Detection of Synchrony in the Activity of Auditory Nerve Fibers by Octopus Cells of the Mammalian Cochlear Nucleus

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 97; no. 22; pp. 11773 - 11779
• Main Authors: Oertel, Donata, Bal, Ramazan, Gardner, Stephanie M., Smith, Philip H., Joris, Philip X.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 24.10.2000; National Acad Sciences; National Academy of Sciences; The National Academy of Sciences
• Series: Colloquium Paper
• Subjects: Acoustic Stimulation; Action potentials; Animals; Aquatic life; Auditory nerve; Cats; Cell nucleus; Cells; Cochlear Nerve - physiology; Cochlear nucleus; Cochlear Nucleus - cytology; Cochlear Nucleus - physiology; Ears & hearing; Electric current; Electric potential; Giant cells; Membrane potential; Nerve Fibers - physiology; Nervous system; Neurons; Octopuses; Papers from the National Academy of Sciences Colloquium on Auditory Neuroscience: Development, Transduction, and Integration; Sound; Synaptic Transmission
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.97.22.11773

The anatomical and biophysical specializations of octopus cells allow them to detect the coincident firing of groups of auditory nerve fibers and to convey the precise timing of that coincidence to their targets. Octopus cells occupy a sharply defined region of the most caudal and dorsal part of the mammalian ventral cochlear nucleus. The dendrites of octopus cells cross the bundle of auditory nerve fibers just proximal to where the fibers leave the ventral and enter the dorsal cochlear nucleus, each octopus cell spanning about one-third of the tonotopic array. Octopus cells are excited by auditory nerve fibers through the activation of rapid, calcium-permeable, α -amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors. Synaptic responses are shaped by the unusual biophysical characteristics of octopus cells. Octopus cells have very low input resistances (about 7 MΩ ), and short time constants (about 200 μ sec) as a consequence of the activation at rest of a hyperpolarization-activated mixed-cation conductance and a low-threshold, depolarization-activated potassium conductance. The low input resistance causes rapid synaptic currents to generate rapid and small synaptic potentials. Summation of small synaptic potentials from many fibers is required to bring an octopus cell to threshold. Not only does the low input resistance make individual excitatory postsynaptic potentials brief so that they must be generated within 1 msec to sum but also the voltage-sensitive conductances of octopus cells prevent firing if the activation of auditory nerve inputs is not sufficiently synchronous and depolarization is not sufficiently rapid. In vivo in cats, octopus cells can fire rapidly and respond with exceptionally well-timed action potentials to periodic, broadband sounds such as clicks. Thus both the anatomical specializations and the biophysical specializations make octopus cells detectors of the coincident firing of their auditory nerve fiber inputs.