Surface Anatomy of the Brainstem · 119 4

Ventral and lateral views. A ventral view of the medulla (Fig. 4.1a) reveals the pyramids, which lend their names to the pyramidal tracts, whose fibers course through them. The pyramidal decussation can also be seen here. Lateral to the pyramid on either side is another protrusion called the olive, which contains the inferior olivary nucleus.

The hypoglossal nerve (XII) emerges from the brainstem in the ventrolateral sulcus between the pyramid and the olive. The nuclei of the hypoglossal nerve, like those of the nerves to the extraocular muscles, are located near the midline in the brainstem, in the so-called basal lamina. Dorsal to the olive, the roots of the accessory (XI), vagus (X), and glossopharyngeal (IX) nerves emerge from the brainstem in a vertically oriented row (Fig. 4.1a and c). Further dorsally, between the exit of these nerves and the dorsolateral sulcus, lies the tuberculum cinereum, formed by the nucleus of the spinal tract of the trigeminal nerve. This is also the site of the posterior spinocerebellar tract, which ascends to the cerebellum by way of the inferior cerebellar peduncle (restiform body).

Pons

Ventral view. The pons (“bridge”) is so called because, when viewed from the front, it appears to connect the two cerebellar hemispheres to each other with a broad band of horizontally disposed fibers, which is bounded caudally by the medulla and rostrally by the cerebral peduncles (crura cerebri) of the midbrain. The descending corticopontine fibers form a synapse with their second neurons on the ipsilateral side of the pons, which give rise to these horizontally disposed pontocerebellar fibers, which then, in turn, cross the midline and travel by way of the middle cerebellar peduncle to the cerebellum. A shallow groove in the midline of the ventral aspect of the pons contains the vertically coursing basilar artery. The groove is not caused by the artery, but rather by the bulges on either side produced by the pyramidal tracts as they descend through the basis pontis.

Lateral view. The lateral view (Fig. 4.1c) reveals the horizontally disposed pontine fibers coming together to form the middle cerebellar peduncle (brachium pontis). The trigeminal nerve (CN V) emerges from the pons just medial to the origin of the middle cerebellar peduncle.

Dorsal view. The dorsal aspect of the pons forms the superior portion of the floor of the fourth ventricle. It takes the form of a triangle whose base is a horizontal line defining the border between the dorsal aspects of the pons and the medulla. At either end of this line, the fourth ventricle opens into the subarachnoid space through a lateral aperture (foramen of Luschka). The unpaired

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4120 · 4 Brainstem

median aperture of the fourth ventricle (foramen of Magendie) is seen at the caudal end of the ventricle (Fig. 4.1c). The roof of the fourth ventricle is formed by the superior cerebellar peduncles (brachia conjunctiva) and the superior medullary velum.

Midbrain

The midbrain (mesencephalon) lies between the pons and the diencephalon.

Ventral view. The ventral view reveals two prominent bundles of fibers converging onto the pons. These are the cerebral peduncles, or, as they are alternatively called, the crura cerebri (singular: crus cerebri). The groove between the peduncles, known as the interpeduncular fossa, is the site of emergence of the two oculomotor nerves (CN III) from the brainstem. The cerebral peduncles disappear caudally as they enter the pons; rostrally, they are encircled by the optic tracts before entering the cerebral hemispheres (Fig. 4.1a).

Dorsal view. The dorsal aspect of the midbrain (the midbrain tectum, i.e., “roof”) contains four protrusions collectively termed the quadrigeminal plate. Visual information is processed in the upper two protrusions (the superior colliculi), while auditory information is processed in the lower two protrusions (the inferior colliculi), which are somewhat smaller. The trochlear nerve (CN IV) emerges from the brainstem just below the inferior colliculus on either side and then courses ventrally around the cerebral peduncle. It is the only cranial nerve that emerges from the dorsal aspect of the brainstem.

Lateral view. The two small protrusions lying lateral to the quadrigeminal plate are the medial geniculate body (an auditory relay area) and the lateral geniculate body (a visual relay area). The geniculate bodies are components of the thalamus and thus belong not to the brainstem but to the diencephalon.

For didactic reasons, the internal structure of the brainstem will be presented after the cranial nerves have been discussed.

Cranial Nerves

Origin, Components, and Functions

Figure 4.2 is a schematic dorsal view of the brainstem, in which the motor and parasympathetic cranial nerve nuclei are shown on the right and the somatosensory and special sensory nuclei are shown on the left. Lateral views showing the anatomical relations of the motor and parasympathetic nuclei, and of

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Sensory

Mesencephalic nucleus and tract of the trigeminal n.

Principal sensory nucleus of the trigeminal n.

Nuclei of the vestibular n.

Nucleus of the cochlear n.

