Organization of postcranial kinesthetic projections to the ventrobasal thalamus in raccoons

Proprioceptive thalamus receiving forelimb and neck muscle spindle inputs via the external cuneate nucleus in the rat

Proprioceptive signals from body muscles have historically been considered to project to the rostrodorsal shell of the ventrobasal thalamic complex [the ventral posterolateral nucleus (VPL) and ventral posteromedial nucleus (VPM)]. However, we have recently found that proprioception from rat jaw-closing muscle spindles (JCMSs) is conveyed via the supratrigeminal nucleus to the caudo-ventromedial edge of the VPM, but not to the rostrodorsal shell of the VPM. Therefore, proprioception from other body muscles may also project to thalamic regions other than the rostrodorsal shell of the VPL. We thus examined the thalamic projection from the rat external cuneate nucleus (ECu), which receives proprioceptive inputs from forelimb and neck muscles. After injection of anterograde tracer into the ECu, axon terminals were contralaterally labeled in the ventromedial part (VPLvm) of the VPL, but not in the rostrodorsal shell of the VPL. After anterograde tracer injection into the cuneate nucleus (Cu), axon terminals were widely labeled in the contralateral VPL including the VPLvm. In the VPLvm, we electrophysiologically confirmed the proprioceptive inputs responsive to electrical stimulation of the ECu or median nerve and to the pressure of forelimb/neck muscles or wrist flexion. After retrograde tracer injection into the VPLvm, neurons were contralaterally labeled in the ECu and Cu. After retrograde tracer injection into the VPL where no such proprioceptive inputs were recorded, no ECu neurons were labeled. These findings indicate that proprioception from forelimb/neck muscle spindles and JCMSs is somatotopically transmitted to the ventromedial floor of the ventrobasal thalamic complex, but not to its rostrodorsal shell.

Somatosensory brainstem, thalamus, and cortex of the California sea lion (Zalophus californianus)

Pinnipeds (sea lions, seals, and walruses) are notable for many reasons, including their ape-sized brains, their adaptation to a coastal niche that combines mastery of the sea with strong ties to land, and the remarkable abilities of their trigeminal whisker system. However, little is known about the central nervous system of pinnipeds. Here we report on the somatosensory areas of the nervous system of the California sea lion (Zalophus californianus). Using stains for Nissl, cytochrome oxidase, and vesicular glutamate transporters, we investigated the primary somatosensory areas in the brainstem, thalamus, and cortex in one sea lion pup and the external anatomy of the brain in a second pup. We find that the sea lion's impressive array of whiskers is matched by a large trigeminal representation in the brainstem with well-defined parcellation that resembles the barrelettes found in rodents but scaled upward in size. The dorsal column nuclei are large and distinct. The ventral posterior nucleus of the thalamus has divisions, with a large area for the presumptive head representation. Primary somatosensory cortex is located in the neocortex just anterior to the main vertical fissure, and precisely locating it as we do here is useful for comparing the highly gyrified pinniped cortex with that of other carnivores. To our knowledge this work is the first comprehensive report on the central nervous system areas for any sensory system in a pinniped. The results may be useful both in the veterinary setting and for comparative studies related to brain evolution.

Proprioceptive and Cutaneous Representations in the Rat Ventral Posterolateral Thalamus

Determining how and where proprioceptive information is represented in the rat ventral posterolateral (VPL) is important in allowing us to further investigate how this sense is utilized during motor control and learning. Here we demonstrate using electrophysiological techniques that the rostral portion of the rat VPL nucleus (rVPL, −2 to −2.5 mm bregma) carries a large amount of proprioceptive information. Caudal to this region is a zone where the cutaneous receptive fields are focal (mVPL for middle VPL, −2.5 to −3.2 mm bregma) with a fine topographic map of the fore- and hindlimbs. The forepaw is represented with digit 1 medial and each subsequent digit increasingly lateral, all of which are dorsal to the pads. The caudal VPL (cVPL, −3.2 to −4.0 mm bregma) has broad receptive fields and is the target of lamina 1 and lamina 2, as well as the dorsal column nuclei, and may represent the flow of nociceptive information through the VPL. Thus we propose that the VPL may be thought of as three subnuclei—the rostral, middle, and caudal VPL—each carrying preferentially a different modality of information. This pattern of information flow through the rat VPL is similar, although apparently rotated, to that of many primates, indicating that these regions in the rat (rVPL, mVPL, and cVPL) have become further differentiated in primates where they are seen as separate nuclei (VPS, VPL, and VPI/VMpo).

