Dorsal Column-medial Lemniscal System Research Articles

Netrin-1 is crucial for the establishment of the dorsal column-medial lemniscal system.

The dorsal column-medial lemniscal system is a significant sensory pathway that mediates touch and limb position sense. In this system, axons from the second-order neurons in the dorsal column nuclei form the internal arcuate fibers, cross the ventral midline (floor plate) within the medulla oblongata, and then project to the thalamus as the medial lemniscus. Here we demonstrate that Netrin-1, which is secreted from the floor plate in the medulla oblongata, is indispensable to the formation of the dorsal column-medial lemniscal system. Axons from the dorsal column nuclei cross the midline at around embryonic day 11 in mice. Concurrently, Netrin-1 mRNA and its receptor DCC (deleted in colorectal cancer) were expressed in the floor plate and commissural axons there, respectively. In our explant culture experiments, the floor plates of the embryonic 11-day-old mutant Netrin-1 homozygous mice did not attract axons from the dorsal column nuclei of ICR mice, while those from the wild type littermates did. Moreover, we observed that although the dorsal column nuclei developed in situ in mutant mice, their axons were not attracted toward the floor plate: they did not cross midline and remained ipsilaterally, without forming the internal arcuate fibers, in embryonic 17-day-old mutant Netrin-1 homozygous mice.

Functional plasticity in the human primary somatosensory cortex following acute lesion of the anterior lateral spinal cord: neurophysiological evidence of short-term cross-modal plasticity

The primary somatosensory cortex (S1) in adult animals and humans is capable of rapid modification after deafferentation. These plastic changes may account for a loss of tonic control by nociceptive inputs over inhibitory mechanisms within structures of the dorsal column-medial lemniscal system. Most studies, however, have been performed under conditions where deafferentation of C and Aδ fibres coexists with large-diameter fibres deafferentation. In this study the effect of the acute lesion of one ascending anterior lateral column on neuronal activity within the dorsal column-medial lemniscal system was assessed by recording somatosensory evoked potentials (SEPs) in seven patients who underwent unilateral percutaneous cervical cordotomy (PCC) as treatment for drug-resistant malignant pain. Spinal, brainstem and cortical SEPs were recorded 2 h before and 3 h after PCC by stimulating the posterior tibial nerve at both ankles. Amplitudes of cortical potentials obtained by stimulation of the leg contralateral to PCC were significantly increased after PCC. No significant changes in spinal or brainstem potentials were observed. PCC did not affect SEP components obtained by stimulation of the leg ipsilateral to PCC. Our results suggest that nociceptive deafferentation may induce a rapid modulation of cortical neuronal activity along the lemniscal pathway, thus providing the first evidence in humans of short-term cortical plasticity across the spinothalamic and lemniscal systems.

Aβ-fiber intensity stimulation of chronically constricted median nerve induces c-fos expression in thalamic projection neurons of the cuneate nucleus in rats with behavioral signs of neuropathic pain

This study was aimed to investigate the possible involvement of neurons in the cuneate nucleus (CN) in the processing of Aβ afferent inputs evoked by electrical stimulation of constricted median nerve in rats with behavioral signs of neuropathic pain. Immunohistochemical localization of Fos protein was used to examine the neuronal activation, and the combination of Fos immunohistochemistry with the retrograde labeling of Fluoro-Gold (FG) injected into the ventrobasal complex of the thalamus was used to characterize the activated neurons. Two weeks after unilateral median nerve constriction injury, the rats exhibited behavioral signs of neuropathic pain in the affected forepaws. In rats after nerve injury but without electrical stimulation, some Fos-like immunoreactive (Fos-LI) neurons were detected in the dorsal horn of the seventh cervical segment (C7) but none was found in the CN. Similar features were also noted when the stimulation of the intact median nerve served as an additional control. After Aβ-fiber intensity stimulation of the previously constricted median nerve, an increase in number of Fos-LI neurons occurred in the medial half of the ipsilateral C7 dorsal horn as well as in the ipsilateral CN. In the latter, the Fos-LI neurons were located in the median nerve projection territory throughout the nucleus. Most of the Fos-LI neurons were distributed in the middle region of the CN, with about 78% of them emitting FG fluorescence indicating that they were cuneothalamic projection neurons. The results of this study suggest that the dorsal column-medial lemniscal system may contribute to the transmission and modulation of Aβ-fiber mediated neuropathic pain signals.

