Brain Stem
A. Medulla
2. Gross Landmarks in the Medulla
The tegmentum of the medulla has two distinct levels: a narrow lower portion that is similar to the spinal cord and an broader upper portion that includes the widened IV ventricle.
Brain Stem Level 1: Spinomedullary Junction at Motor Decussation (Fig. 4.3)
Gross Features. This level resembles the spinal cord, as the dorsal columns are conspicuous; the trigeminal funiculus is more laterally placed.
On the anterior surface of the spinal cord, the medullary pyramids (corticospinal tracts) are evident (Fig. 4.3A). Note that this descending pathway on the right side has crossed first (decussated) with the fibers, shifting from the anterior surface of the medulla, and then entered the lateral funiculus of the spinal cord. The narrow spinal canal is present in the gray matter above the pyramidal decussation.
Motor Cranial Nerve Nuclei (Fig. 4.3A). Only the cranial portion of nerve XI is present at the lateral margin of the reticular formation. It innervates the sterno-cleidomastoid and trapezius muscles that rotate the head and elevate the shoulders, respectively.
Sensory Cranial Nerve Nuclei (Fig. 4.3A). Primary axons convey pain and temperature from the head and neck into the medulla and are located in the spinal tract of the fifth cranial nerve; these fibers have descended from the pons. The sec-ond order axons originate from the underlying descending nucleus and their axons cross and then ascend contralaterally adjacent to the medial lemniscus and termi-nate in the ventral posterior medial nucleus of the thalamus. The nucleus lateral to the pyramidal tract is the inferior reticular nucleus, a portion of the reticular formation.
White Matter (Fig. 4.3A). At this level the tracts are still in the same positions as in the spinal cord.
Posterior columns. Tactile discrimination (fine touch, pressure, vibration sensa-tion, and two-point discrimination) and proprioception from the extremities, thorax, abdomen, pelvis, and neck are carried via the dorsal columns. The nuclei can now be seen that are the second order/2° neurons in this pathway: the gracile and cuneate nuclei.The gracile and cuneate tracts (fasciculi) have reached their maxi-mum bulk with the addition of the last of the fibers from the uppermost cervical levels. The somatotopic arrangement of the tactile fibers in the posterior column is as follows: The most medial fibers are sacral and then the lumbar, thoracic, and, most laterally, cervical fibers.
Spinothalamics/anterolateral column. Pain and temperature from the extremi-ties, abdomen, thorax, pelvis, and neck are carried by the lateral spinothalamic tract located at the lateral surface of the medulla. Light touch from the extremities, thorax, abdomen, pelvis, and neck is carried via the anterior spinothalamic tract, which
is seen on the surface of the medulla just posterior to the corticospinal tract. In the spinothalamic pathways, sacral fibers are on the outside and cervical fibers are on the inside. The dorsal and ventral spinocerebellar tracts are found in the lateral funiculus. The rubrospinal and tectospinal tracts, which are important in supporting voluntary motor movements, are found in the lateral funiculus and near the midline in the medullary and pontine levels).
Brain Stem Level 2: Lower/Narrow Medulla at Sensory Decussation (Fig. 4.4)
Gross Features. At this level, the fourth ventricle is narrow. The funiculus gracilis and the funiculus cuneatus are conspicuous on the posterior surface of the spinal canal and the medullary pyramids are prominent on the anterior surface (Figs. 4.4A and 4.4B).
Ventricular Zone: Motor Cranial Nerve Nuclei (Fig. 4.4A). This section con-tains the inferior extent of the hypoglossal nerve (XII) and the dorsal motor nucleus of the vagus nerve (X). The hypoglossal nucleus innervates the intrinsic and extrin-sic musculature of the tongue, whereas the dorsal motor nucleus of the vagus nerve provides parasympathetic preganglionic innervation of the viscera. This level marks the superior extent of the cranial portion of the 11th cranial nerve in the ambiguus nucleus.
Lateral Zone: Sensory Cranial Nerve Nuclei (Fig, 4.4A). Pain and tempera-ture from the head are conveyed by the descending nucleus and tract of nerve V that is prominent anterior to the cuneate nucleus. Note that myelinated ipsilateral pri-mary axons are on the outside of the second-order neurons while the second-order neurons are leaving the inner surface of the nucleus, crossing and entering the medial lemniscus, and forming the trigeminothalamic (quintothalamic) tract.
White Matter (Fig. 4.4A). Pain and temperature from the extremities, thorax, abdomen, pelvis, and neck are carried via the lateral spinothalamic tract. The spi-nothalamics are located in the lateral funiculus throughout the spinal cord and brain stem! The anterior spinothalamic tract is adjacent to the lateral spinothalamic tract.
