Following tetanic afferent stimulation of a monosynaptic reflex pathway, the transmission through that pathway of isolated reflex volleys is enhanced for some minutes. Post-tetanic potentiation is comparable in the monosynaptic reflex arcs of flexor and extensor muscles. The facilitator and inhibitor actions of monosynaptic reflex afferent fibers, as well as the transmitter action, are potentiated following tetanization. Little post-tetanic change attends reflex transmission through plurisynaptic reflex arcs. Various tests for excitability change made independently of the tetanized afferent fibers reveal none or a slight depression. Hence the potentiating influence of a tetanus is limited to subsequent action on the part of the recently tetanized fibers themselves. Increase in the size of the individual impulses comprising an afferent volley such as might occur during positive after-potential, would accommodate the requirement for a limited process and provide for increased synaptic action. The proposed association between post-tetanic potentiation and positive after-potential (i.e. hyperpolarization) is supported by the following lines of evidence:— 1. Changes in intensity and duration of potentiation with change in frequency and duration of tetanic stimulation are characteristic of, and parallel to, the changes of positive after-potential in similar circumstances. 2. Afferent impulses are increased following a tetanus, and in a fashion that parallels the course of monosynaptic reflex potentiation. Post-tetanic potentiation, as here described, and after-discharge, whatever may be its mechanism, are unrelated phenomena.

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THE SEGMENTAL REFLEX discharge (7,31,21) must be considered of anatomical rather than functional significance in that it contains, in unnatural combination, those elements which constitute the several distinct ipsilateral reflexes. In the present paper are the results of experiments designed to resolve the segmental reflex into its functional components. The observation that a major division of the segmental reflex into its direct (two-neuron-arc) and indirect (multineuron-arc) components followed segregation of muscle afferent and cutaneous afferent fibers for afferent stimulation (21) provides the point of departure for the experiments to be described. Some of the present observations have been mentioned briefly in a preliminary note (23). A general discussion of these and other results will be found in another paper (25). The afferent fibers of the A group (14) exhibit a range of diameters extending from 20~ to 1.5~ (36). In a dorsal root the whole range of fibers is present, but in the peripheral nerves significant segregation .s are found (36, 8, 29, 14) which permit a degree of selective stimulation of the various components (21). For the purposes of the present discussion the afferent fibers will be classified into groups, each group being marked by a peak in the fiber distribution plots of one or another of the several peripheral nerves. Group I consists of the largest afferent fibers, which are to be found only among the afferent fibers arising from muscle. Approximately these fibers range from 20~ to 12~ in diameter (8, 29), with a distribution peak at 15 to 16~. Group II contains fibers of approximately 12~ to 6~ in diameter, with a mode at 8 to 9p. These fibers form a prominent peak in the fiber distribution plots of cutaneous nerves (8, 30, 14), but they are poorly represented among the muscles afferent fibers (8, 29). Group III consists of fibers gathered about a peak at 3 to 4~ (the delta pile). These last are to be found in both muscle and cutaneous nerves. Another category, to consist of the C fibers, the afferent and reflex function of which is proven (3, 2), should be included as group IV. These fibers have not been studied during the course of the present experiments. Since group I and group II fibers are the lowest threshold fibers in muscle and cutaneous nerves respectively, they may be excited in isolation by the simple expedient of selecting the appropriate nerves for stimulation (21). There is no means at present of stimulating group III fibers in isolation but their contribution to reflex action, on stimulation, is easily recognizable as addition to the reflex discharges caused by stimulation of the larger, lower threshold fibers [after-discharge?.] (45)

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The "dorsal root potential" consists of five successive deflections designated for convenience, D.R.I, II, III, IV, and V. Of these, D.R.V alone constitutes the dorsal root potential of prior description. A study has been made of the general properties of those deflections not previously described. Dorsal root potentials are electrotonic extensions into the extramedullary root segment, the result of electrical interactions within the cord comparable to those that have been studied in peripheral nerve. Although the anatomical and electrical conditions of interaction are infinitely more complex in the cord than in nerve, it is seen that the fact of parallel distribution of primary afferent fibers pertaining to neighboring dorsal roots provides a sufficient anatomical basis for qualitative analysis in the first approximation of dorsal root potentials. An extension of the theory of interaction between neighboring nerve fibers has been made to include an especial case of interaction between fibers orientated at right angles to one another. The predictions have been tested in a nerve model and found correct. Given this elaboration, and the stated anatomical propositions, existing knowledge of interaction provides an adequate theoretical basis for an elementary understanding of dorsal root potentials. The study of general properties and the analysis of dorsal root potentials have led to the formulation of certain conclusions that follow. D.R.I, II, and III record the electrotonic spread of polarization resulting from the external field of impulses conducted in the intramedullary segment and longitudinal trajects of primary afferent fibers. D.R.IV arises in part as the result of activity in primary afferent fibers, and in part as the result of activity in secondary neurons. In either case the mode of production is the same, and the responsible agent is residual negativity in the active collaterals, or, more precisely, the external field of current flow about the collaterals during the period of residual negativity. Current flow about active primary afferent collaterals during the period of residual negativity is the agent for residual facilitation of monosynaptic reflex pathways. Since the changes in reflex threshold follow the course of residual negativity there is no need to postulate especial properties for prolonging action at regions the threshold of which is measured by means of monosynaptic test reflexes. D.R.V results from polarization of primary afferent fibers by current flow about secondary neurons. There is indication that somata rather than axons of secondary neurons generate the polarizing currents. Similarity between D.R.V and the positive intermediary potential further indicates that soma gradients established during the recovery cycle are responsible for D.R.V. Little or no net polarization of primary afferent fibers results from activity confined to the contralateral gray substance, the dorsal root potentials in contralateral recording resulting from interaction in the dorsal column or in the ipsilateral gray substance following decussation of contralaterally evoked activity. During the course of asphyxia the initial defect in reflex pathways is the failure of secondary neurons to respond to primary impulses. Subsequently block is established at the branching zone of primary afferent fibers. A relation exists between the sequence of dorsal root potentials and the cord potential sequence, the major departure from exact correspondence occurring in the region of D.R.IV and the negative intermediary potential and being of a nature to suggest that different aspects of internuncial activity are emphasized by the two methods of leading.