Detection of synchrony in the activity of auditory nerve fibers by octopuscells of the mammalian cochlear nucleus

The anatomical and biophysical specializations of octopus cells allow them to detect the coincident firing of groups of auditory nerve fibers and to c onvey the precise timing of that coincidence to their targets. Octopus cell s occupy a sharply defined region of the most caudal and dorsal part of the mammalian ventral cochlear nucleus. The dendrites of octopus cells cross t he bundle of auditory nerve fibers lust proximal to where the fibers leave the ventral and enter the dorsal cochlear nucleus, each octopus cell spanni ng about one-third of the tonotopic array. Octopus cells are excited by aud itory nerve fibers through the activation of rapid, calcium-permeable, alph a -amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors, Synaptic resp onses are shaped by the unusual biophysical characteristics of octopus cell s. Octopus cells have very low input resistances (about 7 M Omega), and sho rt time constants (about 200 mu sec) as a consequence of the activation at rest of a hyper-polarization-activated mixed-cation conductance and a low-t hreshold, depolarization-activated potassium conductance. The low input res istance causes rapid synaptic currents to generate rapid and small synaptic potentials. Summation of small synaptic potentials from many fibers is req uired to bring an octopus cell to threshold. Not only does the low input re sistance make individual excitatory postsynaptic potentials brief so that t hey must be generated within 1 msec to sum but also the voltage-sensitive c onductances of octopus cells prevent firing if the activation of auditory n erve inputs is not sufficiently synchronous and depolarization is not suffi ciently rapid. In vivo in cats, octopus cells can fire rapidly and respond with exceptionally well-timed action potentials to periodic, broadband soun ds such as clicks. Thus both the anatomical specializations and the biophys ical specializations make octopus cells detectors of the coincident firing of their auditory nerve fiber inputs.