Biomedical subjects

Tian Xing Fan

Publications and source records attributed to Tian Xing Fan.

3 recordsLinked to original sources

PubMed2004

Bilateral processing of vestibular responses revealed by injecting lidocaine into the eighth cranial nerve in vitro.

Extracellular unit responses were recorded from the vestibular nucleus (VN) and medial longitudinal fasciculus during horizontal head rotation of an in vitro turtle brainstem in which the temporal bones remained attached. Units were characterized as type I or type II based on the responses to ipsiversive or contraversive rotation, respectively. Lidocaine injections (0.5-2 microl of 0.5%) into the root of the eighth cranial nerve within the cranium caused rapid effects on unit responses to head rotation. Responses of type I units were reduced by ipsilateral injection but enhanced following contralateral injection. On the other hand, type II units had their responses increased by ipsilateral injections yet decreased by contralateral injections. In approximately half of the type II cells, decrease of the contraversive response was accompanied by the appearance of latent ipsiversive activity. Our findings not only confirm that each eighth nerve has afferents that drive ipsiversive excitation of both vestibular nuclei but also suggest that both nerves compete to dominate a central neuron's vestibular response. These results may be inconsistent with the push-pull vestibular model in which each nerve drives the central neuron with a complementary response that enhances the vestibular output. An alternate model is described in which vestibular neurons receive bilateral excitation, and that excitatory input is antagonized by crossed inhibition during contraversive motion.

Action Potentials↗

PubMed2002

Synaptic pharmacology in the turtle accessory optic system.

The accessory optic system of the turtle (the basal optic nucleus, BON) receives both excitatory and inhibitory inputs that are direction-sensitive. When the dorsal midbrain is ablated, only the monosynaptic direction-sensitive input from the retina to the BON remains. To better understand the central visual processing performed by the accessory optic system, this study identifies the neurotransmitters and their receptors that mediate the synaptic excitation and inhibition of BON cells. We used a reduced in vitro turtle brainstem preparation in which the two eyes and brain were isolated pharmacologically. Patch recordings were made on BON neurons while drugs were applied to the brain, with the eyes bathed in control media and either exposed to visual pattern motion or subjected to electrical stimulation. An antagonist of the AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) subtype of glutamate receptor applied within the brain chamber blocked the visual responses. In response to electrical stimulation both excitatory and inhibitory synaptic events were blocked in BON cells, presumably by blocking direct excitation by retinal ganglion cell axons in the BON and indirect excitation of inhibitory interneurons elsewhere in the brainstem. An NMDA receptor antagonist was ineffective, even when the response was measured in a BON cell depolarized in Mg(2+)-free media. A GABA(A) receptor on the BON cell mediates the inhibitory responses to retinal stimulation. Injection of lidocaine into the contralateral eye caused an increase in spontaneous inhibitory post-synaptic potentials (IPSPs), suggesting that a tonic retinal output exists that reduces brainstem inhibition of BON cells. Also, there may be tonic inhibition of an excitatory path to BON neurons from within the brainstem, because bicuculline increased spontaneous excitatory post-synaptic potentials (EPSPs) observed in a BON cell without retinal input. These results indicate that the BON is a site of complex visual processing of competing visual signals and provide insight into how an interaction of excitation and inhibition creates a retinal slip signal in the accessory optic system.

2-Amino-5-phosphonovalerate↗

PubMedDate not supplied

Morphology of the turtle accessory optic system.

Neural signals of the moving visual world are detected by a subclass of retinal ganglion cells that project to the accessory optic system in the vertebrate brainstem. We studied the dendritic morphologies and direction tuning of these brainstem neurons in turtle (Pseudemys scripta elegans) to understand their role in visual processing. Full-field checkerboard patterns were drifted on the contralateral retina while whole-cell recordings were made in the basal optic nucleus in an intact brainstem preparation in vitro. Neurobiotin diffused into the neurons during the recording and was subsequently localized in brain sections. Neuronal morphologies were traced using appropriate computer software to analyze their position in the brainstem. Most labeled neurons were fusiform in shape and had numerous varicosities along their processes. The majority of dendritic trees spread out in a transverse plane perpendicular to the rostrocaudal axis of the nucleus. Neurons near the brainstem surface were often oriented tangential to that surface, whereas more cells at the dorsal side of the nucleus were oriented radial to the brainstem surface. Further analysis of Nissl-stained neurons revealed the largest neurons are located in the rostral and medial portions of the nucleus although neurons are most densely packed in the middle of the nucleus. The preferred directions of the visual responses of the neurons in this sample did not correlate with their morphology and position in the nucleus. Therefore, the morphology of the cells in the turtle accessory optic system appears dependent on its position within the nucleus while its visual responses may depend on the synaptic inputs that contact each cell.

Animals↗