Important Facts
The brain stem consists of the medulla oblongata, pons, and midbrain. Each region has special features, including nuclei of cranial nerves, but long tracts of fibers are present at all levels. The fourth ventricle is partly in the medulla and partly in the pons. This chapter is concerned with the surface landmarks of the brain stem. For more details of the internal features of the brain stem (e.g., certain nuclei and tracts), see Chapter 7 or refer to the Index. The central connections and functions of the cranial nerves are explained in Chapter 8.
Medulla Oblongata
The medulla oblongata (or medulla) is about 3 cm long and widens gradually in a rostral direction. It rests on the midline part of the occipital bone and is covered dorsally by the cerebellum. The junction of the spinal cord and medulla is level with the foramen magnum. The rostral limit of the medulla is marked ventrally by a prominent sulcus (Figs. 6-1 and6-2). On the dorsal surface, the junction is an imaginary transverse line that joins the caudal margins of the middle cerebellar peduncles (Fig. 6-3). The dorsal surface, therefore, contains the caudal half of the fourth ventricle; this rostral end of the medulla is known as the open part because the thin roof of the fourth ventricle is usually removed in the course of dissection. The caudal part of the medulla is called the closed part; it contains a continuation of the central canal of the spinal cord.
The surface of the medulla is marked by several bulges or eminences, outlined by sulci. Ventrally, the pyramid (see Fig. 6-1) consists of corticospinal fibers. This is the origin of the term pyramidal tract as a synonym for corticospinal tract. In the most caudal part of the medulla, most of the pyramidal fibers cross the midline; the decussating fibers obscure the ventral median sulcus at this level. Lateral to the pyramid, the olive (see Fig. 6-2) is an oval swelling that marks the position of the inferior olivary nucleus. Lateral to the olive, theinferior cerebellar peduncle is a body of white matter that connects the medulla with the cerebellum and forms the
P.80
side wall of the caudal half of the fourth ventricle. On the dorsal surface of the closed part of the medulla, the gracile and cuneate fasciculi continue from the spinal cord (see Figs. 6-2 and 6-3). The axons in the fasciculi terminate in the gracile and cuneate nuclei, which cause slight elevations, the gracile and cuneate tubercles. The apex of the V-shaped boundary of the inferior part of the fourth ventricle, which is folded caudally over the most rostral 1 to 2 mm of the central canal, is known as the obex.
|
FIGURE 6-1 Ventral aspect of the brain stem. |
Seven cranial nerves are attached to the medulla or to the junction of the medulla and pons (see Figs. 6-1, 6-2, and 6-3). The abducens nerve emerges near the midline between the pons and the pyramid. The facial and vestibulocochlear nerves are attached to the lateral aspect of the brain stem at the caudal border of the pons. The facial nerve, which is the more medial, has two roots. The smaller sensory and parasympathetic root lies between the larger motor root and the vestibulocochlear nerve; therefore, it is known
P.81
as the nervus intermedius. The cochlear division of the vestibulocochlear nerve ends in the dorsal and ventral cochlear nuclei, which are situated on the base of the inferior cerebellar peduncle. The vestibular division penetrates the brain stem deep to the root of the inferior cerebellar peduncle.
|
FIGURE 6-2 Lateral aspect of the brain stem. |
Roots of the glossopharyngeal and vagus nerves as well as those of the cranial division of the accessory nerve are attached to the medulla rostral and dorsal to the olive. The cranial root of the accessory nerve is joined by the spinal root, and the glossopharyngeal, vagus, and accessory nerves all leave the posterior cranial fossa through the jugular foramen. Rootlets of the hypoglossal nerve emerge along the sulcus between the pyramid and the olive.
