Study guidelines
- 1.
Be able to recognise and label the locations of the ascending and descending pathways in the horizontal sections of brainstem and spinal cord.
- 2.
Be able to describe or trace the four decussations that occur as part of a simple motor action. ( Box 3.1 deserves special attention because it indicates why certain pathways cross the midline and others do not. The brainstem crossings are formally addressed in Chapters 15 and 16 .)
- 3.
Identify the major ‘constituents’ of the midbrain, pons, and medulla (prominent structures) and the location of the DCML pathways and CSTs and their decussations, as well as the superior cerebellar peduncles.
- 4.
List the spinal cord segments, and describe the anatomic reason for the prominent enlargements.
- 5.
Describe the relationships of the three cerebellar peduncles to the fourth ventricle as seen in cross-sections.
The midbrain connects the diencephalon to the hindbrain. As explained in Chapter 1 , the hindbrain is made up of the pons, medulla oblongata, and cerebellum. The medulla oblongata joins the spinal cord at the spinomedullary junction within the foramen magnum of the skull.
In this chapter, the cerebellum (part of the hindbrain) is considered after the spinal cord, for the sake of continuity of motor and sensory pathway descriptions.
Brainstem
Ventral view ( Figures 3.1 and 3.2 A)
Midbrain
The ventral surface of the midbrain shows two massive cerebral peduncles bordering the interpeduncular fossa . The optic tracts wind around the midbrain at its junction with the diencephalon. Lateral to the midbrain is the uncus of the temporal lobe. The oculomotor nerve (III) emerges from the medial surface of the peduncle. The trochlear nerve (IV) passes between the peduncle and the uncus.
Pons
The bulk of the pons is composed of transverse fibres (the pontocerebellar tract ) that raise numerous surface ridges. On each side, the pons is marked off from the middle cerebellar peduncle by the attachment of the trigeminal nerve (V). The middle cerebellar peduncle plunges into the hemisphere of the cerebellum.
At the lower border of the pons are the attachments of the abducens (VI), facial (VII), and vestibulocochlear (VIII) nerves ( Table 3.1 ).
| Number | Name |
|---|---|
| I | Olfactory, enters the olfactory bulb from the nose |
| II | Optic |
| III | Oculomotor |
| IV | Trochlear |
| V | Trigeminal |
| VI | Abducens |
| VII | Facial |
| VIII | Vestibulocochlear |
| IX | Glossopharyngeal |
| X | Vagus |
| XI | Accessory |
| XII | Hypoglossal |
Medulla oblongata
The pyramids are alongside the anterior median fissure. Just above the spinomedullary junction, the fissure is invaded by the decussation of the pyramids , where fibres of the two pyramids intersect while crossing the midline. Lateral to the pyramid is the olive , posterior to which is the inferior cerebellar peduncle . Attached between the pyramid and the olive is the hypoglossal nerve (XII). Attached between the olive and inferior cerebellar peduncle are the glossopharyngeal (IX), vagus (X), and cranial accessory (XIc) (likely caudal or posterior medullary rootlets of the vagus nerve) nerves . The spinal accessory nerve (XIs) arises from the spinal cord and runs up through the foramen magnum to join the cranial accessory nerve.
Dorsal view ( Figure 3.2 B)
The roof or tectum of the midbrain is composed of four colliculi. The superior colliculi process visual information, and the inferior colliculi process auditory information. The trochlear nerve (IV) emerges below the inferior colliculus on each side.
The diamond-shaped fourth ventricle lies posterior to the pons and upper medulla oblongata, under cover of the cerebellum. The upper half of the diamond is bounded by the superior cerebellar peduncles , which are attached to the midbrain. The lower half is bounded by the inferior cerebellar peduncles , which are attached to the medulla oblongata. The middle cerebellar peduncles enter from the pons and overlap the other two.
Near the midline in the midregion of the floor of the fourth ventricle is the facial colliculus , which is created by the facial nerve curving around the nucleus of the abducens nerve. The vestibular area and the vagal and hypoglossal trigones overlie the corresponding cranial nerve nuclei. The obex is the inferior apex of the ventricle.
Below the fourth ventricle, the medulla oblongata shows a pair of gracile tubercles flanked by a pair of cuneate tubercles .
Sectional views
In the midbrain, the central canal of the embryonic neural tube is represented by the cerebral aqueduct . Behind the pons and upper medulla oblongata ( Figure 3.3 ), it is represented by the fourth ventricle, which is tent-shaped in this view. The central canal resumes at midmedullary level; it is continuous with the central canal of the spinal cord, although movement of cerebrospinal fluid into the cord canal is negligible.
The intermediate region of the brainstem is called the tegmentum , which in the midbrain contains the paired red nuclei . Ventral to the tegmentum in the pons is the basilar region . Ventral to the tegmentum in the medulla oblongata are the pyramids.
The tegmentum of the entire brainstem is permeated by an important network of neurons, the reticular formation . The tegmentum also contains ascending sensory pathways carrying general sensory information from the trunk and limbs. Illustrated in Figures 3.4 to 3.6 are the dorsal column–medial lemniscal ( DCML ) pathways , which inform the brain about the position of the limbs in space. At spinal cord level, the label DC ML is used because these pathways occupy the dorsal columns of white matter in the cord. In the brainstem, the label DC ML is used because they continue upward as the medial lemnisci .
The most important motor pathways from a clinical standpoint are the corticospinal tracts (CSTs), the pathways for execution of voluntary movements. The CSTs are placed ventrally, occupying the crura of the midbrain, the basilar pons, and the pyramids of the medulla oblongata.
Note that, in the medulla oblongata, the DCML pathways and CSTs decussate: one of each of the paired tracts intersects with the other to gain the contralateral (opposite) side of the neuraxis (brainstem–spinal cord). The four most important decussations are illustrated in Box 3.1 .
In the following account of seven horizontal sections of the brainstem, the positions of the cranial nerve nuclei are not included.
Midbrain ( Figure 3.4 )
The main landmarks have already been identified. On each side, the medial lemniscal component of the DC ML pathway occupies the lateral part of the tegmentum (upper section) , on its way to the ventroposterolateral (VPL) nucleus of the thalamus immediately above this level. The CST has arisen in the cerebral cortex, and it is descending in the midregion of the cerebral crus on the same side.
The decussation of the superior cerebellar peduncles straddles the midline at the level of the inferior colliculi (lower section).
Pons ( Figure 3.5 )
In the upper section, the cavity of the fourth ventricle is bordered laterally by the superior cerebellar peduncles, which are ascending (arrows) to decussate in the lower midbrain. In the floor of the ventricle is the central grey matter. The medial lemniscus occupies the ventral part of the tegmentum on each side. The basilar region contains millions of transverse fibres , some of which separate the CST into individual fascicles. The transverse fibres enter the cerebellum via the middle cerebellar peduncles and appear to form a bridge (hence, pons ) connecting the cerebellar hemispheres. But the individual transverse fibres arise on one side of the pons and cross to enter the contralateral cerebellar hemisphere. The transverse fibres belong to the giant corticopontocerebellar pathway , which travels from the cerebral cortex of one side to the contralateral cerebellar hemisphere, as depicted in Box 3.1 .
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