Relations between spinocervical and post-synaptic dorsal column neurones in the cat

Mechanoreceptor signal convergence and transformation in the dorsal horn flexibly shape a diversity of outputs to the brain

The encoding of touch in the spinal cord dorsal horn (DH) and its influence on tactile representations in the brain are poorly understood. Using a range of mechanical stimuli applied to the skin, large-scale in vivo electrophysiological recordings, and genetic manipulations, here we show that neurons in the mouse spinal cord DH receive convergent inputs from both low- and high-threshold mechanoreceptor subtypes and exhibit one of six functionally distinct mechanical response profiles. Genetic disruption of DH feedforward or feedback inhibitory motifs, comprised of interneurons with distinct mechanical response profiles, revealed an extensively interconnected DH network that enables dynamic, flexible tuning of postsynaptic dorsal column (PSDC) output neurons and dictates how neurons in the primary somatosensory cortex respond to touch. Thus, mechanoreceptor subtype convergence and non-linear transformations at the earliest stage of the somatosensory hierarchy shape how touch of the skin is represented in the brain.

Comparative study of viscerosomatic input onto postsynaptic dorsal column and spinothalamic tract neurons in the primate

The purpose of the present investigation was to examine, in the primate, the role of the postsynaptic dorsal column (PSDC) system and that of the spinothalamic tract (STT) in viscerosensory processing by comparing the responses of neurons in these pathways to colorectal distension (CRD). Experiments were done on four anesthetized male monkeys (Macaca fascicularis). Extracellular recordings were made from a total of 100 neurons randomly located in the L(6)-S(1) segments of the spinal cord. Most of these neurons had cutaneous receptive fields in the perineal area, on the hind limbs or on the rump. Forty-eight percent were PSDC neurons activated antidromically from the upper cervical dorsal column or the nucleus gracilis, 17% were STT neurons activated antidromically from the thalamus, and 35% were unidentified. Twenty-one PSDC neurons, located mostly near the central canal, were excited by CRD and three were inhibited. Twenty-four PSDC neurons, mostly located in the nucleus proprius, did not respond to CRD. Of the 17 STT neurons, 7 neurons were excited by CRD, 4 neurons were inhibited, and 6 neurons did not respond to CRD. Of the unidentified neurons, 23 were excited by CRD, 7 were inhibited, and 5 did not respond. The average responses of STT and PSDC neurons excited by CRD were comparable in magnitude and duration. These results suggest that the major role of the PSDC pathway in viscerosensory processing may be due to a quantitative rather than a qualitative neuronal dominance over the STT.

Effects of upper cervical spinal cord stimulation on neurons in the lumbosacral enlargement of the cat: spinothalamic tract neurons

Extracellular microelectrode recordings were made from deep spinothalamic tract neurons in the lumbosacral spinal cord of cats anaesthetized with chloralose and paralyzed with gallamine triethiodide. The effects of upper cervical spinal cord stimulation were tested on 43 spinothalamic tract neurons, by stimulation of the ipsilateral dorsolateral funiculus at C3 and rostral C1 using five or six shocks at 333 Hz. The strength of cervical stimulation was adjusted so that the C3 shock was above threshold for antidromic activation of spinocervical tract neurons but the same strength of shock applied at C1 was below threshold for the same neurons. Four of the 43 spinothalamic cells (9%) were not influenced by upper cervical stimulation. The remaining 39 spinothalamic tract cells (91%) were all excited from the upper cervical cord. Twenty-seven of these (63%) were excited more strongly from C3 than from C1, 4 (9%) were excited more strongly from C1 than from C3, and the remaining eight cells (19%) showed no significant differences between their responses to stimulation at C1 and C3. There were no obvious differences between those spinothalamic tract neurons showing differential effects from C1 and C3 and those showing no such effects. The neuronal systems possibly responsible for the differential effects from C3 and C1 on spinothalamic tract neurons are discussed. We conclude that the most likely candidate system for the greater excitation from C3 compared with C1 is the subset of spinocervical tract neurons with axon collaterals in the lumbosacral enlargement and that the spinothalamic tract is a further ascending path, in addition to the postsynaptic dorsal column path, that receives excitatory input from spinocervical axon collaterals. The greater excitation from C1 compared with C3 is interpreted as due to excitation from C1 and a mixture of excitation and inhibition from C3. The responsible neuronal systems seem likely to be either the spinocervical neurons with axon collaterals operating on the spinothalamic tract via inhibitory interneurons, or cells in the lateral cervical nucleus with axons descending to the lumbosacral cord.