Changes in dynamin and actin mRNA expression in the dorsal column-medial lemniscal system following dorsal column lesion.

Transection of the third cervical hindlimb dorsal column nerve fibers in the spinal cord leads to a partial deafferentation atrophy of the neurons of the ascending dorsal column-medial lemniscal neural network (DC-ML) up to the cortex. We now examine the alteration of the steady-state level mRNA coding for the synaptic vesicle protein, dynamin I and the cytoskeletal protein beta-actin as early indicators of direct and trans-synaptic changes in the relay nuclei of the DC-ML. Rats were sacrificed at 6, 24, 72, and 240 hr after C3 hindlimb dorsal column or sham lesion. By 24 hr, there are changes in the steady-state levels of mRNA coding for both dynamin I and beta-actin in regions of the brain containing the first (nucleus gracilis of medulla) and third synaptic relays (cortex). Beta-actin mRNA is increased at both 6 and 24 hr in the nucleus gracilis. The changes in dynamin I mRNA in the nucleus gracilis are early and biphasic, elevated at 6 hr but decreased compared to sham by 24 hr. In both regions, the initial fluctuations of dynamin I and beta-actin mRNA levels are transient. By 72 hr, the levels are no different from those of sham-lesioned animals. In the somatomotor cortex, there is an additional increase in beta-actin mRNA levels at 240 hr. The increased steady-state levels of dynamin and actin mRNA following a hindlimb dorsal column lesion suggest that increased synaptic vesicle recycling and actin cytoskeleton rearrangement are some of the early responses to deafferentation made by the neurons of the DC-ML synaptic relays.

Capsaicin-induced rapid receptive field reorganization in cuneate neurons.

1. To determine whether activity in unmyelinated or thinly myelinated primary afferents affects the organization of cutaneous receptive fields (RFs) in the cuneate nucleus, subcutaneous injections of capsaicin were made into the RFs of cuneate neurons in anesthetized cats. The effects of capsaicin injection on RF size and position, response properties, and spontaneous firing rates were determined during single-unit recordings. 2. Subcutaneous injection of capsaicin (10% dissolved in 70% ethanol) into peripheral RFs produced rapid RF reorganization in all neurons tested (n = 20), even when the original RF remained responsive to tactile stimulation. RF reorganization was marked by the appearance of newly responsive RFs. These RFs appeared as either contiguous expansions of the original RF or as new, noncontiguous fields. Control injections of the vehicle alone never produced RF reorganization (n = 9). RF reorganization was not related to changes in spontaneous activity. 3. Following capsaicin-induced RF reorganization, subcutaneous injections of lidocaine were made into the original RFs (n = 8). These injections produced no additional RF reorganization in seven of the eight neurons, even though in each case the lidocaine injections produced at least partial blocks of responsiveness from within the original RF. 4. In approximately one-half of the neurons (9 of 20), the reorganized RFs had response properties that were different from those of the original RF. This result suggests that, for neurons in the dorsal column nuclei (DCN), physiological mechanisms that normally mask inputs and restrict RF size can give rise to response specificity. 5. The results of these experiments demonstrate that capsaicin-induced RF reorganization, which has been reported previously in ventrobasal thalamus and primary somatosensory cortex, can arise within the dorsal column-medial lemniscal system, at the level of the DCN. The present results also provide evidence that rapid RF reorganization in cuneate neurons can be produced by blockade of a subset of peripheral afferents that are sensitive to capsaicin.

