Biomedical subjects

R F Mark

Publications and source records attributed to R F Mark.

At least 37 records · Page 2Linked to original sources

PubMed1994

Spatiotemporal response properties of direction-selective neurons in the nucleus of the optic tract and dorsal terminal nucleus of the wallaby, Macropus eugenii.

1. The spatial and temporal response characteristics of direction-selective neurons in the nucleus of the optic tract and dorsal terminal nucleus of the accessory optic system (NOT-DTN) of the wallaby were established using moving sinusoidal gratings. This is the first comprehensive investigation of the spatiotemporal response characteristics of NOT-DTN neurons in any species. 2. The analysis revealed two main classes of cells. The first class, referred to as slow neurons, are maximally sensitive to motion at low temporal frequencies (< 1 Hz) and high spatial frequencies (0.5-1.0 cpd). The second class, referred to as fast neurons, are most sensitive to motion at high temporal frequencies (> 10 Hz) and moderate to low spatial frequencies (0.1-0.5 cpd). The fast neurons also have a domain of high sensitivity at low temporal frequencies and high spatial frequencies. As the neurons are tuned to specific temporal frequencies of motion, rather than image velocities, it is suggested that the motion detectors are of the delay-and-compare type and code local motion-related changes in contrast or luminance. 3. Both classes of neuron are highly direction-selective in the midranges of their spatiotemporal tuning curves, i.e., the firing rates increase during motion in the preferred direction (temporonasal movement through the visual field of the contralateral eye) and decrease during motion in the opposite direction. At high temporal and low spatial frequencies, however, the slow neurons are inhibited by motion in both directions along their preferred axis. It is argued that this bidirectional inhibition at high speeds may act to inhibit ocular following during saccades and may act as a gain control mechanism preventing excessive overshoot in eye velocity at motion onset, when retinal-slip velocities are high. 4. The fast neurons probably have two functions. First, they are suited to initiating ocular following responses when image motion is quite fast. Second, their spatiotemporal tuning makes them candidates for supplying a velocity error signal into the velocity storage mechanism, which is most prominent at high stimulus speeds. 5. Fourier analysis of the peristimulus time histograms derived from the response of both the slow and fast neurons revealed that the main frequency components of the responses occurred at the fundamental and second harmonic frequencies of the input signal at low stimulus temporal frequencies (< 3.04 Hz). At higher stimulus frequencies, the responses contained significant frequency components at higher odd-harmonics of the input signal.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Two stages in the development of a mammalian retinocollicular projection.

The retinocollicular projection in the marsupial mammal the wallaby Macropus eugenii, has been investigated anatomically to determine the order in the developing projection and electrophysiologically to determine the time of onset of synaptic transmission by recording evoked potentials in the colliculus in response to stimulation of the optic nerve. There are two clear stages: a protracted period when retinal axons grow into the colliculus in coarse retinotopic order with no recordable electrical activity followed by the formation of terminal zones in retinotopically correct positions, the loss of more widely distributed axons and the onset of evoked potentials. The two stages are not seen in non-mammalian vertebrates where the projection is functional from the beginning.

Animals↗

PubMed1993

Geometry of the representation of the visual field on the superior colliculus of the wallaby (Macropus eugenii). I. Normal projection.

In 13 wallabies (Macropus eugenii, the tammar), microelectrode recordings of the activity of units in the superficial layers of the superior colliculus in response to a flashing light spot were used to make a map of the spatial location of their receptive fields. This article describes the projection of a normal eye to the contralateral colliculus. Ten of the 13 animals had one rotated eye and these projections are analysed in the accompanying paper (James et al., this issue). Units responded briskly to the stimulus at light on and off and had receptive fields about 5 degrees across. The centres of receptive fields from a regular array of recording points on the colliculus were plotted with a perimeter and fitted to a flattened representation of the colliculus according to a spline technique. The visual field of each colliculus extends from 25 degrees ipsilateral to the vertical meridian to 120 degrees temporal contralaterally. The lines of isoazimuth are regularly spaced and parallel and run mediolaterally on the colliculus. The horizon is represented by a line running rostrocaudally and the parallels are more widely spaced near the horizon and become compressed in the superior and inferior fields. The variation of areal magnification factor fits the distribution of density of retinal ganglion cells very well. Anisotropy of the projection means that the increased ganglion cell density of the retinal visual streak is entirely accommodated by magnification in the vertical direction, while the magnification of the azimuthal projection is equal over the whole field. No responses were recorded from the ipsilateral eye even though anatomically there is a direct retinal ipsilateral projection.