Nucleus of the tractus solitarius

Spinal nucleus and tract of the trigeminal n.

Cranial Nerves · 121 4

Motor

Accessory (autonomic) nucleus = Edinger– Westphal nucleus

Nucleus of the oculomotor n. Nucleus of the trochlear n.

Motor nucleus of the trigeminal n.

Superior and inferior salivatory nuclei Nucleus ambiguus

Nucleus cuneatus

Dorsal nucleus of the vagus n.

Nucleus of the hypoglossal n.

Nucleus gracilis

Nucleus of the accessory n.

Fig. 4.2 Cranial nerve nuclei, dorsal view (schematic drawing). The somatosensory and special sensory nuclei are shown on the left side of the figure, the motor and parasympathetic nuclei on the right.

the somatosensory and special sensory nuclei, are found in Figures 4.3 and 4.4, respectively.

The origin, components, and function of the individual cranial nerves are listed in Table 4.1. Figure 4.5 provides a synoptic view of the sites of emergence of all 12 cranial nerves from the brainstem, their functional components, and their peripheral sites of origin and termination. All 12 cranial nerves are seen in the figure, from I (olfactory nerve) to XII (hypoglossal nerve); it should be borne in mind, however, that the second cranial “nerve”—the optic nerve—is actually not a peripheral nerve at all, but rather a tract of the central nervous system.

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Nervus intermedius (secretory)

IIIAccessory (autonomic) nucleus = Edinger–Westphal nucleus

III Nucleus of the oculomotor n.

IV Nucleus of the trochlear n.

VMotor nucleus of the trigeminal n.

VI Nucleus of the abducens n.

VII Nucleus of the facial n.

Superior salivatory nucleus Inferior salivatory nucleus

X Dorsal nucleus of the vagus n.

XII Nucleus of the hypoglossal n.

Nucleus ambiguus

XI Nucleus of the accessory n.

Fig. 4.3 Motor and parasympathetic cranial nerve nuclei, lateral view (schematic drawing)

Recall that spinal nerve fibers can be classified as somatic afferent, somatic efferent, vegetative afferent, and vegetative efferent. The classification of cranial nerve fibers is a little more complicated, for two reasons. Some of the cranial nerve fibers are special sensory fibers arising from the sense organs of the head (vision, hearing, taste, smell). Furthermore, some of the efferent cranial nerve fibers arise in nuclear areas that are embryologically derived from the branchial arches; these fibers innervate muscles of branchial origin.

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Cranial Nerves · 123 4

V Mesencephalic nucleus and tract of the trigeminal n.

V Principal sensory nucleus of the trigeminal n.

VIII Nucleus of the vestibular n.

Trigeminal (gasserian) ganglion

VIII Nucleus of the cochlear n.

Nervus intermedius (somatosensory, taste)

Nucleus of the tractus solitarius

V Spinal nucleus and tract of the trigeminal n.

Fig. 4.4 Somatosensory and special sensory cranial nerve nuclei, lateral view (schematic drawing)

There results a sevenfold classification of cranial nerve fibers, as follows:

Somatic afferent fibers (pain, temperature, touch, pressure, and proprioceptive sense from receptors in the skin, joints, tendons, etc.)

Vegetative afferent fibers (or, alternatively, visceral afferent fibers), which carry impulses (pain) from the internal organs

Special somatic afferent fibers carrying impulses from special receptors (eye, ear)

Special visceral afferent fibers carrying impulses related to taste and smell

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Table 4.1 The Cranial Nerves

Name		Components	Origin
I.	Olfactory nerve	Special visceral	Olfactory cells of the
	(or olfactory	afferent	olfactory epithelium
	fasciculus)
II	Optic nerve	Special somatic	Retina, retinal
	(or optic	afferent	ganglion cells
	fasciculus)
III.	Oculomotor	(a) Somatic efferent	Nucleus of the
	nerve		oculumotor nerve
			(midbrain)
		(b) Visceral efferent	Edinger−Westphal
		(parasympathetic)	nuclei
		(c) Somatic afferent	Proprioceptors in the
			extraocular muscles
IV.	Trochlear nerve	(a) Somatic efferent	Nucleus of the
			trochlear nerve
			(midbrain)
		(b) Somatic afferent	Proprioceptors
V.	Trigeminal nerve	(a) somatic afferent	Bipolar cells in the
			semilunar ganglion
	1st branchial arch	(b) Branchial efferent	Motor nucleus of the
			trigeminal nerve
		(c) Somatic afferent	Proprioception
VI. Abducens nerve		Somatic efferent	Nucleus of the
			abducens nerve
VII. Facial nerve		(a) Branchial efferent	Nucleus of the facial
			nerve
	Nervus inter-	(b) Visceral efferent	Superior salivatory
	medius		nucleus
	2nd branchial arch
		(c) Special visceral	Geniculate ganglion
		afferent
		(d) Somatic afferent	Geniculate ganglion