Pons
This part of the brain stem is about 2.5 cm long. It owes its name to the appearance of its ventral surface (see Fig. 6-1), which is that of a bridge connecting the right and left cerebellar hemispheres. (This appearance is deceptive, as noted later.) The
P.82
pons consists of quite different basal (ventral) and dorsal parts (see Figs. 7-9 and 7-10).
|
FIGURE 6-3 Dorsal aspect of the brain stem. |
The basal part is indented along its ventral surface in the midline by a shallow groove, the basilar sulcus, which accommodates the basilar artery. The basal pons blends laterally into the middle cerebellar peduncles, with the attachment of the trigeminal nerve marking the transition between the pons and the peduncle (see Figs. 6-1 and 6-2). The motor root of the trigeminal nerve is rostromedial to the larger sensory root. Fibers from the cerebral cortex terminate ipsilaterally on neurons that compose the pontine nuclei, and axons of the latter cells cross the midline and then constitute the contralateral middle cerebellar peduncle. The basal pons thus serves as a large synaptic relay station, providing a connection from the cortex of each cerebral hemisphere to the opposite cerebellar hemisphere. The corticospinal tracts traverse the basal part of the pons before they enter the pyramids (seeFig. 7-9).
The dorsal part or tegmentum of the pons is similar to much of the medulla and midbrain in that it contains ascending and descending tracts and nuclei of cranial nerves. The dorsal surface of
P.83
the pons is formed by the floor of the fourth ventricle. The most rostral part of the pons is known as the isthmus of the brain stem; it is situated immediately below the cerebral peduncles and inferior colliculi of the midbrain (see Fig. 6-2).
Fourth Ventricle
When the cerebellum is removed by cutting through its six peduncles, the thin roof of the fourth ventricle comes off with it, exposing the floor of the ventricle on the dorsal aspect of the brain stem (see Fig. 6-3).
The diamond-shaped floor of the fourth ventricle, also called the rhomboid fossa, narrows toward the obex caudally and the aqueduct of the midbrain rostrally (see Fig. 6-3). The floor is divided into symmetrical halves by a median sulcus. The sulcus limitans further divides each half into medial and lateral areas. The lateral area is known as the vestibular area because there the vestibular nuclear complex lies beneath most of the floor of the ventricle.
Motor and parasympathetic nuclei are located beneath the floor of the medial area. The caudal part of the rhomboid fossa is marked by two triangles or trigones. The vagal trigone(or ala cinerea) marks the site of the rostral ends of the dorsal nucleus of the vagus nerve and the rostral end of the solitary nucleus. The hypoglossal trigone is a landmark for the rostral end of the hypoglossal nucleus. The facial colliculus, a swelling at the lower end of the medial eminence (see Fig. 6-3), is formed by fibers from the motor nucleus of the facial nerve looping over the abducens nucleus.
|
FIGURE 6-4 Median aperture of the fourth ventricle (foramen of Magendie) opening from the fourth ventricle into the cerebellomedullary cistern of the subarachnoid space (x2.5). |
A pigmented area, the locus coeruleus, is situated at the rostral end of the sulcus limitans, indicating the site of a cluster of noradrenergic neurons that contain melanin pigment. In the middle of the floor of the fourth ventricle, delicate strands of nerve fibers emerge from the median sulcus, run laterally as the striae medullares, and enter the inferior cerebellar peduncle. The connections of these fibers, which are more conspicuous in some brains than in others, are explained in Chapter 7.
The tent-shaped roof of the fourth ventricle protrudes dorsally toward the cerebellum. The rostral part of the roof is formed on each side by the superior cerebellar peduncles, which consist mainly of fibers proceeding from cerebellar nuclei into the midbrain. The V-shaped interval between the converging peduncles is bridged by the superior medullary velum, a thin sheet of white matter. The remainder of the roof consists of a thinner pial-ependymal membrane, the inferior medullary velum, which adheres to the undersurface of the cerebellum. A deficiency of variable size in the inferior medullary velum constitutes the median aperture of the fourth ventricle, also called the foramen of Magendie. This hole provides the principal communication between the ventricular system and the subarachnoid space (Fig. 6-4).
P.84
The lateral walls of the fourth ventricle include the inferior cerebellar peduncles, which curve from the medulla into the cerebellum on the medial aspects of the middle peduncles (see Fig. 6-3). Lateral recesses of the ventricle extend around the sides of the medulla and open ventrally as the lateral apertures of the fourth ventricle (the foramina of Luschka), which are two other channels through which cerebrospinal fluid enters the subarachnoid space (Fig. 6-5). These foramina are situated at the junction of the medulla, pons, and cerebellum (the cerebellopontine angles) near the attachment to the brain stem of the vestibulocochlear and glossopharyngeal nerves.
