The Brain

Thomas F. Fletcher and Alvin J. Beitz

The brain (encephalon) and spinal cord (medulla spinalis) constitute the central nervous system. Twelve pairs of cranial nerves (nervi craniales) emerge from the brain and exit the cranial cavity to innervate the head, certain neck muscles, and viscera of the thoracic and abdominal cavities. Terms such as body, colliculus, peduncle, pyramid, lobe, gyrus, and folium are used to refer to various elevations of the brain surface.

Brain tissue is composed of billions of neurons and glial cells that form gray matter (substantia grisea) and white matter (substantia alba). Localized accumulations of gray matter are designated nuclei, and the gray matter covering the surface of the cerebrum or cerebellum is called cortex. Concentrations of myelinated axons form white matter, which generally can be subdivided into tracts or fasciculi or striae. Regions (in the brainstem) where white and gray matter mix together are designated reticular formation (formatio reticularis).

The brain develops from three enlargements of the rostral end of the embryonic neural tube (Table 18-1). The enlargements become the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). Subsequently the forebrain and hindbrain differentiate further, producing five primary divisions of the brain: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon (Fig. 18-1).

TABLE 18-1

Divisions, Major Components, and Brain Cavities Derived from Primary Embryonic Vesicles That Develop into the Brain

EMBRYONIC BRAIN DIVISION	DERIVED BRAIN STRUCTURES	DEFINITIVE BRAIN CAVITIES
Forebrain
Telencephalon	Cerebral hemispheres	Lateral ventricles
Diencephalon	Thalamus, hypothalamus, epithalamus, subthalamus, metathalamus	Third ventricle
Midbrain
Mesencephalon	Cerebral peduncles and tectum	Mesencephalic aqueduct
Hindbrain
Metencephalon	Pons and cerebellum	Fourth ventricle
Myelencephalon	Medulla oblongata	Fourth ventricle

The brain may also be divided into three large regions: cerebrum, cerebellum, and brainstem (Fig. 18-2). The cerebrum is the telencephalon, the cerebellum is the dorsal part of the metencephalon, and the brain stem encompasses the remaining primary divisions. This chapter is organized to present first the brainstem, then the cerebrum, and finally the cerebellum.

The Brainstem

The brainstem occupies the fossae of the floor of the cranial cavity, caudal to the optic canals. All of the cranial nerves arise from the brainstem, except for olfactory nerves (including those to the vomeronasal organ). The rostral end of the brainstem is connected to the cerebrum by the internal capsule, a mass of myelinated axons. The brainstem is connected to the cerebellum by axons within three cerebellar peduncles. Caudally, the brainstem is continuous with the spinal cord.

A ventral view of the brainstem reveals the primary brain divisions that compose it and the cranial nerves that emerge from it (Fig. 18-3). The medulla oblongata (myelencephalon) is the most caudal region. It is distinguished by bilateral longitudinal bands of white matter, the pyramids, that parallel the ventral midline. Seven cranial nerves (CN VI-XII) arise from the medulla oblongata. A transversely running trapezoid body demarcates the rostral extent of the medulla.

FIGURE 18-3 Ventral view of the brain and the first cervical segment of the spinal cord.

1. Olfactory bulb

2. Olfactory peduncle

3. Medial olfactory tract

4. Olfactory tubercle

5. Lateral olfactory tract

6. Lateral olfactory gyrus

7. Rostral part of lateral rhinal sulcus

8. Tuber cinereum

9. Piriform lobe

10. Mamillary bodies

11. Caudal part of lateral rhinal sulcus

12. Crus cerebri

13. Transverse fibers of pons

14. Ventral paraflocculus

15. Flocculus

16. Cerebellar hemisphere

17. Ansiform lobule

18. Trapezoid body

19. Pyramids

20. Median fissure

21. Decussation of pyramids

22. Interpeduncular fossa

23. Infundibulum

24. Optic tract

25. Optic chiasm

26. Medial rhinal sulcus

II Optic nerve

III Oculomotor nerve

IV Trochlear nerve

V Trigeminal nerve

VI Abducent nerve

VII Facial nerve

VIII Vestibulocochlear nerve

IX Glossopharyngeal nerve

X Vagus nerve

XI Accessory nerve

XII Hypoglossal nerve

CI Spinal ventral root (C1)

