Fornix system efferent projections in the squirrel monkey: An experimental degeneration study

Abstract

The efferent projections of the fornix system have been studied in the squirrel monkey using modified silver staining techniques to delineate degP*.mating fibers and terminal boutons after inducing careful lesions of the subcallosal fornix and dorsal fornix.Subcallosal fornix projections, after supplying the medial and lateral septa1 nuclei, are traced via precommissural fibers to terminal degeneration in the nu&us of the diagonal band of Broca, nucleus accumbens, ~Y N S rectus, olfactory tubercle, medial and lateral preoptic areas, and medial hypothalamus. Degenerating precommissural subcallosal fornix fibers course with the medial forebrain bundle throughout the extent of the lateral hypothalamus, extending caudally to the medial mammillary nucleus with specific hypothalamic offsets to the tuberal region and nucleus intercalatus (''lateral cortico-hypothalamic tract"). Subcallosal postcommissural fornix fibers are traced to terminal degeneration in the bed nucleus of the stria terminalis and medial preoptic and anterior hypothalamic structures (''medial cortico-hypothalamic tract"); fornix column fibers disperse throughout the perifornical nucleus; these fibers more caudally supply nucleus intercalatus and medial mammillary nucleus. In the thalamus postcommissural fibers supply the nucleus reuniens, the paraventricular, anterior ventral, and lateral dorsal nuclei. In the mammillary nucleus a laminated radiation is described with some fibers extending caudally to the prerubral field.A "midline subcallosal stria," derived from the subcallosal fornices, distributes fibers to the subfornical organ, the dorsal part of the medial septum, the precommissural fornix and especially the medial cortico-hypothalamic tract.The dorsal fornix projection is predominantly pericallpsal to the ~Y N S rectus and parolfactory area, with only a minor callosal-penetrating contribution to pre-and postcommissural fibers.The composite projections of the hippocampal formation fornix system appear more extensive than those of the amygdala to basal forebrain, preoptic, and hypothalamic areas.A review of anatomical descriptions of fornix system projections, comprising a voluminous literature, reveals significant discrepancies. Although G. Elliot Smith concluded in 1897 that I'. . . the fornix conforms to one and the same fundamental plan in the arrangement of its fibers in all vertebrates," five years later Edinger and Wallenberg ('02) found that postmammillary fornix connections differed significantly in different strains of rabbits. Thereafter, many of the seeming discrepancies of the early studies were explained by further work which showed that there is significant interspecific variability in the distribution and extent of direct fornix fibers (Tello, '36; Simpson, '52; Cowan and Powell, '55; Valenstein and Nauta, '59).Recently further clarification of fornix system projections has been achieved using the newer technique of studying axonal transport CHARLES E. POLETTI AND GEORGE CRESWELLof radioactive amino acids (Swanson and...

Selective disconnection of the hippocampal formation projections to the mammillary bodies produces only mild deficits on spatial memory tasks: Implications for fornix function

“…As the fornix descends into the septal region, the tract splits into two components: the precommissural fornix passes rostral to the anterior commissure, while the postcommissural fornix passes behind. The precommissural fornix innervates the prefrontal cortex and ventral striatum, while fibers in the postcommissural fornix either turn caudally to run directly into the anterior thalamus or form a compact bundle that descends through the posterior hypothalamus to reach the mammillary bodies (Raisman et al, 1966;Poletti and Creswell, 1977). This study examined the importance of this latter, descending tract.…”

The Frequency and Extent of Mammillary Body Atrophy Associated with Surgical Removal of a Colloid Cyst

“…This correlational analysis was extended to include a number of temporal lobe regions, particularly the hippocampus (including the subiculum) and the entorhinal cortex, because these areas provide almost all of the fornical input to the mammillary bodies. [21][22][23][24][25] These 2 aims led to the need to develop an MR imaging sequence that would improve the reliability of volume estimates for the mammillary bodies-a second subsidiary goal. We, therefore, developed an 0.8-mm-thick-section scan for the measurement of these small structures, which we compared with the standard 1-mm-volume scans.…”

Topographic separation of fornical fibers associated with the anterior and posterior hippocampus in the human brain: An MRI‐diffusion study

“…A previous study employing a similar methodology (Christiansen, Aggleton, et al., 2016) demonstrated that the fornix can also be divided between its precommissural and postcommissural fiber subdivisions (Poletti & Creswell, 1977). The precommissural fornix contains hippocampal connections with the basal forebrain, ventral striatum, and prefrontal cortex, whereas the postcommissural fornix connections reach the medial diencephalon and hypothalamus (Poletti & Creswell, 1977).…”

“…The precommissural fornix contains hippocampal connections with the basal forebrain, ventral striatum, and prefrontal cortex, whereas the postcommissural fornix connections reach the medial diencephalon and hypothalamus (Poletti & Creswell, 1977). Using diffusion imaging, the precommissural and postcommissural fibers were found to occupy different locations within the body of the fornix: the postcommissural fornix fibers were located dorsally, whereas the precommissural fibers were located ventrally (Christiansen, Aggleton, et al., 2016).…”

“…The fornix is the principal white matter tract connecting the hippocampal formation with areas beyond the temporal lobe, including prefrontal cortex, the anterior thalamic nuclei, the mammillary bodies, the ventral striatum, and the basal forebrain (Aggleton, 2008; Poletti & Creswell, 1977). Neuropsychological investigations of patients with fornix damage first highlighted the importance of this tract for episodic memory (Aggleton et al., 2000; D'Esposito, Verfaellie, Alexander, & Katz, 1995; Gaffan & Gaffan, 1991; Vann et al., 2008).…”