Animals↗

PubMed1993

Geometry of the projection of the visual field onto the superior colliculus of the wallaby (Macropus eugenii). II. Stability of the projection after prolonged rearing with rotational squint.

At about the time of eye opening, one eye of seven tammar wallaby pouch young was surgically rotated about the optic axis by approximately 90 degrees. In adulthood the projection of the visual field through the rotated eye onto the contralateral superior colliculus was mapped electrophysiologically. Although apparently distorted, the projection could be coherently re-rotated mathematically to a reasonable copy of the normal projection from the opposite eye of the same animal, including details such as regional variations of the magnification factor. The same was true of three adult animals in which the eye rotation was done after anaesthesia immediately before the electrophysiological mapping. In animals in which the visual field seen through one eye, the other being normal, was rotated for the entire period of visual experience, there was no sign of compensation or rearrangement of the topographic map. Retinocollicular synaptic connections appear unmoved by such discordant visual experience.

Animals↗

PubMed1992

Cytoarchitecture and visual field representation in area 17 of the tammar wallaby (Macropus eugenii).

Tritiated proline was injected into one eye in the tammar wallaby and transported label was studied in the cortex after transneuronal passage through the lateral geniculate nucleus. The autoradiographic label and cytoarchitecture were used to anatomically demarcate the borders of area 17. Electrophysiological recordings from single units were done to obtain a retinotopic map of area 17. Single units in area 17 were found to have orientation sensitivity comparable to those seen in placental mammals such as cat and monkey. They could also be classified as simple, complex, and hypercomplex cells. Changes in the cortical areal magnification factor with eccentricity were found to match the drop off in retinal ganglion cell density only along the vertical meridian representation. Along the horizontal meridian, the cortical magnification falls off significantly with eccentricity, whereas the ganglion cell density shows only a mild reduction. Thus central vision, especially the binocular segment, is heavily represented at the cost of the periphery.

Animals↗

PubMed1992

Australian marsupials as models for the developing mammalian visual system.

This article makes two points. First, the diprotodont marsupials, including the kangaroos, wallabies and the Australian possum are not primitive mammals, and their brains make as good a general model of the higher mammals such as monkeys and humans as do those of the more common laboratory mammals such as cats and rats. Second, the peculiarities of marsupial reproduction, which comprises a very short period of intrauterine development, followed by a relatively protracted period of development in the pouch, provide unparalleled advantages for research into mammalian neuroembryology. Examples will be provided of how such research has made a contribution to our understanding of neural development, concentrating primarily on the visual system.

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PubMed1992

Partition of function in the morphological subdivisions of the lateral geniculate nucleus of the tammar wallaby (Macropus eugenii).

Extra-cellular recordings from single cells in the dorsal lateral geniculate nucleus (dLGN) of the tammar wallaby, Macropus eugenii, were made to find out whether the stratification of the nucleus could be correlated with the receptive field properties of units. Retinofugal fibres terminate in the lateral geniculate nucleus of the wallaby in nine interleaved eye-specific layers. These may be grouped into a lateral alpha segment of six laminae and a medial beta segment of three, in which the cells are less densely packed. Ninety percent of the geniculate neurons recorded from in the alpha segment gave brisk responses to stimulation of their receptive fields. Cells with sluggish responses predominated in the beta segment, but there was also a sizable minority of cells with brisk responses that were indistinguishable from those recorded in the alpha segment. In contrast, other response properties were rarely differentiated in individual layers. Thus, in most layers, the numbers of cells with transient or sustained responses were not significantly different, and this was also true for cells with ON- or OFF- responses. For each of these response pairings, however, the numbers of one type (ON- and transient) predominated in every layer. The accumulation of this laminar distinction lead to significant differences in the alpha and beta segments and in the nucleus as a whole. We conclude that cells in the individual layers of the dLGN of the tammar wallaby show no evidence of having receptive field properties in common that might correlate with separate functional streams. There is a functional segregation of receptive field properties between the alpha and beta segments. The organization of these two segments resembles that of the A and the C layers of the dLGN in cats and, possibly, the magnocellular and koniocellular components of the dLGN in primates. These broad similarities in functional partition of the dLGN of different species suggests that this aspect of the organization of the nucleus is independent of lifestyle.

