The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

No abstract

Four variants of the Golgi method have been used, in the rabbit, to reveal the morphological attributes of neurons within the periaqueductal gray. Of these methods, the Golgi-Cox version provided the most satisfactory results in terms of both quality and quantity of cell impregnation. In order to make comparison with other descriptions of Golgi characteristics of the periaqueductal gray, statistical analysis was carried out on the distinguishing features of individual neurons, following two different rationales. One method, primarily based on soma characteristics (shape, area, length, and width) and basic features of the primary dendrites (number, length, and number of end-points) resulted in nine different categories of neurons being recognized: round, ovoid, spindle, pyriform, triangular, pyramidal, rhomboidal, polygonal, and stellate shaped cells. The alternative method principally characterized neurons by an assessment of the degree of dendritic spine development and prominent features of the dendritic tree (number of primary dendrites, length, number of branches, end-points, and degree of spine development). This approach resulted in nine subgroups within three major classes being identified: three spiny, four moderately spiny, and two aspiny classes (subdivision of each of the groups being resultant on neuronal size and/or the degree of dendritic branching). There was no similarity between the nine groups found by the two methods. Some, though little, correlation of neuron type was evidenced with respect to four zonal subdivisions of the periaqueductal gray complex. It remains to be seen how any of these readily recognizable morphological features, or the subgroups (derived on a statistical basis) into which they fall, might be shown to relate to function.

The lateral olfactory tract (LOT) of the ferret has been shown to project to most of the pyriform lobe, as in the cat. Only a small medio-posterior region of the pyriform cortex (the angular cortex), which has a distinctly different appearance in Nissl stained sections, proves to be devoid of olfactory connections. Despite the fact that sub-areas can be recognized within olfactory cortex, there is an underlying constancy in design throughout. Latency measurements indicate that fine collateral branches of theLOT reach the posterior olfactory cortex, whereas mainly larger diameter (faster conducting) fibres comprise the contribution to anterior olfactory cortex. Thus in the present context it is sufficient to recognise just an anterior and posterior subdivisionof olfactory cortex, contingent on the above criterion and correlated with cytoarchitectural features, chiefly variations in cortical layers II and III of the regions concerned. Pyriform cortex directly in receipt of LOT fibres, and associated deeper cortical zones, generated potentials in the olfactory bulbs which can only be attributed to centrifugal input, thus further substantiating an olfactory role for this cortex. In addition, non-cortical regions in receipt of LOT fibres, namely the anterior olfactory nucleus, olfactory tubercle, cortical amygdaloid nucleus and nucleus of the lateral olfactory tract, also contribute centrifugal input to the bulbs. All these regions are thereby capable of providing a rather direct feedback on olfactory bulb activity. The exact pathways concerned have yet to be determined.

No abstract