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.

The preceding paper considered the characteristics of touch receptors in the skin of cat, the analysis being made on certain of the fibres that were isolated in the course of the study now to be described. The present report is concerned with the relation between fibre diameter and receptor characteristics, as determined in 421 individual myelinated afferent fibres from the sural nerve of cat.Several considerations caused us to undertake this problem. Although there have been many studies on perception of cutaneous sensation in man, comparatively few investigations have been concerned with the receptor characteristics of individual myelinated cutaneous afferent fibres in mammals (see Discussion). Another factor was that recent anatomical studies (Sinclair, Weddell & Zander, 1952) have suggested re-evaluation of the generally held 'doctrine of specific nerve energies', i.e. the specificity of a given receptor for a particular type of stimulus. Although it has long been recognized that the specificity of receptors for particular types of stimulus was not absolute, the concept that sensory information for a particular type of natural stimulus is transmitted exclusively in certain afferent fibres has been widely accepted.Theuse ofvolleys in cutaneous nerves, graded in sizebystimulus strength, for the study of spinal reflexes and of sensory systems has also posed a need for more information on the relation between receptor characteristics and fibre diameters. Similarly, the detailed information now available on responses to natural stimuli of cortical neurones of the primary sensory receiving area (Mountcastle, 1957) requires, for its fuller interpretation, more information as to receptive fields and response characteristics as recorded in primary afferent fibres.It will be shown that several distinct categories of receptors may be recognized by their responses to adequate or natural stimulation, and the relation of fibre diameter to these categories has been defined. Also, 7-2

The flexor longus digitorum (FLD) in the cat is an unusual muscle in several respects. It consists of two separate heads, of which the lateral is the larger (the homologue of flexor longus hallucis in certain other species). The tendons of the two heads merge in the foot and the combined tendon splits into four subdivisions which are inserted into the terminal phalanges of the four digits (Reighard & Jennings, 1934). Thus the two heads of FLD have an identical action consisting of plantar flexion of the digits and protrusion of the claws. While this muscle behaves reflexly as an extensor, acting to oppose the force of gravity, it also has the unique function of claw protrusion. Possibly in relation to the latter function, it appears to be the only hind-limb muscle subject to long spinal inhibition from the forelimb (Lloyd & McIntyre, 1948). A recent study by Eccles, Eccles & Lundberg (1957) suggests that FLD might be atypical in still another regard, namely, in that volleys presumably confined to afferent fibres of the largest diameter range evoke unusually powerful reflex effects in a pattern oflimb flexion. It was suggested that such actionwas produced by impulses in fibres from tendon organs. This flexor action has been considered to be significant in initiation of the flexor phase of stepping movements (Lundberg, 1959;Eccles & Lundberg, 1959).Another point of interest in relation to FLD is that a small nerve from the region of the interosseous membrane courses between the medial and lateral heads of the muscle and merges with the nerve to FLD medial head. Since it seems likely that this interosseous nerve might frequently have been included with the FLD nerves in both anatomical and physiological studies, analysis of the receptor characteristics of its fibres is of obvious interest.The present study had the twofold purpose of analysing the afferent-fibre diameter-receptor relation in the nerves to FLD, and of exploring the types of receptor served by fibres in the interosseous nerve. The findings indicate

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SUMMARY1. Afferent responses were recorded from filaments of the trigeminal nerve in each of two platypuses (Ornithorhynchus anatinus) anaesthetized with a-chloralose. All receptive fields were located along the lateral border of the upper bill. Discrete receptive fields could be identified as belonging to two distinct classes of sensory receptor.2. The most prominent response was an irregular resting discharge which could be increased or decreased by weak electric pulses. These receptors were insensitive to moderately strong mechanical stimulation, and it was concluded that they were electroreceptors.3. Each electroreceptor had a single spot of maximum sensitivity on the bill surface. When the stimulating electrode over this spot was the cathode it excited the receptor for the duration of the stimulating pulse, using stimulus strengths as low as 20 mV. When it was the anode, it inhibited the discharge. Cathodal excitation was followed by rebound inhibition and anodal inhibition by rebound excitation.4. Receptors responded to cathodal steps with an initial high-frequency burst of impulses, followed by a lower maintained rate of discharge. Rapidly changing pulses were similarly effective in exciting receptors, adding support to the claim that platypuses are able to detect moving-prey by the electrical activity associated with muscle contraction.5. The centres of the receptive fields of two electroreceptors were marked by the insertion of fine entomological pins. Histological examination established the presence of a large mucus-secreting gland at the marked spot. The epidermal duct of the gland contained an elaborate myelinated innervation, with morphologically distinct axon terminals that we identify as the electroreceptors.6. As well as electroreceptors, the skin of the bill contained three kinds of mechanoreceptors: slow-adapting receptors, rapidly adapting, vibration-sensitive receptors and receptors with an intermediate adaptation rate. The slowly adapting receptors were characterized by their low threshold to mechanical stimuli, irregular discharge and significant dynamic sensitivity. Vibration receptors showed maintained responses to sinusoidal vibration of the skin up to 600 Hz. 7. These experiments confirm an earlier report that the platypus bill is an electrodetector organ. The presence of electroreceptors of a unique structure and J. E. GREGORY AND OTHERS supplied by the trigeminal nerve indicates that electroreception has evolved independently in monotremes. This in turn emphasizes that monotremes are a highly evolved group which split off from the main mammalian stem a long time ago.