Receptive fields of fibres in the central ends of divided dorsal roots.

1. Action potentials were recorded from single fibres in the central ends of divided lumbar dorsal root filaments in anaesthetized cats. 2. Six classes of receptive field could be identified in such fibres, which corresponded very closely to the receptive fields of single primary afferent fibres. 3. For fibres with superficial receptive fileds (cutaneous or hair), a percutaneous electrical stimulation was used to drive the afferents at sustained high frequencies (100, 200 or 300 Hz), and the ability of the recorded fibre to follow the stimulation monitored. About one half of fibres with superficial receptive fields could be driven at sustained high frequencies. 4. The shape of the action potential from the fibre was analysed at different distances from the spinal cord. Close to the spinal cord a clear action potential could be recorded, but further out along the filament the action potential was often attenuated or absent. 5. It is suggested that some at least of these fibres are recurrent collaterals of primary afferent fibres, that taper in size after leaving the spinal cord. The function of such fibres is discussed.