Showing papers on "Cuneate nucleus published in 1973"

Projections of cervicothoracic dorsal roots to the cuneate nucleus of the rat, with observations on cellular "bricks".

Abstract: The distribution of cervicothoracic (C1–T2) dorsal roots to the cuneate and accessory cuneate nuclei was studied in the rat with the Fink- Heimer I technique following single extradural rhizotomies. Cytoarchitectural analysis of the cuneate nucleus revealed an anatomically distinct caudal and rostral region. The caudal region was characterized by discrete aggregates of cells arranged as “slabs” or “bricks” which continue vertically almost to the ventral limit of the cuneate nucleus. In contrast, the cells of the rostral region were organized in a non-focal manner. The projection of the cervicothoracic dorsal roots to the cuneate nucleus reflected the cytoarchitectural pattern observed; non-focal terminal fields of degeneration in the rostral region and discrete “terminal field bands” of degeneration isolated from one another by degeneration-free zones in the caudal region. However, the projections of dorsal roots C5 and T2 to the caudal region is non-focal. In both regions of the cuneate nucleus, the distribution of dorsal root degeneration was topographically organized with cranial roots terminating ventrolaterally and more caudal roots, dorsomedially. The amount of intersegmental overlap of dorsal root terminal fields was greater in the rostral than in the caudal region. Individual dorsal roots projected differentially to the two regions of the cuneate nucleus. Roots C3 and C4 distributed primarily to the rostral region whereas C5 to T1 distributed to both regions. T2 root projected primarily to the caudal region. Dorsal roots C1 and C2 did not terminate in either region of the cuneate nucleus. All roots studied projected heavily and topographically to the accessory cuneate nucleus. Extensive overlap of the very dense terminal fields characterized the dorsal root projections to the accessory cuneate nucleus. On the basis of cytoarchitectonics and dorsal root projections, a dual organization of the cuneate nucleus was revealed. This organization reflected previous anatomical and electrophysiological studies in the rat and paralleled the organization described in the dorsal column nuclei of the cat. The significance of the dual organization with respect to dorsal column function was discussed, as was the finding of the lack of C1 and C2 dorsal root projections to the cuneate nucleus.

Organization of group I activated cells in the main and external cuneate nuclei of the cat: identification of muscle receptors.

Abstract: Extracellular recording was made from 77 primary afferent fibres, 106 cells in the external cuneate nucleus, and 60 cells in the main cuneate nucleus, all activated by slowly adapting muscle stretch receptors. The nature of the muscle receptors responsible for the activation was determined by various types of receptor stimulation.

On the pharmacology of ascending, descending and recurrent postsynatic inhibition of the cuneo‐thalamic relay cells in the cat

Abstract: 1. In cats decerebrated or anaesthetized with pentobarbitone, cells of the middle third of the cuneate nucleus that were excited by tactile stimulation of the ipsilateral forelimb (responding to displacement of hairs, skin or joints) and inhibited by electrical stimulation of the contralateral pyramid, were invariably inhibited by electrical stimulation of the ipsilateral forepaw and the contralateral forelimb nerves.2. In 50% of the cats, the cells were more fully identified by placing electrodes stereotaxically in the contralateral medial lemniscus. Recurrent inhibition was always a concomitant of the antidromic action potential.3. The intensity and the duration of inhibition evoked by all of these pathways was totally resistant to iontophoretic and intravenous strychnine in doses at least 5 times that required to block completely the response of the same cells to iontophoretic glycine and was extremely sensitive to either iontophoretic bicuculline or picrotoxin.4. Although the inhibition was invariably sensitive to intravenous picrotoxin, no significant change occurred in the duration or intensity of the inhibition when bicuculline was administered intravenously (5 or 6 times) as repeated doses of 0.2 mg/kg.5. Postsynaptic inhibition in the cuneate may be mediated by gamma-aminobutyric acid released from the nerve terminals of a common pool of interneurones shared by ascending, descending and recurrent pathways. Since the receptors involved in this pathway are resistant to intravenous bicuculline, they may well be distinct from those responsible for changes in the primary afferent terminal excitability, usually believed to be associated with presynaptic inhibition.

Organization of group I activated cells in the main and external cuneate nuclei of the cat: convergence patterns demonstrated by natural stimulation.