The choroid plexus of the fourth ventricle is suspended from the inferior medullary velum; the plexus extends into the lateral recesses, and a small tuft usually protrudes through each lateral aperture. Choroid plexus is the tissue that secretes cerebrospinal fluid (see Chapter 26). Most of the fluid is produced in the lateral and third ventricles and flows into the fourth ventricle by way of the cerebral aqueduct. The choroid plexus of the fourth ventricle makes a small addition to the volume of the cerebrospinal fluid in the cavity of the ventricle and directly into the subarachnoid space of the pontocerebellar angle (see Fig. 6-5).
|
FIGURE 6-5 Lateral apertures of the fourth ventricle (formina of Luschka). Tufts of choroid plexus (arrows) occupy the foramina, into which metal marker sticks have been inserted. |
Midbrain
The midbrain is about 1.5 cm long. Its ventral surface extends from the pons to the mamillary bodies of the diencephalon (see Fig. 6-1). The robust column of white matter on each side is the basis pedunculi (crus cerebri), composed of corticospinal, corticobulbar, and corticopontine fibers. The deep depression between these two columns is theinterpeduncular fossa. Many small blood vessels penetrate the midbrain in the floor of the interpeduncular fossa; therefore, this region is known as the posterior perforated substance. The oculomotor nerve emerges from the side of the interpeduncular fossa.
The lateral surface of the midbrain (see Fig. 6-2) is formed mainly by the cerebral peduncle, which constitutes the major part of this region of the brain stem on each side. The cerebral peduncle comprises the basis pedunculi and some internal structures, the substantia nigra, and the tegmentum, which are described in Chapter 7.
P.85
The dorsal surface of the midbrain bears four rounded elevations, the paired inferior and superior colliculi (also called the corpora quadrigemina). These colliculi (see Figs. 6-2 and6-3) make up the tectum and indicate the extent of the midbrain on the dorsal surface. Fibers that connect the inferior colliculus with the medial geniculate body (part of the thalamus) form a ridge known as the inferior brachium (see Figs. 6-2 and 6-3). The superior colliculus is involved in the control of movements of the eyes and head in response to visual and other stimuli. The superior brachium contains fibers proceeding from the cerebral cortex and the retina to the superior colliculus. Other fibers in the superior brachium terminate in the pretectal area ventral and just rostral to the superior colliculi; these fibers are part of a pathway from the retina for the pupillary light reflex. The trochlear nerveemerges from the brain stem immediately caudal to the inferior colliculus and curves around the midbrain on its way toward the orbit.
The posterior part of the thalamus projects caudally beyond the plane of transition between the diencephalon and the midbrain (see Fig. 6-3). Consequently, transverse sections at the level of the superior colliculi include thalamic nuclei, in particular those of the medial and lateral geniculate bodies, and a prominent part of the thalamus known as the pulvinar (see Figs. 6-3, 7-14, and 7-15).
Suggested Reading
Barr ML. Observations on the foramen of Magendie in a series of human brains. Brain 1948;71:281-289.
England MA, Wakely J. Color Atlas of the Brain and Spinal Cord. An Introduction to Normal Neuroanatomy, 2nd ed. Orlando, FL: Mosby, 2005.
Haines DE. Neuroanatomy: An Atlas of Structures, Sections and Systems, 5th ed. Philadelphia: Lippincott, Williams & Wilkins, 2000.
Montemurro DG, Bruni JE. The Human Brain in Dissection, 2nd ed. New York: Oxford University Press, 1988.
Noback CR, Strominger NL, Demarest RJ, et al. The Human Nervous System: Structure and Function, 6th ed. Totowa, NJ: Humana Press, 2005.
Smith CG. Serial Dissections of the Human Brain. Baltimore: Urban & Schwarzenberg, 1981.