Rostral to medulla oblongata, the pons (ventral metencephalon) is distinguished by transverse fibers along its ventral surface. The trigeminal nerve (CN V) connects to the pons. Rostral to the pons, the ventral surface of the midbrain (mesencephalon) features a median interpeduncular fossa between bilateral cerebral peduncles. The ventral surface of each peduncle is capped by the white matter called crus cerebri. The oculomotor nerve (CN III) and the trochlear nerve (CN IV) arise from the midbrain, the latter exits from the dorsal surface.

The diencephalon is the rostral extent of the brainstem. Its ventral surface features mamillary bodies caudally, optic chiasm (chiasma opticum) rostrally, and between these an infundibulum connecting the brainstem to the hypophysis. Rostral to the optic chiasm, the optic nerve (CN II) runs to the eyeball.

Brain divisions are also evident in a dorsal view of the brainstem (Fig. 18-4). The dorsal surface of the medulla oblongata and pons features a rhomboid fossa (fossa rhomboidea), which is the floor of the fourth ventricle. Paired rostral and caudal colliculi mark the dorsal surface of the midbrain. Bilaterally, the diencephalon features a prominent thalamus and, more caudally, a metathalamus composed of medial and lateral geniculate bodies.

FIGURE 18-4 The brainstem from a dorsolateral perspective.

1. Stria habenularis thalami

2. Dorsal aspect of thalamus

3. Habenular commissure

4. Lateral geniculate body

5. Medial geniculate body

6. Rostral colliculus

7. Commissure of caudal colliculi

8. Caudal colliculus

9. Decussation of trochlear nerves in rostral medullary velum

10. Middle cerebellar peduncle

11. Caudal cerebellar peduncle

12. Rostral cerebellar peduncle

13. Acoustic stria

14. Median sulcus in fourth ventricle

15. Nucleus cuneatus lateralis

16. Fasciculus cuneatus

17. Fasciculus gracilis

18. Spinal tract of trigeminal nerve

19. Superficial arcuate fibers

20. Cochlear nuclei (dorsal and ventral)

21. Brachium of caudal colliculus

22. Optic tract

23. Brachium of rostral colliculus

24. Cut surface of internal capsule

25. Pineal body

26. Stria terminalis

II Optic nerve

IV Trochlear nerve

V Trigeminal nerve

VIII Vestibulocochlear nerve

The Spinomedullary Junction

The caudal extent of the medulla oblongata has some features resembling the spinal cord, with which it is continuous (Fig. 18-8). In a transverse section, one can see a central canal, superficial white matter, laterally expanded central gray matter, a ventral median fissure, and a dorsal median sulcus and septum. A dominant feature of the spinomedullary junction is the pyramidal decussation (decussatio pyramidum).

FIGURE 18-8 Transverse section at the level of the pyramidal decussation, juncture of the canine brain and spinal cord. (Luxol Fast Blue–Cresyl Violet stain.)

Each pyramid (pyramis) consists of myelinated axons that originate from neuronal cell bodies in the cerebral cortex. Axons within the pyramids go to the medulla oblongata (corticonuclear and corticoreticular axons) or to the spinal cord (corticospinal axons). The axons synapse on interneurons that regulate both efferent neurons (motor units) and projection neurons (cranial projecting pathways) (Davidoff, 1990).

Most corticospinal axons turn dorsally and cross the midline (pyramidal decussation) to reach the dorsal half of the contralateral lateral funiculus, where they project caudally as the lateral corticospinal tract. A minority of axons delay decussation until they terminate in spinal gray matter; these run in the ipsilateral lateral corticospinal tract or in the ventral funiculus as the ventral corticospinal tract. The decussation of pyramidal axons and other caudally projecting tracts explains why one side of the brain controls voluntary movement on the contralateral side of the body.