Abstract: Patterns of inhibitory and excitatory convergence were investigated for 70 group I activated cells in the main cuneate nucleus (MCN) (34 identified cuneo-thalamic relay cells) and 102 group I activated cells in the external cuneate nucleus (ECN), using natural stimulation of stretch receptors in various wrist muscles. MCN cells as well as ECN cells showed a high degree of spatial specificity. Most of the cells were activated by receptors in one wrist muscle only. A few cells were activated from a couple of adjacent synergists. Inhibitory effects were rarely produced in group I activated MCN cells, but were common among the ECN cells (40%). Many different combinations of excitation and inhibition from the various wrist muscles were found for ECN cells. There was no particular tendency of reciprocal effects from antagonistic muscles.

A study of the functional relationship between the dorsal column and spinocervical system of the cat

Abstract: Electrophysiological experiments have shown that stimulation of the ipsilateral DLF, caudal to a transection of the dorsal columns (at C4), inhibits cells in the dorsal column nuclei. The presence of such inhibition was shown both by focal potential, and single unit, recording. It was found that the amplitude of a volley, conducted down the dorsal columns in response to a shock applied to the cuneate nucleus and recorded from the killed ends of dorsal column fibres at the rostral and of a transection of the dorsal columns, was increased by a preceding tetanus applied to the ipsilateral DLF. This evidence of depolarisation of dorsal column fibre terminals suggests that presynaptic inhibition contributes to the inhibition produced by DLF stimulation. A contribution from postsynaptic inhibitory mechanisms was suggested by the observation, made on several occasions, that antidromic activation of a unit from the contralateral mid-brain could be prevented by a preceding shock applied to the ipsilateral DLF. In some oases inhibition of units was produced by squeezing one or more of the cat's paws; some of these units projected into the contralateral mid-brain. In addition to an inhibitory action from the ipsilateral DLF on the cells of the dorsal column nuclei, electrophysiological experiments have also demonstrated the presence of an excitatory action. The majority of the cells transynaptically activated stimulation of the DLF were found in the region andminus;3 to +2andnbsp;mm (the obex being aero); however this may have been because the majority of electrode penetrations were also made in this region. Many of the unite so excited were shown to project into the contralateral mid-brain, thereby demonstrating the existence of a genuine excitatory effect on the dorsal column nuclei. Approximately one third of the units excited by stimulation of the DLF were also excited by peripheral mechanical stimulation. In the majority of cases the effective stimulus was cutaneous stimulation. A number of units, deep in the cuneate nucleus, ware found to be excited by "noxious" mechanical stimulation; no such units wore found in the gracile nucleus. Only one of those tested was found to project into the contralateral mid-brain, suggesting that these units may be inhibitory interneurones. The excitation and inhibition, produced by stimulation of the ipsilateral DLF, were abolished by transection of the DLF. They were net affected by transection of the brain stem at a level immediately rostral to the dorsal column nuclei. None of the cells transynaptically activated by stimulation of the DLF were also activated by stimulation of the anterior lobe of the cerebellum; this suggests that dorsal column nuclei cells were not excited by collaterals of direct spinocerebellar tract fibres. The presence of direct connections between the dorsal column nuclei and the DLF was demonstrated electrophysiologically by the recording, in the dorsal column nuclei, of short constant latency responses with properties indicative of external recordings from fibres. Whether such fibres were afferent to, or efferent from, the cells of the dorsal column nuclei could not be established from such recordings. However, evidence for the existence of both such categories was found. Some cells in the dorsal column nuclei were antidromically activated by stimulation of the ipsilateral DLF. Some of these cells were orthodromically activated by afferents coursing in the DLF. The presence of such orthodromic activation of those cells led to the speculation that the cells with descending projections may synapse, directly or indirectly, with the cells of origin of the afferent pathway, and thereby form a feed-back circuit within this system. Anatomical experiments have demonstrated the existence of direct afferent fibre connections between the DLF and the dorsal column nuclei. Terminal degeneration was found, after lesions of the DLF, in the rostral parts of both cuneate and gracile nuclei. The terminal degeneration extended from 1andnbsp;mm caudal to the obex up to the rostral poles of both nuclei. Such anatomical evidence of a direct afferent pathway does not, of course, exclude the presence of indirect pathways; such pathways, if synapsing before reaching the dorsal column nuclei, would not be demonstrated by anatomical methods. It is not possible, from the results described, to determine whether the direct pathway is formed by collaterals from a tract already known, such as the spinocervical tract, or whether it exists as a separate entity.