Corticospinal tract collaterals to the dorsal column nuclei of cats. An anatomical single and double retrograde tracer study

Modulation of tactile feedback for the execution of dexterous movement

[Figure: see text].

Converging integration between ascending proprioceptive inputs and the corticospinal tract motor circuit underlying skilled movement control

Converging interactions between ascending proprioceptive afferents and descending corticospinal tract projections are critical in the modulation and coordination of skilled motor behaviors. Fundamental to these processes are the functional inputs and the mechanisms of integration in the brain and spinal cord between proprioceptive and corticospinal tract information. In this review, we first highlight key connections between corticospinal tract motor circuit and spinal interneurons that receive proprioceptive inputs. We will also address corticospinal tract access to the presynaptic inhibitory system in the spinal cord and its role in modulating proprioceptive stimuli. Lastly, we will focus on the corticospinal neuron influences on the dorsal column nuclei complex, an integration hub for processing ascending somatosensory information.

Corticofugal projections induce long-lasting effects on somatosensory responses in the trigeminal complex of the rat

The sensory information flow at subcortical relay stations is controlled by the action of topographic connections from the neocortex. To determinate the functional properties of the somatosensory corticofugal projections to the principal (Pr5) and caudal spinal (Sp5C) trigeminal nuclei, we performed unitary recordings in anesthetized rats. To examine the effect of these cortical projections we used tactile stimulation of the whisker and electrical stimulation of somatosensory cortices. Corticofugal anatomical projections to Pr5 and Sp5C nuclei were detected by using retrograde fluorescent tracers. Neurons projecting exclusively to Pr5 were located in the cingulate cortex while neurons projecting to both Sp5C and Pr5 nuclei were located in the somatosensory and insular cortices (>75% of neurons). Physiological results indicated that primary somatosensory cortex produced a short-lasting facilitating or inhibiting effects (<5 min) of tactile responses in Pr5 nucleus through activation of NMDA glutamatergic or GABA_A receptors since effects were blocked by iontophoretically application of APV and bicuculline, respectively. In contrast, stimulation of secondary somatosensory cortex did not affect most of the Pr5 neurons; however both cortices inhibited the nociceptive responses in the Sp5C nucleus through activation of glycinergic or GABA_A receptors because effects were blocked by iontophoretically application of strychnine and bicuculline, respectively. These and anatomical results demonstrated that the somatosensory cortices projects to Pr5 nucleus to modulate tactile responses by excitatory and inhibitory actions, while projections to the Sp5C nucleus control nociceptive sensory transmission by only inhibitory effects. Thus, somatosensory cortices may modulate innocuous and noxious inputs simultaneously, contributing to the perception of specifically tactile or painful sensations.

Retrograde labeling, transduction, and genetic targeting allow cellular analysis of corticospinal motor neurons: implications in health and disease

Corticospinal motor neurons (CSMN) have a unique ability to receive, integrate, translate, and transmit the cerebral cortex's input toward spinal cord targets and therefore act as a “spokesperson” for the initiation and modulation of voluntary movements that require cortical input. CSMN degeneration has an immense impact on motor neuron circuitry and is one of the underlying causes of numerous neurodegenerative diseases, such as primary lateral sclerosis (PLS), hereditary spastic paraplegia (HSP), and amyotrophic lateral sclerosis (ALS). In addition, CSMN death results in long-term paralysis in spinal cord injury patients. Detailed cellular analyses are crucial to gain a better understanding of the pathologies underlying CSMN degeneration. However, visualizing and identifying these vulnerable neuron populations in the complex and heterogeneous environment of the cerebral cortex have proved challenging. Here, we will review recent developments and current applications of novel strategies that reveal the cellular and molecular basis of CSMN health and vulnerability. Such studies hold promise for building long-term effective treatment solutions in the near future.

