PubMed Health⌕ Search

Biomedical subjects

L R Britto

Publications and source records attributed to L R Britto.

63 records · Page 4Linked to original sources
PubMed1983

Retinal ganglion cells of the pigeon accessory optic system.

After the demonstration of its involvement in visuomotor coordination, the accessory optic system has been the subject of renewed interest. An interesting feature of its functional organization is that the retinal ganglion cells generating projections to accessory optic nuclei appear to be, at least in birds, the displaced ganglion cells of Dogiel. Retinal extracellular unit recordings were performed for 21 pigeons. Neurons were identified by antidromic activation from primary visual relays, including the nucleus of the basal optic root of the accessory optic system, the optic tectum, and the thalamic dorsolateral nuclear complex. Ganglion cells (N = 68) of the accessory optic system were always found deeper in the retina, close to the boundary between the inner plexiform and the inner nuclear layers, i.e. displaced from the ganglion cell layer itself. Their latencies to antidromic activation ranged from 1.3 to 4.6 ms (mean +/- SD, 2.3 +/- 0.5 ms). The receptive fields were very large, peripheral and apparently non-organized. Responses to stationary photic stimuli were weak, and the main feature of these cells appeared to be high sensitivity to slowly moving targets, with some degree of directional selectivity. These response properties indicate that the ganglion cells of the accessory optic system in pigeons could well detect visual image displacement on the retina. Together with other anatomical, physiological and behavioral data, the present study provides information linking the accessory optic system to oculomotor control functions.

Animals↗
PubMed1981

The accessory optic system in pigeons: receptive field properties of identified neurons.

Visual receptive fields of neurons in the nucleus of the basal optic root were investigated in pigeons. Their projections could be traced to vestibulo-cerebellum and oculomotor complex by means of antidromic activation. Units in that nucleus showed large peripheral receptive fields and appeared highly sensitive to moving targets, with the majority displaying axis specificity and direction selectivity. The results seem consistent with the proposed role of the accessory optic system in oculomotor reflexes.

Afferent Pathways↗
PubMed1978

Inhibition of tectal neurons from telencephalic visual areas in pigeons.

The author investigated the influence of efferent projections of visual telencephalic areas upon the optic tectum, in locally anesthetized, paralyzed and artificially ventilated pigeons, using extracellular unit recording and electrical stimulation by means of stereotaxically implanted bipolar electrodes. Despite the fact that some neurons located at superficial tectal layers could be driven by telencephalic shocks, the most consistent data are the inhibitory effects observed throughout the tectum, but mainly at deep layers. This effect was observed either on spontaneous activity or on electrically elicited spikes, by stimulating the optic tract. In the latter case, a critical interval (25--60 msec) was necessary to obtain maximal inhibition, when testing Wulst shocks previous to optic tract stimuli. These results show that the higher telencephalic visual area (the so-called Wulst) can modulate tectal activity, and so control the output of this region, by an example, to thalamic nucleus rotundus, second station of the tectofugal visual pathway. The Wulst-tectal relationship is discussed in its possible role in visual information processing and visually guided behaviours in pigeons, and also in comparison to mammalian corticotectal systems.

Animals↗
PubMedDate not supplied

The retinal targets of centrifugal neurons and the retinal neurons projecting to the accessory optic system.

In birds, neurons of the isthmo-optic nucleus (ION), as well as "ectopic" neurons, send axons to the retina, where they synapse on cells in the inner nuclear layer (INL). Previous work has shown that centrifugal axons can be divided into two anatomically distinct types depending on their model of termination: either "convergent" or "divergent" (Ramon y Cajal, 1889; Maturana & Frenk, 1965). We show that cytochrome-oxidase histochemistry specifically labels "convergent" centrifugal axons and target neurons which appear to be amacrine cells, as well as three "types" of ganglion cells: two types found in the INL (displaced ganglion cells) and one in the ganglion cell layer. Labeled target amacrine cells have distinct darkly labeled "nests" of boutons enveloping the somas, are associated with labeled centrifugal fibers, and are confined to central retina. Lesions of the isthmo-optic tract abolish the cytochrome-oxidase labeling in the centrifugal axons and in the target amacrine cells but not in the ganglion cells. Cytochrome-oxidase-labeled ganglion cells in the INL are large; one type is oval and similar to the classical displaced ganglion cells of Dogiel, which have been reported to receive centrifugal input; the other type is rounder. Rhodamine beads injected into the accessory optic system results in retrograde label in both types of cells, showing that two distinct types of displaced ganglion cells project to the accessory optic system in chickens. The ganglion cells in the ganglion cell layer that label for cytochrome oxidase also project to the accessory optic system. These have proximal dendrites that ramify in the outer inner plexiform layer.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

Cholera toxin mapping of retinal projections in pigeons (Columbia livia), with emphasis on retinohypothalamic connections.

Anterograde transport of cholera toxin subunit B (CTb) was used to study the retinal projections in birds, with an emphasis on retinohypothalamic connections. Pigeons (Columbia livia) were deeply anesthetized and received unilateral intraocular injections of CTb. In addition to known contralateral retinorecipient regions, CTb-immunoreactive fibers and presumptive terminals were found in several ipsilateral regions, such as the nucleus of the basal optic root, ventral lateral geniculate nucleus, intergeniculate leaflet, nucleus lateralis anterior, area pretectalis, and nucleus pretectalis diffusus. In the hypothalamus, CTb-immunoreactive fibers were observed in at least two contralateral cell groups, a medial hypothalamic retinorecipient nucleus, and a lateral hypothalamic retinorecipient nucleus. To compare retinorecipient hypothalamic nuclei in pigeons with the mammalian suprachiasmatic nucleus, double-label experiments were conducted to study the existence of neurophysin-like immunoreactivity in the retinorecipient avian hypothalamus. The results showed that only cell bodies in the medial hypothalamic nucleus contained neurophysin-like immunoreactivity. The results demonstrate CTb to be a sensitive anterograde tracer and provide further anatomical information on the avian equivalent of the mammalian suprachiasmatic nucleus.

Animals↗
PubMedDate not supplied

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization.

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMedDate not supplied

Enkephalin-immunoreactive ganglion cells in the pigeon retina.

A small number of enkephalin-like immunoreactive cells were observed in the ganglion cell layer of the pigeon retina. Many of these neurons were identified as ganglion cells, since they were retrogradely labeled after injections of fluorescent latex microspheres in the contralateral optic tectum. These ganglion cells were mainly distributed in the inferior retina, and their soma sizes ranged from 12-26 microns in the largest axis. The enkephalin-containing ganglion cells appear to represent only a very small percentage of the ganglion cells projecting to the optic tectum (less than 0.1%). Two to 7 weeks after removal of the neural retina, there was an almost complete elimination of an enkephalin-like immunoreactive plexus in layer 3 of the contralateral, rostrodorsal optic tectum. These data provide evidence for the existence of a population of enkephalinergic retinal ganglion cells with projections to the optic tectum.

Animals↗