Somatosensory Cortex: Descending Influences on Ascending Systems. In: Iggo A, editor. Somatosensory System. Berlin: Springer Berlin Heidelberg; 1973. pp. 701-18. [DOI: 10.1007/978-3-642-65438-1_18]

Primary afferent plasticity following deafferentation of the trigeminal brainstem nuclei in the adult rat

Alpha-tyrosinated tubulin is a cytoskeletal protein that is involved in axonal growth and is considered a marker of neuronal plasticity in adult mammals. In adult rats, unilateral ablation of the left facial sensorimotor cortical areas induces degeneration of corticotrigeminal projections and marked denervation of the contralateral sensory trigeminal nuclei. Western blotting and real-time-PCR of homogenates of the contralateral trigeminal ganglion (TG) revealed consistent overexpression of growth proteins 15 days after left decortication in comparison with the ipsilateral side. Immunohistochemical analyses indicated marked overexpression of alpha-tyrosinated tubulin in the cells of the ganglion on the right side. Cytoskeletal changes were primarily observed in the small ganglionic neurons. Application of HRP-CT, WGA-HRP, and HRP to infraorbital nerves on both sides 15 days after left decortication showed a significant degree of terminal sprouting and neosynaptogenesis from right primary afferents at the level of the right caudalis and interpolaris trigeminal subnuclei. These observations suggest that the adaptive response of TG neurons to central deafferentation, leading to overcrowding and rearrangement of the trigeminal primary afferent terminals on V spinal subnuclei neurons, could represent the anatomical basis for distortion of facial modalities, perceived as allodynia and hyperalgesia, despite nerve integrity.

Corticofugal action on somatosensory response properties of rat nucleus gracilis cells

Single unit recordings were performed in the nucleus gracilis (Gr) of anesthetized rats to study the influences of the sensorimotor corticofugal projections on sensory responses of those cells. The effects of electrical stimulation of contralateral primary sensory cortex were studied in two conditions: when the receptive fields of the stimulated cortical area and the gracilis cells overlapped (matched) or when they were completely different (unmatched). Cortical stimulation at low intensities (<50 microA) evoked spike firing only in gracilis neurons with matched receptive fields. When the receptive fields were unmatched, the intensity of the stimulation had to be increased above 50 microA to elicit spike firing. To study the corticofugal actions on the responses of Gr neurons, the onset of peripheral stimulation was likened to a single cortical shock in the sensorimotor cortex. When receptive fields matched, cortical stimulation facilitated the cellular responses to the natural sensory stimulation of their RF in most of the Gr neurons (86%). In the unmatched receptive fields, cortical stimulation could either inhibit (66.7%), facilitate (20.8%) or did not modify (12.5%) the sensory response at all. Trains of cortical shocks during sensory stimulation demonstrated that the facilitatory and inhibitory effects on Gr neurons outlasted the period of stimulation by 30-60 s. Results indicate that the sensorimotor cortex exercises a very precise control of sensory transmission throughout the Gr nucleus and suggest that the corticofugal projection may play an important role in the plasticity of the sensorimotor system.

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.

Sensorimotor cortical projections to the primate cuneate nucleus

The organization of the corticocuneate pathway was investigated in monkeys by using the anterograde and retrograde axonal transport of either horseradish peroxidase (HRP) or wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injection of either tracer into the precentral cortex (centered on area 4) results in heavy anterograde labeling in the tegmental region, which lies immediately ventrolateral to the cuneate nucleus, particularly at levels caudal to the obex. On the other hand, injections of the same tracers involving areas 3b, 1, and 2 cause anterograde labeling mainly within the core (pars rotunda of Ferraro and Barrera, '35, Arch. Neurol. Psychol. 33:262-75) of the cuneate nucleus. Anterograde labeling is also evident in the rostral parts of the cuneate nucleus, especially after injections involving areas 1 and 2. Injections restricted largely to area 3b cause anterograde labeling preferentially in the core of the cuneate nucleus. After injection of HRP or WGA-HRP into the dorsal medulla, retrogradely labeled neurons are present both in the pre- and postcentral gyrus, but their location depends upon the sites and extent of the injection site. When the tracer diffuses into the underlying tegmental area, many retrogradely labeled neurons appear in the precentral motor cortex, principally in area 4 although some of them also occur in area 6. With smaller injections, largely restricted within the cuneate nucleus, most labeled neurons are present in the postcentral gyrus, with the largest population in areas 1 and 2; a smaller number of small neurons in area 3b are best demonstrated with WGA-HRP; and area 3a contains the smallest complement of retrogradely labeled neurons. The data from these studies suggest a segregation of pre- and postcentral afferents in the ventral tegmental region and the cuneate nucleus, respectively. These findings pertaining to the corticocuneate projection in the monkey are discussed in relation to the parallelism between monkeys and cats possible physiological implications of the anatomical organization described, and conflicting evidence in the neurophysiological observations obtained, by earlier investigators, by antidromic and orthodromic activation of this pathway.

Operant conditioning of somatosensory evoked potential (SEP) amplitude in rats. I. Specific changes in SEP amplitude and a naloxone-reversible somatotopically specific change in facial nociception

The aim of this experiment was to investigate possible endogenous opioid modulation of innocuous somatosensory activity. Somatosensory activity was measured by recording cortical somatosensory evoked potential (SEP) and reflex movement amplitude evoked by innocuous electrical stimulation of the spinal trigeminal tract in awake rats. Putative endogenous opioid activity was blocked using the opiate antagonist naloxone (1 mg/kg). The amplitude of midlatency SEP components (14-50 ms latency) increased following administration of naloxone and repeated stimulus presentations. The amplitude of these components decreased following administration of the opiate agonist morphine (3 mg/kg). An early cortical component (10 ms latency) habituated following the administration of saline but did not habituate following naloxone. Naloxone also enhanced habituation of the late SEP components (60-120 ms latency) and reflex movement evoked at higher stimulus intensities. Morphine decreased the amplitude of the early cortical component but had no consistent effect on the amplitude of the late SEP components.

Dual dorsal columns: a review

Recent evidence indicates that Wall (1970) may have been premature in concluding that dorsal column lesions produce no discernable sensory defects. Much of the negative evidence Wall presented to support this view is inconclusive. In addition several studies have reported significant sensory deficits in animals with severed dorsal columns. On the other hand, the literature strongly supports Wall's view that dorsal column lesions cause motor disturbances. A review of the anatomical and electrophysiological literature reveals growing evidence for the dissociation of two major subsystems relaying in the dorsal column nuclei. The possible functions of these two systems are discussed.