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Biomedical subjects

K E Webster

Publications and source records attributed to K E Webster.

At least 37 records · Page 2Linked to original sources
PubMed1985

Cytoarchitectural fields and retinal termination: an axonal transport study of laminar organization in the avian optic tectum.

The cytoarchitecture in the retinoreceptive zone of the pigeon optic tectum has been studied in Nissl-stained sections taken in four planes. As suggested by a previous study, two cytoarchitectural fields are present. Reconstructed views of the tectum show that the fields are separated by a narrow transition zone approximating to the tectal representation of the retina's horizontal meridian. In field 1 (which is upper and rostral), sublayer IIb is wide, IIc wide and trilaminate, IId narrow and IIe continuous; in field 2, IIb and c are narrow, IId wide and IIe discontinuous. The distribution of retinal terminals was investigated by the anterograde axonal transport of [3H]proline or horseradish peroxidase from intravitreal injections. The depth distribution of grains or reaction product throughout the entire tectum was quantified by scanning with a microdensitometer. Both autoradiography and horseradish peroxidase transport show two patterns of lamination separated by a narrow transition zone and these two terminal fields correspond closely to the cytoarchitectural fields. In field 1 optic terminals are concentrated in sublayer IIb, superficial c, d, and to a lesser extent in f; in field 2 concentrations are present at the IIb/c boundary, across deep IIc and d, and a small concentration is found IIf. The patterns of retinal termination with depth in the tectum found by axonal transport are compatible with those found by electron microscopy, and are discussed in relation to the optic termination found by other techniques. Study of the time course of axonal transport shows that both radioactive material and horseradish peroxidase are fast transported to all the bands of optic terminals at about 150 mm/day. Horseradish peroxidase gradually accumulates in the retinoreceptive zone, filling clusters of terminals and horizontal processes. At 12 days, it has begun to disappear from the zone and a few diffusely filled profiles, that may be transcellularly labelled, are present. Electron microscope autoradiography of fast transported material shows clusters of grains over optic terminals and preterminals and a percentage density analysis confirms that these profiles are specifically labelled. The two tectal fields each contain the projection from specialized areas of the retina, suggesting functional specialization in the tectum for the processing of different kinds of visual information.

Animals↗
PubMed1984

The effects of capsaicin applied to peripheral nerves on responses of a group of lamina I cells in adult rats.

In adult rats, the sciatic and saphenous nerves on one side were treated topically with capsaicin. The capsaicin treatment had the effect of increasing the latency for withdrawal of the foot from hot water; 11-22 days later, the animals were decerebrated, and cells in the superficial dorsal horn of the lumbar cord with axons projecting in the contralateral dorsolateral funiculus (DLF) were examined electrophysiologically on the treated and untreated sides of the cord. HRP was applied to cut axons of the DLF at C4, in other rats, and retrograde labelling of cells in the lumbar cord indicated that most or all of the recordings in the capsaicin-treated animals were likely to originate from lamina 1. The dorsal horn cells, with receptive fields on the foot, showed decreased responses to electrically evoked afferent impulses in C fibres and grossly altered receptive fields. After capsaicin treatment, the proportion of cells responding to C afferents fell from 83% to 14%. The proportion responding only to C afferents and not to A afferents fell from 9% to 0%. The receptive fields (RFs) of these cells showed two gross abnormalities; 32% of the cells on the treated side had no apparent RF or an ill-defined, intermittent RF, whereas such cells were rare on the untreated side or in intact animals. By contrast 49% of the cells had grossly expanded RFs with an average area of 430 mm2 against the normal average size of 130 mm2.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed1981

Neurones situated outside the isthmo-optic nucleus and projecting to the eye in adult birds.

The centrifugal projection to the eye has been studied by retrograde horseradish peroxidase (HRP) transport in adult pigeons and chickens. About 1500 large neurones outside the contralateral isthmo-optic nucleus and 20 or so ipsilateral ectopic cells contain HRP 0.5-3.5 days after intravitreal HRP injections. The number of ectopic neurones which project to the contralateral eye is more than 20% of the number of labelled cells within the isthmo-optic nucleus. In contrast to the monopolar isthmo-optic neurones, the ectopic cells are a distinct population of large multipolar cells. Previously it has been suggested that many of these cells degenerate during the development of the chick. This study shows that they persist in the adult avian brain where they can be detected in large numbers by the presence of retrogradely transported HRP, provided that an HRP chromogen reaction of sufficient sensitivity is used. The exact target for their axons is unknown.

Animals↗
PubMed1975

The organisation of the spinotectal projection. An experimental study in the rat.

The spinal grey projects upon the contralateral superior colliculus via two overlapping pathways in the lateral funiculus. One, more ventrally placed, is the classical spinotectal tract, which crosses immediately in the spinal cord and remains crossed. The other, lying more dorsally, ascends homolaterally but is subject to delayed crossing in the brainstem (especially the intertectal) commissures to reach the contralateral colliculus. Both sets of fibers end only in the caudal half of the colliculus, predominantly in the stratum album intermedium. The projections from individual cord segments distribute in an ordered fashion as a series of transverse, overlapping bands, the cervical cord projecting most rostrally, the sacro-coccygeal most caudally. Additional fibers, which distribute with less marked topography, end in the lateral extreme of the stratum griseum profundum and stratum album profundum.

Animals↗
PubMed1975

The projection of the retina upon the optic tectum of the pigeon.

The projection of the retina onto the optic tectum of the pigeon has been investigated using silver impregnation methods for degenerating axons and terminals, autoradiographic tracing, and the Golgi methods. Degenerating optic afferents distribute to sublaminae a-d and f of the stratum griseum et fibrosum superficiale over the whole tectum, but two major fields are observed. One occupies the caudal and ventral tectum (in which region laminar cytoarchitecture is marked), and the other rostral and dorsal tectum (where demarcation of cell laminae is relatively poor). Degeneration in the latter field is coarse and clearly distributes in a distinctly laminated fashion within the stratum griseum et fibrosum superficiale. In contrast, degeneration in the caudo-ventral region is fine, and laminated distribution less clear. The evolution of the degeneration pattern over survival periods from 3 to 56 days suggests that these laminar distributions reflect the existence of several different types of optic terminal ramification present in all parts of the tecum. A selective laminar distribution of such optic afferent types correlates well with our own and other Golgi studies. The results of the autoradiography experiments are broadly compatible with these findings.

Animals↗
PubMed1975

An electron microscope study of the retino-receptive layers of the pigeon optic tectum.

The fine structure of the superficial layer of the optic tectum of the pigeon, Columba livia, has been examined both in normal animals and after unilateral eye removal. Optic afferent terminals distributed throughout sublaminae IIa-f, are principally concentrated at three depths in the retinoreceptive zone. At first, degeneration of optic afferent terminals occurs as swelling of synaptic vesicles and accumulation of neurofilaments and later as dense degeneration. There appear to be several classes of optic afferent terminals in the retino-receptive zone, as would be expected from most previous studies of the avian tectum. The non-optic components of the neuropil, which include glomerular and pleomorphic vesicle terminals, and the synaptic interrelationships between optic and non-optic components are described. Optic afferent terminals mainly make synaptic contact with the branches of radial dendrites but also, in sublaminae IIb and IIc, with the dendrites of non-radial cells. In sublamina IIc afferent terminals synapse onto the horizontal presynaptic dendrites of spindle cells and triplet synaptic arrays including optic afferent terminals, horizontal dendrites and radial dendrite profiles and radial dendrite profiles are found. This optic projection to non-radial elements in IIb c, is discussed in relation to light microscope and electrophysiological studies.

Animals↗