Ultrastructural study of external cuneothalamic neurons and their synaptic relationships with primary afferents in the gerbil

The present study examined the synaptic organization of external cuneothalamic neurons and their relationships with primary afferents in the gerbil external cuneate nucleus (ECN) following an injection of horseradish peroxidase (HRP) into the anterodorsal cap of the ventrobasal thalamus in conjunction with a simultaneous injection of HRP into the contralateral brachial and cervical nerve plexuses. The thalamus-projecting neurons have been shown to be confined to the intermediate portion of the caudal half of the ECN at the light microscopic level (Lan et al., 1994c). In this study, HRP-labelled external cuneothalamic neurons were ultrastructurally characterized by their relatively small-sized soma bearing a variable number of somal spines. Their nucleus had a slightly indented contour with an eccentric nucleolus. The HRP-labelled somata were postsynaptic to many axon terminals, which were classified into round (Rs type; 53.0%), pleomorphic (Ps type; 32.7%), and flattened (Fs type; 14.3%) vesicle-containing boutons. The HRP-labelled dendritic elements were postsynaptic to a greater number of axon terminals, which were also classified into the round (Rd; 64.7%), pleomorphic (Pd; 25.2%), and flattened (Fd; 10.1%) type boutons. These presynaptic axonal boutons tended to synapse on distal and secondary dendrites of external cuneothalamic neurons. In the present simultaneous HRP labelling study, some of the primary afferent terminals made direct synaptic contacts with the dendrites of the external cuneothalamic neurons. In view of the multiple inputs onto the external cuneothalamic neurons, impinging particularly on their somata and secondary dendrites, it is suggested that the proprioceptive information reaching these neurons is intensively modulated and integrated before transmission ultimately to the cerebral sensorimotor cortex.

Changes in the organization of the ventroposterior lateral thalamic nucleus after digit removal in adult raccoon

The ventroposterior lateral nucleus of the thalamus was studied in seven raccoons that had undergone amputation of the fourth digit between 2 and 5 months previously. Extracellular recordings were made in a series of closely spaced penetrations through the thalamus in chloralose anesthetized animals. The responses to cutaneous stimulation of the forepaw were used to reconstruct the somatotopic organization of the thalamus and to identify recording sites believed to be located in the digit zone that had lost its peripheral input. Twelve penetrations that passed through both of the adjacent fifth and third digit regions were analyzed in detail to delineate this deafferented region. None of the recording sites in this region were completely silent, indicating that the deafferented thalamus had undergone significant reorganization of its inputs. At most sites, the neurons had receptive fields on the skin surrounding the amputation wound and including one of the adjacent digits. Approximately half of the sites had low thresholds in the range of normal thalamic neurons. These results indicate that the ventroposterior thalamus is capable of substantial reorganization, which may account for much of the reorganization seen in somatosensory cortex.

Cells of origin, thalamic relay and termination of the external cuneothalamocortical tract in the gerbil

The present study is concerned with the connections of the external cuneate nucleus (ECN) in the gerbil following an injection of horseradish peroxidase (HRP) into the ventralis posterior pars oralis (VPLo) or adjacent nuclei of the thalamus. The number, soma size and distribution of the retrograde-labelled ECN neurons were studied and quantified. The application of two retrograde fluorescent tracers was also used to determine whether the ECN neurons would project to the thalamus as well as to the cerebellum through their collaterals. The HRP-positive ECN neurons projecting to the thalamic VPLo were confined to the contralateral caudal half of the ECN, primarily within the intermediate portion represent the forearm and arm territories with a small part of the thoracic and shoulder areas. Labelled neurons were classified into small and medium-sized cells. The majority (96%) of the external cuneothalamic neurons were of the small variety. No double-labelled cells were detected in the ECN following injections of Rhodamine-labelled latex microspheres and Fast blue into the cerebellum and thalamus respectively, suggesting that the ECN neurons projecting to the thalamus form a separate cell group different from those projecting to the cerebellum. The injected HRP into the VPLo was also transported in an anterograde direction by the thalamocortical fibers. The HRP-labelled axonal terminals were distributed within motor area 4 and the dysgranular zones (DZs) of the primary somatosensory cortex (SmI), reaching the deep layers IV and VI as well as superficial layer I. The external cuneothalamocortical pathway shown in the present study may be related to the proprioceptive feedback control of the coordinating motor activity, especially during forelimb muscle movement.