Animals↗

PubMed1991

Development of the laminar distribution of thalamocortical axons and corticothalamic cell bodies in the visual cortex of the wallaby.

The distribution of afferents from the dorsal lateral geniculate nucleus (LGNd) and the lateral posterior nucleus (LP) and of cell bodies projecting to these nuclei has been studied in the visual cortex of the wallaby (Macropus eugenii) throughout development to determine how the characteristic laminar distribution of afferents and efferents of the mature cortex is achieved. Young are born after 26-28 days of gestation and do not open their eyes until around 140 days after birth. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected in the visual thalamus in adults and in pouch young aged from 22 days after birth, just after thalamic axons first reach the visual cortex, to 118 days, when cortical lamination resembles the adult. From 22 to 65 days, the developing visual cortex consists of a marginal zone (MZ), cortical plate (CP), and intermediate zone (IZ) including the superficial subplate (SP), subventricular zone, and ventricular zone. There is a thin compact cell zone (CCZ) at the top of the CP and below it a less densely packed region that increases in thickness with age. Retrogradely labelled cells in two bands were first seen at 40 days, one in the CCZ and the other at the base of the CP. Two bands of cells were seen at all subsequent times if the injection covered both LGNd and LP, and by 76 days, these cells were located within cytoarchitectonically recognizable layers V and VI. Anterograde label prior to 45 days was distributed densely and evenly throughout the IZ and the CP up to the CCZ. Label in MZ was first seen at 25 days and was substantial by 54 days. Anterograde label than became gradually reduced in the IZ, whereas in the CP it remained evenly and densely distributed until 82 days. At this age, coincident with the emergence of layer IV, label within the CP first showed variations in density and by 99 days was concentrated over layer IV and, to a lesser extent, over layer VI. By 118 days label resembled the adult after injections covering both LGNd and LP, with label concentrated in layer I, IV, and VI with a less dense projection to lower layer III and upper layer V. There is a relatively earlier initial ingrowth of axons into the visual cortex in the wallaby and throughout development thalamocortical axons appear to be more widely distributed in the depth of the visual cortex than has been demonstrated for placental mammals.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗

PubMed1991

Development of whisker representation in the cortex of the tammar wallaby Macropus eugenii.

The somatosensory cortex associated with the whiskers has been studied in adult tammar wallabies (Macropus eugenii) and in pouch young from 60-120 days of pouch life. The time course of anatomical changes examined with succinic dehydrogenase (SDH) histochemistry and Nissl staining has been correlated with the maturation of electrically evoked cortical responses to stimulation of the whisker follicles. The earliest signs of aggregates of SDH reaction product in layer IV of the cortex were seen at 85 days, coincident with the first recordings of an immature cortical evoked potential. Aggregates, in a pattern corresponding to that of the facial whiskers, were most clearly seen from 90-140 days. At later stages, and in the adult, they were present but their arrangement was less clearly seen. By 186 days the electrical activity resembled the mature pattern. Patches of SDH activity in layer IV were not associated with changes in soma density characteristic of true barrels.

Animals↗

PubMed1990

Development of connections to and from the visual cortex in the wallaby (Macropus eugenii).

