Wally Welker

Fractured Somatotopy in Granule Cell Tactile Areas of Rat Cerebellar Hemispheres Revealed by Micromapping; pp. 94–105

We defined spatial patterns of tactile projections to cerebellar cortex of anesthetized albino rats using microelectrode micromapping methods. Low threshold natural stimulation of cutaneous mechanorec

Why Does Cerebral Cortex Fissure and Fold

The most striking, interesting, yet poorly understood gross morphological features of the cerebral hemispheres in mammals are the diverse and complex arrangements of their cortical gyri and sulci (Fig. 1). Among mammals, the spinal cord and brain-stem nuclei are morphologically quite similar, despite variations in size. However, during evolution, the cerebrum and cerebellum have undergone marked variations in size, shape, and convolutional complexity (Fig. 2). External morphological features of mammalian brains have long been utilized to judge not only the degree of phylogenetic development, but also the nature and level of complexity of brain functions. The great variety of living mammals exhibit a corresponding variety of brain shapes, sizes, and patterns of fissuration and convolution of the cerebral neocortex. The view has consistently been expressed that animals with brains having greater amounts of convoluted cerebral neocortex were more intelligent as well as perceptually and behaviorally more complex.

Reevaluation of motor cortex and of sensorimotor overlap in cerebral cortex of albino rats

The organization of motor cortex and the sensorimotor overlap zone was examined by in-depth electrical stimulation using micromapping procedures in rats. The cutaneous somatic sensory, as well as the efferent motor projections to the hindlimb and forelimb sensorimotor overlap zone were studied in the same animals. Low-threshold movements were elicited from portions of 3 architectonic areas: the lateral agranular, dysgranular and granular areas. Cutaneous light touch projections occur only within the granular area. Cutaneous projections to, and motor projections from individual punctures in the granular overlap zone did not always involve homologous body parts. The total motor cortex exhibits a general musculotopic pattern of organization.

Principles of organization of a cerebro-cerebellar circuit. Micromapping the projections from cerebral (SI) to cerebellar (granule cell layer) tactile areas of rats.

We defined spatial patterns of organization of projections from somatosensory cerebral cortex (SI) to the somatosensory cerebellar cortex of anesthetized albino rats using microelectrode (stimulation and recording) micromapping methods and low-threshold cutaneous (tactile) stimulation. Two sampling strategies were used: (1) a single cerebral SI locus in layers V-VI was stimulated electrically, while a responding region of the cerebellar granule cell (GC) layer was systematically mapped with a recording electrode; (2) the SI stimulating electrode was used as the mapping electrode while the cerebellar GC electrode remain fixed. We found highly specific patterns of connections between somatotopically organized SI cortex and the somatotopically fractured tactile cerebellar cortex. Using threshold stimulating currents in SI, the projections from small populations of neural elements were found to be highly restricted, terminating within the confines of only those tactile cerebellar hemispheric locations having the same receptive fields (RFs). These SI-GC projections conform to the patchy mosaic pattern of organization previously shown for peripheral tactile projections. SI projections to GC patches were either contralateral or ipsilateral, depending on the laterality of the peripheral projections to that patch. Each SI focus projected to only a portion of a patch; projections from several adjacent SI loci overlapped serially within a patch. As with the peripherally evoked GC layer responses, SI-evoked GC responses were organized in a columnar fashion and were maximal at middle levels of the GC layer; SI-GC latencies were 5-8 ms. These data reveal that this tactile-related cerebro-cerebellar circuit exhibits precisely organized patterns of projection.

Quantitative studies of stimulus coding in first-order vibrissa afferents of rats. 1. Receptive field properties and threshold distributions.

We examined stimulus-response relationships of vibrissa-activated mechanosensory neurons of the rats fifth (trigeminal) ganglion. Single-unit activity was recorded with tungsten microelectrodes. The vibrissae were deflected with a variety of parametrically controlled stimulus waveforms. We found that the receptive field of each vibrissa-activated neuron consisted of a single vibrissa. Few, if any, unambiguous examples of spontaneous activity were observed in these neurons. Even if true spontaneous activity was present, its observed incidence was low, as were the measured discharge rates. Thresholds of individual neurons were usually quite discrete; often a 1-2% increase in pulse magnitude (angular displacement) above a level to which the neuron did not respond caused it to discharge on every trial. The distribution of thresholds for the sample was continuous with a median of about 1 degree and a range of over three orders of magnitude. The most sensitive neurons responded to deflections of less than 0.1 degrees. Many neurons responded to a single suprathreshold pulse with more than one spike. We found no consistent relationships among the thresholds of the additional evoked discharges of an individual neuron other than that the total number of evoked spikes either increased or stayed the same, but never decreased, as stimulus magnitude increased. About one-third of the neurons examined had velocity thresholds below 3 degrees/sec. Above that value, thresholds were distributed continuously throughout a range of over three orders of magnitude. The median velocity threshold was about 100 degrees/sec. The broad and continuous distributions of both magnitude and velocity thresholds suggest that a population of vibrissa-activated neurons can code stimulus strength smoothly and continuously over a wide range, even though individual neurons may be poorly suited to do so.

Tactile projections to granule cells in caudal vermis of the rat's cerebellum.