Dorsally at the midline, fasciculus gracilis axons terminate in the nucleus gracilis. Further laterally, fasciculus cuneatus axons terminate in the medial cuneate nucleus (nucleus cuneatus medialis) (Fig. 18-8). The fasciculi are composed of cranial branches of primary afferent axons associated with encapsulated receptors located in skin or in muscles, tendons, and joints. The nuclei relay sensory information from primary afferent neurons to neurons in the thalamus. Axons from the nuclei decussate as deep arcuate fibers (fibrae arcuatae profundae), and project rostrally as medial lemniscus (lemniscus medialis).

The fasciculus cuneatus and medial cuneate nucleus are concerned with discriminative touch and kinesthesia (sense of position and movement) from the thoracic limb and neck. Kinesthesia is relayed by neurons located ventrally in the medial cuneate nucleus.

The fasciculus and nucleus gracilis are concerned with discriminative touch from the caudal half of the body. Neurons situated medial and rostral to the nucleus gracilis are referred to as nucleus Z. Kinesthesia from the caudal half of the body reaches nucleus Z through a spinomedullary tract. Most kinesthetic input is from collateral branches of the dorsal spinocerebellar tract; only scant input arrives via the fasciculus gracilis (Hand, 1966).

Lateral to fasciculus cuneatus, the spinal tract of the trigeminal nerve (tractus spinalis n. trigemini) is visible (Fig. 18-8). It is superficial to the nucleus of the spinal tract of the trigeminal nerve (nucleus tractus spinalis n. trigemini). The tract is composed of small, myelinated and nonmyelinated axons from neuronal cell bodies located in the trigeminal ganglion, plus a minority of somatic afferent axons from the vagus, glossopharyngeal, and facial nerves. The nucleus is divisible into rostral, interpolar, and caudal parts. The tract and nucleus extend into the first two cervical segments of the spinal cord overlapping with dorsolateral fasciculus, marginal nucleus, and substantia gelatinosa.

Axons constituting the spinal tract of the trigeminal nerve convey noxious, temperature, and crude touch information from the face and nasal and oral cavities, including the teeth. Within the tract, axons from the dorsal face (ophthalmic division of the trigeminal nerve) travel ventrally, and those from the ventral face (mandibular division) travel dorsally. The nucleus has a comparable somatotopic organization. Additionally, the nasal and oral cavities are represented in rostral regions of the nucleus, while the surrounding perimeter of the face is represented caudally.

Caudal Half of the Medulla Oblongata

The olivary nucleus (nucleus olivaris) is a prominent feature of the caudal medulla oblongata (Fig. 18-9). It is located dorsolateral to the pyramid and lateral to the medial lemniscus; it presents a distinctive serpentine profile in the ventrolateral medulla. The nucleus receives axonal input from the cerebellum, from the cerebral cortex via the pyramids, and from the red nucleus and PAG via the central tegmental tract. Dorsal and medial accessory olivary nuclei receive afferents from the spinal cord.

FIGURE 18-9 Transverse section at the level of the hypoglossal nucleus at the caudal end of a canine medulla oblongata. Choroid plexus is evident in the roof of the fourth ventricle (IV). (Luxol Fast Blue–Cresyl Violet stain.)

Efferent axons from the olivary nuclei decussate and reach the cerebellum via the caudal cerebellar peduncle (pedunculus cerebellaris caudalis). Olivocerebellar fibers climb along dendritic trees of Purkinje (piriform) neurons in the cerebellar cortex and intensely activate small, localized regions of cortex for adjusting movement and posture. Other nuclei that project to the cerebellum terminate as mossy endings on granule neurons that diffusely influence the cerebellar cortex.

