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, nucleus cuneatus; NG, nucleus gracilis; Vn, spinal trigeminal nucleus; Vt, spinal trigeminal tract.

Figure 11-3 Rostral medulla (pyramids, fourth ventricle). 4V, Fourth ventricle; IAF, internal arcuate fibers; ICP, inferior cerebellar peduncle; IO, inferior olivary nucleus; MLF, medial longitudinal fasciculus (explained in Chapter 12); Vn, spinal trigeminal nucleus; Vt, spinal trigeminal tract.

Every level of the pons contains part of the basal pons and fourth ventricle; the caudal pons is the part physically attached to the cerebellum, primarily by the middle cerebellar peduncles (Fig. 11-4). Here the medial lemniscus starts to flatten out and move laterally, and axons emerging from deep cerebellar nuclei start to form the superior cerebellar peduncle. The rostral pons is no longer connected to the cerebellum (Fig. 11-5); the middle cerebellar peduncles haven’t formed yet, and the superior cerebellar peduncles have left the cerebellum and are traveling rostrally through the brainstem. The trigeminal nerve is attached to the brainstem at the caudal pons-rostral pons junction.

Figure 11-4 Caudal pons (basal pons, middle cerebellar peduncle). MCP, Middle cerebellar peduncle; MLF, medial longitudinal fasciculus; PN, pontine nuclei (clumps of gray matter scattered all through the basal pons, just one neuron shown here); SCP, superior cerebellar peduncle; Vn, spinal trigeminal nucleus; Vt, spinal trigeminal tract.

Figure 11-5 Rostral pons (basal pons, no attachment to cerebellum). 4V, Fourth ventricle; MLF, medial longitudinal fasciculus; SCP, superior cerebellar peduncle.

The cerebral aqueduct continues the ventricular system through the midbrain, surrounded by a distinctive area of periaqueductal gray that participates in many of the autonomic control functions discussed in Chapter 23. The caudal midbrain is the part containing the inferior colliculi (Fig. 11-6). Here the superior cerebellar peduncles decussate, so inputs to the cerebellum from each cerebral hemisphere cross in the basal pons and outputs cross back in this decussation of the superior cerebellar peduncles. The rostral midbrain is the part containing the superior colliculi (Fig. 11-7); it also contains two other distinctive areas of gray matter, the red nucleus and substantia nigra. The red nucleus is hooked up in cerebellar circuitry (see Chapter 20), and the substantia nigra is part of the basal ganglia (see Chapter 19). The trochlear nerve emerges at the pons-midbrain junction, and the posterior commissure is located at the midbrain-diencephalon junction.

Figure 11-6 Caudal midbrain (aqueduct, inferior colliculi). A, Cerebral aqueduct; CP, cerebral peduncle (the middle part contains the corticospinal tract, most of the rest contains corticopontine fibers); MLF, medial longitudinal fasciculus; PAG, periaqueductal gray; PN, pontine nuclei (the last few, as the cerebral peduncles replace the basal pons).

Figure 11-7 Rostral midbrain (aqueduct, superior colliculi). A, Cerebral aqueduct; CP, cerebral peduncle; MLF, medial longitudinal fasciculus; PAG, periaqueductal gray; RN, red nucleus (the superior cerebellar peduncles seem to have disappeared, but their fibers mostly pass through or around this nucleus); SN, substantia nigra.

Figures 11-2 through 11-7 indicate just the major features of each of these brainstem levels. Information about additional brainstem functions and connections (e.g., cranial nerve nuclei) can be found in THB6 and in Chapters 12–14 of this book. The same figures are shown again in Chapter 15, with the structures discussed in Chapters 12–14 added in.

The Reticular Core of the Brainstem Is Involved in Multiple Functions

The reticular formation forms a core of neural tissue in the brainstem, surrounded by cranial nerve nuclei and the tracts mentioned thus far. It samples the information carried by most sensory, motor, and visceral pathways. The reticular formation uses some of this information in various reflexes (e.g., circulatory and respiratory reflexes, swallowing, coughing). It also sends outputs caudally to the spinal cord and rostrally to the diencephalon and cerebral cortex. The outputs to the spinal cord mediate some aspects of movement, control the sensitivity of spinal reflexes, and regulate the transmission of sensory information (especially pain) into ascending pathways. The outputs to the cerebrum from a portion of the reticular formation called the ascending reticular activating system (ARAS) modulate the level of cortical activity and hence the level of consciousness; the ARAS is important in sleep-wake cycles.

