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P C Murphy

Publications and source records attributed to P C Murphy.

25 records · Page 2Linked to original sources
PubMed1989

The binocular input to cells in the feline dorsal lateral geniculate nucleus (dLGN).

1. Cells in the A laminae of the dorsal lateral geniculate nucleus receive their primary innervation from either the contralateral (A) or ipsilateral (A1) eye. This paper provides evidence concerning the responses they give to visual stimulation of what is commonly regarded as the ineffective or non-dominant eye. It also examines the contribution of the corticofugal input to these responses. 2. Cells were identified and classified according to their responses to stimulation of the dominant eye receptive field. This was then occluded and the corresponding location in the non-dominant eye field stimulated by a moving bar. Out of fifty-seven cells examined forty-three (75%) gave a response to stimulation of the non-dominant eye. There was no obvious difference between the effects on X and Y cells in these experiments. 3. In most cases (thirty-seven) the response involved an inhibition of the resting discharge level, but three cells gave a mixed excitatory and inhibitory response and three a pure excitatory response. All the responses were weak and only revealed by prolonged periods of averaging (20-100 trials). 4. Ionophoretic application of the GABA antagonist N-methyl-bicuculline (NMB) blocked the visually elicited inhibitory effects and in most cases (twenty-seven out of thirty-two tested) revealed an excitatory response. Out of a further eight cells previously unresponsive to the non-dominant eye, NMB application revealed excitatory responses in three. 5. Increasing background discharge levels and cell excitability by ionophoretic application of either acetylcholine or the excitatory amino acid, quisqualate, did not eliminate inhibitory responses and did not reveal excitatory responses. We suggest that the visually driven non-dominant eye suppression of the background discharge involves a GABA-mediated inhibitory input which masks an underlying excitatory input. 6. An excitatory non-dominant eye response could potentially derive from the influence of the corticofugal projection. However, removal of the corticofugal input by aspiration of areas 17 and 18 did not reduce either the excitatory or the inhibitory components of the response. 7. In the absence of corticofugal input all cells tested (fourteen) exhibited a non-dominant eye response and all studied during NMB application (eleven) gave an excitatory response. The primary effect of removing the corticofugal input appeared to involve the loss of a 'damping' influence on the excitatory and inhibitory responses, such that they were more easily revealed. The significance of these findings is discussed.

Action Potentials↗
PubMed1988

The modulation of the retinal relay to the cortex in the dorsal lateral geniculate nucleus.

The translation of the retinal input through the dorsal lateral geniculate nucleus (dLGN) to the visual cortex is highly dependent on a range of influences. This article reviews the available evidence. One of the influences, the corticofugal projection to the dLGN from layer VI of the visual cortex, provides a synaptic input which in magnitude exceeds that from the retina. This makes direct synaptic contact on relay cells and the intrinsic and perigeniculate inhibitory interneurones influencing their activity. The corticofugal system appears to be spatially organised in such a way that for any given region in the dLGN, there is a central zone comprising an overlying field with facilitatory effect, and a surrounding zone with inhibitory influence. The extent to which these overlap is open to question at present. The inhibitory effect of the corticofugal projection can be clearly seen in its contribution to the length tuning of dLGN cells when tested with drifting bars. On average dLGN cells exhibit a very high degree of length tuning, matching that of cortical hypercomplex cells. Removal of the corticofugal influence causes a radical reduction in this, shifting the mean reduction in peak response with increasing bar length from 71% to 43%. One consequence of this corticofugal effect is that the selectivity of the dLGN cell receptive field towards stimuli spatially restricted to the vicinity of the centre mechanism, is as good for moving bars as it is for stationary flashing spots. The retinal output to dLGN relay cells appears to be mediated by excitatory amino acid receptors, of both NMDA and non-NMDA categories. The non-NMDA receptors appear to provide an initial level of depolarisation which enables the operation of the voltage dependent NMDA receptor channels. The NMDA receptor however sits as a critical gate regulating the transmission of retinal information in the dLGN, when it is blocked visual responses are virtually eliminated. Its voltage dependency makes it crucially dependent on the complex pattern of excitatory and inhibitory influences from the cortex and the "non-specific" modulatory influence of the cholinergic system.

Animals↗
PubMed1986

Continuity of orientation columns between superficial and deep laminae of the cat primary visual cortex.

