Morphology of physiologically characterized medial lemniscal axons terminating in cat ventral posterior thalamic nucleus

Human Thalamic Somatosensory Nucleus (Ventral Caudal, Vc) as a Locus for Stimulation by INPUTS from Tactile, Noxious and Thermal Sensors on an Active Prosthesis

The forebrain somatic sensory locus for input from sensors on the surface of an active prosthesis is an important component of the Brain Machine Interface. We now review the neuronal responses to controlled cutaneous stimuli and the sensations produced by Threshold Stimulation at Microampere current levels (TMIS) in such a locus, the human thalamic Ventral Caudal nucleus (Vc). The responses of these neurons to tactile stimuli mirror those for the corresponding class of tactile mechanoreceptor fiber in the peripheral nerve, and TMIS can evoke sensations like those produced by the stimuli that optimally activate each class. These neuronal responses show a somatotopic arrangement from lateral to medial in the sequence: leg, arm, face and intraoral structures. TMIS evoked sensations show a much more detailed organization into anterior posteriorly oriented rods, approximately 300 microns diameter, that represent smaller parts of the body, such as parts of individual digits. Neurons responding to painful and thermal stimuli are most dense around the posterior inferior border of Vc, and TMIS evoked pain sensations occur in one of two patterns: (i) pain evoked regardless of the frequency or number of spikes in a burst of TMIS; and (ii) the description and intensity of the sensation changes with increasing frequencies and numbers. In patients with major injuries leading to loss of somatic sensory input, TMIS often evokes sensations in the representation of parts of the body with loss of sensory input, e.g., the phantom after amputation. Some patients with these injuries have ongoing pain and pain evoked by TMIS of the representation in those parts of the body. Therefore, thalamic TMIS may produce useful patterned somatotopic feedback to the CNS from sensors on an active prosthesis that is sometimes complicated by TMIS evoked pain in the representation of those parts of the body.

Neuronal responses to tactile stimuli and tactile sensations evoked by microstimulation in the human thalamic principal somatic sensory nucleus (ventral caudal)

The normal organization and plasticity of the cutaneous core of the thalamic principal somatosensory nucleus (ventral caudal, Vc) have been studied by single-neuron recordings and microstimulation in patients undergoing awake stereotactic operations for essential tremor (ET) without apparent somatic sensory abnormality and in patients with dystonia or chronic pain secondary to major nervous system injury. In patients with ET, most Vc neurons responded to one of the four stimuli, each of which optimally activates one mechanoreceptor type. Sensations evoked by microstimulation were similar to those evoked by the optimal stimulus only among rapidly adapting neurons. In patients with ET, Vc was highly segmented somatotopically, and vibration, movement, pressure, and sharp sensations were usually evoked by microstimulation at separate sites in Vc. In patients with conditions including spinal cord transection, amputation, or dystonia, RFs were mismatched with projected fields more commonly than in patients with ET. The representation of the border of the anesthetic area (e.g., stump) or of the dystonic limb was much larger than that of the same part of the body in patients with ET. This review describes the organization and reorganization of human Vc neuronal activity in nervous system injury and dystonia and then proposes basic mechanisms.

Psychophysical elements of place and modality specificity in the thalamic somatic sensory nucleus (ventral caudal, vc) of awake humans

Discrete anatomic structures in the monkey somatic sensory thalamus may segregate input arising from different peripheral receptors and from different parts of the body. It has been proposed that these structures serve as components of modality- and place-specific pathways from the periphery to the cortex. We now test this hypothesis by examining the modality- and place-specific segregation of sensations at sites where microstimulation (microA currents) within the region of ventral caudal (Vc; human principal somatic sensory nucleus) evokes somatic sensations. Microstimulation was delivered in an ascending staircase protocol consisting of different numbers of pulses (4-100) presented at different frequencies (10-200 Hz) during awake thalamic surgery for movement disorders. The results demonstrate that the part of the body where microstimulation evoked sensation (projected field) and the descriptors of nonpainful sensations were usually uniform across the staircase. These results strongly support the existence of psychophysical elements of place and modality specificity in the Vc thalamus. The proportion of sites at which the sensation included more than one part of the body almost always stayed constant over current intervals (plateaus) of 10 microA. Similar plateaus were not found for sites with more than one descriptor, suggesting that elements of modality-specificity are smaller than and located within those for place-specificity. The intensity of sensations varied with the number of stimulation pulses for mechanical/tingle and cool sensations. The results provide strong evidence for psychophysically defined elements that are responsible for modality specificity of nonpainful sensations, place specificity, and intensity coding of somatic sensation in the human thalamus.