The immediate effects of peripheral denervation on inhibitory mechanisms in the somatosensory thalamus

Multiunit recordings were made in the ventroposterior lateral nucleus of the thalamus in anesthetized raccoons. During recording from cells responding to cutaneous stimulation of a forepaw digit, the corresponding digit was denervated permanently (by cutting its four digital nerves) or temporarily (by injecting lidocaine into the base of the digit). Both procedures resulted in immediate increases in the inhibition that could be induced by stimulation of the adjacent digits when the original cutaneous receptive field was on the glabrous skin. In each case with temporary denervation, this enhanced off-focus inhibition decreased when the excitatory responses returned to normal. In contrast, temporary denervation of the digit during recording at sites in the hairy skin representation did not reveal this increased inhibition from adjacent digits. When capsaicin was applied to the digital nerves in two animals, the excitatory receptive fields of thalamic neurons increased in area, but were still restricted to the same part of the digit. These data indicate that the immediate unmasking of inhibitory responses, previously reported in primary somatosensory cortex of the raccoon, is also present in the thalamus. The capsaicin-induced expansion of excitatory receptive fields confirms previous experiments in other species, and suggests that C fibers play a role in modulating the size of cutaneous receptive fields. However, the enlargement of excitatory receptive fields by capsaicin is much less than the unmasking of inhibitory fields induced by digit denervation, and indicates that different mechanisms are involved in controlling these various inputs to thalamic neurons.

Functional regions within the map of a single digit in raccoon primary somatosensory cortex

Electrophysiological recordings were made at a large number of sites in the primary somatosensory cortex of six anesthetized raccoons. A high density of penetrations (110-229 per animal), within or near the representation of the fourth digit, allowed identification of three cortical regions with different physiological properties: a glabrous zone, containing a highly detailed, somatotopically ordered representation of the glabrous surface of the digit; rostral to this a claw-dominant zone, in which the neurons at most penetrations respond to stimulation of the claw of the fourth digit, but may also receive input from the hairy skin or surrounding glabrous skin; and a more rostral multidigit zone, in which the neurons respond to stimulation of two to five digits, with the dominant digit usually being the one represented caudally (i.e., the fourth digit at most of the sites sampled here). Claw-dominant zones with receptive fields restricted to digit three or five are also found rostral to the representations of the glabrous skin of the corresponding digit. The glabrous and claw-dominant zones constitute a complete map of the fourth digit. The multidigit region presumably is a separate map, since its neurons have different spatial convergence, higher thresholds, and a lower incidence of slowly adapting inputs than those in the claw-dominant and glabrous zones. A comparison between animals with lesions of the basal forebrain and intact animals found no differences in the organization of these zones or in the responses to peripheral input, suggesting that cholinergic inputs to the cortex are not essential to these properties. The detailed description of these regions and the proposed terminology should resolve some inconsistencies in the use of the term "heterogeneous zone" in this species.

Raccoon forelimb motorsensory cortex: I. Somatic afferent inputs to different cytoarchitectonic areas

The distribution of potentials evoked in and around forelimb MsI by graded electrical stimulation of forelimb nerves has been studied in the raccoon (Procyon lotor). These data have been correlated with cytoarchitectonic characteristics of pericruciate cortical tissue. Potentials evoked by cutaneous nerve stimulation were widely distributed in MsI and SmI, but were smaller in amplitude and of longer latency in MsI than in SmI. Stimulation of ulnar, median or deep radial nerve at 1-1.4T, a strength considered to activate only Group I muscle afferent fibers, caused evoked potentials in a localized region mostly confined to posterior sigmoid gyrus. On the basis of cytoarchitectonic features it is concluded that: a) Anterior sigmoid gyrus, to near the level of the tip of the cruciate sulcus, is area 6 cortex; b) The lateral portion of the posterior sigmoid gyrus, cortex comprising the caudal bank of the cruciate sulcus and cortex surrounding the lateral tip of the cruciate sulcus is area 4 cortex; c) The middle portion of the posterior sigmoid gyrus, almost to the lip of the cruciate sulcus rostrally and extending onto the rostral bank of the ascending coronal and postcruciate sulci caudally, is area 3a cortex. The cortical focus for Group I afferent-evoked potentials coincides with area 3a cortex. It is concluded that forelimb MsI of raccoon is organized in a fashion similar to MsI of cats and monkeys.

Thalamic and extrathalamic connections of the dysgranular unresponsive zone in the grey squirrel (Sciurus carolinensis)

The connections of the cortical dysgranular "unresponsive zone" (UZ) (Sur et al.: J. Comp. Neurol. 179:425-450, '78) in the grey squirrel were studied with horseradish peroxidase and autoradiographic techniques. The results of these experiments show that the major subcortical connections of the unresponsive zone are in large part reciprocal. Connections are distributed within the thalamus in a poorly defined region including restricted portions of several nuclei that lie along the rostral, dorsal, and caudal borders of the ventral posterior nucleus. Additional thalamic connections of the UZ terminate in the reticular nucleus and are reciprocally related to the paralaminar and central median nuclei. Extrathalamic terminations were observed in the zona incerta, the intermediate and deep layers of the superior colliculus, the red nucleus, and several subdivisions of the pontine nuclei. The similarity between the pattern of subcortical connections of the UZ in the grey squirrel and patterns reported for the parietal septal region in rats (Chapin and Lin: J. Comp. Neurol. 229:199-213, '84) and for area 3a in primates (Friedman and Jones: J. Neurophysiol. 45:59-85, '81), suggests that the UZ in the grey squirrel may represent a counterpart of at least part of area 3a as described in primates. The results are further discussed with respect to a possible role of the thalamus in control or modulation of interhemispheric circuits and of the UZ in the modulation of nociceptive and kinesthetic pathways through the thalamus. Finally, the term parietal dysgranular cortex (PDC) is proposed as an alternative to denote the region currently called the unresponsive zone.

