Using isolated frog spinal cords, treated with pentobarbital to silence internuncial discharge, intracellular recordings from motor neurons reveal differences in dendritically initiated EPSP evoked by dorsal root stimulation and somatically initiated EPSP evoked by lateral column stimulation. Under these conditions, dorsal root EPSP never reached motor neuron threshold whereas threshold was easily reached by lateral column EPSP. EPSP's initiated by dorsal root volleys were slower in their time course and smaller in their amplitude than those initiated by lateral column volleys. EPSP's initiated by lateral column volleys reduced the amplitude of antidromic spikes, while those produced by dorsal root stimulation did not. Lateral column induced EPSP was also capable of reducing the amplitude of orthodromic spikes. Some observations on duration of transmitter action are reported. It is concluded that dendritic excitation following a dorsal root volley influences the level of polarization of the cell body by electrotonic propagation of the resulting EPSP.

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It has been noted that the patterns of discharge evoked in frog spinal motoneurones are significantly different when a presynaptic volley is initiated in dorsal root fibres and when the excitation arrives over a system of fibres in the lateral column . Anatomical studies (Liu & Chambers, 1957) raise the possibility that the differences may be ascribed to variations in the site of synaptic terminations on the motoneurones or to differences in the number of interneurones involved in the two pathways, or both. The present study was undertaken in order to determine which of these possibilities is the more likely. The technique of focal recording from within the spinal cord, and comparison of the temporal relations of simplified responses, have permitted a test of the hypothesis suggested by anatomical findings. METHODSThe experiments have been performed on isolated spinal cords of frogs (Rana pipiens) during winter and spring months. The arrangement has been similar to that described previously . Micropipettes of 5-15 MQ resistance, filled with potassium citrate, were used for depth recording. Ipsilateral evoked focal potentials were recorded unipolarly through an impedance-matching preamplifier, a high-gain, RC-coupled amplifier (time constant 1 sec), and photographed from an oscilloscope screen. A second oscilloscope was usually used to monitor ipsilateral ventral root responses. A thermistor thermometer in contact with the cord made possible temperature control in the range from 15-17°C.The method of preserving function in isolated dorsal and ventral roots was important to the success of these experiments. The root was extended laterally on a small slip of ifiter paper coated with thinned petroleum jelly. The root was laid over hooked platinum electrodes and a complete covering of petroleum jelly delivered from a syringe and hypodermic needle was built around the entire root. This procedure not only preserved the fibres of the root, but also prevented troublesome electrical instability and provided the necessary insulation of the ventral root from the fluid surrounding the cord. In recording from the ventral root, the distance between proximal lead and cord varied in different experiments from 3 to 5 mm; the distal lead was always applied as far as possible from the cord on the crushed end of the root.

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