Brainstem

6 Brainstem

The brainstem comprises the medulla, pons, and mid-brain. Like the spinal cord, with which it is caudally contiguous, it contains multiple long ascending and descending pathways that are oriented parallel to the long axis of the brainstem, as well as nuclei whose axons are oriented parallel to the transverse plane. There are four major elements of the brainstem: (1) long ascending and descending tracts, (2) cranial nerve nuclei and their fascicles, (3) cerebellar nuclei and their connections, and (4) reticular neurons and their processes.

Because brainstem dysfunction is both common and potentially life threatening, prompt recognition of its cardinal manifestations is imperative. An understanding of the principles of the brainstem’s organization simplifies the study of its complex regional anatomy and offers the clinician a logical approach to anatomic diagnosis.

This chapter outlines the basic organization of the brainstem, then discusses the regional and functional anatomy of the cranial nerve nuclei, the long ascending and descending tracts, the cerebellar connections, and the reticular formation. A description of the blood supply of the brainstem is followed by a discussion of the localization of brainstem lesions, and finally by a description of classic brainstem syndromes.

The Brainstem Is Organized into Four Major Parts

Grossly, four major parts of the brainstem are contiguous throughout the length of the medulla, pons, and mid-brain (Fig. 6.1):

Ventricular cavity The ventricular cavity of the brainstem is contiguous with the central canal of the spinal cord. It comprises the central canal (caudal medulla), the fourth ventricle (rostral medulla and pons), and the cerebral aqueduct of Sylvius (mid-brain).
Roof Overlying the ventricular cavity is the roof of the brainstem, which comprises the choroid plexus and tela choroidea of the fourth ventricle (medulla), the cerebellum (pons), and the tectum (midbrain). The tectum consists of the rostral pretectum, the paired superior colliculi of the optic system, and the paired inferior colliculi of the auditory system. The superior and inferior colliculi are collectively known as the corpora quadrigemina.
Tegmentum Just ventral to the ventricular cavity is the tegmentum of the brainstem, which contains, among other structures, the cranial nerves and their nuclei, the major long ascending tracts, and the reticular formation.
Base The basilar part of the brainstem comprises the pyramids of the medulla, the ventral pons, and the crura cerebri of the midbrain. It contains the major long descending tracts, including the corticospinal, corticobulbar, and corticopontine tracts.

Fig. 6.1 Brainstem.

Cranial Nerve Nuclei Are Organized into Longitudinal Columns

Ten of the 12 cranial nerves have their nuclei in the brain-stem. (Cranial nerves I and II are the exceptions.) Like the spinal gray nuclei, the cranial nerve nuclei are grouped into longitudinal columns. These columns are both anatomically and functionally distinct: medial columns contain exclusively motor nuclei, and lateral columns contain exclusively sensory nuclei. This organization is explained by developmental events, as follows.

The alar and basal plates of the developing neural tube give rise to sensory afferents and motor efferents, respectively. Early in development, these plates are positioned in a dorsal/ventral orientation, an orientation that is maintained in the mature spinal cord. In the developing brainstem, however, this organization is changed: the lateral spread of the fourth ventricle causes the dorsal alar plate to rotate laterally in relation to the ventral basal plate. As a result, the lateral columns of the mature brainstem contain strictly sensory cranial nerve nuclei; the medial columns contain strictly motor cranial nerve nuclei.

Medial Columns Contain Three Types of Motor Nuclei

The medial columns are composed of somatic and visceral motor nuclei. Two types of visceral motor nuclei are distinguished on the basis of the two types of muscles their neurons innervatestriated muscle derived from primitive branchial arches and smooth muscle associated with viscera and glands. Three types of motor nucleione somatic, two visceralthus comprise the three medial columns. The individual nuclei that form these columns, in medial to lateral order, are described in the following sections.

Column 1

See Fig. 6.2.

