Lateral Zone
C. Midbrain
1. Blood Supply: Basilar Artery and Posterior Cerebral Arteries 2. Gross Landmarks in the Midbrain
On the anterior surface of the midbrain we find the principal landmark of this level:
the cerebral peduncles with the third cranial nerve exiting from their medial surface in the interpeduncular fossa. On the posterior surface of the midbrain we find the corpora quadragemini: the superior and inferior colliculi. The fourth cranial nerve exits from the posterior surface of the midbrain and the junction with the pons. In a coronal section through the midbrain, one finds the red nucleus in the tegmentum of the midbrain (Fig. 4.8B). Also in the inferior levels in the tegmentum just before the red nucleus, one finds the crossing of the superior cerebellar peduncle.
Brain Stem Level 6: Inferior Colliculus and Pontine Basis (Fig. 4.8)
In this section the midbrain forms the roof and floor of the narrow cerebral aque-duct, whereas the pons makes up the basilar portion. The roof is the inferior collicu-lus, an important nucleus in the auditory pathway.
Tectum
Inferior Colliculus (Figs. 4.8A and 4.8B). The gray matter of the inferior collicu-lus forms the tectum at this level. The inferior collicucollicu-lus is divided into three nuclei:
a large central nucleus, a thin dorsal nucleus )the paracentral or cortical nucleus), and an external nucleus. The central and cortical nucleus functions as a relay center for the cochlea. The external nucleus functions for the acousticomotor reflexes
through the tectospinal pathway. The brachium of the inferior colliculus carries auditory information onto the medial geniculate nucleus of the diencephalon. The tectospinal pathway descends from this region and terminates on the motor nuclei of the lower cranial nerves and upper cervical ventral horn cells.
Fig. 4.8 A Brain stem level 6 at midbrain inferior collicular level, Coronal (From EM Marcus and S Jacobson, Integrated neuroscience, Kluwer, 2003)
Tegmentum
Ventricular Zone (Figs. 4.8A and 4.8B). The periaqueductal gray is conspicuous in midbrain levels. Descending and ascending tracts associated with the visceral brain are found here. In the midbrain, the lower half of the periaqueductal gray and some other nuclei in the tegmentum are part of the midbrain limbic area. This zone is important in our level of attentiveness. Bilateral lesions to the periaqueductal gray and midbrain tegmentum usually produce comatose patients.
Motor Cranial Nerve Nuclei (Fig. 4.8A). The nucleus of cranial nerve IV is seen indenting the medial longitudinal fasciculus in lower regions of the midbrain. The axons of this nucleus pass posteriorly (unique for a cranial nerve) and exit the brain at the midbrain pontine junction, where the fibers then proceed anteriorly to reach the superior oblique muscle through the superior orbital fissure.
Sensory Cranial Nerve Nuclei (Fig. 4.8A). Proprioception is conveyed from the muscles in the head and neck. The mesencephalic nucleus of nerve V is located at the lateral margin of the periaqueductal gray. Remember that these are the only pri-mary sensory neurons in the central nervous system; their axons join the medial lemniscus and ascend to the ventral posterior medial nucleus in the thalamus.
Lateral Zone (Fig. 4.8A). At this level, the medial lemniscus and the other major ascending tracts are stretched over the lateral surface of the tegmentum of the midbrain. The spinothalamic and trigeminothalamic fibers are found lateral to the medial lemniscus and the auditory fibers are located above them. Thus, the follow-ing sensory modalities for the entire body are located here: tactile discrimination, proprioception, pain and temperature, gustatory, and visceral sensations.
Auditory Fibers lateral lemniscus (Fig. 4.8A). The lateral lemniscus is seen superior to the medial lemniscus. These secondary auditory fibers are now entering
Fig. 4.8 B Brain stem at inferior collicular level. Coronal MRI, T1
the inferior colliculus. Many of these fibers synapse in the inferior colliculus and then continue up into the medial geniculate nucleus MGN,. as the brachium of the inferior colliculus.
Medial Zone (Fig. 4.8A). The raphe nuclei of the reticular formation are very conspicuous in these levels. The medial longitudinal fasciculus is prominent in the floor of the ventricle as it nears cranial nerve III; the fibers located within it are important in coordinating ocular movements.
Central Zone (Fig. 4.8A). This zone in the lower tegmentum of the midbrain is nearly completely filled with the crossing fibers of the superior cerebellar peduncle.
Ultimately, all of these fibers decussate and continue up to the red nucleus, where some fibers synapse, but the majority of these axons bypass the red nucleus and terminate on the ventral lateral thalamic nuclei.
Basilar Zone (Fig. 4.8A). The bulk of this region is taken up by the pontine gray and descending fibers from the cerebrum to the pontine gray (the corticopontine system). The corticospinal and corticobulbar fibers are also present here.
Brain Stem Level 7: Midbrain Superior Collicular Level and Pontine Basis (Fig. 4.9)
Tectum
Superior Colliculus (Fig. 4.9). The superior colliculi form the rostral portion of the midbrain tectum. The parenchyma of the superior colliculus is organized in layers. There are four layers containing gray and white matter from inside out:
stratum zonale, stratum cinereum, stratum opticum, and stratum lemnisci. The superficial layers of the superior colliculus receive their input from the retina and visual cortex, with the contralateral upper quadrant medially and the contralateral lower quadrants laterally. In contrast, the deeper layers receive their input from polysensory sources, including the cerebellum, inferior colliculus, spinal cord, reticular formation gracile, cuneate, trigeminal nuclei, and visual regions. A portion of the tectospinal pathway originates at this level and descends in the lower brain stem to terminate on the lower cranial nerves and ventral horn cells of the spinal cord. (Remember that the tectospinal and tectobulbar fibers terminate on the same motor nuclei as the corticonuclear and corticospinal pathways). The zone of the tectum above the superior colliculus (the pretectal zone) is important in light reflexes.
