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Six The Human Nervous Syslem: An Analomical ViewoinL Sixlh Edilion, Mufiay L. Barr and John A. Kieman J,B, Lippincott Company, Philadelphia, O I993 + & 6 94 &r el :l :': : f i! 't. ortant Facts hoceeding laterally from the ventral midline, the anatomical landmarks at different levels are as follows, Each represents a functionally important nucleus or tract within the brain stem. The student must also know the sites of emergence of cranial nerues III to XII in relation to these landmarks. Medulla: furamid, olive, inferior cerebellar peduncle; cuneate and gracile tubercles (below obe><); floor of fourth ventricle (above obe><). Pons: Basal part of pons, middle cerebellar peduncle, superior cerebellar peduncle, floor of fourth ventricle. Midbrain: Interpeduncular frossa; basis pedunculi, inferior or superior colliculus. In the floor of the fourth ventricle motor nuclei of cranial nerves are typically medial and sensory nuclei lateral to the sulcus limitans. There are named areas for hypoglossal, vagal and vestibular nuclei. The,Tacial colliculus overlies the abducens nucleus. The superior and inferior medullaryvela form the roof of the fourthventricle, which narrows into the central canal caudallv and the cerebral aoueduct rostrally. Cerebrospinal fluid enters the fourth ventricle from the aqueduct and leaves by way of the median and lateral apertures. brain stem consists of the medulla oblongata, pons, and midbrain. Although each of The the three regions has special features, they have certain fiber tracts in common. and each region includes nuclei of cranial nerves. The fourth ventricle is partly in the medulla and partly in the pons. It is advantageous therefore 88 to describe the medulla, pons, and midbrain together. This chapter is concerned with the sutface landmarks of the brain stem. For more details ofthe internal features ofthe brain stem (such as certain nuclei and tracts) that are mentioned in this chapter, see Chapter 7 or refer to the Index. The central connections and 4 5 : 't ! 'j I Chapter 6: Brain Stem: External B functions of the cranial nerve s are explained in Chapter 8. Medulla Oblongata The medulla oblongata (ormedulla) is about 3 cm long and widens gradually in a rostral di- It rests on the basilar portion of the occipital bone and is concealed from above by rection. Anatomy g9 the cerebellum. The junction of the spinal cord and medulla is at the upper rootlet of the first cervical nerve, num. The rostr marked on the sulcus (Fig. 6-l and 6-2). On the dorsal surface, the junction between the pons and medulla is an imaginary transverse line that passes between the caudal margins of the mid- Optic tract Mamillary body Interpeduncular fossa Oculomotor n. Basis pedunculi of cerebral peduncle Trigeminal n. Basilar sulcus Middle cerebellar peduncle Abducens Facial Basal portion of pons n, n. Vestibulocochlear n. Clossopharyngeal n. Ove Vagus n. Pyramid Hypoglossal n Ventrolateral sulcus Ventral median fissure Cranial root of accessory n. Spinal root of accessory n. Ventral root of f irst cervical n. Figure 6-1. Ventral aspect of the brain stem. 90 Regional Anatomy of the Central Neryous System Lateral geniculate body Medial geniculate body Optic tract Superior colliculus lnferior brachium Inferior colliculus Superior brachium Trochlear n. Cerebral peduncle Position of lateral lemniscus Superior cerebel lar peduncle Basal portion of pons Trigeminal n. Middle cerebellar peduncle I nferior cerebellar peduncle Vestibulocochlear n. Abducens n. Facial Pyramid n, Clossopharyngeal n. Cuneate tubercle Vagus n. Cracile tubercle Hypoglossal n, Dorsolateral sulcus Olive Fasciculus cuneatus Cranial root of accessory n Tuberculum cinereum Spinal rootof accessory n, Fasciculus gracilis Ventrolateral sulcus First cervtcal n. Figure 5-2. Lateral aspect of the brain stem. SCTT dle 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 portion, That part of the medulla between the obex (see below) and the first cervical segment of the spinal cord is called the closed portion; ir contains a continuation of the central canal of the spinal cord, The ventricle results from a flexure of the embryonic brain with a dorsal concavity (the pontine flexure) and subsequent development of the large cerebellum with its thick peduncles. These events caused divergence of the dorsal halves of the maturing brain stem, so that its lumen widened out to form the fourth ventricle, The longitudinal grooves previously de- me( rup dles nal me( ther The late suri Chapter 6: Brain Stem: External Anatomy Pineal body Superior colliculus Medial geniculate body Superior brachium Lateral geniculate body Inferior brachium lnferior colliculus Trochlear n Superior medullary velum Position Medial em of lateral lemniscus Superior cerebellar peduncle Sulcus limltans I nferior cerebellar peduncle Striae medullares Middle cerebellar peduncle Area vestibuli Hypoglossal triangle Dorsal cochlear nucleus Vestibulocochlear Clossopharyngeal Obex n n Vagal triangle Cuneate tubercle Area postrema Cracile tuberc Vagus n Dorsolateral sulcus Dorsal median sulcus Fasciculus cuneatus Cranial root of accessory Tuberculum cinereum Spinal root of accessory Fascicgrlus gracilis n. n. Dorsal root of first cervical n. Figure 5-3. Dorsal aspect of the brain stem. scribed for the spinal cord continue on the medulla. The ventral median fissure is interrupted at the spinomedullary junction by bundles ofdecussating fibers, These are corticospi- nal fibers that cross from the pyramid of the medulla to the opposite side of the cord, where they constitute the lateral corticospinal tract. The dorsal median, dorsolateral, and ventrolateral sulci also extend from the cord onto the surface of the medulla. On each side, several bulges or eminences are outlined by the sulci. Ventrally, the pyramid (see Fig. 6-l) consists of corticospinal fibers. This is the origin of the term ,,pyramidal ftact" as a synonym for corticospinal tract. Laterally (Fig. 6-2), the olive is a prominent oval swelling that marks the position of the inferior olivary nucleus. Rootlets of the glossopharyngeal, vagus, and accessory nerves are attached to the medulla just dorsal to the olive, 9l 92 Regional Anatomy of the Central Nervous System The tuberculum cinereum, immediately dorsal to these nerve rootlets, is a rather incon- nerves as well as those of the cranial division of the accessory nerve are attached to the pa spicuotis ridge that marks the position of the dorsal spinocerebellar tract and the more deeply situated spinal tract of the trigeminal nerve and its associated nucleus, The latter are comparable to the dorsolateral tract (of Lis- medulla along a line between the olive and the tuberculum cinereum, The accessory nerve is tre motor, whereas the glossopharyngeal and vagus nerves are mixed, having sensory and motor components. The cranial root of the accessory nerve is joined by the spinal root, and the glossophanTngeal, vagus, and accessory nerves leave the posterior cranial fossa through the tic jugular foramen, Roots of the hypoglossal nerve, a motor nerve, emerge along the ventrolateral sulcus between the pyramid and the m, olive, and the nerve leaves the posterior fossa rh sauer) and the outer laminae of the dorsal horn in the spinal cord. The gracile and cuneate fasciculi continue from the spinal cord into the dorsal area of the medulla (see Fig. 6-2 and 6-3), The gracile and cuneate tubercles are slight elevations at the rostral ends of the corresponding fasciculi of the spinal cord. They contain the gracile and cuneate nuclei, in which the fibers of the fasciculi end. The apex of the V-shaped boundary of the inferior portion of the fourth ventricle, which is folded caudally over the most rostral I to 2 mm of the central canal, is known as the obex. 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 be- tween the pons and the pyramid of the J ' medulla, The nerve passes forward in the subarachnoid space beneath the pons, traverses the cavernous venous sinus (described in Clls. 25 and 26), and enters the orbit through the superior orbital fissure. The facial and vestibulocochlear nerves are attached to the brain stem at the caudal border of the pons well out laterally. The facial nerve, which is the more medial, has two roots, motor and sensory, The sensory root, which includes some efferent parasympathetic fibers, lies be- tween the larger motor root and the vestibulocochlear nerve; it is therefore known as the nervus intermedius (of Wrisberg). 