White matter of brainstem

The white matter of brainstem is a complex network of nerve fiber tracts that traverse through the medulla oblongata, pons, and midbrain, interconnecting these regions and serving as vital pathways for communication between the spinal cord and various cortical and subcortical areas.

One of the key motor pathways in the white matter of brainstem is the corticospinal tract. This tract descends through the cerebral peduncles of the midbrain, accompanied by the corticonuclear fibers and the frontopontine and temporopontine fibers. As it continues into the pons, the anterior section contains numerous pontocerebellar fibers that run transversely. These fibers converge bilaterally to form the middle cerebellar peduncles, which establish a connection between the pons and the cerebellum. The descending corticospinal fibers thread through these structures, continuing their journey to the medulla where they form the pyramids on its anterior surface, and eventually progress into the spinal cord as the lateral and anterior corticospinal tracts.

In terms of sensory pathways, the Dorsal Column comprises fasciculus gracilis and fasciculus cuneatus, which cross over as internal arcuate fibers and then ascend as the medial lemniscus, close to the midline and posterior to the pyramids. The spinal lemniscus, comprising the lateral and anterior spinothalamic tracts and spinotectal tracts, is positioned laterally in the medulla. As these tracts ascend into the pons, the medial lemniscus shifts slightly laterally, paralleled by the spinal and lateral lemnisci, which are further laterally positioned relative to the medial lemniscus.

In the lateral medullary region, the spinal tract of the trigeminal nerve carries pain and temperature information from the face to the spinal trigeminal nucleus. Proprioceptive information is transmitted by the mesencephalic tract of the trigeminal nerve to the mesencephalic nucleus. Sensory information from the trigeminal nuclei then ascends through the trigeminal lemniscus to the thalamus.

Higher up in the midbrain, the various lemniscal tracts—including the medial, spinal, lateral, and trigeminal lemnisci—align closer together, forming a curved band posterior to the substantia nigra. The lateral lemniscus terminates at the level of the inferior colliculus, not extending into the upper midbrain.

Several tracts connect to the inferior and superior colliculi located at the back of the midbrain. The lateral lemniscus connects to the inferior colliculus, while fibers from the optic nerve, visual cortex, and spinotectal pathways connect to the superior colliculus, facilitating visual reflexes. Motor pathways such as the tectospinal and tectobulbar tracts originate from the superior colliculi, coordinating eye movements and body balance.

In the lateral medulla and pons, the anterior and posterior spinocerebellar tracts relay proprioceptive information to the cerebellum, but are absent in the midbrain. Additionally, the medial longitudinal fasciculus, which runs in front of the fourth ventricle on either side of the midline, is essential for conjugate eye movements and is found anterior to the cerebral aqueduct in the midbrain.

Cerebellar peduncles, namely the inferior, middle, and superior cerebellar peduncles, connect different parts of the brainstem to the cerebellum. Another significant structure, the reticular formation, a diffuse network of nerve fibers, resides posterior to the inferior olivary nuclei in the medulla and is associated with arousal and consciousness. This formation extends through the brainstem, reaching the tegmentum of the midbrain.

Within the midbrain, notable decussating fibers can be observed in the anterior region near the midline, including the crossing fibers of the red nuclei at the inferior colliculus level and the decussating fibers of the superior cerebellar peduncles at the superior colliculus level.

This intricate network of tracts within the white matter of brainstem ensures seamless communication and coordination among various brain regions, vital for motor control, sensory processing, and maintaining essential bodily functions.