This column, which is not continuous longitudinally, is immediately adjacent to the midline, just below the floor of the ventricular system. It contains nuclei composed of neurons that innervate the striated muscles of the head and neck derived from embryonic myotomes (i.e., the extraocular muscles and the muscles of the tongue). In rostrocaudal order, the nuclei contained in this column include the following four structures:

Oculomotor nucleus (III) This nucleus contains neurons that project to the oculomotor nerve. They innervate four of the six extraocular muscles (i.e., all but the lateral rectus and the superior oblique) as well as the levator palpebrae superioris. The oculomotor nucleus is located in the midbrain at the level of the superior colliculus, just ventral to the cerebral aqueduct.
Trochlear nucleus (IV) This nucleus contains neurons that form the trochlear nerve. They innervate the superior oblique muscle. The trochlear nucleus is located in the midbrain at the level of the inferior colliculus, just ventral to the cerebral aqueduct.
Abducens nucleus (VI) This nucleus contains neurons that form the abducens nerve. They innervate the lateral rectus muscle. The abducens nucleus is located in the pons, just ventral to the floor of the fourth ventricle.
Hypoglossal nucleus (XII) This nucleus contains neurons that form the hypoglossal nerve. They innervate the muscles of the tongue. The hypoglossal nucleus is located in the medulla, just ventral to the floor of the fourth ventricle.

Fig. 6.2 Somatic motor nuclei.

Column 2

See Fig. 6.3.

This column is located lateral and ventral to column 1. It contains nuclei composed of neurons that innervate the striated muscles of the head and neck derived from the branchial arches (i.e., the muscles of mastication, the muscles of facial expression, the muscles of the pharynx and larynx, and the sternocleidomastoid and trapezius muscles). In rostrocaudal order, the nuclei in this column include the following four structures:

Motor nucleus of the trigeminal nerve (V) This nucleus contains neurons that form the motor component of the trigeminal nerve. They innervate the muscles of mastication. The motor nucleus of the trigeminal nerve is located in the pons.
Facial motor nucleus (VII) This nucleus contains neurons that form the motor component of the facial nerve. They innervate the muscles of facial expression. The motor nucleus of the facial nerve is located in the pons.
Nucleus ambiguus (IX and X) This nucleus contains neurons that are projected in the glossopharyngeal (IX) and vagus nerves. They innervate the muscles of the larynx and pharynx responsible for speech and swallowing. The nucleus ambiguus, so-called because of its unclear borders in histological sections, is located in the medulla.
Nucleus of the spinal accessory nerve (XI) This nucleus contains neurons that form the spinal accessory nerve. They innervate the sternocleidomastoid and trapezius muscles. This nucleus extends from the medulla into the cervical regions of the spinal cord.

Fig. 6.3 Visceral motor nuclei (striated muscle).

Column 3

See Fig. 6.4.

This column is located immediately lateral to column 1. It contains nuclei of preganglionic parasympathetic neurons that innervate the smooth muscles and glands of the head and neck, as well as the thoracic and parts of the abdominal viscera. (Preganglionic parasympathetics originating in sacral segments of the spinal cord supply the rest of the abdominal and pelvic viscera.) In rostrocaudal order, these parasympathetic nuclei include the following three structures:

Edinger-Westphal nucleus (III) This nucleus contains preganglionic parasympathetic neurons that are projected in the oculomotor nerve (III) to terminate in the ciliary ganglion. The ciliary ganglion contains postganglionic neurons that travel in the short ciliary nerve to innervate the pupillary constrictor and ciliary muscles of the eye. The Edinger-Westphal nucleus is located in the midbrain.
Superior and inferior salivatory nuclei (VII and IX, respectively) The superior salivatory nucleus contains preganglionic parasympathetic neurons that are projected in the facial nerve (VII) to terminate in the submandibular and pterygopalatine (sphenopalatine) ganglia. The submandibular ganglion contains post-ganglionic neurons that innervate the sublingual and submandibular glands, and the pterygopalatine ganglion contains postganglionic neurons that innervate the lacrimal gland. The inferior salivatory nucleus contains preganglionic parasympathetic neurons that are projected in the glossopharyngeal nerve (IX) to terminate in the otic ganglion. The otic ganglion contains postganglionic neurons that innervate the parotid gland. The superior and inferior salivatory nuclei are located in the medulla.
Dorsal motor nucleus of the vagus (X) This nucleus projects fibers carried in the vagus nerve that innervate the heart, lungs, and gastrointestinal tract. It is located in the medulla.