The lateral cuneate nucleus (nucleus cuneatus lateralis) is situated most dorsally in the medulla oblongata. It receives proprioceptive input from the thoracic limb and neck via the fasciculus cuneatus. Axons from the nucleus form superficial arcuate fibers (fibrae arcuatae superficiales). The fibers merge with the dorsal spinocerebellar tract to form the caudal cerebellar peduncle, located at the dorsolateral margin of the medulla oblongata (Fig. 18-9). The dorsal spinocerebellar tract conveys proprioceptive information from the caudal half of the body.

The fourth ventricle is located in the medulla oblongata and pons. Caudally, the region where the fourth ventricle narrows to a point is called the obex. Immediately rostral to the obex, the wall of the ventricle is formed by the area postrema, a densely vascularized gray matter that has fenestrated capillaries and serves as an emetic center (activated by apomorphine). As one of the circumventricular organs, it is a region that lacks a blood brain barrier.

The floor of the fourth ventricle, designated rhomboid fossa (fossa rhomboidea), has a median sulcus (sulcus medianus) (Fig. 18-4). Bilaterally, a sulcus limitans marks the transition from floor to wall; also, it is the demarcation between the embryonic alar and basal plates. The roof of the fourth ventricle (tegmen ventriculi quarti) is formed by the tela choroidea (tela choroidea ventriculi quarti) a layer of ependyma and pia mater that attaches to the medullary wall along a line, called tenia of the fourth ventricle (tenia ventriculi quarti).

Rostrally, tela choroidea that connects to rostral cerebellar peduncles and contains trochlear nerve axons, constitutes rostral medullary velum (velum medullare rostrale). Caudally, the caudal medullary velum (velum medullare caudale) forms the roof of the fourth ventricle. Tela choroidea of the caudal medullary velum gives rise to two longitudinal proliferations of blood vessels, forming the choroid plexuses of the fourth ventricle (plexus choroideus ventriculi quarti). The paired choroid plexuses produce cerebrospinal fluid.

Some cerebrospinal fluid enters the central canal, but most of it flows outward to the subarachnoid space, exiting the fourth ventricle bilaterally through a lateral recess (recessus lateralis) that leads to a lateral aperture (aperturae laterales). The recess and aperture are located immediately caudal to the caudal cerebellar peduncle. Some choroid plexus extends through the lateral recess and aperture to secrete directly into the subarachnoid space (Fig. 18-11).

The motor nucleus of the hypoglossal nerve (nucleus motorius n. hypoglossi) is evident dorsally beside the midline (Fig. 18-9). Axons from the nucleus run ventrally and then angle through the lateral region of the olivary nucleus. They leave the medulla oblongata as roots of the hypoglossal nerve and innervate muscles of the tongue (somatic efferent axons).

The parasympathetic nucleus of the vagus nerve (nucleus parasympathicus n. vagi) is located dorsolateral to the hypoglossal nucleus. Preganglionic parasympathetic visceral efferent axons from the nucleus run laterally to join the vagus nerve and innervate thoracic and abdominal viscera. Rostrally, two small nuclei of this cell column contribute axons to the glossopharyngeal and facial nerves (Fig. 18-9). The parasympathetic nucleus of the glossopharyngeal nerve (nucleus parasympatheticus n. glossopharyngei) innervates parotid and zygomatic salivary glands. Further rostrally, the parasympathetic nucleus of the facial nerve (nucleus parasympatheticus n. facialis) innervates mandibular and sublingual salivary glands and nasal, palatine and lacrimal glands.

The nucleus intercalatus, positioned between the hypoglossal and the parasympathetic nuclei, sends axons to the cerebellum. The nucleus receives input from vestibular nuclei and there is clinical evidence that it is involved in holding vertical gaze position (Munro et al 1993).

The solitary tract (tractus solitarius) is distinct dorsolateral to the parasympathetic nucleus of the vagus (Fig. 18-9). The tract contains axons from visceral afferent cell bodies located in distal ganglia of the vagus and glossopharyngeal nerves and the geniculate ganglion of the facial nerve. The axons synapse in the nucleus of the solitary tract (nucleus tractus solitarii). Caudally, right and left nuclei merge dorsal to the central canal, forming a commissural nucleus.