Kinesthetic cortical area anterior to primary somatic sensory cortex in the raccoon (Procyon lotor)

Extracellular microelectrode recording of cortical unit activity, with subsequent histological examination, was used to determine the extent, organization, and cytoarchitecture of the zone of muscle afferent projections (kinesthetic cortex) anterior to the primary somatic sensory cortex in anesthetized raccoons. Activity was evoked in response to mechanical stimulation of muscles from which the overlying skin had been dissected away. Most kinesthetic responses were elicited in a contiguous cortical area, which included: the anterior bank of the lateral arm, and the fundus and posterior bank of the medial arm of the medial central sulcus; and the anterior two-thirds of the interfundic rise within the interbrachial sulcus. Some responses were recorded in a separate small area of the anterior bank at the medial end of the lateral central sulcus. Somatotopy was evident with forelimb represented lateral to hindlimb. Proximal limb muscles were represented in the center of the medial central sulcus; distal muscle projections were medial (hindlimb) or lateral (forelimb) in the same sulcus. Most representations were of flexor and extensor muscles of the contralateral carpus and forepaw digits. Activity at a given recording locus in the kinesthetic area could be elicited by both flexor and extensor muscles, which acted about a common joint. Low amplitude units evoked by cutaneous stimulation of the dissected skin were recorded in the kinesthetic area; these were from receptive fields of skin that normally overlay the muscles whose higher-amplitude evoked kinesthetic units were represented in that same recording locus. The kinesthetic zone was anterior to primary somatic sensory cortex, where the outer stripe of Baillarger and granular layer IV become attenuated. In the hindlimb muscle representation area, the additional criterion of area 3a (large pyramidal cells in layer V) was seen. However, no cytoarchitecture could be identified that was consistently associated with the kinesthetic cortex.

Medullary sources of projections to the kinesthetic thalamus in raccoons: external and basal cuneate nuclei and cell groups x and z

In raccoons and other mammals, a pathway for kinesthetic sensation (from muscles, fascia, tendons, and joints) reaches the anterodorsal cap of the ventrobasal thalamus and the anteriormost part of the somatic sensory cerebral cortex. To find the medullary component of this kinesthetic pathway in raccoons, small injections of horseradish peroxidase were made in the thalamus under guidance of simultaneous electrophysiological recording from kinesthetic projections. As determined by retrograde labeling following these injections, kinesthetic thalamic subregions receive projections as follows: caudomedial from cells in the external cuneate nucleus and its medial tongue, rostromedial from cells in basal cuneate nucleus, and rostrolateral from cells in cell group z and the reticular division of cell group x. Electrophysiological recording showed kinesthetic representations in each of these medullary regions. Labeled cells were also observed in the infratrigeminal subnucleus of the lateral reticular nucleus. Cats have kinesthetic projections to the thalamus from the basal cuneate and cell group z; raccoons (and monkeys) have these plus projections from the external cuneate and cell group x. This suggests that the kinesthetic projection system in raccoons and monkeys is expanded in correlation with their more dextrous use of the hand.

Demarcations of the mechanosensory projection zones in the raccoon thalamus, shown by cytochrome oxidase, acetylcholinesterase, and Nissl stains

To determine anatomically the boundaries and internal organization of the kinesthetic and cutaneous mechanosensory regions of the ventrobasal thalamus, alternate section series from electrophysiologically mapped tissues from 14 raccoons were stained for cytochrome oxidase, myelinated fibers, acetylcholinesterase, and Nissl substance. Microelectrode tracks, along with electrolytic lesions placed as tissue markers, reveal that the mechanoreceptor projection zones have higher cytochrome oxidase and lower acetylcholinesterase staining than some neighboring regions. Both these enzymatic stains reveal particularly sharp boundaries separating the mechanoresponsive region, from the lateral posterior nucleus dorsally and from the ventroposterior inferior nucleus ventrally. The kinesthetic projection zone is often separated from other mechanoreceptor projections by bundles as well as laminae of myelinated fibers, similar to those separating cutaneous projections from distinct body parts. These subdivisions are particularly well marked by the cytochrome oxidase stain. The combination, in neighboring sections, of the use of the several stains adds considerably to the visible delineation of these functionally distinct regions, beyond what can be seen in Nissl-stained sections.