title: Morphology, intrinsic membrane properties, and rotation-evoked responses of trochlear motoneurons in the turtle.
abstract: Intrinsic properties and rotation-evoked responses of trochlear motoneurons were investigated in the turtle using an in vitro preparation consisting of the brain stem with attached temporal bones that retain functional semicircular canals. Motoneurons were divided into two classes based on intrinsic properties. The first class exhibited higher impedance (123.0 +/- 11.0 MOmega), wider spikes (0.99 +/- 0.05 ms), a single spike afterhyperpolarization (AHP), little or no spike frequency adaptation (SFA), and anomalous rectification, characterized by an initial "sag" in membrane potential in response to hyperpolarizing current injection. The second class exhibited lower impedance (21.8 +/- 2.5 MOmega), narrower spikes (0.74 +/- 0.03 ms), a double AHP, substantial SFA, and little or no rectification. Vestibular responses were evoked by horizontal sinusoidal rotation (1/12-1/3 Hz; peak velocity: 30-100 degrees /s). Spiking in higher-impedance cells was recruited earlier in the response and exhibited a more limited dynamic range relative to that of lower impedance cells. Spiking evoked by injecting depolarizing current during rotation was blocked during contraversive motion and was consistent with a shunting inhibition. No morphological features were identified in neurobiotin-filled cells that correlated with the two physiological classes. Recovered motoneurons were multipolar but exhibited a less-complex dendritic morphology than ocular motoneurons of similarly sized mammals. The two physiologically defined cell classes have homologues in other vertebrates, suggesting that intrinsic membrane properties play an important role in oculomotor processing.
authors.affiliation: Department of Pharmacological and Physiological Science, Saint Louis University, 1402 S. Grand Blvd., St. Louis, MO 63104, USA.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Jones
authors.forename: Michael
authors.initials: M
authors.lastname: Ariel
mesh_terms: D000818:Animals; D001933:Brain Stem / Q000166:cytology; D002452:Cell Count / Q000379:methods; D002462:Cell Membrane / Q000502:physiology / Q000528:radiation effects; D048429:Cell Size; D004307:Dose-Response Relationship, Radiation; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000502:physiology* / Q000528:radiation effects; D006735:Horseradish Peroxidase / Q000378:metabolism; D009046:Motor Neurons / Q000145:classification / Q000166:cytology / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology* / Q000528:radiation effects; D018408:Patch-Clamp Techniques; D012399:Rotation; D014321:Trochlear Nerve / Q000502:physiology*; D014426:Turtles / Q000033:anatomy & histology* / Q000502:physiology
chemical_list: D006735:Horseradish Peroxidase