The Brainstem Has Conduit, Cranial Nerve, and Integrative Functions

The brainstem is another part of the CNS whose importance is out of proportion to its size. All the long tracts on their way to or from the spinal cord traverse the brainstem, so it has conduit functions. In addition, through connections with cranial nerves the brainstem takes care of the same basic sensory and motor functions for the head that the spinal cord does for the body, and takes care of some special senses as well (hearing, equilibrium, taste). Finally, the brainstem contains an anatomically diffuse reticular formation whose activity is crucial for a variety of functions, including the maintenance of consciousness.

The Medulla, Pons, and Midbrain Have Characteristic Gross Anatomical Features

Various nuclei and fiber bundles form surface features at different levels of the brainstem. The most prominent of these are listed in this section (Fig. 11-1).

Figure 11-1 Major surface features of the brainstem. IC, Inferior colliculus; ICP, inferior cerebellar peduncle; MCP, middle cerebellar peduncle; SC, superior colliculus; SCP, superior cerebellar peduncle. The dashed line indicates the approximate transition from medulla to spinal cord. (Thanks to Grant Dahmer.)

Major surface features of the medulla include the pyramids and the olives. Each pyramid is a longitudinal bundle of fibers on the ventral surface of the medulla, made up of the corticospinal tract of that side of the brainstem. The olive is an ovoid bump dorsolateral to each pyramid in the rostral medulla, underlain by the inferior olivary nucleus, an important component of cerebellar circuitry. The central canal of the spinal cord continues into the medulla and opens up into the fourth ventricle at a mid-medullary level.

The pons is dominated by its basal part, a large transverse band of fibers and nuclei for which the pons is named (pons is Latin for “bridge”). The basal pons looks like it interconnects the two halves of the cerebellum, but instead it’s the site of a transfer station through which each cerebral hemisphere talks to the contralateral half of the cerebellum. The pontine nuclei in each side of the basal pons receive cerebral inputs via the ipsilateral corticopontine tract. Axons from these nuclei then travel transversely, cross the midline, funnel into the contralateral middle cerebellar peduncle, and then fan out into the cerebellar cortex. The fourth ventricle is at its widest near the pontomedullary junction; it narrows progressively at more rostral pontine levels.

The surface of the midbrain includes the inferior colliculi, two rounded elevations on the dorsal surface of the caudal midbrain that are part of the auditory pathway; superior colliculi, two rounded elevations on the dorsal surface of the rostral midbrain, involved in eye movements and the direction of visual attention; and cerebral peduncles, large paired fiber bundles on the ventral surface of the midbrain, each carrying fibers descending from the cerebral cortex to the brainstem and spinal cord (mostly corticopontine and corticospinal fibers). The narrow fourth ventricle of the rostral pons is continuous with the cerebral aqueduct of the midbrain.

The Internal Structure of the Brainstem Reflects Surface Features and the Position of Long Tracts

The medulla, pons, and midbrain are commonly divided into rostral and caudal halves using some of the surface elevations just described and several other features. Each of these six brainstem levels has a few major, characteristic structures. All through the brainstem, the corticospinal tract is in a ventral location and the medial lemniscus is medial to the spinothalamic tract.

The caudal or closed medulla is the part that does not contain any portion of the fourth ventricle (Fig. 11-2); it extends from the pyramidal decussation to the beginning of the fourth ventricle. The posterior columns start to terminate in nuclei gracilis and cuneatus in the caudal medulla; axons of these second-order neurons arch through the reticular formation as internal arcuate fibers, cross the midline, and turn upstream as the medial lemniscus. The rostral or open medulla is the part that contains a portion of the fourth ventricle (Fig. 11-3); it extends from the caudal end of the fourth ventricle to the point at which the brainstem becomes attached to the cerebellum by the inferior and middle cerebellar peduncles. The pyramids are still there, and now the inferior olivary nucleus gets added. Axons of these neurons curve across the midline as more internal arcuate fibers and form most (but not nearly all) of the inferior cerebellar peduncle, which turns up into the cerebellum right at the pontomedullary junction.

Figure 11-2 Caudal medulla (pyramids, central canal). As explained in Chapter 12, the spinal trigeminal tract and nucleus are the parts of the trigeminal system that take care of pain and temperature information from the head. CC, Central canal; IAF, internal arcuate fibers; NC

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