Search

Florida manatees have an extensive, well-developed system of vibrissae distributed over their entire bodies and especially concentrated on the face. Although behavioral and anatomical assessments support the manatee's reliance on somatosensation, a systematic analysis of the manatee thalamus and brainstem areas dedicated to tactile input has never been completed. Using histochemical and histological techniques (including stains for myelin, Nissl, cytochrome oxidase, and acetylcholinesterase), we characterized the relative size, extent, and specializations of somatosensory regions of the brainstem and thalamus. The principal somatosensory regions of the brainstem (trigeminal, cuneate, gracile, and Bischoff's nucleus) and the thalamus (ventroposterior nucleus) were disproportionately large relative to nuclei dedicated to other sensory modalities, providing neuroanatomical evidence that supports the manatee's reliance on somatosensation. In fact, areas of the thalamus related to somatosensation (the ventroposterior and posterior nuclei) and audition (the medial geniculate nucleus) appeared to displace the lateral geniculate nucleus dedicated to the subordinate visual modality. Furthermore, it is noteworthy that, although the manatee cortex contains Rindenkerne (barrel-like cortical nuclei located in layer VI), no corresponding cell clusters were located in the brainstem ("barrelettes") or thalamus ("barreloids")., (Copyright 2007 Wiley-Liss, Inc.)

I propose the general hypothesis that each individual folium in the cerebellum is an integrative module that is involved in unique sets of sensorimotor transactions. Although the basic types of operations carried out by cerebellar cortex may be similar in all folia, the mosaic of afferent sources, intrinsic organization and efferent destinations appear to be unique for each folium. I believe that this conception is supported by: 1) comparative data which illustrate species-typical folial patterns, 2) neuroanatomical data which reveal not only different structural features of folial crowns and fundi, but differential afferent and efferent connectivity of different folia as well, 3) physiological data, which demonstrate unique patterns of afferent activity in different folia, and especially by 4) ontogenetic data which establish that each folial crown expands and differentiates into an architecturally distinct cortical entity. Taken together, all these lines of evidence suggest that the numbers and patterns of folia exhibited by the cerebellar cortex of different mammals are morphological indicators of differential organization of sensorimotor control functions in each animal. Even intraspecific individual variations in folial number, size and pattern may signify structural-functional determinants of some individual differences in sensorimotor transactions. Since so little research has addressed the many testable ideas embodied in these general hypotheses, it seems to me that neuroscientists have a long way to go to clarify how the many different folia and lobules of cerebellar cortex actually function in the common, everyday, orderly, dynamic and ongoing reflex, postural, learned and deliberate behavioral sequences that characterize the normal behavioral repertoires of different animals. The enormous advances in understanding brought forth by the extensive research and writings of Professor Brodal and his colleagues have expanded our horizons to avail us of an enormous range of new vistas into cerebellar functional morphology. It is now the task of neurobiologists to explore these diverse new domains in ever greater depth and detail.

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.

Mechanosensory neurons are often classified as either rapidly adapting or slowly adapting. We examined response decay (adaptation) during constant deflection of the vibrissae with quantitative, repeatable, ad hoc measures. We found that first-order vibrissa-activated neurons of the fifth ganglion exhibit a variety of adaptation rates that appear to be distributed continuously between the rapidly and slowly adapting extremes. Also, adaptation rate is influenced markedly by stimulus magnitude. We found no evidence for a dichotomy within the more slowly adapting neurons on the basis of discharge regularity. Threshold tuning curves were used to evaluate vibration sensitivity. Both the best frequencies and 1:1 discharge thresholds for sinusoidal stimulation ranged over two orders of magnitude and were continuously distributed. First-order vibrissa-activated afferents exhibit a broad variety of response patterns to constant-velocity stimulation. The pattern of discharge varied both as a function of time during constant-velocity (ramp) deflection and as a function of stimulus velocity. Although information about the parameters of a stimulus may be conveyed by any of several features of the response pattern, it appears that few if any neurons function as "pure" encoders of any particular stimulus parameter. We examined quantitatively the relationship between discharge rate and both velocity and amplitude of vibrissa deflection with the aid of a computer-based curve-fitting procedure. We found that about half the observed rate-level functions were best described by a power function; the remainder were best fit by a logarithmic function. The parameters of the best-fitting functions varied widely and continuously, emphasizing further the diversity of coding properties of the rat's vibrissa afferents. Rate-level curves for stimulus magnitude generally exhibited saturation; some were nonmonotonic. None were described adequately by either a logarithmic function or a power function.

