Biomedical subjects

Paul R Martin

Publications and source records attributed to Paul R Martin.

22 records · Page 2Linked to original sources

PubMedDate not supplied

Expression and distribution of ionotropic glutamate receptor subunits on parasol ganglion cells in the primate retina.

The response properties of postreceptoral sensory neurones are determined by the properties of their input neurones, by intrinsic membrane properties, and by the properties of neurotransmitter receptors on the soma and dendritic tree. We previously showed that inhibitory neurotransmitter (GABA(A) and glycine) receptors on a well-characterised sensory neurone, the parasol ganglion cell in the primate retina, are segregated towards the distal part of the dendritic tree. Here we studied the distribution of excitatory ionotropic glutamate receptor subunits on the dendrites of parasol cells in the retina of a New World monkey, the marmoset, Callithrix jacchus. Individual ganglion cells were intracellularly injected in an in vitro retinal wholemount preparation. Ionotropic glutamate receptor subunits. including AMPA (GluR1-4), kainate (GluR6/7), NMDA (NR1C2') subunits, and the orphan receptors delta1 and delta2 were visualized with immunocytochemical methods. Immunoreactive puncta that colocalized with the dendrites of ganglion cells were analyzed using standard and/or confocal light microscopy. Colocalized puncta were present on parasol dendrites for all subunits studied, but their density was much lower (approximately 1/5) than previously reported for inhibitory (GABA and glycine) receptors. Segregation of the glutamate receptor clusters (GluR1, GluR6/7 subunits) to the peripheral dendrites was less marked than that shown for GABA and glycine receptor clusters. No sign of segregation of colocalized puncta to the peripheral part of the dendritic field was seen with antibodies to the GluR2, GluR2/3, GluR4, delta1/2, or NR1C2' subunits. The results suggest that although there is diverse expression of glutamate receptor subtypes, the glutamatergic synapses form only a small proportion of the total synaptic input to primate ganglion cells. They further suggest that the processes which control distribution of excitatory and inhibitory synapses on the dendritic field of ganglion cells are, at least to some extent, independent.

Animals↗

PubMedDate not supplied

Mosaic properties of midget and parasol ganglion cells in the marmoset retina.

We measured mosaic properties of midget and parasol ganglion cells in the retina of a New World monkey, the common marmoset Callithrix jacchus . We addressed the functional specialization of these populations for color and spatial vision, by comparing the mosaic of ganglion cells in dichromatic ("red-green color blind") and trichromatic marmosets. Ganglion cells were labelled by photolytic amplification of retrograde marker ("photofilling") following injections into the lateral geniculate nucleus, or by intracellular injection in an in vitro retinal preparation. The dendritic-field size, shape, and overlap of neighboring cells were measured. We show that in marmosets, both midget and parasol cells exhibit a radial bias, so that the long axis of the dendritic field points towards the fovea. The radial bias is similar for parasol cells and midget cells, despite the fact that midget cell dendritic fields are more elongated than are those of parasol cells. The dendritic fields of midget ganglion cells from the same (ON or OFF) response-type array show very little overlap, consistent with the low coverage of the midget mosaic in humans. No large differences in radial bias, or overlap, were seen on comparing retinae from dichromatic and trichromatic animals. These data suggest that radial bias in ganglion cell populations is a consistent feature of the primate retina. Furthermore, they suggest that the mosaic properties of the midget cell population are associated with high spatial resolution rather than being specifically associated with trichromatic color vision.

Animals↗

PubMedDate not supplied

Spatial coding and response redundancy in parallel visual pathways of the marmoset Callithrix jacchus.

Many neurons in the primary visual cortex (area V1) show pronounced selectivity for the orientation and spatial frequency of visual stimuli, whereas most neurons in subcortical afferent streams show little selectivity for these stimulus attributes. It has been suggested that this transformation is a functional sign of increased coding efficiency, whereby the redundancy (or overlap in response properties) is reduced at consecutive levels of visual processing. Here we compared experimentally the response redundancy in area V1 with that in the three main dorsal thalamic afferent streams, the parvocellular (PC), koniocellular (KC), and magnocellular (MC) divisions of the dorsal lateral geniculate nucleus (LGN) in marmosets. The spatial frequency and orientation tuning of single cells in the LGN and area V1 were measured, using luminance contrast sine-wave gratings. A joint spatial frequency-orientation response selectivity profile was calculated for each cell. Response redundancy for each population was estimated by cross-multiplication of the joint selectivity profiles for pairs of cells. We show that when estimated in this way, redundancy in LGN neurons is approximately double that of neurons in cortical area V1. However, there are differences between LGN subdivisions, such that the KC pathway has a spatial representation that lies between the redundant code of the PC and MC pathways and the more efficient sparse spatial code of area V1.

Action Potentials↗

PubMedDate not supplied

Effects of noise and a stressor on head pain.

OBJECTIVE: Can causal relationships be established between negative affect (NA) and headaches, and noise (N) and headaches? Do NA and N interact to cause headaches? Do NA and N cause headaches by means of the same or different physiological mechanisms? Are the answers to these questions a function of diagnostic status? BACKGROUND: A functional model of chronic headaches has been proposed that seeks to understand the variance in headaches by focusing on the controlling variables, that is, the antecedents and consequences. This study is one in a series investigating the immediate antecedents of headaches, namely the trigger factors. DESIGN: Twenty-four subjects with migraine and 44 subjects with tension-type headache were randomly assigned to one of four experimental conditions defined by the presence or absence of two antecedent challenges, a stressor (S) designed to induce NA and N. METHODS: The S challenge consisted of difficult-to-solve anagrams accompanied by failure feedback. The N challenge consisted of 50 dB of white N. Laboratory sessions were divided into adaptation, baseline, challenge, and recovery phases. Responses were measured in terms of headache intensity ratings, forehead electromyographic activity, heart rate, blood pressure, and temporal pulse amplitude (TPA). RESULTS AND CONCLUSIONS: Both NA and N precipitated headaches. These two factors did not interact in triggering headaches. Headaches induced by N were associated with elevated TPA but headaches induced by NA were not associated with significant physiological changes. Diagnosis was not related to any of the outcomes.

Adolescent↗