Medial Lemniscus

Medial Lemniscus

This fiber system includes the most impor-tant ascending pathways of the exteroceptive sensibility from the spinal cord and the brainstem. It is subdivided into the spinal lemnis-cus and the trigeminal lemniscus. The spinallemniscus contains the sensory pathways for trunk and limbs (bulbothalamic tract, spinothalamic tract, spinotectal tract), while the trigeminal lemniscus contains the sensory pathways for the face (anterior tegmental fasciculus).

1.        Bulbothalamic tract (B3). The fibers rep-resent the extension of the posterior funiculi of the spinal cord (B4) (epicritic sen-sibility). They originate in the gracile nu-cleus (B5) and the cuneate nucleus (B6), cross as arcuate fibers (decussation of lem-nisci) (B7), and form the medial lemniscus in the narrower sense. The cuneate fibers initially lie dorsally to the gracile fibers, while they lie medially to them in the pons and midbrain. They termi-nate in the thalamus.

2.        Spinothalamic tract (lateral and ante-rior) (B8). The fibers (protopathic sensibility,pain, temperature, coarse tactile sensation) have already crossed to the contralateral side at various levels of the spinal cord and form slightly spread, loose bundles (spinal lemniscus) in the medulla oblongata. They join the medial lemniscus  in the midbrain.

3.        Spinotectal tract (B9). The fibers runtogether with those of the lateral spinothalamic tract. They form the lateral tip of the lemniscus in the midbrain and ter-minate in the superior colliculi (pupillary re-flex on sensation of pain).

4.        Anterior tegmental fasciculus (Spitzer)(B10). The fibers (protopathic and epicriticsensibilities of the face) cross in smallbundles from the spinal nucleus of the trigeminal nerve (principle nucleus) to the contralateral side (trigeminal lemniscus) and join the medial lemniscus at the level of the pons. They terminate in the thalamus.

5.        Secondary taste fibers (B11). These origi-nate from the rostral part of the solitary nu-cleus (B12), probably cross to the con-tralateral side, and occupy the medial mar-gin of the lemniscus. They terminate in the thalamus.