Changes in c-fos expression in the dorsal column-medial lemniscal system following dorsal column lesions.

Transection of the hindlimb dorsal column fibers leads to a partial deafferentation of the neurons of the nucleus gracilis, the first relay of the ascending dorsal column-medial lemniscal (DC-ML) neural network. In response to the deafferentation, a synaptic renewal cycle is initiated and the neurons of the nucleus gracilis atrophy. The present study examines the molecular changes that occur in synaptic relays of the ascending DC-ML following a hindlimb dorsal column transection. Rats were sacrificed 0.5, 1, 24, or 72 hours postlesion. Steady-state levels of mRNA coding for c-fos were significantly elevated only at 24 hours postlesion in caudal dorsal medulla, which contains the nucleus gracilis. The increased c-fos is neuronal in origin since there is an increased level of c-fos immunoreactivity in both the cluster neurons and the interneurons of the nucleus gracilis. In the third relay of the DC-ML system, the somatomotor cortex, levels of c-fos mRNA were significantly decreased 72 hours postlesion. These data indicate that lesions of the hindlimb dorsal column fibers have transneuronal effects on gene expression that extend to at least the third synaptic relay in the DC-ML system.

Distribution of the postsynaptic dorsal column projection in the cuneate nucleus of monkeys

Cells in the spinal cord that are postsynaptic to primary afferent fibers project to the dorsal column nuclei in the postsynaptic dorsal column pathway. The projection of cells in the cervical spinal cord of monkeys to the cuneate nucleus has been reported to avoid pars rotunda of that nucleus, the part that contains the somatotopic representation of the ipsilateral hand. We used the sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin to reexamine this projection. We made multiple iontophoretic injections into the cervical enlargements of three monkeys (two Macaca fascicularis and one Macaca mulatta). Control injections were made in the contralateral dorsal columns of one of these and in the dorsal roots of a fourth animal (M. fascicularis) to test for transport by fibers of passage. After 28-39 days, the animals were deeply anesthetized and perfused, and the tissue was processed for immunohistochemical detection of the label. In all cases (excluding control injections), labeled fibers and varicosities were distributed widely in the ipsilateral cuneate and external cuneate nuclei, including pars rotunda. The dorsal column nuclei ipsilateral to control injections contained no label or only very few poorly labeled fibers, indicating that labeling through fibers of passage did not contribute importantly to the results. This study indicates that the postsynaptic projection to the cuneate nucleus is widespread and includes pars rotunda. Such projections may contribute to transmission of information originating in nociceptors through the dorsal column-medial lemniscal system to the ventrobasal thalamus.

Giant somatosensory evoked potentials in a patient with the anterior spinal artery syndrome

We studied a previously healthy 25-year-old woman with the anterior spinal artery syndrome, a rare thoracocervical myelopathy with multiple potential etiologies. Quantitative and clinical sensory examination showed dissociated loss of pin-prick and temperature discrimination below the level of the lesion, with normal light touch, vibratory, and position sense. Magnetic resonance imaging was consistent with cervical spinal cord infarction. Median SEPs showed normal Erb's potential with absent spinal N13- and normal scalp N20- latency. Tibial SEPs showed normal lumbosacral responses and normal scalp P30- latency. Both median and tibial nerve stimulation produced cortical responses of unusually large amplitude (median 38 microV, tibial 17 microV). We hypothesize that large SEP amplitudes in this patient resulted from loss of anterolateral inhibitory influences on the dorsal column-medial lemniscal system.