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, whereas the vestibular division penetrates the brain stem deep to the root of the inferior cerebellar peduncle. The facial and vestibulocochlear nerves enter the internal acoustic meatus in the petrous temporal bone, Roots of the glossopharyngeal and vagus cr( rh te) sit co m' CC hr th through the hypoglossal canal. pc m ns de This part of the brain stem, which is about 2,5 cm long, owes its name to the appearance presented on its ventral surface (see Fig. 6- l), which is that of a bridge connecting the right and left cerebellar hemispheres. The appearance is deceptive as far as the constituent nerve TT fibers are concerned, as noted below. The pons consists of quite different basal (ventral) and dorsal portions (see Figs. 7 -9 and fe. 7- r0), The basal portion is distinctive of this flc is lll SO TI ni th part of the brain stem. A shallow groove, the basilar srilcus, runs along its ventral surface in the midline. The pons merges 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- I and 6-2). The motor root of the F, T1 b( in ol flt ffigeminal nerve is rostromedial to the larger sensory root. The trigeminal nerve enters the middle cranial fossa at the medial end of the petrous temporal bone, where the trigeminal m di ganglion is located. The three divisions of the nerve diverge from the ganglion, embedded in the dura mater, The ophthalmic division passes through the superior orbital {issure to reach the orbit. The maxillary division traverses the foramen rotundum, and the mandibular division traverses the foramen ovale. Fibers from the cerebral cortex terminate ipsilaterally on nerve cells that compose the tt T] rt rt nl pi tr d nl tt lir Chapter 6: Bfain Stem: F.xternal pontine nuclei, and axons of the latter cells cross the midline and then constitute the contralateral middle cerebellar peduncle. In effect, the basal pons is a large synaptic or relay station, providing a connection between the cortex of each cerebral hemisphere and the opposite cerebellar hemisphere as part of a circuit contributing to efficient voluntary movements. The cerebral cortex, basal pons, and cerebellum all increased in size during mammalian evolution and are best developed in the human brain. The corticospinal tracts traverse the basal portion of the pons before they enter the pyramids (see Fig. 7-9). The dorsal portion or tegmentum of the pons is similar to much of the medulla and midbrain, in that it contains,ascending and descending tracts and nuclei ofcranial nerves. The dorsal surface of the pons is formed by the flooi of the fourth ventricle . The rostral part of the pons is known as the isthmus of the brain stem. A slight bandlike elevation runs obliquely across the dorsolateral surface of the isthmus toward the inferior colliculus of the midbrain (see Fig. 6-2). This elevation is produced by the lateral lemniscus, which carries auditory fibers through the pons. Fourth ntricle The floor of the fourth ventricle (rhom- boid fossa) is broad in its midportion, narrowing 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 vestibular nuclear complex lies beneath the floor of most of the lateral area. This area is therefore known as the vestibular area of the rhomboid fossa. Motor 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 rostral end of the dorsal nucleus of the vagus nerve lies beneath the vagal triangle (or ala cinerea), and the rostral end of the hypoglossal nucleus Iies beneath the hypoglossal triangle. The area Anatomy 93 postrema is a narrow strip between the vagal triangle and the most caudal part of the margin of the ventricle. Because the appeararrce of this part of the rhomboid fossa suggested the tip of a pen to early anatomists, the term calamus scriptorius was applied to it, The facial colliculus, a slight swelling at the lower end of the medial eminence (see Fig. 6-3), is formed by fibers from the moror nucleus of the facial nerve looping over the abducens nucleus. There is a pigmented area, the locus coeruleus, at the rostral end of the sulcus limitans, indicating the site of a cluster of noradrenergic nerve cells that contain