We discovered a small tactile area in a single a folium of the uvula of the cauday vermis of the rats cerebellum. Gentle mechanical stimulation of relatively small cutaneous receptive fields (RFs) activated multiple units in the granule cell (GC) layer in a portion of a single folium in rats anesthetized with sodium pentobarbital. The total size of this area on each side of the midline is about 1.5 mm2, yet micromapping within this tiny region using tungsten ball microelectrodes and a high puncture sampling density (about 75 punctures/mm2) revealed a highly differentiated pattern of cutaneous projections to the GC layer. All peripheral projections are ipsilateral; the two homologous areas from each side adjoining at the midline of folium 9a. The larger projection areas from cutaneous RFs are mostly from mystacial vibrissae and upper lip, but small projection sites from the remainder of the head, neck and forelimb also are present. The pattern of projections were patch-like, forming a fractured somatotopic pattern or mosaic, with some somatotopic and some nonsomatotopic features. Each RF activated units in a vertical column in the GC layer. This area has not been described in any mammal, and its functional role can now be studied.

Somatotopic organization of the external cuneate nucleus in albino rats

Abstract The pattern of representation of muscle receptive fields was delineated in the external cuneate nucleus (EC) of anesthetized, cerebellectomized albino rats. This nucleus was explored systematically using ball-tipped tungsten microelectrodes and closely-spaced electrode punctures. Single neurons were recorded from and the peripheral source and modality of their activating receptive fields were identified using natural stimulation of exposed muscles of the rats forequarter. Each muscle was stimulated by pulling the tendon, pressure to its belly with fine probes and/or by stretch induced by joint rotation. Of 585 single units recorded from 411 punctures in 31 animals, 243 were localized to EC and were activated by stretch at low threshold or punctate pressure of muscles of the ipsilateral forequarter. All cells definitely localized within EC by histological verification were activated only from receptive fields located in muscle. The musculotopic pattern of organization of peripheral projections within the three-dimensional confines of EC was rather detailed, with neck muscles represented in its rostrolateral pole, arm and shoulder muscles more caudomedially, and forearm and hand muscles progressively more caudally. There was no overlap of representations from muscles located in different anatomical segments. EC units for which receptive field identification was certain were always activated from a single muscle. However, those units whose fields could not be identified precisely may have received convergent activation (or inhibition) from different muscles which our procedures did not allow us to detect. No units in EC were activated from cutaneous receptive fields, nor did we find convergence of activation from cutaneous and muscle receptive fields. EC cells were never activated from receptive fields in the hindlimb or contralateral forelimb.

Patterns of afferent projections to transitional zones in the somatic sensorimotor cerebral cortex of albino rats.

The organization of somatosensory projections to the dysgranular areas of somatic sensory cortex was mapped in albino rats. Receptive fields that activate layer IV granule cells in these dysgranular zones were: cutaneous and deep (including muscle), roughly somatotopic, larger, and required stronger stimulation (tap) than the cutaneous light touch RFs of the adjacent granule cell zones.

Manatee cerebral cortex: cytoarchitecture of the frontal region in Trichechus manatus latirostris

Members of the order Sirenia are unique among mammals in being the only totally aquatic herbivores. They display correspondingly specialized physiological, behavioral and anatomical features. There have been few reports concerning sirenian neuroanatomy, and most of these have consisted of gross anatomical observations. Our interest in Sirenia stems from the desire to understand neuroanatomical specializations in the context of behavior and the effort to elucidate trends in mammalian brain evolution. The architecture of frontal regions of cerebral cortex was investigated in several brains of the Florida manatee, Trichechus manatus latirostris. Through observation of sections stained for Nissl substance or myelinated fibers, several distinct cortical areas were identified on the basis of laminar organization. These range from areas with poorly defined laminae to those having 6 well-defined layers, some of which exhibit sublayers. Two cortical areas exhibit pronounced cell clusters in layer VI, and these stain positively for acetylcholinesterase and cytochrome oxidase. We hypothesize that these clusters may be involved in perioral tactile bristle function. Certain of our findings are consistent with previous observations in the literature on the brains of dugongs. On the basis of their lamination patterns, these frontal cortical areas appear to be organized into concentric zones of allocortex, mesocortex and isocortex.

Comparisons between Brains of a Large and a Small Hystricomorph Rodent: Capybara, Hydrochoerus and Guinea Pig, Cavia;Neocortical Projection Regions and Measurements of Brain Subdivisions

Somatic sensory, auditory and visual areas of cerebral neocortex were mapped in anesthetized capybaras using surface macroelectrode-evoked potential recording methods. The cortical motor area was mapped using electrical stimulation methods. The results of these experiments in the largest living rodent were similar to those found for the cortical sensory and motor areas of guinea pigs, a small rodent in a closely related family. The representation of the perioral skin in SI cortex was relatively large in capybaras and guinea pigs. In capybara, several cortical sulci reliably demarcate different cortical projection areas from one another. Quantitive neuroanatomical comparisons of volumes and neuron numbers in several major prosencephalic nuclei revealed that all nuclear masses are larger in capybara than in guinea pig, but that different nuclei are enlarged to different degrees. Possible causes of larger brains in larger animals are discussed.