Synaptic organization of excitatory and inhibitory boutons associated with spinal neurons which project through the dorsal columns of the cat

The cell bodies and proximal dendrites of postsynaptic dorsal column neurons were examined for synaptic boutons which displayed immunoreactivity for the principal excitatory and inhibitory neurotransmitters, glutamate and GABA. The neurons were labelled by retrograde transport of horseradish peroxidase and GABA or glutamate-containing boutons were revealed by performing postembedding immunogold reactions on electron microscope sections. Five neurons were examined and all of them were postsynaptic to boutons which contained either GABA or glutamate. Quantitative analysis of two of the cells revealed that more than 90% of the synaptic profiles associated with them displayed immunogold reactions for these transmitters. Analysis of series of alternate sections, which were reacted for either GABA or glutamate, showed that there was no overlap in the populations of immunoreactive boutons. Furthermore, GABA and glutamate immunoreactions were associated with boutons which had different morphological characteristics. In addition, some large glutamate-enriched boutons were postsynaptic to small boutons which displayed immunogold reactions for GABA. This study demonstrates morphological bases for direct excitation, postsynaptic inhibition and presynaptic inhibition of postsynaptic dorsal column cells.

Lumbosacral spinal neurons in the cat that are candidates for being activated by collaterals from the spinocervical tract

Lumbosacral spinal neurons activated via the spinocervical tract were stained by intracellular injection of horseradish peroxidase in cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The neurons were activated orthodromically by single shock stimulation of the ipsilateral dorsolateral funiculus at the second to third cervical segment, but not from the rostral part of the first cervical segment. Twenty nine cells were recovered from the histological material and subsequently reconstructed from transverse sections. Sixteen cells (55%) had axons that projected ipsilaterally to the lateral funiculus and their somata were located in two regions of the spinal cord, one group in the dorsal horn (laminae IV-V) and the other in the intermediate gray matter (laminae VI-VII). The axons of 10 of these cells gave off collaterals, and in seven of them the collaterals ramified in the grey matter deep to the cell body. The axons of five cells (17%) projected medially towards the central canal, four crossing the mid line in the ventral white commissure and ascending in the contralateral ventral funiculus. Only one of these cells had an axon collateral that crossed into the contralateral dorsal horn. Of the remaining eight cells, three had no obvious long axons but had many local axon collaterals, the axons of three cells were not stained, one had an axon projecting towards the ipsilateral ventral funiculus and one was a motoneuron and its axon projected into a ventral root. A feature of the dendritic trees of many cells was their wide spread in the mediolateral and/or the dorsoventral directions, although no dendrites reached dorsally into lamina II. Twenty-two cells (76%) were excited by moving hairs and by noxious pinch, three (10%) by hair movement alone, two (7%) by noxious pinch and pressure, and for two cells (7%) no receptive field could be found. It is concluded that not only postsynaptic dorsal column neurons receive input from the spinocervical tract but also other cells in the dorsal and ventral horns and the intermediate gray matter. Possible identities for these cells are discussed.

Calcium-binding proteins, parvalbumin- and calbindin-D 28k-immunoreactive neurons in the rat spinal cord and dorsal root ganglia: a light and electron microscopic study