Morphological features of cat cervicothalamic tract terminations in different target regions

Using biotinylated dextran amine to label cervicothalamic tract terminations of cats, three types of terminal arrangements were recognized. The ventral posterior lateral nucleus contains the largest proportion of the cervicothalamic tract terminals (79%) and most (72%) of these are type I terminals (form compact clusters of 5-30 boutons). In contrast, type II (form less compact clusters of 3-10 boutons) and type III (widely spaced boutons along thin axons) terminals dominate in the medial nucleus of the posterior complex (78%) and in the ventral periphery of the ventrobasal complex (86%). In the magnocellular medial geniculate nucleus, type I terminals (38%) are found close to medially located clusters of Cat-301 immunoreactive neurons, whereas type II and type III terminals locate in the surrounding Cat-301-negative regions. These findings indicate a high degree of synaptic security in the transmission between cervicothalamic tract fibers and neurons in the ventral posterior lateral nucleus and highlight the role of this nucleus in faithful transmission of cervicothalamic tract input to the cerebral cortex. Also, the Cat-301-positive neurons in the magnocellular medial geniculate nucleus may faithfully transmit cervicothalamic tract signals. The domination of type II and type III terminals in the medial nucleus of the posterior complex and in the ventral periphery of the ventrobasal complex indicates a more divergent cervicothalamic input to these regions, in line with the large receptive fields and multimodal responses of neurons in the posterior complex.

Effects of a dorsal column lesion on temporal processing within the somatosensory system of primates

A dorsal column (DC) lesion has lasting effects on behavioral tasks that require temporal processing of tactile information (e.g., frequency and duration discrimination). The present experiments describe physiological correlates of these deficits in temporal discrimination. Compound action potentials evoked by electrocutaneous stimulation were recorded from the major white matter subdivisions of the spinal cord in anesthetized monkeys, and relationships between stimulation frequency and evoked potential (EP) amplitude were determined for the ascending pathways. At 10 pulses per second (Hz) EPs recorded in the lateral spinal columns were attenuated slightly (by 15% or less, relative to 1.5 Hz), whereas potentials recorded from the DCs were not attenuated. The attenuation increased with stimulation frequencies up to 50 Hz, reaching 80% for the anterolateral column and 38% for the dorsolateral column, but only 15% for the DC. Epidural EPs were recorded, before and after interruption of the contralateral DC, from awake animals with electrodes chronically implanted over primary somatosensory cortex (SI). Following the lesion. EP responses to 1.5-Hz stimulation were 46% of preoperative responses to the same stimulus. At 10 Hz, EP amplitudes were attenuated even more, to 27% of the preoperative amplitude at 1.5 Hz. Principal components analysis was employed to quantify alterations in EP conformation and stimulus frequency was varied from 1.5 to 10 Hz, before and after a DC lesion. Interruption of the DC resulted in a significant decrease in the information provided by the EP about changes in stimulus frequency. EPs were also recorded from different locations along the anterior-posterior dimension of the hindlimb region of SI in lightly anesthetized animals. Principal components analysis revealed that there was less information present in the EP about changes in stimulus frequency (1.5-10 Hz) at all recording locations in animals with a DC lesion, compared with the cortex of normal animals. The DC lesion significantly decreased the amplitude of cortical EPs evoked by repetitive stimulation. At 10 Hz the EP was nearly buried in noise, consistent with behavioral deficits in discrimination of the duration of 10 Hz stimulation following interruption of the DC. Also, significantly less information was present in the cortical EPs about changes in stimulus frequency in the absence of intact DCs, which is consistent with deficits in frequency discrimination. This reduction could be explained in part by a lesser capacity of spinal pathways in the lateral column to follow repetitive stimulation above 10 Hz. However, more rostral manifestations of a DC lesion, at either the thalamus or the cortex, are likely to contribute to the reduced capacity of animals with DC lesions to make temporal discriminations.