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

New corticocuneate cellular mechanisms underlying the modulation of cutaneous ascending transmission in anesthetized cats

The ascending cutaneous transmission through the middle cuneate nucleus is subject to cortico-feedback modulation. This work studied the intracuneate cellular mechanisms underlying the corticocuneate influence. Single unit extracellular records combined with iontophoresis showed that the corticocuneate input activates cuneo-lemniscal (CL) and noncuneo-lemniscal (nCL) cells via N-methyl-D-aspartate (NMDA) and non-NMDA receptors as shown by the decrease of the cortical-induced activation on ejection of CNQX and APV, either alone or in combination. These results were confirmed by in vivo intracellular recordings. Two subgroups of nCL cells were distinguished according to their sensitivity to iontophoretic ejection of glycine and its antagonist, strychnine. Finally, the corticalevoked activation of CL cells was decreased by GABA and increased by glycine acting at a strychnine-sensitive site, indicating that glycine indirectly affects the cuneo-lemniscal transmission. A model is proposed whereby the cortex influences CL cells through three different mechanisms, producing 1) activation via non-NMDA and NMDA receptors, 2) inhibition through GABAergic nCLs, and 3) disinhibition via serial glycinergic-GABAergic nCL cells. These corticocuneate feedback effects serve to potentiate the activity of CL cells topographically aligned through direct activation and disinhibition, while inhibiting, via GABAergic cells, other CL neurons not topographically aligned.

Tonic and bursting activity in the cuneate nucleus of the chloralose-anesthetized cat

Whole-cell recordings were obtained from cuneate neurons in anesthetized, paralysed cats. Stimulation of the contralateral medial lemniscus permitted us to separate projection cells from presumed interneurons. Pericruciate motor cortex electrical stimulation inhibited postsynaptically all the projection cells (n=57) and excited all the presumed interneurons (n=29). The cuneothalamic cells showed an oscillatory and a tonic mode of activity. Membrane depolarization and primary afferent stimulation converted the oscillatory to the tonic mode. Hyperpolarizing current steps applied to projection neurons induced a depolarizing sag and bursts of conventional spikes in current-clamp records. This indicates the probable existence of low-threshold and hyperpolarization-activated inward currents. Also, the hyperpolarization induced on projection cells by motor cortex stimulation deinactivated a low-threshold conductance that led to bursting activity. The presumed cuneate interneurons had larger and more proximally located peripheral receptive fields than the cuneothalamic cells. Finally, experiments specifically designed to test whether motor cortex-induced presynaptic inhibition could be postsynaptically detected gave negative results. These results demonstrate, for the first time, that the cuneothalamic cells possess both bursting and tonic firing modes, and that membrane depolarization, whether produced by injection of positive current or by primary afferent stimulation, converts the oscillatory into the tonic mode.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Multiple inputs of GABA-immunoreactive neurons in the cuneate nucleus of the rat

Using anterograde transport of WGA-HRP and the experimental degeneration method for identification of corticocuneate (CCT) and primary afferent (PAT) terminals in conjunction with gamma-amino butyric acid (GABA) and glutamate immunocytochemistry, this study has demonstrated that the GABA-immunoreactive (GABA-IR) neurons in the rat cuneate nucleus were post-synaptic to PATs (some of them being glutamate-IR), GABA-IR and GABA-negative terminals. The HRP-labelled CCTs did not make any synaptic contacts with GABA-IR neurons but with some GABA-negative dendrites. PATs labelled by HRP or showing degenerating features made direct synaptic contacts with the dendrites of GABA-IR neurons. Beside the above GABA-IR boutons also showed axosomatic and axodendritic synapses with the GABA-IR neurons. In 'triple labeling' method for GABA, PAT and glutamate, it was found that the PATs which were usually glutamate-positive were presynaptic to the dendrites of GABA-IR neurons. Furthermore, some glutamate-IR terminals which were of non-PAT's origin also synapsed with the dendrites and somata of GABA-IR neurons. It is concluded from this study that the major inputs of GABA-IR neurons were from glutamate immunopositive PATs and glutamate terminals of non-PATs origin; other GABA-IR terminals either intrinsic or extrinsic also contributed to the afferent sources of GABA-IR neurons. The CCTs contributed very little, if any, to this input. It is suggested that the PATs and glutamate-IR terminals on GABA-IR neurons may be involved in lateral inhibition for increase of spatial precision. The synaptic contacts between GABA-IR boutons and dendrites or somata of GABA-IR neurons may provide a possible means for disinhibition.

