Biophysical properties of descending brain neurons in larval lamprey

In the brains of larval lamprey, biophysical properties of reticulospinal (RS) neurons were determined by applying depolarizing and hyperpolarizing current pulses under current clamp conditions. In response to above threshold depolarizing current pulses, almost all RS neurons produced an initial relatively high spiking frequency (Fi) followed by a variable decay to a steady-state firing frequency (Fss). Spike-frequency adaptation (SFA), defined as [(Fi - Fss)/Fi] x 100%, was minimal at the lowest currents and more pronounced with larger applied current pulses. Some RS neurons, particularly those in the posterior rhombencephalic reticular nucleus (PRRN), either adapted very quickly, and stopped firing, or fired in short bursts during a constant depolarizing current pulse. Several types of RS neurons, including some Muller cells and unidentified neurons in the middle rhombencephalic reticular nucleus (MRRN), displayed delayed excitation (DE) in which spiking in response to a depolarizing current pulse was delayed if preceded by a hyperpolarizing prepulse. Very few neurons fired action potentials following a hyperpolarizing pulse, such as in the case of post-inhibitory rebound (PIR), and no neurons were found that displayed plateau potentials. The possible contributions of these properties to the descending activation of spinal locomotor networks is discussed.