1. Recent reports of a marked and consistent dislocation between orientation columns in the superficial and deep layers of cat striate cortex (Bauer, 1982, 1983) directly contradict the traditional view of the system (Hubel & Wiesel, 1962). This has considerable implications for our current understanding of cortical organization, and in order to clarify the issue we have carried out experiments to test the continuity of the columnar system with depth, in central regions of area 17. 2. In twenty-four penetrations, eighteen of which were placed as perpendicular as possible to the surface of the cortex, orientation preference was assessed at regular intervals both qualitatively and using a randomly interleaved quantitative technique. The distribution of preferred orientations was analysed with reference to a detailed histological reconstruction of the electrode track, including the location of laminar boundaries and the course of radial tiers of cells and capillaries. 3. From a further series of eighteen near-perpendicular penetrations, the change in average orientation between one superficial and one deep layer recording site was compared with the deviation of the track from perpendicular to the surface and hence parallel to the orientation columns. 4. In penetrations perpendicular to the surface of the cortex, orientation preference showed little variation between superficial and deep laminae. In oblique penetrations, preferred orientation generally changed according to a single, smooth trend. Those irregularities that were encountered were confined to oblique penetrations, and were distributed throughout the cortical laminae. 5. In conclusion, our evidence does not support the presence of a systematic discontinuity with depth within the orientation columnar system. It is therefore entirely consistent with earlier evidence on the subject.

Animals↗
PubMed1985

Nerve entrapment of the foot and ankle in runners.

In the 10 years 1972 through 1982, the senior author performed 21 operations on 15 runners with persistent foot and ankle pain. The operative procedures involved decompression of peripheral nerves in the foot and ankle, consisting of release of soft tissues in the tarsal tunnel and foot or removal of abnormal bony excrescences that were irritating these nerves. All 15 runners had good to excellent results and all returned to their preinjury running status, including the competitive athletes. Foot and ankle pain is best treated conservatively, but when signs and symptoms culled from a careful history and physical examination reflect a nerve entrapment syndrome, surgical intervention has its place in the armamentarium of the surgeon.

Adult↗
PubMedDate not supplied

Brain-stem modulation of the response properties of cells in the cat's perigeniculate nucleus.

Transmission through the lateral geniculate nucleus is facilitated following activation of the cholinergic input from the brain stem, which is thought to reflect activity patterns seen during arousal. One of the underlying mechanisms is the suppression of inhibitory circuits local to the lateral geniculate nucleus. However, evidence exists that some visually driven inhibitory inputs to geniculate nucleus. preserved or even enhanced under conditions of arousal, and during electrical activation of the parabrachial region of the brain stem. We have therefore reexamined the effect of brain-stem activation on the visual responses of one group of local inhibitory inputs to geniculate relay cells, those emanating from the adjacent perigeniculate nucleus. We recorded single perigeniculate cells in anesthetized, paralyzed cats. Axons innervating the lateral geniculate and perigeniculate nuclei from the parabrachial region of the brain stem were electrically activated, and the effect of this activation was assessed on both spontaneous and visually evoked responses. Visual stimulation consisted of sinusoidally modulated sine-wave gratings of varying spatial and temporal frequency. For the great majority of perigeniculate cells (32 of 40), brain-stem activation inhibited spontaneous activity, while one cell was excited, three showed a mixed effect and four were unaffected. Nevertheless, the responses of most cells (30 of 40) were facilitated when brain-stem activation was paired with certain spatio-temporal patterns of visual stimulation. Spatial tuning curves were constructed for 17 cells and temporal tuning curves for 14, before and during parabrachial activation. The responses of any one cell could be facilitated, unchanged, or suppressed, depending on the visual stimulus used. In some cases, this substantially modified the cell's spatial and temporal tuning properties. We conclude that activation of the brain stem disinhibits geniculate relay cells in the absence of visual stimulation, but it has the potential to enhance either the magnitude or specificity of visually driven inhibition arising from the perigeniculate nucleus.

Animals↗
PubMedDate not supplied

Corticofugal feedback influences the generation of length tuning in the visual pathway.

In the feline visual system, neurons exhibiting sensitivity to the length of a moving contour were first observed in the cortex and described as 'hypercomplex cells'. In these cells an increase in stimulus length beyond an optimal value leads to a rapid decline in response. This decline has been attributed to an intracortical inhibitory input which may be driven by layer VI cells with very long receptive fields. It is now clear, however, that cells in the dorsal lateral geniculate nucleus (dLGN), exhibit a degree of length tuning similar to that of cortical 'hypercomplex cells', suggesting that this response property could be generated subcortically. Alternatively, as the dLGN receives a massive corticofugal projection from layer VI cells in the visual cortex, it is possible that this input has a function in generating length tuning in the dLGN. We have investigated this issue by comparing the length tuning of dLGN cells with and without corticofugal feedback. The data show that corticofugal feedback makes a highly significant contribution to the length tuning of dLGN cells. This raises the possibility that length tuning is an emergent property of the geniculo-cortical loop.

Animals↗