operation. So comes doom to the hope of abstracting pattern of connec-tion from pattern of single unit discharge.B. ELECTROMETER CATHODE FOLLOWERFigure XI-1 illustrates a circuit that is now in use in our laboratory. WVhen the 1 Kvariable resistance is adjusted so that the collector of T 2 sits at approximately -3. 5volts, the variational resistance of the emitter load circuit of T 1 is greater than1 megohm at approximately 3 ma. Thus thecathode current of 5886 (electrometer tube)s_ i kent t nnrnorimatelv 23.5 amn. At the col-V lector of T 2 there is an inphase gain slightlygreater than 2 with respect to any signal atthe emitter of T . This gain-of-two pointsupplies the positive feedback (via the emitterfollower T 3 ) through the condenser to thegrid of 5886 to neutralize shunt capacitanceat the input, and also, when divided by thepair of 10-K resistances, the unity gainfeedback from cathode to screen of 5886.Thus the electrometer tube is bootstrappedover a reasonable dynamic range (approx-Vimately 4 volts). Specifications are as fou-lows:Fig. XI-1. Circuit for electrometer lcathode follower. I < 1 x 1013 amp.g1630(XI. NEUROPHYSIOLOGY)Frequency response with a 1-inch 20-megohm series resistor to the grid of 58836,flat to 40 kc, after suitable adjustment of C.Output; approximately 0. 6 V+ with respect to input; drift, approximately 20-30microvolts/minute, and less if the circuit is embedded in a silicone 550 bath.C. REPETITIVE FIRING OF NEURONSTwo types of repetitive firing have been examined. The first is the echo phenomenoncalled the dorsal root reflex which occurs in spinal afferent fibers. The second is therepetitive firing of the sensory nerve cells in the spinal cord after the arrival of avolley of sensory nerve impulses. A paper on this subject, which will be published inthe Journal of Neurophysiology, can be summarized as follows.The pattern of repetitive discharge of the dorsal root reflex and of sensory inter-neurons in the unanesthetized cat spinal cord has been examined by microelectrodes anda new method of recording.1. The dorsal root reflex was set off by stimulation of peripheral nerves. Theinitial latency and the variation of latency throughout the discharge depended on thenerve stimulated and the strength of stimulation. The pattern of the response differedmarkedly from that seen in the interneurons.2. The time of arrival of the afferent impulse in an afferent fiber did not affect thepattern of the dorsal root reflex.3. The interjection of an impulse into a dorsal root reflex resets the rhythm, andit is concluded that the site of origin of the rhythm is within the fiber.4. "Spontaneous" activity of sensory endings is similarly reset by the introductionof an additional impulse.5. The pattern of discharge of the sensory interneurons varied between three typesof response: a low-frequency short discharge with great variation of latency, a high-frequency prolonged discharge, and a short abrupt discharge.6. The interjection of an additional impulse into the repetitive discharge of aninterneuron fails to reset the rhythm and we conclude that the repetitive firing is prob-ably driven by bombardment from other neurons.7. The "spontaneous" activity of the interneurons is not reset by interjectedimpulses.In subsequent work on this problem we have examined the variability of the patternof discharge. In the case of the dorsal root reflex, the answer is clear-cut. The fre-quency of the discharge is unaffected by the number of impulses in the afferent volley;but the larger the volley, the smaller the variability in the pattern of discharge of thedorsal root reflex. However, when the discharge of the postsynaptic cells is examinedthis simple situation is no longer found, and an increase in size of the input volley164(XI. NEUROPHYSIOLOGY)frequently results in an increase in the variability of the output pattern. The work onthe behavior of the sensory internuncials is being continued in an attempt to relate thepattern of discharge to the nature and position of a touch stimulus to the skin.D. THE ROLE OF REFLEXES IN MOVEMENTA. R. Johnson has started experiments to determine the manner in which theneuromuscular reflexes play a part in the maintenance of position against a varyingpressure or in the maintenance of a steady pressure against a varying position. In thefi-rst stage of this experiment, J. V. Cichon completed studies on the variation of tensionin a single muscle while the muscle was being forced to stretch and relax at variousfrequencies. The results showed that reflex adjustment continues to play a part whenoscillations up to 25 cps are impressed on the muscle. Highly characteristic patternsof response were seen at various frequencies. The effect of drugs on this reaction wasstudied. The results of this first stage were described in Mr. Cichon's undergraduatethesis for the Department of Physics.165

Neurophysiology

Contains research objectives and reports on three research projects.Bell Telephone Laboratories, IncorporatedThe Teagle Foundation, IncorporatedNational Science Foundatio

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