Tegmentum
Gross Features. In this section, the roof and floor are formed by the midbrain, whereas the basilar portion is made up of peduncles and the pons. The roof is the superior colliculus, an important station in the visual pathway. The cerebral aque-duct forms the narrow ventricular lumen (Figs. 4.9A and 4.9B). The superior
col-Fig. 4.9 A Brain stem level 7; midbrain: superior collicular level
liculus is a laminated structure important in relating eye movements and body position. The tectospinal tract originates from its deepest layer and provides con-nections onto certain cranial and spinal neurons.
Ventricular Zone
Sensory Cranial Nerve Nuclei (Fig. 4.9A). Proprioception is conveyed from the muscles in the head and neck by the mesencephalic nucleus of nerve V, located at the lateral margin of the periaqueductal gray in the superior collicular levels as well as in the upper pontine and inferior collicular levels.
Motor Cranial Nerve Nuclei (Fig. 4.9A). Cranial nerve III is visible in the floor of the cerebral aqueduct, adjacent to the medial longitudinal fasciculus. This nerve supplies the medial, inferior, and superior rectus muscles and the inferior oblique and superior levator muscles of the eyelid. The Edinger–Westphal nucleus of nerve III is also present; it provides preganglionic parasympathetic innervation to the constrictor muscle of the pupil via the ciliary ganglion.
Lateral Zone
White Matter (Fig. 4.9A). Medial lemniscus (At this level, in close proximity to the medial lemniscus, the white matter includes most of the ascending sensory fibers).
Fig. 4.9 B Brain stem at superior collicular level. Coronal, MRI, T1
This tract is stretched out over the inferior and lateral surface of the tegmentum, with the fibers that mediate pain and temperature from the extremities, thorax, abdomen, and pelvis located at its superior extent. The trigeminal fibers form its middle portion, and the fibers that mediate tactile, proprioceptive, and visceral sen-sations are placed medially.
Anterolateral Column: Lateral Spinothalamic. Pain and temperature from the limbs, thorax, and pelvis are found in this pathway.
Anterior spinothalamic. Light touch fibers from the limbs, abdomen, and neck are seen in close proximity to the lateral spinothalamic pathway.
Auditory Pathway. The fibers from the inferior colliculus (called, at this level, the brachium of the inferior colliculus) are seen at the inferior surface of the supe-rior colliculus. On the left side, they enter the medial geniculate nucleus of the metathalamus, at which point they reach their final subcortical center.
Cerebellar Fibers. The superior cerebellar peduncle is ascending and continues to cross in the tegmentum of the midbrain. Just above this level, many of these fibers syn-apse in the red nucleus, whereas others will continue to the VL nucleus in the thalamus.
Central Zone (Fig. 4.9A). The central tegmental tract is conspicuous in the midbrain tegmentum in the reticular formation. The red nucleus is found in the medial edge of the reticular formation. In the human, it consists of a small magno-cellular region and a large parvomagno-cellular region. Fibers from all cerebellar nuclei, but especially the dentate, synapse here.
Corticorubral fibers run bilaterally from the motor cortex synapse also terminate here in a somatotopic relationship. Fibers from the red nucleus terminate directly or indirectly in the cerebellum and on many of the motor nuclei in the brain stem and spinal cord, which also receive input from the corticospinal and corticonuclear pathways. The red nucleus has minimal, if any, projections onto the thalamus in the human, although it most likely influences the cerebellar input onto the thalamus.
The substantia nigra is also located in this level, just behind the cerebral pedun-cle. Its functions are discussed in the relationship between the basal ganglia and movement (see Chapter 11).
Medial Zone (Fig. 4.9A). The serotinergic raphe nuclei of the reticular forma-tion are present in this level. The medial longitudinal fasciculus in the floor of the cerebral aqueduct is conspicuous and connected across the midline. The oculomo-tor complex indents the MLF and receives ascending fibers from cranial nerves VI and VIII through the MLF. This might be significant if herniation of the temporal lobe produces compression of the cerebral peduncle and cranial nerve III, with the accompanying cranial nerve and upper motor neuron signs. The sympathetic nucleus of Edinger–Westphal, associated with cranial nerve III to the pupillary constrictor, is also present. The interstitial nucleus of Cajal and the nucleus of Darschewtiz, which are important pupillary reflexes, are also found here.
The tectospinal fibers at this level are primarily from the superior colliculus.
Basilar Zone (Figs. 4.9A and 4.9B): Cerebral Peduncles. At this level, the frontopontine fibers, which occupy the most medial part of the cerebral peduncles, enter the pons. The corticospinal fibers and many of the corticobulbar fibers are still in the peduncle. In Figure 4.9B, a coronal MRI section through the midbrain, the
relationship of the cerebral peduncle to the medial temporal lobe demonstrates that the cerebral peduncles are adjacent to the medial temporal lobe and that herniation of the temporal lobe will affect the cerebral peduncle signs in the III cranial nerves and descending pathways.
Figure 4.10 is a myelin-stained section through the cerebral peduncles demon-strating the relationship between the peduncles and the III cranial nerve.