The raccoon lateral cervical nucleus: a single-unit analysis

1. Properties of 90 lateral cervical nucleus (LCN) neurons responsive to light tactile stimulation of ipsilateral body surfaces were examined in pentobarbital sodium-anesthetized raccoons. Peripheral receptive fields (RFs) of 60 of these lay totally or partially on glabrous skin of the forepaw. There were 71 neurons antidromically activated from the contralateral thalamic ventro-basal complex (VB) or medial lemniscus. Results were compared with previous findings in the raccoon spinocervical tract (SCT) and the dorsal column-medial lemniscal system (DC-MLS). 2. RFs located on glabrous skin of the digits were significantly smaller than those located on glabrous skin of the palm. All RFs, whether on glabrous skin of the forepaw or elsewhere, tended to be larger than those of either SCT or DC-MLS neurons. LCN units with glabrous forepaw RFs tended to be located ventrally within the nucleus. 3. Of those LCN neurons for which the RFs lay totally or partially on glabrous skin of the forepaw, relative numbers that were rapidly adapting (RA; 77%) versus slowly adapting (SA; 23%) were comparable with those found in the SCT and in VB. Relative numbers of LCN neurons that were classed as light touch (87%) versus multireceptive (13%) were comparable with those found in the SCT. 4. In contrast to both the SCT and VB, but in common with the prethalamic DC-MLS, indentation velocity coding functions of both RA and SA units fell within homogeneous groupings, power function exponents for RA units tending to be steeper than those for SA units (range of b = 0.710 - 0.919 vs. 0.448 - 0.883). 5. It is concluded that the raccoon spinocervicothalamic system (SCTS) as a whole lacks the "modality and place specificity" associated with the DC-MLS. Although the SCTS probably makes significant contributions to properties of VB neurons, these properties primarily reflect those of neurons of the DC-MLS.

POTENTIALS EVOKED IN HUMAN AND MONKEY CEREBRAL CORTEX BY STIMULATION OF THE MEDIAN NERVE

Somatosensory evoked potentials (SEPs) are generated in afferent pathways, subcortical structures and various regions of cerebellar and cerebral cortex by stimulation of somatic receptors or electrical stimulation of peripheral nerves. This review summarizes current knowledge of SEPs generated in cerebral cortex by stimulation of the median nerve, the most common form of stimulation for human research and clinical investigations. Major sources of data for the review are intracranial recordings obtained from patients during diagnostic or neurosurgical procedures, and similar recordings in monkeys. Short-latency cortical SEPs in the 20-40 ms latency range consist of P20 and N30, recorded from motor cortex and frontal scalp; P25 and N35, recorded from cortex near the central sulcus and central scalp; and N20 and P30, recorded from somatosensory cortex and parietal scalp. Several lines of evidence including cortical surface and intracerebral recordings, neuromagnetic recordings and lesion studies in humans and monkeys, strongly support the conclusion that these potentials are generated in contralateral somatosensory cortex in areas 3b and 1, in contrast to the conclusion of many previous studies that SEPs recorded from the frontal scalp are generated in motor cortex and other frontal lobe areas. These potentials are primarily mediated by cutaneous afferents of the dorsal column-medial lemniscal system; the contribution of muscle afferents has not been completely resolved but appears to be small. There is currently no evidence that short-latency SEPs are generated in cortex other than primary somatosensory cortex. Recordings from the vicinity of the second somatosensory area, from the supplementary motor and sensory areas and from surface cortex other than sensorimotor cortex have not detected reliable short-latency activity, although some of these regions generate long-latency potentials. Consequently, short-latency SEPs recorded from the scalp are similar to those recorded from the surface of sensorimotor cortex. Old World monkeys such as Macaca mulatta and M. fascicularis provide an excellent model for human short-latency SEPs. All the potentials described above in humans have corresponding monkey analogues, with similar distributions over the cortical surface. The squirrel monkey, a New World species, exhibits the same potentials, but due to the different morphology of sensorimotor cortex, the surface distribution of SEPs is quite different.

