BIOPHYSICAL PROPERTIES OF DESCENDING BRAIN NEURONS IN LARVAL LAMPREY

In the brains of larval lamprey, biophysical properties of reticulospi nal (RS) neurons were determined by applying depolarizing and hyperpol arizing current pulses under current clamp conditions. In response to above threshold depolarizing current pulses, almost all RS neurons pro duced an initial relatively high spiking frequency (F-i) followed by a variable decay to a steady-state firing frequency (F-ss). Spike-frequ ency adaptation (SFA), defined as [(F-i-F-ss)/F-i] x 100%, was minimal at the lowest currents and more pronounced with larger applied curren t pulses. Some RS neurons, particularly those in the posterior rhomben cephalic reticular nucleus (PRRN), either adapted very quickly, and st opped firing, or fired in short bursts during a constant depolarizing current pulse. Several types of RS neurons, including some Muller cell s and unidentified neurons in the middle rhombencephalic reticular nuc leus (MRRN), displayed delayed excitation (DE) in which spiking in res ponse to a depolarizing current pulse was delayed if preceded by a hyp erpolarizing prepulse. Very few neurons fired action potentials follow ing a hyperpolarizing pulse, such as in the case of post-inhibitory re bound (PIR), and no neurons were found that displayed plateau potentia ls. The possible contributions of these properties to the descending a ctivation of spinal locomotor networks is discussed. (C) 1998 Elsevier Science B.V.