Showing papers on "Cuneate nucleus published in 1968"

Somatotopic organization of raccoon dorsal column nuclei.

Abstract: Somatotopic organization was demonstrated by means of microelectrode mapping studies of three somatic sensory nuclear regions in the raccoon medulla. (1) Projections of peripheral receptive fields in the cuneate-gracile nuclear complex occur iteratively in rostro-caudal columns. These projections are organized into a detailed 3-dimensional pattern, in contrast to the more diffuse intermingling of such projections in the spinal dorsal roots and columns. This indicates that afferent fibers are re-sorted before synaptic termination. In the extensive representation of the volar hand, there are distinct subnuclei, delineated by intervening fiber laminae, each containing the projections from one of the forepaw digits. (2) Units in the external-cuneate nucleus responded only to stimulation of deeperlying tissues in anterior body regions. These projections were also somatotopically organized. Retrograde degeneration studies indicated that some cells, in that region where cuneate-gracile adjoins external-cuneate, may project through both the ipsilateral cerebellar peduncles and the contralateral midbrain. (3) The spinal trigeminal nucleus adjoining the cuneate-gracile complex displayed many characteristics similar to those of the latter. Neighboring, although anatomically distinct, regions contained projections from neighboring peripheral receptive fields.

Patterns of firing in cuneate neurones and some effects of flaxedil

Abstract: In cats under pentobarbitone anaesthesia, single cuneate cells excited directly by microiontophoresis of glutamate (or ATP) reproduce the temporal patterns of firing seen during spontaneous activity or during activity evoked by peripheral stimulation. In particular, the glutamate-evoked discharges of hair or touch cells show their characteristic tendency to fire in high-frequency pairs or bursts of impulses. Since glutamate acts mainly on the post-synaptic cell, the explanation for the multiple discharges must lie in special, repetitive properties of the hair and touch cells. Cuneate neurones are strongly excited by microiontophoretic applications of Flaxedil. The most prominent effect is the appearance of prolonged bursts of spikes at a high frequency. Even systemic Flaxedil can alter the discharge of cuneate hair cells; short intervals occur more frequently, and, in some cases, there is an acceleration in spontaneous firing.

Antagonism of Presynaptic Inhibition in the Cuneate Nucleus by Picrotoxin

Abstract: AN afferent volley in a cutaneous forelimb nerve produces on the surface of the cuneate nucleus a characteristic electrical response. This consists of a brief initial spike potential indicating the arrival of the primary afferent volley, followed by a negative wave (N wave) lasting about 4–5 msec and a prolonged positivity (P wave) reaching a maximum in 20–30 msec and lasting for about 200 msec1. Similarly, N and P waves result from electrical stimulation of the contralateral sensorimotor cortex2. The N wave produced by the afferent stimulus is thought to be caused by synaptically induced depolarization of the cuneate cells by the ascending volley in the dorsal column3. Systematic investigation of the P waves produced by cutaneous volleys and by cortical stimulation led Andersen et al.3 to suggest that both are caused by prolonged depolarization of cuneate tract fibres close to their synaptic terminals in the cuneate nucleus. This suggestion was further supported by excitability testing of these presynaptic fibres at their terminals and along their length, and by intracellular recording from these fibres4. With the technique of extracellular single cell recording, Jabbur and Towe5–8 had shown earlier that the sensorimotor cortex (primarily the contralateral) produced excitatory and inhibitory influences on cuneate and gracile neurones. With the additional technique of antidromic activation of dorsal column neurones after medial lemniscal stimulation9,10, two types of neurones have been identified. These are direct projection neurones, or dorsal column-thalamic relay neurones, which are depressed by antecedent descending volleys from the sensorimotor cortex, and interneurones, through which the cortex exerts presynaptic (and sometimes post-synaptic11) inhibition on the afferent inflow passing through the dorsal column nuclei. There is histological evidence for the presence of these corticofugal projections12.

Synaptic transmission of proprioceptive and cutaneous impulses through the cuneate nucleus

Abstract: 1. Responses of 100 neurons of the cuneate nucleus to stimulation of cutaneous and proprioceptive afferents of the ipsilateral forelimb were investigated. 2. From analysis of the latent periods of responses of these cells they can be divided into two groups: one with latencies from 3.5 to 6.5 msec, and a second with latencies from 7 to 18.3 msec. These two groups can be regarded as activated monoand polysynaptically by peripheral nerve stimulation. The great majority of cells of the first group lie at depths of between 0.8 and 1.2 mm, the great majority of cells of the second group at depths of between 1.2 and 1.6 mm from the dorsal surface of the nucleus. 3. The number of cells activated monosynaptically during stimulation of cutaneous afferents is much greater than the number activated monosynaptically by stimulation of proprioceptive afferents. 4. A characteristic feature of responses of cells connected with cutaneous afferents is their ability to respond by grouped discharges to single afferent impulses. Cells connected with proprioceptive afferents, on the other hand, respond to a single impulse by one, or sometimes two, action potentials. 5. A group of cells has been found which is activated during stimulation of both cutaneous and proprioceptive afferents with approximately the same latency. These cells are distributed diffusely throughout the depth of the nucleus.

Neurophysiological methods in anaesthesia research

Abstract: SaxroiES by Larrabee and Posternak 1 on sympathetic ganglia demonstrated that some anaesthetics depress synaptic transmission more readily than the process by which the action potentials are propagated. Recent experiments on the spinal cord monosynaptic reflex have confirmed these observations. =-~ However, the mechanism responsible for this depression is not known. The synaptic process is complicated and anaesthetics might act at different sites, e for instance, at the afferent terminals, at the synaptic gap, or at the post-synaptic membrane. Furthermore, synapses can be classified as excitatory and inhibitory. The latter type regulates the input and output of specialized pathways in the form of preand post-synaptic inhibition. Studies of the effect of anaesthetics on synaptic transmission are frequently difficult to interpret because: (a) the synapse is not located in the central nervous system (cHs) or (b) most experiments are performed on anaesthetized animals or (c) the administration of the anaesthetic, per se, alters the animal's systemic conditions. This paper describes some methods of studying the effect of anaesthetics on synaptie transmission in the cNs of unanaesthetized cats. A summary of the action of local and systemic administrations of procaine, pentobarbitone, and halothane is presented. METHOD Synaptic transmission was studied in the cuneate nucleus. This nucleus represents the first synaptic relay of the main sensory pathway originating in the forelimb and terminating in the cerebra] cortex. This pathway is also known as the dorsal column-medial lemniseal system. Most of the axons of euneate relay cells form part of the medial lemniscus that ends in the ventroposteral-lateral nucleus of the thalamus. Other axons and/or collaterals terminate in the cerebellum. The cuneate nucleus is located in the dorsum of the medulla; it has a rostrocaudal length of approximately 10 ram. and a dorsoventral thickness of 9. ram. at its maximum. The terminal fibres of the dorsal columns cover the dorsal surface of the nucleus before they bend, almost at a right angle, to terminate in the nucleus itself. The two cuneates are separated from each other by the more medial gracilis nuclei.