The Perception of Painful and Nonpainful Stimuli during Voluntary Motor Activity in Man

Previous studies have shown that voluntary movement diminishes the transmission of cutaneous afferent input through the dorsal column-medial lemniscal system, and also raises the threshold for detecting nonpainful, cutaneous stimuli (electrical shocks). Although there is some evidence that pain elicited by electrical stimulation is diminished during movement, no studies have tested the effect of movement on the perception of pain produced by natural stimulation. For this reason, we tested the effects of voluntary motor activity on the perception of noxious thermal stimuli in human volunteers. We first developed a motor paradigm in which the thermal stimulation could be applied to the immobile limb (isometric elbow flexion-extension). Both isometric and isotonic muscle contractions about the elbow increased the threshold for detecting weak cutaneous stimuli (electrical shocks) applied to the forearm, and to a lesser extent the detection of stimuli applied to the dorsum of the hand. Afterwards, noxious and innocuous heat stimuli were applied to the forearm during isometric contractions and at rest. Magnitude estimates for the intensity of the pain, as well as latency measures of the onset of pain, were recorded. We found no evidence that isometric motor activity diminished either the threshold for pain or the subjective intensity of the noxious and innocuous thermal stimuli. Thus, motor activity decreases the ability to detect weak low-threshold cutaneous inputs, but has no effect on the perception of warmth and heat pain.

Somatotopic organization of forelimb representation in cervical enlargement of raccoon dorsal horn.

1. Somatosensory representation of the forelimb in the dorsal horn of spinal segments C5-T2 was examined in 13 North American raccoons anesthetized with pentobarbital sodium. Single- or multiple-unit responses to light mechanical stimulation were recorded at a total of 504 loci, which were subsequently reconstructed from stained, transverse sections. From these, dorsal view maps of forelimb representation in Rexed's laminae III and IV were synthesized. 2. There was a shifting, serial overlap of representations of different forelimb regions in both the rostrocaudal and mediolateral axes of the dorsal horn. The rostrocaudal progression of receptive fields was from preaxial forelimb to forepaw to postaxial forelimb, whereas the mediolateral progression was from the volar glabrous forepaw toward the trunk. 3. Representations of the glabrous surfaces of the digits and palm pads showed considerable overlap, with the digital representations extending more laterally, but the palmar representations extending more rostrally and caudally. One-third of all recording loci were devoted exclusively to glabrous skin representation. 4. Comparison with results of earlier studies in raccoons indicates that representation of a given digit or palm pad is more restricted in rostrocaudal extent in the dorsal horn than in the dorsal roots, and that, compared with various nuclear regions of the dorsal column-medial lemniscal system, the glabrous surfaces of the forepaw are underrepresented in the dorsal horn. 5. The results suggest that there is a dorsoventral modular organization of forelimb representation in the dorsal horn. These wedge-shaped modules from larger aggregates which represent major body subdivisions and which course sinuously in the rostrocaudal dimension.

Spinocervical tract neurons responsive to light mechanical stimulation of the raccoon forepaw.

1. The extracellular activity of 45 antidromically identified spinocervical tract (SCT) neurons responsive to light mechanical stimulation of the glabrous surfaces of the forepaw was examined in raccoons anesthetized with pentobarbital sodium. An additional seven neurons had peripheral receptive fields (RFs) located on hairy skin of the forelimb, and three had deep RFs. 2. All recording sites were histologically verified as falling within Rexed's laminae III and IV in spinal cord segments C6-T1. Antidromic conduction velocities of the 55 neurons ranged between 8.3 and 64.2 m/s. 3. Units with glabrous skin RFs were classified according to their response to a maintained mechanical stimulus as either rapidly adapting (n = 39) or slowly adapting (n = 6). Of 11 cells tested, 2 displayed enhanced responses to noxious stimuli and were classed as multireceptive. 4. RF areas were significantly smaller on digits (range = 0.4-45.0 mm2) than on palm pads (range = 5.6-76.0 mm2), and comparable in size to RF areas previously reported in raccoon cuneate nuclear cells (32). 5. RA neurons fell into three distinct categories with respect to the relationship between instantaneous spike frequency during displacement ramp stimulation, and ramp velocity, steep functions (as defined by the value of power function exponents), flat functions, and discontinuous functions; SA neurons fell into two categories, continuous, and discontinuous. 6. The results, in conjunction with those of previous studies, lead to two major conclusions: 1) raccoon and primate spinocervicothalamic systems are more similar to each other than either is to that of the cat and 2) the ability of the raccoon SCT to convey information from the glabrous skin of the forepaw regarding characteristics of light mechanical stimuli is at least as precise as that of neurons of the dorsal column-medial lemniscal system.

