A

Figure 5 Transverse sections through the developing medulla oblongata of a chicken embryo illustrating the initial two longitudinal fiber bundles [A; medial longitudinal fascicle (mlf) and lateral longitudinal fascicle (lat)] and the splitting of the lateral longitudinal fascicle into separate tracts (B; 1, spinocerebellar tract; 2, spinal trigeminal tract; 3, lateral vestibulospinal tract) (modified from Glover and Petursdottir, 1991).

Figure 5 Transverse sections through the developing medulla oblongata of a chicken embryo illustrating the initial two longitudinal fiber bundles [A; medial longitudinal fascicle (mlf) and lateral longitudinal fascicle (lat)] and the splitting of the lateral longitudinal fascicle into separate tracts (B; 1, spinocerebellar tract; 2, spinal trigeminal tract; 3, lateral vestibulospinal tract) (modified from Glover and Petursdottir, 1991).

fascicle (llf) (Fig. 5). Each contains ascending and descending axons, which tend to be segregated into subfascicles containing axons of a given type. As the transversal area of the hindbrain expands, the mlf maintains its coherence alongside the raphe and remains as the tract of the same name in the mature brain. The llf, on the other hand, splits into several tracts, including various ascending sensory tracts, the spinal trigeminal tract, and the lateral vestibulospinal tract. This splitting is accompanied by intercalation of increasing numbers of neurons and other axons. Thus, the early formation of longitudinal axon tracts occurs on a simple scaffold that increases in complexity by a process of fission. Later formed tracts, such as the corticospinal tract and the pontocerebellar projection, are layered externally to the early formed tracts.

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