Coupled slow and delta oscillations between cuneothalamic and thalamocortical neurons in the chloralose anesthetized cat

Simultaneous recordings were obtained from cuneothalamic (extracellular) and thalamocortical (intracellular) cells in chloralose anesthetized cats. It was found that cuneothalamic neurons present slow rhythmicity (0.1-1 Hz) tightly coupled to slow oscillations of thalamocortical neurons. This coupling was not due to a direct synaptic linkage but rather produced by other (s) structure (s) probably the cortex. Furthermore, the cuneothalamic neurons also showed delta rhythms (1-4 Hz) coherently oscillating with the delta rhythms of thalamocortical cells which suggests that these rhythms are more widespread than previously thought, and may be a general phenomenon characterizing quiet sleep in multiple structures.

Pyramidal tract and corticospinal neurons with branching axons to the dorsal column nuclei of the cat

Extracellular single activity was recorded from pericruciate neurons in anaesthetized, paralysed, artificially ventilated cats. A total of 309 neurons were identified antidromically by stimulation of the dorsal column nuclei (229 from the nuneate nucleus and 80 from the gracile nucleus). The study addressed the question whether pericruciate-dorsal column nuclei neurons (corticonuclear cells) sent collaterals to the ipsilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. Also, the ipsilateral pyramidal tract was stimulated at mid-olivary level, as was the crossed corticospinal tract at C2, Th2 and L2 levels in order to know whether the corticonuclear cells sent their axons to the spinal cord and if so to which level. It was found that more than 95% of the corticonuclear fibres coursed through the pyramidal tract. A significant (28.4%; 88/309) proportion of the the corticonuclear neurons sent collaterals to the red nucleus and/or to the nucleus reticularis gigantocellularis. About 68% (209/309) of the corticonuclear cells did not send their axons to the spinal cord and the remainder were corticospinal neurons. Most of the corticospinal fibres terminated at the cervical level (72/100) and the remaining ended at thoracic (18/100) and lumbar (10/100) segments of the cord. While 63.4% (123/194) of the corticonuclear fibres coursing through the pyramidal tract and ending at supraspinal levels were slow conducting, the great majority of the corticospinal neurons were fast conducting (91/100). The non-corticospinal neurons were significantly slower conducting than the corticospinal cells. The corticogracile neurons were slower conducting than the corticocuneate cells. Of the 88 corticonuclear neurons that sent at least a branch to the sites tested, 50% branched into the red nucleus, 35.2% into the nucleus reticularis gigantocellularis and 14.7% into both nuclei, without significant difference between non-corticospinal and corticospinal cells. Most of the main axons of the corticonuclear cells ended at bulbar and cervical levels (281/309 or 90.9%). The data indicate that pericruciate-dorsal column nuclei neurons form a particular substrate within pyramidal tract cells. They can serve precise functions in motor coordination associated with the selection of their own sensory input. The results are discussed from this point of view.

Timing of corticofugal actions on the gracile and cuneate nuclei of the cat

A comparison is presented of the latencies of corticofugal effects from the contralateral somatosensory cortex (SI) onto the cat's dorsal column nuclei (d.c.n.) under pentobarbitone anaesthesia. The latencies for transmission in the ascending pathway from d.c.n. to SI after stimulation within the gracile and cuneate nuclei were found to be 3.3 ms for the former and 2.8 ms for the latter. The time courses of inhibition of a medial lemniscal mass response following cortical conditioning and evoked by stimulation of peripheral nerves were measured. All latencies were corrected to exclude the different times taken for stimuli to reach the nuclei from the two limbs. The optimal condition-test interval was 12 ms with a duration of 14.3 ms for the superficial radial nerve (s.r.n.) and 45 ms and 30 ms respectively for the medial plantar nerve (m.p.n.). In each case cortical conditioning inhibited the wave by about 50%. The effect of cortical conditioning upon spontaneously firing d.c.n. single units was investigated. For cuneate cells the mean latency was 6.8 ms and the mean duration 36.8 ms. For gracile cells the latency of onset of inhibition was 17.2 ms and its duration 129 ms. In 75% of cells mixed effects were seen with facilitation preceding inhibition. The latencies of 'corticofugal reflex' action on the gracile and cuneate nuclei after stimulation of the s.r.n. and m.p.n. were determined. The gracile response had a latency approximately 4 times that for the cuneate response. The temporal asymmetry of these corticofugal effects suggests that the pathway is not purely a simple feed-back loop, but may be concerned in other physiological contexts, some of which are discussed.