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L Maffei

Publications and source records attributed to L Maffei.

At least 37 records · Page 2Linked to original sources
PubMed1988

The axon initial segment as a possible determinant of retinal ganglion cell dendritic geometry.

In wholemounted retinae of cat, rat and monkey, in which ganglion cells were retrogradely labelled with horseradish peroxidase, a quantitative analysis of the direction of the axon initial segment with respect to the optic disc and of the relationship between the axon initial segment and the direction and distribution of primary dendrites was performed on the class of largest ganglion cells. The results show the following. (1) In all 3 species, the majority of primary dendrites of ganglion cells are directed away from the axon initial segment. (2) Primary dendrites arise with a greater frequency from the region of the cell body opposite to the axon initial segment than close to it. (3) In cat the direction of the axon initial segments show less variance in their initial direction with respect to the optic disc than in rat or monkey. In adult cats the nucleus of alpha-ganglion cells occupies a central position. In the kitten the position of the nucleus is eccentric and lies in a part of the cell body opposite to the axon initial segment. The nucleus moves to a central position over the next 3 weeks. The position of the axon initial segment is discussed as a possible determinant of ganglion cell dendritic geometry.

Animals↗
PubMed1988

Dendritic competition: competition for what?

A lesion to the retina of a newborn rat results in the retrograde degeneration of ganglion cells in a sector of retina peripheral to the lesion. The dendritic tree of ganglion cells bordering the region depleted of ganglion cells have their dendrites preferentially directed into this area. We have examined the factors which play a role in this rearrangement of the dendritic tree. The results show that the lesion in neonates selects for or produces a population of cells with the axon directed away from the depleted area and primary dendrites directed towards the depleted area. The abnormal dendritic bias cannot be accounted for solely on the basis of a decrease in contact inhibition since a reduction in the density of all ganglion cells by 30% prior to making the retinal lesion does not attenuate the abnormal dendritic bias into the depleted area. The abnormal dendritic bias is present in animals operated on up to 15 days of age postnatally but not in more mature animals. The abnormal dendritic bias develops prior to the formation of a large number of synapses in the inner nuclear layer. Our results cannot be easily accounted for by competition for synaptic contacts or a loss of contact inhibition as previously suggested. We propose that chemotropic factors produced within the area depleted of ganglion cells induce the abnormal dendritic bias and the number of synaptic contacts may limit the size of the dendritic field.

Aging↗
PubMed1979

A perceptual phenomenon and its neurophysiological correlate.

A square-wave grating from which the fundamental harmonic has been subtracted (missing fundamental grating) when viewed from a suitable distance appears similar to another grating of periodicity three times higher. The neurons of the visual cortex of the cat, in a given range of spatial frequencies characteristic of each cell, give similar responses to the two gratings.

Animals↗
PubMed1979

Responses of visual cortical cells to periodic and non-periodic stimuli.

1. The activity of neurones of the visual cortex (area 17) has been recorded in anaesthetized cats in response to gratings of different profile and to single light and dark bars. 2. At very low spatial frequencies, outside the frequency response range to sinusoidal gratings, the response to square-wave drifting gratings is obtainable from a combination of the response to the single bars of the grating presented in isolation. At higher spatial frequencies this is no longer true. 3. At very low spatial frequencies the responses to square-wave gratings and to missing-fundamental gratings (obtained by subtraction from the square-wave grating of its fundamental gratings (obtained by subtraction from the square-wave grating of its fundamental harmonic) are very similar. 4. At spatial frequencies near the peak of the spatial frequency tuning curve of the cell, the responses to square-wave grating and to sinusoidal gratings are very similar. At these spatial frequencies the response to the missing-fundamental grating is practically zero. 5. At spatial frequencies lower than that of best sensitivity for the cell, the response to square-wave gratings is correlated with the 1st and 3rd harmonic of the stimulus. 6. We conclude that at very low spatial frequencies of the grating the response of cortical cells is correlated with the light or dark edges (or light or dark bars) of the stimulus, because the edges contain high frequencies within the range of sensitivity of the cells. At higher spatial frequencies the results are interpreted best by assuming that cortical cells respond to the harmonics of the visual periodic stimulus. 7. When a background of dynamic visual noise is added to increase the spontaneous discharge of simple cells, their response to visual stimuli becomes linear or quasi-linear. The stimuli could be either single bars or gratings.

Action Potentials↗
PubMed1979

Erythropoietin-dependent primary pure erythrocytosis.

We investigated the pathogenesis of isolated erythrocytosis of 14 yr duration in a 28-yr-old man. The increase in red cell mass was attributed to increased erythropoietin production. An extensive search for recognized causes of secondary erythrocytosis was unrevealing. Family members were found to be hematologically normal. After reduction of the circulating red cell mass by 20%, erythropoietin activity nearly quadrupled, thus suggesting a normal erythropoietin response to phlebotomy. When bone marrow cells of the patient were cultured in plasma clots in the absence of added erythropoietin, endogenous erythroid colony formation was observed, a pattern previously believed to be specific for polycythemia vera bone marrow cells. Our observations suggest that the erythrocytosis in this individual is best explained by an abnormal "servoregulatory" mechanism of erythropoietin production. In addition, this is the first instance in which the rule that endogenous erythroid colony formation is correlated with the diagnosis of polycythemia vera has not held.

Adult↗
PubMed1978

Selective impairment of contrast sensitivity in kittens exposed to periodic gratings.

1. Kittens were exposed to a periodic grating orientated vertically during the critical period of their life. 2. At the end of the critical period the amplitude of the response to drifting gratings was found to be reduced at the spatial frequency of exposure in multi-unit and in some single unit recordings at lateral geniculate level. 3. The contrast sensitivity measured with behavioural methods also showed a depression at the spatial frequency of exposure. 4. An analysis of the preferred stimulus orientation of cortical units in area 17 showed that all orientations were represented.

Action Potentials↗