The nucleus of the solitary tract contains interneurons and projection neurons concerned with reflexes and sensation from the auditory tube, pharynx, larynx, esophagus, trachea, and other thoracic and abdominal viscera. The rostral end of the nucleus receives taste (special visceral afferent) input from three nerves: vagus (pharynx and larynx), glossopharyngeal (caudal third of tongue), and facial (rostral two thirds of tongue).

The nucleus ambiguus is a column of sparse neurons located ventral to the nucleus of the spinal tract of the trigeminal nerve (Fig. 18-9). Except for some visceral efferent neurons that innervate the heart (Fig. 18-7), the nucleus ambiguus contains somatic efferent neurons. Via vagus and glossopharyngeal nerves, the neurons send axons to striated muscles of the pharynx, larynx, and esophagus.

A caudal continuation of the ambiguus cell column extends through the cervical spinal cord as the motor nucleus of the accessory nerve (nucleus motorius n. accessorii) (Fig. 18-5). Its axons form the spinal root of the accessory nerve, which innervates certain muscles of the neck (cleidocephalicus, mastoid part of sternocephalicus, omotransversarius, and trapezius). The accessory nerve has a cranial root that arises from the caudal pole of the nucleus ambiguus. The root immediately joins the vagus nerve and eventually becomes recurrent laryngeal nerve.

The lateral reticular nucleus (nucleus reticularis lateralis), also referred to as nucleus of the lateral funiculus (nucleus funiculi lateralis), is located lateral to the olivary nucleus (Fig. 18-9). It receives input from the red nucleus and the spinal cord. Its axons join superficial arcuate fibers to reach the cerebellum via the caudal cerebellar peduncle.

Level of the Facial Nucleus

The motor nucleus of the facial nerve (nucleus motorius n. facialis) is located ventrally in the medulla oblongata (Fig. 18-11). The nucleus contains somatic efferent neurons that innervate muscles of facial expression. Neurons are topographically arranged within the nucleus: rostral to caudal positioned neurons innervate rostral to caudal muscles, dorsal neurons innervate ventral muscles, and vice versa (Berman, 1968).

Axons from the facial nucleus stream dorsally and collect in a bundle that arcs, from medial to lateral, dorsally around the abducent nucleus, before proceeding ventrolaterally to exit passing through the trapezoid body (Fig. 18-5). The loop, located dorsal to the abducent nucleus, is referred to as the genu of the facial nerve (genu n. facialis).

The facial nerve also contains visceral efferent and afferent fibers. The parasympathetic nucleus of the facial nerve (nucleus parasympathicus n. facialis) is located caudal to the genu of the facial nerve. The nucleus is a rostral satellite of the parasympathetic column that supplies the glossopharyngeal and vagus nerves. Preganglionic cell bodies of the nucleus innervate neuronal cell bodies of postganglionic axons that supply lacrimal, nasal, and palatine glands, and the mandibular and sublingual salivary glands. Special visceral afferent axons in the facial nerve convey taste from the rostral two thirds of the tongue. The axons join the solitary tract and terminate in the rostral pole of its nucleus.

The facial nerve has a small contingent of general somatic afferent fibers that supply the concave surface of the auricle of the ear (Whalen & Kitchell, 1983). These axons join the spinal tract of the trigeminal nerve.

White matter at the lateral edge of the medulla oblongata constitutes the caudal cerebellar peduncle (pedunculus cerebellaris caudalis) (formerly restiform and juxtarestiform bodies). The axons of the peduncle pass deep to the acoustic stria and turn abruptly dorsad to join the cerebellum (Fig. 18-10). The lateral recess of the fourth ventricle is located immediately caudal to the abrupt turn of the peduncle (Fig. 18-11).