We studied single units innervating the glabrous forepaw skin of 35 domestic kittens ranging in age from 1 to 52 postnatal days. There aspects of coding were emphasized: (a) size and force thresholds of receptive fields, (b) time course of recovery between stimulus presentations, and (c) electrical conduction properties of the afferent nerve fibers. Receptive field (RF) size and force thresholds were (a) positively correlated with age for palmar but not digital RFs, and (b) were significantly larger on palm than on digits. Unit responsiveness was highly dependent on intertrial interval, complete recovery requiring at least 30 sec. Conduction velocity increased more rapidly than nerve fiber length, thus conduction time decreased with age. Refractory period decreased with age, but the conduction velocities of sequential spikes were proportional, regardless of age. These changes which we observed can more readily be ascribed to alterations of the mechanical properties of skin and conduction properties of nerve fibers than to changes in the coding mechanisms themselves.

Somatic sensory (SI) cortex was mapped in hyraxes by micromapping methods developed and used earlier in slow lorises and llamas. The somatotopic pattern of organization was similar in its general features to that found in other mammals. The perioral and intraoral representations are relatively large in SI of hyrax and exhibit considerable distortion. Sulci are reliable landmarks demarcating boundaries of SI in hyrax as they are in other mammals.

In order to gain insight into the functional properties of the tail in North American beavers (Castor canadensis) we (1) examined morphological features of skeletal, muscular, vascular, cutaneous and neural structures; (2) determined the segmental organization of spinal roots and certain stimulus-response features of receptive fields of single dorsal root fibers; (3) mapped the main somatic sensory area (SI) of the cerebral neocortex, and (4) attempted to relate these findings to observations of tail behavior in living beavers. The behavioral observations revealed the beaver capable of forceful yet discrete movements of the tail in water. A morphological correlate of these motor skills was the distinct segmentally organized pattern of serial muscle tendon arrangements and spinal sensory and motor innervation. Neither the receptors innervating the scales or hairs of the tail, the stimulus-response properties of single dorsal root afferents, nor the representation of the tail in SI suggested unusual cutaneous sensory capabilities associated with the morphological and behavioral specializations exhibited by the beaver's tail.

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.

We examined stimulus-response relationships of vibrissa-activated mechanosensory neurons of the rat's 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.

In order to understand the coding and information processing capabilities of mechanosensory neurons, it is necessary to examine stimulus--response relationships under a wide variety of stimulus conditions, using a comprehensive set of quantitative analytic procedures. We employ the methodology of stimulus--response profile analysis, which is based on 4 principles: (1) the use of a broad-based battery of quantitatively controlled mechanical stimuli; (2) maintenance of comprehensive records of experiments; (3) detailed, quantitative analysis of single-unit responses; and (4) the application of these principles uniformly and consistently to all units studied. In addition to conventional graphical portrayals of single-unit activity, we employ a set of quantitative response indices, each of which represents a particular aspect of a unit's overall responsiveness. The distribution of a response index for the entire sample of neurons examined in a particular population offers insight into the manner in which a specific stimulus feature is presented within that population. The distributions of response indices obtained from different neural populations can be compared statistically in order to evaluate interpopulation differences in stimulus-response relationships. This comprehensive, quantitative approach is capable of demonstrating significant, although subtle, interpopulation differences which are not revealed by more cursory, qualitative methods.

Microelectrode mapping methods and strategies were developed to map fine details of the somatotopic organization of somatic sensory cortex (SI) in slow loris. In-depth, multiple-unit recording, and threshold natural stimulation of skin and hari revealed three modality-specific cortical areas. The centrally located one was activated by extremely light touch and the somatopic organization of peripheral projections to it was mapped in detail. As many as 25 punctures/mm2 were made in delineating these details. The microelectrode mapping method used revealed smaller receptive fields, lesser degrees of overlap, a more differentiated pattern of peripheral projections, and a more precise correlation of electrophysiological results with underlying cytoarchitectonic details than did the usual macroelectrode surface recording method. These results and the correlations found between different physiologically defined areas and cortical sulci are discussed as they pertain to methodological advances in studies of cerebral localization of function.

The somatic sensory cortex (SI and SII) was mapped in llamas using microelectrode mapping methods developed earlier in a study of SI of the slow loris. Projections to SI from the llama's prehensile browsing lips were differentially enlarged when compared to those reported for sheep. In llama, SII was reversed in its mediolatreal pattern from that reported for SII in most other mammals. Fissural landmarks reliably demarcated different projections within SI, between SI and SII and between SI or SII and other surrounding nonsensory areas. The use of microelectrode mapping methods in different mammals to determine gyral and fissural homologies is discussed.