The time course of the development of connections between the visual cortex and the main subcortical visual structures, as well as intrahemispheric and interhemispheric connections, has been studied in the marsupial wallaby (Macropus eugenii) to compare its development with that of placental mammals. Pouch young are born prior to retinal innervation of the primary visual centers and spend a protracted period of development in the pouch, making them ideal for visual, developmental studies. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected into either the presumptive visual cortex or the superior colliculus in young of varying ages. Thalamocortical projections from the dorsal lateral geniculate and lateral posterior nuclei reach the presumptive visual cortex between 12 and 15 days after birth. Descending cortical connections form later. Corticogeniculate axons are first detected in the geniculate and lateral posterior nucleus at 48 days after birth, while corticocollicular axons first reach the superior colliculus at 71 days and, by 81 days, have innervated the superficial layers. Intrahemispheric and interhemispheric connections form even later. By 99 days intrahemispheric axons from area 17 have accumulated in visual association areas but are yet to invade layers III and IV, their major termination zones in adult, while axons projecting back to area 17 have also reached their target area. At this time interhemispheric axons from area 17 have begun to accumulate in the opposite visual cortex, although they have not invaded the cortical layers. By 111 days cortical cells projecting to the opposite visual cortex are first labelled. These have a more widespread distribution in area 17 at 111 and 122 days compared to the adult, where they are confined to the 17/18 border. The results show that the marsupial wallaby has a timetable of similar sequence, but different relative timing, in the formation of cortical connections compared to that of placental mammals. In the first half of the period between conception and eye opening, the timing in the wallaby precedes considerably that in placental mammals. Ascending connections from the thalamus develop relatively earlier in the wallaby but descending collicular connections are delayed until the same relative time that they appear in placental mammals.

Afferent Pathways↗

PubMed1990

Effects of muscle history on the stretch reflex in cat and man.

1. This is a report of experiments on cat and man which demonstrate effects of a muscle's previous history of contraction and length changes on the size of the stretch reflex. 2. In adult human subjects the size of the tendon jerk was measured in ankle extensor muscles by tapping the Achilles tendon. Muscle conditioning consisted of a maximum voluntary contraction with the foot dorsiflexed or plantarflexed by 30 deg from the test position, after which the subject was asked to relax while the foot was held still for several seconds before being returned to the test position and a tendon tap given. After a contraction of the lengthened muscle the tendon jerk was smaller than after a contraction of the shortened muscle. 3. The experiment was then repeated, but instead of a tendon jerk an H (Hoffmann) reflex was elicited by transcutaneous electrical stimulation of the tibial nerve in the popliteal fossa. The reflex after a conditioning contraction of the lengthened muscle was larger than after a contraction of the shortened muscle. In other words muscle conditioning produced opposite effects on the tendon jerk and H reflex. 4. These findings were confirmed in cats anaesthetized with chloralose. After a conditioning contraction of triceps surae at a length 5 mm longer than the test length (hold-long) a quick tendon stretch produced a smaller reflex response than following a conditioning contraction with the muscle 5 mm shorter than the test length (hold-short). The reverse trend was seen with a reflex elicited by direct electrical stimulation of the muscle nerve, which stimulates the H reflex. 5. One consequence of a conditioning contraction is that it leads to an alteration of the level of resting discharge of muscle spindles. We propose that the larger tendon jerk after a contraction of the shortened muscle is the result of changes in stretch sensitivity of muscle spindles. The reverse effect on the H reflex we attribute to a rise in the level of resting discharge of muscle spindles, which, we propose, leads to reflex inhibition of motoneurones. 6. We support this conclusion with evidence from an experiment in which the size of the conditioning step was systematically altered. Even quite small hold-short conditioning steps led to depression of the H reflex in man and the monosynaptic reflex in cats. Recordings from single afferents showed that such small steps were also accompanied by a detectable rise in spindle resting discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1989

Effects of very early monocular and binocular enucleation on primary visual centers in the tammar wallaby (Macropus eugenii).