Some Brainstem Nuclei Have Distinctive Neurochemical Signatures

Most of the neurons described so far in this book have discrete connections that seem to suit them for preserving the details of information—e.g., somatotopic projections from motor cortex to lower motor neurons, or from nucleus cuneatus to a particular small part of the thalamus. In contrast, there are some collections of brainstem neurons with extremely widespread connections, not designed for point-to-point transmission of information, but designed instead to have modulatory effects that regulate the background level of activity in large parts of the CNS. Each of these collections is made up of neurons using a distinctive small-molecule transmitter with slow, second-messenger effects on their targets. The most prominent examples are norepinephrine, dopamine, serotonin, and acetylcholine. The diffuse projections of the central noradrenergic, dopaminergic, serotonergic, and cholinergic neurons make them better suited for more general roles in adjusting the background level of activity or sensitivity of large parts of the CNS, each of the four in a somewhat different way.

Neurons of the Locus Ceruleus Contain Norepinephrine

Neurons that use norepinephrine as a transmitter (noradrenergic neurons) are found in the peripheral nervous system as postganglionic sympathetic neurons. In the CNS, some are located in the medullary reticular formation, but most are pigmented neurons of the locus ceruleus in the rostral pons (Fig. 11-8). CNS noradrenergic neurons collectively project to practically every part of the CNS.

Figure 11-8 Noradrenergic neurons of the locus ceruleus.

Neurons of the Substantia Nigra and Ventral Tegmental Area Contain Dopamine

Most of the neurons that use dopamine as a transmitter (dopaminergic neurons) are located in the midbrain (Fig. 11-9), either in the compact part of the substantia nigra or closer to the midline in the ventral tegmental area. Nigral dopaminergic neurons project to the caudate nucleus and putamen, and their degeneration causes Parkinson’s disease. Ventral tegmental neurons project to an assortment of limbic structures and (mostly frontal) cortical areas, and malfunction of these neurons or their targets plays a role in some forms of mental illness.

Figure 11-9 Brainstem dopaminergic neurons. As discussed in Chapter 19, the substantia nigra is a two-part structure. The part farther from the cerebral peduncle (the compact part) is where the dopaminergic neurons live. SNc, Substantia nigra (compact part); VTA, ventral tegmental area.

Neurons of the Raphe Nuclei Contain Serotonin

Neurons that use serotonin as a transmitter (serotonergic neurons) are located primarily in the raphe nuclei, a collective name for a series of nuclei near the midline of the brainstem reticular formation (Fig. 11-10). Like the noradrenergic neurons of the locus ceruleus, the serotonergic neurons of the raphe nuclei project practically everywhere in the CNS, suggesting that they too may be involved in adjusting levels of attention or arousal.

Figure 11-10 Serotonergic raphe nuclei in the caudal pons. Similar serotonergic neurons are distributed in midline raphe nuclei in most of the brainstem.

Neurons of the Rostral Brainstem and Basal Forebrain Contain Acetylcholine

Neurons that use acetylcholine as a transmitter (cholinergic neurons) play an important role in the peripheral nervous system. Some are located entirely in the periphery (postganglionic parasympathetic neurons), whereas others have their cell bodies in the CNS and axons that travel through spinal or cranial nerves (lower motor neurons, preganglionic sympathetic and parasympathetic neurons).

Some CNS cholinergic neurons are local interneurons in various structures (such as the putamen and caudate nucleus). Others have longer axons that project from one part of the CNS to another. Some of these are located in the midbrain reticular formation, but the largest collection is in the basal nucleus (of Meynert), a group of large cholinergic neurons in the basal forebrain (Fig. 11-11) that project to widespread areas of the cerebral cortex. Nearby cholinergic neurons in the septal nuclei project to the hippocampus. These basal forebrain neurons degenerate in victims of Alzheimer’s disease.

Figure 11-11 CNS cholinergic neurons in the basal nucleus. Other cholinergic neurons in the midbrain reticular formation modulate the activity of the thalamus.

The Brainstem Is Supplied by the Vertebral-Basilar System

The vertebral arteries run along the lateral and anterior surfaces of the medulla and fuse to form the basilar artery, which runs along the anterior surface of the pons until it bifurcates into the posterior cerebral arteries on the anterior surface of the midbrain (THB6 Figures 6-3 and 11-29, pp. 126 and 291). Hence, nearly the entire supply of the brainstem comes from the vertebral-basilar-posterior cerebral system (THB6 Figure 11-30, p. 292). Knowing where each of the major branches from this system arises lets you surmise the supply of different levels of the brainstem (Fig. 11-12), because even if a branch (e.g., PICA) is headed for someplace like the cerebellum it needs to wrap around the brainstem to get there.

Figure 11-12 Vertebral-basilar branches on the surface of the brainstem. ACA, Anterior cerebral artery; ACoA, anterior communicating artery; AICA, anterior inferior cerebellar artery; BA, basilar artery; Di, diencephalon; ICA, internal carotid artery; PCA, posterior cerebral artery; PCoA, posterior communicating artery; PICA, posterior inferior cerebellar artery; SCA, superior cerebellar artery; VA, vertebral artery.