Function

Olfaction

Vision

Innervates superior, inferior, and medial rectus muscles, inferior oblique muscle, and levator palpebrae muscle Sphincter pupillae muscle, ciliary muscle Proprioception

Superior oblique muscle

Proprioception

Sensation on the face and in the nasal and oral cavities

Muscles of mastication Proprioception

Lateral rectus muscle

Muscles of facial expression, platysma, stylohyoideus muscle, digastric muscle Nasal and lacrimal glands, salivation, sublingual and submandibular glands Taste (anterior 2/3 of tongue)

External ear, portions of the auditory canal, external surface of the tympanic membrane (somatosensory)

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				Cranial Nerves · 125	4
Table 4.1 The Cranial Nerves (Continued)
Name		Components	Origin	Function
VIII. Vestibulo-		Special somatic	(a) Vestibular	Equilibrium, cristae of
	cochlear nerve	afferent	ganglion	the semilunar canals,
				maculae of the utricle
				and saccule
			(b) Spiral ganglion	Hearing, organ of
				Corti
IX.	Glossopharyn-	(a) Branchial efferent	Nucleus ambiguus	Stylopharyngeus
	geal nerve			muscle, pharyngeal
				muscles
	3rd branchial arch	(b) Visceral efferent	Inferior salivatory	Salivation
		(parasympathetic)	nucleus	Parotid gland
		(c) Special visceral	Inferior ganglion	Taste (posterior 1/3 of
		afferent		the tongue)
		(d) Visceral afferent	Superior ganglion	Somatosensory: post-
				erior 1/3 of the
				tongue and pharynx
				(gag reflex)
		(e) Somatic afferent	Superior ganglion	Middle ear, eustachian
				tube (somatosensory)
X.	Vagus nerve	(a) Branchial efferent	Nucleus ambiguus	Muscles of the larynx
				and pharynx
4th branchial arch		(b) Visceral efferent	Dorsal nucleus of the	Thoracic and abdom-
			vagus nerve	inal viscera (parasym-
				pathetic)
		(c) Visceral afferent	Inferior (nodose)	Abdominal cavity
			ganglion	(somatosensory)
		(d) Special visceral		Taste: epiglottis
		afferent
		(e) Somatic afferent	Superior (jugular)	Auditory canal, dura
			ganglion	mater (somato-
				sensory)
XI.	Accessory nerve	(a) Branchial efferent	Nucleus ambiguus	Muscles of the larynx
				and pharynx
		(b) Somatic efferent	Anterior horn cells	Sternocleidomastoid
				and trapezius muscles
XII. Hypoglossal		Somatic efferent	Nucleus of the hypo-	Muscles of the tongue
	nerve		glossal nerve

General somatic efferent fibers carrying motor impulses to the skeletal musculature (oculomotor, trochlear, abducens, and hypoglossal nerves)

Visceral efferent fibers innervating the smooth muscles, the cardiac musculature, and the glands (both sympathetic and parasympathetic)

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Fig. 4.5 Cranial nerves: sites of exit from the brainstem, components, and distribution

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Cranial Nerves · 127	4

Cribriform plate (I)

Optic canal (II)

Superior orbital fissure (III, IV, VI, V [ophthalmic n.])

Carotid canal

Foramen rotundum (V [maxillary n.])

Foramen ovale

(V [mandibular n.])

Foramen lacerum

Internal acoustic meatus (VII, VIII)

Jugular foramen (IX, X, XI)

Hypoglossal canal (XII)

Fig. 4.6 Sites of exit of the cranial nerves from the skull. the transected cranial nerves on the right.

	I	Olfactory bulb
	II	Optic n.
	III	Oculomotor n.
	IV Trochlear n.
	Ophthalmic n.
	Maxillary n.	V
	Mandibular n.
	Trigeminal	Trigeminal n.
	ganglion

Motor root

VI Abducens n. VII Facial n. and nervus

intermedius

VIII Vestibulo-

cochlear n.

IX Glossopha-

ryngeal n.

X Vagus n.

XI Accessory n.

XII Hypoglossal n.

The exit foramina are shown on the left,

Special branchial efferent fibers innervating muscles that are derived from the mesodermal branchial arches, i.e., the motor portions of the facial nerve (2nd branchial arch), glossopharyngeal nerve (3rd branchial arch), and vagus nerve (4th branchial arch and below)

The cranial nerves exit from the skull through the openings (foramina, fissures, canals) depicted on the left side in Figure 4.6. The cut-off nerve stumps in their corresponding openings are shown on the right.

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