The role of retinal afferents and their binocular interactions in the development of mammalian primary visual centers has been studied in the marsupial wallaby. Monocular and binocular enucleation was performed prior to any retinal innervation of the visual centers. After monocular enucleation retinal projections were traced by horseradish peroxidase histochemistry and compared with those in normal animals and those during development. The topography of retinal projections to the superior colliculus and the dorsal lateral geniculate nucleus after monocular enucleation was determined by making retinal lesions and tracing the remaining projections with horseradish peroxidase. The position and nature of the filling defects in terminal label were compared with controls with similarly placed lesions. The superior colliculus and dorsal lateral geniculate nucleus ipsilateral to the remaining eye were shrunken. Projections to the ipsilateral superior colliculus, ipsilateral accessory optic nuclei, and ipsilateral suprachiasmatic nucleus, although enlarged, never approached the density contralaterally, as was also the case during normal development. The expanded projection in the ipsilateral superior colliculus came primarily from temporal and ventral retina. In the dorsal lateral geniculate nucleus, terminal bands and cellular laminae, although not identical to normal, did develop. During normal development overlap of afferents from the two eyes occurs in the binocular region. The decrease in volume of the nucleus ipsilateral to the remaining eye after monocular enucleation suggests that the monocular region disappears in the absence of appropriate input and the binocular region survives. Contralaterally there was no decrease in volume, compatible with this idea. The topography of retinal projections supports this interpretation. It was normal contralaterally while ipsilaterally it was appropriate for the normal binocular region. There was an expansion of the projection along the lines of projection in what would normally be binocular regions of the nucleus, where retinal afferents failed to segregate in the absence of binocular competition. After binocular enucleation the alpha and beta segments of the dorsal lateral geniculate nucleus were still recognizable but cell-sparse zones were absent, as was the characteristic orientation of primary dendrites of geniculocortical cells. There are rigid developmental constraints operating on the innervation of territory by retinal afferents from the two eyes, and many features of the mature pattern arise without binocular interactions during development.

Afferent Pathways↗

PubMed1988

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): I. Normal topography.

The topography of retinal projections to the superior colliculus and dorsal lateral geniculate nucleus of a wallaby, the tammar (Macropus eugenii), was investigated by an anatomical method. Small laser lesions were made in the retinas of experimental animals, and the remaining retinal projections were visualized by means of horseradish-peroxidase histochemistry. The position of each lesion was correlated with the position of the filling defects in the terminal label. The whole of the retina projects to the contralateral superior colliculus. The nasal retina is represented caudally, and the temporal retina rostrally. The ventral retina is represented medially, and the dorsal retina laterally. There is a projection to the ipsilateral superior colliculus, but it is patchy and its topography could not be determined by this method. The retinotopic map in the contralateral dorsal lateral geniculate nucleus has the nasal retina represented rostrally and the temporal retina caudally in the nucleus. The dorsal retina is represented ventrally, and the ventral retina is represented dorsally. It appears that the whole of the retina projects contralaterally, and in addition the temporal retina projects ipsilaterally. The maps of visual space through the two eyes were shown to be in topographic register in the binocular region by making a deposit of HRP in the visual cortex. This resulted in a column of retrogradely labeled cells in the nucleus. This column crossed the laminae, which are innervated by the ipsilateral and contralateral eye at right angles.

Animals↗

PubMed1988

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): II. Topography after rotation of an eye prior to retinal innervation of the brain.

Retinal projections to visual centers in a marsupial mammal, the tammar wallaby (Macropus eugenii), have been investigated after an eye rotation prior to retinal innervation of the brain. Retinal topography to the superior colliculus and dorsal lateral geniculate nucleus was mapped by using laser lesions of the retina and horseradish peroxidase histochemistry. Despite the change in orientation of optic axon outgrowth from the developing eye after rotation, retinal ganglion cells made orderly connections in the colliculus and geniculate according to their original retinal position within the eye and not their rotated position. Axons must have corrected their pathways at some point between the back of the eye and their targets. The optic chiasm was one such site. Optic axons from the rotated eye took an abnormal course at the caudal end of the chiasm. Growth of optic axons through aberrant pathways in the brain did not preclude specific innervation of targets. When by chance optic axons entered through the oculomotor nerve root they specifically innervated their correct visual centers, albeit in reduced density, and did not innervate inappropriate targets. These results support the idea of specific interactions between growing axons, the pathways they grow along, and their targets.

Animals↗

PubMed1988

Mammalian motoneuron development: effect of peripheral deprivation on motoneuron numbers in a marsupial.