The distribution of two calcium-binding proteins, parvalbumin (PV) and calbindin-D 28K (CaBP), was studied by the peroxidase-anti-peroxidase immunohistochemical method at the light and electron microscopic level in the rat spinal cord and dorsal root ganglia. The possible coexistence of these two proteins was also investigated. PV-positive neurons were revealed in all layers of the spinal cord, except lamina I, which was devoid of labelling. Most of the PV-positive cells were found in the inner layer of lamina II, lamina III, internal basilar nucleus, central gray region, and at the dorsomedial and ventromedial aspects of the lateral motor column in the ventral horn. Neuronal processes intensely stained for PV sharply delineated inner lamina II. With the electron microscope most of them appeared to be dendrites, but vesicle containing profiles were also found in a smaller number. CaBP-positive neurons appeared to be dispersed all over the spinal gray matter. The great majority of them were found in laminae I, II, IV; the central gray region; the intermediolateral nucleus; and in the ventral horn just medial to the lateral motor column. Laminae I and II were densely packed with CaBP-positive punctate profiles that proved to be dendrites and axons in the electron microscope. A portion of labelled neurons in lamina IV and on the ventromedial aspect of the lateral motor column in the ventral horn disclosed both PV- and CaBP-immunoreactivity. All of the funiculi of the spinal white matter contained a large number of fibres immunopositive for both PV and CaBP. The highest density of CaBP-positive fibres was found in the dorsolateral funiculus, which was also densely packed with PV-positive fibres. PV-positive fibres were even more numerous in the dorsal part of the dorsal funiculus. The territory of the gracile funiculus in the brachial cord and that of the pyramidal tract in its whole extent were devoid of labelled fibres. In the thoracic cord, the dorsal nucleus of Clarke received a large number of PV-positive fibres. Dorsal root ganglia displayed both PV- and CaBP-immunopositivity. The cell diameter distribution histogram of PV-positive neurons disclosed two peaks--one at 35 microns and the other at 50 microns. CaBP-positive cells in the dorsal root ganglia corresponded to subgroups of small and large neurons with mean diameters of 25 microns and 45 microns, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

The morphology of cat spinal neurons projecting to both the lateral cervical nucleus and the dorsal column nuclei

Morphological features of physiologically characterized doubly projecting spinocervical tract-dorsal column postsynaptic (SCT-DCPS) neurons were examined following intracellular injections of horseradish peroxidase. Their cell bodies and majority of dendrites were located in laminae III-IV. The SCT-DCPS neurons have rostrocaudally elongated dendritic trees with spiny fine caliber dendrites, and issue local axon collaterals that are studded with varicosities. The results show that the doubly projecting SCT-DCPS neurons do not represent a morphologically distinct subset of the spinocervical tract (SCT) or dorsal column postsynaptic (DCPS) cells.

Postsynaptic dorsal column neurons in the cat: a study with retrograde transport of horseradish peroxidase

The numbers, laminar position, perikaryal and dendritic morphology, and axonal trajectories of postsynaptic cells ascending the dorsal column have been studied after implantation of HRP pellets in either the dorsal columns or dorsal column nuclei after destruction of the dorsolateral fascicle on one side. Observations made throughout the spinal cord gave estimated figures of 800-1000 and 1700-2000 cells in lumbosarcal and brachial enlargements respectively on the side of the implant. The commonest type (C), centred on lamina IV, had dendritic trees greatly extended rostrocaudally and restricted mediolaterally in the lateral dorsal horn, the extension and restriction diminishing for more medial cells. Type B cells differed dramatically, with large straight dendrites in the transverse plane and large perikarya in medial lamina V. Type A cells, distinguished by both rostrocaudal and mediolateral restriction in dendritic trees, were only found medially in laminae III and IV. Outside the enlargements, in high lumbar and thoracic cord, many fewer cells were found, corresponding to Type C but with dendrites much elongated rostrocaudally and little mediolateral variation. Many small fusiform cells were found in medial lamina VI in the upper cervical cord, distinct from any of the above. A few cells were found in the cord enlargements in lamina VII of the contralateral ventral horn, with axons crossing through the ventral commissure. The axons of all cell types were tortuous, and some entered the dorsolateral fascicle before crossing into the dorsal column: collaterals were often seen but could not be followed far. A complementary study of cells with axons ascending in the dorsolateral fascicle is reported in the following paper.

