The central-peripheral transitional regions of cranial nerves. Trochlear and abducent nerves

Unlike all other nerves containing somatic efferent fibres, the trochlear nerve emerges from the dorsal aspect of the brainstem. It generally emerges as a single trunk which resembles a dorsal rather than a ventral spinal nerve rootlet in terms of its size and of the morphology and position of the central tissue projection which it contains. The morphology of the central-peripheral transition of the trochlear nerve is therefore correlated with its dorsal location rather than with the nature of its constituent fibres. By contrast, abducent nerve rootlets emerge from the ventral aspect of the neuraxis, in line with other cranial and with spinal ventral nerve rootlets which also contain somatic efferent fibres. Its rootlets resemble the latter in terms of their size, being much smaller than those of dorsal rootlets or the trochlear nerve. They possess two distinct types of central-peripheral transitional zone: those of the rostral rootlets resemble zones of cervical ventral spinal rootlets. Many of these emerge through a circumscribed thickening of the astrocytic glia limitans. Caudal abducent rootlets emerge through a tongue-shaped glial elevation projecting above the level of the surrounding brainstem surface, resembling Type 1 oculomotor rootlets.