Bouton renewal patterns in rat hindlimb cortex after thoracic dorsal funicular lesions.

The transneuronal effect of bilateral, dorsal funicular lesions (T 12) on the frequency of boutons on cells in layer IV of hindlimb cortex was studied. Adult rats were utilized 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperative (DPO), and tissue was processed for the light microscopic visualization of silver-impregnated boutons (Rasmussen method). Bouton counts were taken on soma, or along 5- and 10-micrometers segments or proximal dendrite branching from soma. The soma diameter also was measured on those neurons chosen for bouton counts on the circumference of the soma. Statistically significant, increased afferentation on soma and proximal dendrite occurred during the first postlesion week relative to longer survival times; bouton counts on the proximal dendrite showed a trend (not statistically significant) toward increases above normal. These data mirror similar, consistent increases in bouton counts reported in thalamic ventroposterolateral nucleus of these same cases. At 14 DPO, bouton counts on the soma decreased below normal (P less than 0.005) and, except at 60 DPO, remained so through 120 DPO (P less than 0.025). Bouton counts on the proximal dendrite also decreased below normal at 14 DPO (P less than 0.005), thereafter exhibiting either periodic (along 5-micrometers) or extended (along 10-micrometers) periods in significant decreases from normal. Correlations in lesioned cases between the number of boutons on the soma and either bouton counts on proximal dendrite or soma diameter were positive and statistically significant (P less than 0.005 in all correlations). Possible anterograde (via the dorsal column-medial lemniscal system) and/or retrograde (via the corticospinal tract) transneuronal mediation of these effects in hindlimb cortex is discussed.

Dorsal column input into the reticular formation.

Somatic sensory information, such as touch, pressure, pain and body position, is conveyed to the mammalian thalamus and ultimately the cerebral cortex along three major ascending pathways in the spinal cord: (1) the dorsal column medial lemniscal (DCML) system, (2) the ventral column (VC) system, including the ‘neospinothalamic’, ‘palaeospinothalamic’ and spinoreticular tracts, and (3) the spino-cervico-lemniscal (SCL) system1,2. The phylogenetically recent DCML system has been traditionally viewed as an exclusive and homogeneous one-to-one relay of somatic sensory information to the thalamic ventralis posterolateralis nucleus. The emerging evidence, however, shows the gracile and cuneate nuclei as non-homogeneous structures which contain neurones that are subject to many kinds of central and peripheral influences, and which include neurones that do not project to the thalamus3. The relation of the ascending spinal pathways to the brain stem reticular formation (RF) is of particular interest in view of the numerous important functions subserved by this region. The spinal input into the RF is generally considered to reach it exclusively through the VC4. In view of their intraspinal connections5–8, it is difficult to separate and interact the three ascending spinal pathways. We now demonstrate a DCML input into the gigantocellular nucleus (Rgc) of the medial medullary RF in cats, and show inhibitory as well as excitatory interactions between DCML and VC tracts in RF cells. We used a combination of careful spinal surgical lesions aimed at disconnecting the DCML from the other afferent systems and of localized spinal and peripheral stimulations. Persistence of the response of Rgc neurones to DCML stimulation in decerebrate and decerebellate cats rules out the possibility of descending activation via a feedback loop from the cerebral and cerebellar cortices. In view of the possible involvement of Rgc in processing nociceptive input, these findings are of interest to our understanding of the DCML system in somatosensory function and, in particular, pain mechanisms.

