Brain stem reticular units: synaptic responses to stimulation within the ascending reticular pathways

A re-examination of the spino-reticulo-diencephalic pathway in the cat

One commonly accepted idea is that affective aspects of pain sensation are derived from a flow of information from the spinal cord through the reticular formation to the intralaminar thalamus and subthalamus. Little is known, however, about the extent to which spinoreticular terminations and reticulodiencephalic neuronal cell bodies overlap. This study used a combination of anterograde and retrograde tracing techniques to compare these distributions in the cat. Whereas spinoreticular terminations were concentrated caudally and laterally, neurons projecting to intralaminar thalamus and subthalamus were concentrated rostrally and medially. Thus, information conveyed from the spinal cord to the reticular formation appears to have direct access to intralaminar thalamus and subthalamus only by way of a few widely scattered neurons. When considered with the results of others, these results encourage less emphasis on a putative spino-reticulo-diencephalic pathway for pain. Rather, the reticular formation's role in pain is more likely to involve its full complement of interconnected descending and ascending connections.

Cross-correlation analysis of midbrain reticular neuron pairs during sleep-waking cycle of the cat

By simultaneously inserting three extracellular microelectrodes, separated by 1.0 mm from each other, into the cat's midbrain reticular formation (MRF), temporal cross-correlation of firing of neuron pairs was measured to investigate the mode of interaction among the MRF neurons during different states of sleep and wakefulness. None of 97 neurons pairs studied gave clear-cut cross-correlograms suggesting cascade connection between two neurons. However, 29 neuron pairs showed weakly synchronized firing which occurred periodically with a mean interval of 1.23 s. This interval was different from the respiratory cycle. The rhythmic synchronization appeared most obviously during slow wave sleep. The synchronized firing was encountered more often in the pairs of adjacent neurons picked up with a single electrode and in the pairs picked up with separate electrodes positioned along the mediolateral axis, when compared with the neuron pairs located along the rostrocaudal axis. About 80% of the neurons (37 our of 46) which showed periodically synchronized discharge had no corresponding periodicity in their auto-correlograms. Simple synaptic linkage conceivable for the observed periodic synchronization of a neuron pair would be shared inhibition with the common inhibitory source activated periodically. Some neurons in the dorsal raphe nucleus and in the solitary tract nucleus were also examined for the cross-correlation with the MRF neurons. However, no positive relationship was found.

Trigeminocerebellar mossy fiber branching to granule cell layer patches in the rat cerebellum

Recent micromapping studies of tactile areas in rat cerebellum have revealed a spatially precise mosaic of granule cell layer areas termed 'patches', with multiple patches on several folia resulting in multiple representations of the same body part. The research reported in the present paper tested the hypothesis that trigeminocerebellar mossy fiber branching is patch-related, so that collaterals terminate only within the boundaries of one or more of the tiny (often less than 1 sq. mm) patches having similar receptive fields. This hypothesis was supported by 3 separate series of electrophysiological experiments: (1) electrical stimulation in a patch resulted in short latency, low threshold responses confined within the boundaries of a second patch having a similar receptive field; (2) antidromic collison tests demonstrated that collaterals of axons originating in nucleus interpolaris reach pairs of patches with similar receptive fields; and (3) micromapping of the area from which a particular interpolaris neuron could be antidromically activated revealed that local terminal branching of a given mossy fiber axon appeared to be confined within the boundaries of a single patch. Together these results indicate that trigeminocerebellar mossy fiber branching is related to the fractured somatotopical organization of the granule cell layer. Possible functional implications of such mossy fiber branching are discussed.