Fig. 6.4 Visceral motor nuclei (smooth muscle).

Lateral Columns Contain Three Sensory Nuclei

The lateral columns are composed of sensory nuclei. Unlike most motor nuclei, whose axons are carried in a single corresponding cranial nerve, each sensory nucleus receives input from several different cranial nerves. There are three major sensory nuclei in the brainstem (Fig. 6.5):

The trigeminal sensory nucleus comprises three distinct nuclei: the mesencephalic nucleus, the main sensory nucleus, and the spinal trigeminal tract nucleus. These nuclei receive input from the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X) nerves, mediating proprioceptive (mesencephalic), light touch (main sensory), and pain and temperature (spinal trigeminal) sensation.
The vestibular and cochlear nuclei extend from the rostral medulla into the pons, receiving input from fibers of the vestibulocochlear nerve (VIII).
The solitary nucleus, which is located in the medulla, receives general and special visceral afferents carried in the facial (VII), glossopharyngeal (IX), and vagus (X) nerves. These nerves mediate taste sensation as well as general visceral sensations of the heart, lungs, and gastrointestinal tract. The cell bodies of these visceral afferents are located in sensory ganglia outside the brainstem; they possess central connections with the thalamus (taste sensation), the reticular formation, and the limbic system of the forebrain.

Fig. 6.5 Sensory nuclei.

Long Ascending and Descending Tracts Traverse the Brainstem

Four long tracts, two ascending, two descending, provide landmarks along the transverse axis of the brainstem. The spinothalamic tract lies laterally in the brainstem, whereas the medial lemniscus, the corticospinal tract, and the corticobulbar tract lie medially.

Two Ascending Tracts Occur Laterally and Medially

Spinothalamic Tract

See Fig. 6.6.

The lateral and anterior spinothalamic tracts are responsible for pain, temperature, and light touch sensation. They are located in the lateral aspect of the teg-mentum throughout the brainstem, adjacent to the descending sympathetic tract. They occur in essentially the same position they occupy in the spinal cord. The spinothalamic tract consists of second-order neurons that originate in the dorsal gray horn of the spinal cord, cross the midline in the anterior white commissure, and project to the ventral posterolateral (VPL) nucleus of the thalamus. Third-order neurons in the VPL thalamus send axons to the postcentral gyrus. Because of the close proximity of the spinothalamic tract to the descending sympathetic fibers, both systems are typically impaired as a result of damage to the lateral tegmentum, where they represent important landmarks. An ipsilateral Horner syndrome (descending sympathetic lesion) is thus often associated with a contralateral hemisensory loss (spinothalamic lesion), which may be caused by a lesion in the lateral medulla or pons.

Fig. 6.6 Spinothalamic tract.

Medial Lemniscus

See Fig. 6.7.

The medial lemniscus, which is the rostral continuation of the dorsal columns of the spinal cord, mediates position sense and discriminative touch. It consists of second-order neurons that originate in the nucleus cuneatus and nucleus gracilis. These nuclei receive input from the spinal cord via the cuneate and gracile fasciculi (dorsal columns), which carry impulses from the upper and lower extremities, respectively. After synapse in the ipsilateral cuneate and gracile nuclei, these axons act as the internal arcuate fibers and ascend to the contralateral VPL thalamus. From here they ascend to the sensory cortex. The medial lemniscus is situated in the medulla close to the midline between the posteriorly situated medial longitudinal fasciculus (MLF) and the anteriorly situated corticospinal and corticopontine tracts. In its rostral ascent, the medial lemniscus moves laterally but remains an important landmark of the medial aspect of the medulla and pons.