Descending influences on the cutaneous receptive fields of postsynaptic dorsal column neurones in the cat

1. The influence of activity in descending systems on the cutaneous receptive field properties of postsynaptic dorsal column (PSDC) neurones has been investigated in chloralose-anaesthetized cats. The main aim of the study was to determine whether the receptive field boundaries of PSDC neurones are under the control of systems descending from the brain. 2. Single-unit recordings were made from the ascending axons of PSDC units in the dorsal columns. Receptive fields were analysed using light tactile and noxious mechanical and thermal stimuli, both before and during a reversible block of spinal conduction produced by cooling the cord rostral of the recording site. 3. The light tactile excitatory fields of PSDC neurones were largely unaffected by the cold-block procedure. 4. In contrast, both the sensitivity of PSDC neurones to noxious stimuli and the area of skin from which they could be effectively excited by such stimuli were found to be profoundly modified by interruption of descending activity. Two-thirds of the units excited by noxious pinch responded more vigorously in the cold-blocked state and one-half from an expanded area of skin. Responses to noxious radiant heat were similarly modified. 5. Inhibition evoked in PSDC neurones, whether by light tactile or noxious stimuli, involved predominantly segmental mechanisms since it remained effective in the cold-blocked state. 6. It is concluded that neurones of the PSDC system are amongst those dorsal horn neurones with receptive field geometries which may be modified by activity in descending systems.

Morphological evidence for newly discovered double projection spinal neurons

A novel population of spinal neurons is shown to terminate in two nuclei: the lateral cervical nucleus and the dorsal column nuclei. Nuclear yellow and Fast blue injected respectively into these nuclei are retrogradely transported to common neurons in the lumbosacral dorsal horn. The bifurcation of these neurons' axons appears to occur at the cervico-thoracic junction. These results indicate that some dorsal horn neurons transmit sensory information to two distinct nuclei. The two projections and their branching points may play a special role in neuronal communication.

Cutaneous excitatory and inhibitory input to neurones of the postsynaptic dorsal column system in the cat

1. In chloralose-anaesthetized cats single-unit microelectrode recordings were made from axons in the dorsal columns, at the lumbar level, identified as belonging to the postsynaptic dorsal column (PSDC) system. 2. Excitatory and inhibitory receptive field arrangements of a sample of seventy-five PSDC neurones were examined in detail using natural cutaneous stimuli. 3. The sample was characterized by a high degree of convergent input: 80% of units were activated by both light tactile and noxious mechanical stimuli and more than half of those examined were excited by noxious radiant heat. In addition, three-quarters of the units had inhibitory receptive fields on the ipsilateral limb. 4. Twenty-three units (27%) were influenced by input from areas of both hairy and glabrous skin covering the foot and distal limb. Neurones in this group had complex receptive fields, many of which occupied several discontinuous areas of skin. Background and evoked activity of these units could frequently be inhibited by light tactile and/or noxious stimuli. Their inhibitory receptive fields occupied small areas of skin overlapping or adjacent to excitatory fields. 5. Fifty-two units (73%) had receptive fields restricted to areas of hairy skin on the thigh and upper hindlimb. Half the units in this group had coextensive low- and high-threshold excitatory areas but about one-third had a concentric receptive field organization; a high-threshold excitatory component extending beyond, or around, a central low-threshold area. The discharge of these units could be inhibited only by light tactile stimuli. Their inhibitory receptive fields covered extensive areas of skin, sometimes completely surrounding the excitatory field. 6. The complex receptive field arrangements observed for neurones of the postsynaptic dorsal column system are discussed in relation to previous observations on dorsal horn neurones of other ascending tracts.

Fine structure of synapses associated with characterized postsynaptic dorsal column neurons in the cat

Fourteen dorsal horn neurons with axons projecting through the dorsal columns were identified either by electrophysiological methods (and subsequently injected with horseradish peroxidase) or by retrograde labelling with horseradish peroxidase in cats. All neurons were contacted by small (less than 2 micron) boutons containing spherical or elongated agranular vesicles. One neuron with its soma located in lamina III received additional contacts from central elements of glomerular complexes. Neurons with somata located more ventrally (deep lamina IV and V) were also postsynaptic to large (greater than 2 microns) electron lucent profiles which formed multiple synapses with the labelled cells. Some boutons presynaptic to postsynaptic dorsal column neurons were themselves postsynaptic to profiles containing pleiomorphic agranular vesicles at axoaxonic synapses. They also occasionally participated in triadic complexes. It is concluded that the synaptic arrangements formed by boutons in association with postsynaptic dorsal column neurons differ significantly from those associated with spinocervical neurons. Such differences might provide the anatomical substrate for the observed receptive field characteristics of these neurons.