Distribution of four types of synapse on physiologically identified relay neurons in the ventral posterior thalamic nucleus of the cat

This study was aimed at providing quantitative data on the thalamic circuitry that underlies the central processing of somatosensory information. Four physiologically identified thalamocortical relay neurons in the ventral posterior lateral nucleus (VPL) of the cat thalamus were injected with horseradish peroxidase and subjected to quantitative electron microscopy after pre- or postembedding immunostaining for gamma-aminobutyric acid to reveal synaptic terminals of thalamic inhibitory neurons. The four cells all had rapidly adapting responses to light mechanical stimuli applied to their receptive fields, which were situated on hairy or glabrous skin or related to a joint. Their dendritic architecture was typical of cells previously described as type I relay cells in VPL, and they lacked dendritic appendages. Terminals ending in synapses on the injected cells were categorized as RL (ascending afferent), F (inhibitory), PSD (presynaptic dendrite), and RS (mainly corticothalamic) types and were quantified in reconstructions of serial thin sections. RL and F terminals formed the majority of the synapses on proximal dendrites (approximately 50% each). The number of synapses formed by RL terminals declined on intermediate dendrites, but those formed by F terminals remained relatively high, declining to moderate levels (20-30%) on distal dendrites. RS terminals formed moderate numbers of the synapses on intermediate dendrites and the majority (> 60%) of the synapses on distal dendrites. Synapses formed by PSDs were concentrated on intermediate dendrites and were few in number (approximately 6%). They formed synaptic triads with F terminals and rarely with RL terminals. On somata, only a few synapses were found, all made by F terminals. The total number of synapses per cell was calculated to be 5,584-8,797, with a density of 0.6-0.9 per micrometer of dendritic length. Of the total, RL terminals constituted approximately 15%, F terminals approximately 35%, PSD terminals approximately 5%, and RS terminals approximately 50%. These results provide the first quantitative assessment of the synaptic architecture of thalamic somatic sensory relay neurons and show the basic organizational pattern exhibited by representatives of the physiological type of relay neurons most commonly encountered in the VPL nucleus.

Quantitative analyses of intracellularly characterized and labeled thalamocortical projection neurons in the ventrobasal complex of primates

This study describes the architecture of neurons and individual dendritic arbors of thirteen intracellularly labeled thalamocortical projection neurons that respond to non-noxious stimuli from the primate (Macaca fascicularis or Macaca mulatta) ventrobasal complex (VB). The neurons compose a homogeneous morphological class with total dendritic lengths from 10,169 microns to 21,711 microns (mean 17,615 microns +/- 3,705). The labeled neurons were remarkably similar in most measured parameters including the number of dendrites (7.5 +/- 1.2), percentage of dichotomous branching (89.8% +/- 3.4), and contribution of terminal branches to total dendritic length (88.4% +/- 2.0). The individual dendrites ranged in total length from 443 microns to 7,657 microns with a mean of 2,346 microns (+/- 137, n = 98). There was a positive correlation between stem dendrite diameter and total dendrite length, making it possible to estimate the total size of an individual dendrite by measuring the stem dendrite diameter. There was only a small increase in mean path distance with increasing dendritic size at the whole neuron and individual dendritic levels, so that for individual dendrites the mean path distance of a dendrite consisting of only two segments was 199 microns, while the mean path distance for a dendrite with eight segments was only 45 microns longer. Analysis of dendrite diameter, segment order, and path distance shows that dendritic diameter is not reliable for determining the location of synaptic contacts viewed by electron microscopy onto dendritic trees. The small variation of measured parameters between these neurons presents a powerful tool for future developmental, plasticity and comparative studies of VB neurons.