Dorsal column nuclei and ascending spinal afferents in macaques.

Cell populations and thalamic projections of the dorsal column nuclei in macaques have been investigated in the medullae of normal animals and of animals with injections of horseradish peroxidase in the nucleus ventralis posterolateralis. In the same species, the course, distribution and origin of ascending non-primary pathways to the dorsal column nuclei have been demonstrated with the aid of degeneration methods, 3H-amino acid autoradiography and retrograde axonal transport of horseradish peroxidase. Non-primary afferents to the gracile and cuneate nuclei ascend mainly in the dorsal columns and, to a lesser extent, in the dorsal part of the lateral funiculus. Afferents originating from lumbar segments and ascending in the lateral funiculus terminate mainly in the rostral part of the gracile nucleus while those ascending in the dorsal columns distribute throughout most of the rostrocaudal extent of the same nucleus. Afferents from brachial levels terminate mainly in the cuneate nucleus and in the external cuneate nucleus. Degeneration and autoradiographic material concurrently demonstrate that non-primary afferents to the cuneate nucleus terminate preferentially within certain cytoarchitectonic subdivisions of this nucleus. Ascending spinal afferents to the dorsal column nuclei originate mainly from the ipsilateral dorsal horn, particularly from its medial part at upper cervical levels and from a band of gray, throughout the cord, largely corresponding to lamina IV and adjacent laminae. Large neurons along the lateral border of the ventral horn at lumbar levels may also contribute non-primary afferents to the ipsilateral dorsal column nuclei. These anatomical results provide some cues to a revised view of the organization of the dorsal column nuclei in monkeys and, taken together with recent electrophysiological and clinical data, contribute to a re-evaluation of some functional aspects of the dorsal column-medial lemniscal system of primates.

Tactile discrimination study of the dorsal column-medial lemniscal system and spino-cervico-thalamic tract in cat.

The role of two spinal sensory pathways, the dorsal column (DC) pathway and the spino-cervico-thalamic tract (SCTT), involved in tactile discrimination was studied. Pour lesion groups of cats (DC lesion, SCTT lesion, combined DC and SCTT lesion, and sham-operated) were tested to discriminate various degrees of roughness. Animals with either the SCTT or the combined lesions performed at around 50–60% level under all the conditions tested. On the other hand, those with the DC lesion and sham-operations generally attained 80–100% level of performance. A difference between the DC group and sham-operated group was found in their rate of learning and ability for finer discrimination. These results are interpreted as an indication that a duplicity in transmission of tactile sensation exists, the DC pathway being more specific than the SCTT.

The role of cerebral cortex in evoked somatosensory activity in cat cerebellum

Gross responses were recorded at various cerebellar loci following forepaw electrical stimulation. In chloralose-anesthetized cats responses are large over posterior culmen and adjacent simplex, somewhat reduced in paramedian lobules bilaterally, and considerably smaller in posterior vermis and cerebellar hemispheres. Although variable, responses are usually positive-negative and appear to contain components indicated by notches or slope changes in the main wave. Following decerebration, responses in culmen and simplex are greatly reduced and simplified; responses elsewhere are abolished. Similar results follow administration of barbiturates, KCl-induced depression of cerebral cortex, ablation of somatosensory cortex, lesions of dorsal column, medial lemniscus or nucleus ventralis posterolateralis. In addition to “direct afferent pathways” from forepaw to cerebellum, activity travels via the dorsal column-medial lemniscal system to somatosensory cortex and thence to cerebellum and provides a “cortical loop pathway” which acts to enhance the spinocerebellar responses, resulting in increase of response amplitude and duration as well as enlargement of the responsive area. The major route from cortex to cerebellum appears to be via the corticopontocerebellar pathway, and the wide distribution of the forepaw response beyond forelimb area in culmen is presumably due to the wider distribution of this pathway.