melanin pigment, In the middle of the floor of the fourth ventricle, delicate strands of nervc fibers emerge from the median sulcus, run laterally as the striae medullares, and enter the inferior cerebellar peduncle. The connections of these fibers are explained in Chapter 7. The tent-shaped roof of the fourth ventricie protrudes 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 sheet of tissue that consists of a layer of pia mater and one of ependyma with nerve fibers in between. The remainder of the roof consists of a thinner pial-ependymal membrane, the inferior medullary velurn, which often 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, alternatively known as the foramen of Magendie. This hole provides the principal communication between the ventricular system and the subarachnoid space (Fig. 6-4). The lateral walls of the fourth ventricle in- clude 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 fbrarnina 94 Regional Anatomy of the Central Nervous System 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. of Luschka), which are two other channels through which cerebrospinal fluid enters the subarachnoid space (Fig. 6- 5 ). These foramina are at the junction of the medulla, pons, and cei:ebellum (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 velurrr, the plexus extends into the lateral recesses, and a small tuft usually protrudes through each foramen of Luschka. Midbrain The midbrain is about I.5 cm long. Its ventral surface extends from the pons to the mamillarybodies of the diencephalon (see Fig. 6-I). The robust column of white matter on each side is the basis pedunculi (crus cerebri), which consists of fibers of the pyramidal motor system and corticopontine fibers. The deep depression between these two columns is the interpeduncular fossa. Many small blood vessels penetrate the midbrain in the floor of the interpeduncular fossa; this region is there- fore known as the posterior perforated sqbstance. The oculomotor nerve emerqes (x 2.5) from the side of the interpeduncular fossa and passes forward through the cavernous venous sinus and then through the superior orbital fissure into the orbit. The lateral surface of the midbrain (see Fig. 6-2) is formed mainly by the cerebral peduncle, which constitutes the major portion of tair tex otl 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 d.escribed in Chapter 7. 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 and 6-3) make up the tectum and indicate the extent of the midbrain on the dorsal surface. The inferior colliculus is a relay nucleus on the auditory pathway. Fibers that con - nal nect the inferior colliculus with the specific thalamic nucleus for hearing (medial genicuIate nucleus) form an elevation known as the thr inferior brachium (see Figs. 6-2 and 6-3). The superior colliculus is involved in the control of ocular movements and related movements of the head in response to visual and other stimuli. The superior brachium con- IOS are pul em dal mi, the Ldt tw Fig thr Chapter 6: Bfain Stem: External Anatouy 95 C lossopharyngeal, vagus, and accessory nerves Figure 6-5. Lateral apertures of the fourth ventricle (foramina of Luschka). Tlrfts of choroid plorus (arcows) occupy the foramina, into which marker sticks (black) have been inserted. (x 1.5) tains 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 dial and lateral geniculate bodies, and a prominent part of the thalamus known as the pulvinar (see Figs. 7-14 and 7-15). are pafi of a pathway from the retina for the pupillary light reflex, The trochlear nerve SUGGESTED READING emerges from the brain stem immediately cau- Bertram EGM, Moore I(Ll An Atlas of the Human Brain and Spinal Cord, Baltimore, Williams & dal to the inferior colliculus, curves around the midbrain, and enters the orbit after traversing the cavernous venous sinus. 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 me- Wilkins, 1982 Montemurro DG, Bruni JE: The Human Brain in Dissection, 2nd ed. New York, Oxford University Press, 1988 Noback CR, Strominger NL, Demarest RJ: The Human Nervous System: Introduction and Review, 4th ed. Philadelphia, Lea 6 Febiger, l99t Smith CG: Serial Dissections of the Human Brain. Baltimore, Urban & Schwarzenberg, lgBI