The tract carrying unconscious proprioception from the upper extremity originates from the external cuneate nucleus and its fibers enter the cerebellum through the posterior spinocerebellar tract. The function of this nucleus is similar to that of Clark’s column in the spinal cord. The vestibulospinal tract is found internal to the spinothalamic tracts. The dorsal and ventral spinocerebellar tracts are seen on the surface of the medulla covering the spinal tract of nerve V and the spinothalamic tracts. The rubrospinal tract is an important afferent relay to the alpha and gamma neurons in the spinal cord. It originates from the red nucleus in the tegmentum of the midbrain, crosses the midline, and descends. It is found in the lateral funiculus of the medulla internal to the spinocerebellar tract and anterior to spinal nerve V throughout the pons and medulla. It is important in postural reflexes. The tectospi-nal tract is phylogenetically an old tract, being the equivalent of the corticospitectospi-nal
tract in nonmammalian vertebrates. It originates from the deep layers of the supe-rior colliculus and, to some extent, from the infesupe-rior colliculus. It is seen antesupe-rior to the medial longitudinal fasciculus throughout the medulla, pons, and midbrain and is important in coordinating eye movements and body position.
Medial Zone. The medial longitudinal fasciculus is located in the midline of the tegmentum just below the hypoglossal nucleus and above the tectospinal and
Fig. 4.4 A Brain Stem Level 2 at lower medullary level. Coronal (From EM Marcus and S Jacobson, Integrated neuroscience, Kluwer, 2003)
medial lemniscal pathways. The medial lemniscus is the largest pathway in the medial zone. However, as we progress superiorly in the brain stem, these fibers migrate laterally to finally enter the lateral zone in the midbrain. This migration is necessary for the medial lemniscus to be correctly positioned as it enters the thalamus. Tactile discrimination and proprioception are conveyed from the extremities, thorax, abdomen, pelvis, and neck by the axons in the fasciculus gracilis and cuneatus.
The nucleus gracilis and the nucleus cuneatus form the second-order neurons in this pathway and are conspicuous in this section. Internal arcuate fibers are seen leaving the inner surface of the gracile and cuneate nuclei, curving around the ventricular gray, crossing the midline (sensory decussation), and accumulating behind the pyramid and beginning the formation of one of the major ascending highways—the medial lemniscus. The serotonin-containing raphe nucleus of the reticular formation is found in this zone throughout the brain stem. In all levels of the brain stem, it will be in this subventricular position.
Central Zone. The central core of the medulla, pons, and midbrain consists of the reticular formation, which is important for many vital reflex activities and for the level of attentiveness.
Basilar Zone. This level contains the corticospinal fibers just before they cross in the medullospinal junction. Note that the pyramidal system is named for the passage of the corticospinal fibers through this pyramidal-shaped region respec-tively (Figs. 4.4A and 4.4B). One of the important concepts from this section is that axons that form the volitional motor pathway, the pyramidal system, originate in the cerebral cortex. These tracts are located in the basis of the brain stem and include the corticospinal and corticonuclear fibers that run in the medullary pyramid.
Motor system fibers that do not run in the medullary pyramid are considered extrapyramidal (e.g., cerebellar peduncles, rubrospinal tectospinal, etc.).
Fig. 4.4 B MRI, T2 Brain stem at lower medullary level MRI-T1. (From EM Marcus and S Jacobson, Integrated neuroscience, Kluwer, 2003)
Brain Stem Level 3: Wide Medulla at Level of Inferior Olive (Fig. 4.5)
Gross Features. At this level the medullary tegmentum expands laterally as the fourth ventricle enlarges. The prominent hallmark of this level is the inferior olive located above the pyramids.
Ventricular Zone. The floor of the fourth ventricle contains the median eminence and the vestibular and cochlear tubercles. The median eminence consists of aque-ductal gray and cranial nerve nuclei, whereas the vestibular tubercle is formed by the medial vestibular nucleus and the descending root and nucleus of cranial nerve VIII. The dorsal cochlear nucleus forms the cochlear tubercle. The ventricle at this level is at its widest extent. (Figs. 4.5A and 4.5B). The sulcus limitans in the ven-tricular floor separates the medially placed motor cranial nuclei from the laterally placed sensory cranial nuclei.
Motor Cranial Nerve Nuclei (Fig. 4.5A). This level marks the superior extent of nerve XII and the dorsal motor nucleus of nerve X. The ambiguous nucleus in the lateral margin of the reticular formation cotains cell bodies innervating the pharynx and larynx (nerve X).