Responses of irregularly discharging chinchilla semicircular canal vestibular-nerve afferents during high-frequency head rotations.
Mammalian vestibular-nerve afferents innervating the semicircular canals have been divided into groups according to their discharge regularity, gain at 2-Hz rotational stimulation, and morphology. Low-gain irregular afferents terminate in calyx endings in the central crista, high-gain irregular afferents synapse more peripherally in dimorphic (bouton and calyx) endings, and regular afferents terminate in the peripheral zone as bouton-only and dimorphic endings. The response dynamics of these three groups have been described only up to 4 Hz in previous studies. Reported here are responses of chinchilla semicircular canal vestibular-nerve afferents to rotational stimuli at frequencies up to 16 Hz. The sensitivity of all afferents increased with increasing frequency with the sensitivity of low-gain irregular afferents increasing the most and matching the high-gain irregular afferents at 16 Hz. All afferents increased their phase lead with respect to stimulus velocity at higher frequencies with the highest leads in low-gain irregular afferents and the lowest in regular afferents. No attenuation of sensitivity or shift in phase consistent with the presence of a high-frequency pole over the range tested was noted. Responses were best fit with a torsion-pendulum model combined with a lead operator (tau(HF1)s + 1)(tau(HF2)s + 1). The discharge regularity of individual afferents was correlated to the value of each afferent's lead operator time constants. These findings suggest that low-gain irregular afferents are well suited for encoding the onset of rapid head movements, a property that would be advantageous for initiation of reflexes with short latency such as the vestibulo-ocular reflex.
Hullar
Timothy E
TE
Department of Otolaryngology--Head and Neck Surgery, Washington University School of Medicine, 660 S. Euclid Ave. #8115, Saint Louis, MO 63110, USA. hullart@ent.wustl.edu
Della Santina
Charles C
CC
Hirvonen
Timo
T
Lasker
David M
DM
Carey
John P
JP
Minor
Lloyd B
LB
D000818:Animals; D002682:Chinchilla; D006257:Head; D019416:Head Movements; D009475:Neurons, Afferent / Q000502:physiology*; D012027:Reflex, Vestibulo-Ocular / Q000502:physiology*; D012399:Rotation; D012665:Semicircular Canals / Q000502:physiology*; D013997:Time Factors; D014725:Vestibular Nerve / Q000166:cytology* / Q000502:physiology