In nonmammalian vertebrates, the survival of developing motoneurons is dependent on their contacting appropriate target cells. It is generally accepted that developing mammalian motoneurons have a similar dependency on their target, but as yet there is little experimental evidence to support this contention. This is mainly because of the difficulty of experimenting on eutherian embryos. We have, therefore, been studying neuronal development in the tammar (an Australian marsupial) as its nervous system is immature at birth. Radical or partial removal of hindlimb buds from newborn tammars resulted in an increased motoneuron cell death. The motoneurons which survived in the operated tammars did so by innervating muscle remnants. In the instances where a group of muscles was totally removed, the corresponding motonuclei appeared to be totally lost. This study supports the hypothesis that mammalian motoneurons must contact their appropriate muscle in order to survive through the period of natural neuronal cell death.

Age Factors↗

PubMed1987

Retinotopic organization in the dorsal lateral geniculate nucleus of the tammar wallaby (Macropus eugenii).

Electrophysiological recordings were made from 187 single cells in the tammar wallaby (Macropus eugenii) dorsal lateral geniculate nucleus (LGNd). The results show that it is topographically organized such that the superior visual field is represented dorsally, the inferior field is represented ventrally, the nasal visual field is represented caudally, and the temporal visual field is represented rostrally. The visual field of one eye ranges from -30 degrees nasal to +179 degrees temporal in azimuth and +73 degrees superior to -49 degrees inferior in elevation. Ganglion cells that had receptive field positions between -9 degrees and +179 degrees projected to the contralateral LGNd while the ganglion cells that projected to the ipsilateral LGNd had visual fields from 0 to +30 degrees. The binocular visual field extends 60 degrees in azimuth. This representation in the LGNd is expanded relative to the monocular representation. There is also an increased representation of the horizon in the temporal field corresponding to the visual streak of retinal ganglion cells. The binocular visual field is located where contralateral and ipsilateral laminae are shown to interdigitate by proline autoradiography. There are nine eye-specific laminae in the LGNd. Four receive afferents from the contralateral eye and five receive afferents from the ipsilateral eye. The lines of isoelevation are perpendicular to the coronal plane of section while the lines of isoazimuth are nearly parallel to the coronal plane. The lines of projection representing one visual direction are inferred to be perpendicular to the tangent of curvature of the laminae as in the LGNd of other mammals. The majority of cells (85%) recorded had on- or off-centre responses. On- and off-centre responses were not apparently segregated in the LGNd but segregation may not have been revealed by the single-unit recording technique.

Animals↗

PubMed1987

Mammalian motoneuron cell death: development of the lateral motor column of a wallaby (Macropus eugenii).

We have investigated the development of the lumbar lateral motor column of the tammar as a model of mammalian motoneuron cell death that is accessible to experimental manipulation. The tammar is an Australian marsupial, belonging to the subfamily of wallabies and kangaroos. After a gestational period of 26-28 days, the pup crawls to its mother's pouch using its forelimbs. The major morphometric events that shape the formation of the hindlimb occurred between 21 days gestation and birth. At birth the premuscle masses had divided and motor nerves had begun to penetrate the muscles of the thigh and shank. The period of motoneuron cell death was biphasic and occurred entirely postnatally. During phase I, between birth and 40 days, 59% of motoneurons were lost. Cell numbers then stabilised before falling a further 24%, to give an overall loss of 70%. Most of phase II cell loss occurred between 90 and 150 days. The possibility that a second period of motoneuron cell death may be a common feature of mammals is discussed.

Animals↗

PubMed1987

Retinotectal reorganization in goldfish--IV. Effects of retinal ganglion cells after half tectal ablation.

During compression of the entire retinotectal projection into the rostral half of the tectum after ablation of the caudal half there is widespread sprouting of ganglion cell axons, not only those cut during the operation but also those left intact. However, unlike cut axons those left intact sprout without their cell bodies showing chromatolysis or swelling. Chromatolysis and swelling of the cell bodies of cut axons are more prolonged than after optic nerve section and resolve in more central regions of retina first. The cut axons of cell bodies in these regions tend to be the first to form terminal arborizations during the compression process as judged electrophysiologically. However, there is no clear correlation in individual fish between these measures and the state of compression assessed electrophysiologically. Large areas of retina may contain chromatolysed cells even after compression has occurred. Electrophysiological mapping alone may give a misleading picture of the interactions occurring between retinal and tectal cells during reorganization.

Animals↗