Role of intrinsic conductances underlying responses to transients in octopus cells of the cochlear nucleus

Recognition of acoustic patterns in natural sounds depends on the transmiss ion of temporal information. Octopus cells of the mammalian ventral cochlea r nucleus form a pathway that encodes the timing of firing of groups of aud itory nerve fibers with exceptional precision. Whole-cell patch recordings from octopus cells were used to examine how the brevity and precision of fi ring are shaped by intrinsic conductances. Octopus cells responded to steps of current with small, rapid voltage changes. Input resistances and membra ne time constants averaged 2.4 MR and 210 mu sec, respectively (n = 15). As a result of the low input resistances of octopus cells, action potential i nitiation required currents of at least 2 nA for their generation and never occurred repetitively Backpropagated action potentials recorded at the som a were small (10-30 mV), brief (0.24-0.54 msec), and tetrodotoxin-sensitive . The low input resistance arose in part from an inwardly rectifying mixed cationic conductance blocked by cesium and potassium conductances blocked b y 4-aminopyridine (4-AP). Conductances blocked by 4-AP also contributed to the repolarization of the action potentials and suppressed the generation o f calcium spikes. In the face of the high membrane conductance of octopus cells, sodium and c alcium conductances amplified depolarizations produced by intracellular cur rent injection over a time course similar to that of EPSPs. We suggest that this transient amplification works in concert with the shunting influence of potassium and mixed cationic conductances to enhance the encoding of the onset of synchronous auditory nerve fiber activity.