Cranial Nerves and Their Nuclei

Keywords

oculomotor, trochlear, facial, glossopharyngeal, abducens, trigeminal, vagus nerves, vestibulocochlear, hypoglossal, accessory

Chapter Outline
Cranial Nerve Nuclei Have a Generally Predictable Arrangement, 72
Cranial Nerves III, IV, VI, and XII Contain Somatic Motor Fibers, 72
- The Abducens Nucleus Also Contains Interneurons That Project to the Contralateral Oculomotor Nucleus, 74
- The Hypoglossal Nerve (XII) Innervates Tongue Muscles, 75
Branchiomeric Nerves Contain Axons From Multiple Categories, 75
- The Trigeminal Nerve (V) Is the General Sensory Nerve for the Head, 75
- The Facial Nerve (VII) Innervates Muscles of Facial Expression, 77
- The Vagus Nerve (X) Is the Principal Parasympathetic Nerve, 77
- The Accessory Nerve Innervates Neck and Shoulder Muscles, 78
Brainstem Damage Commonly Causes Deficits on One Side of the Head and the Opposite Side of the Body, 78

Cranial nerves and their central connections often look bewilderingly complicated, but for the most part they are actually arranged systematically.

Cranial Nerve Nuclei Have a Generally Predictable Arrangement

Key Concept

The sulcus limitans intervenes between motor and sensory nuclei of cranial nerves.

The olfactory nerve (I) is a series of thin filaments that attach directly to the olfactory bulb, part of the telencephalon. Fibers of the optic nerve (II) proceed through the optic chiasm and tract (see Chapter 17 ), and most end in the lateral geniculate nucleus of the thalamus, part of the diencephalon. The 10 remaining cranial nerves originate or terminate in the brainstem (or upper cervical spinal cord), as indicated in Table 12.1 and Fig. 12.1 . The taste afferents in the facial , glossopharyngeal , and vagus nerves ( VII , IX , and X ) are considered separately in Chapter 13 , along with the olfactory nerve (I), and the vestibulocochlear nerve ( VIII ) has its own chapter (see Chapter 14 ). The rest of the brainstem cranial nerves are introduced in this chapter.

TABLE 12.1

Major Functions of the Brainstem Cranial Nerves

Cranial Nerve	Main Functions	Attachment Point
III. Oculomotor	Eye movements, pupil, lens	Rostral midbrain (V)
IV. Trochlear	Eye movements (superior oblique)	Pons/midbrain junction (D)
V. Trigeminal	Facial sensation, chewing	Midpons (L)
VI. Abducens	Eye movements (lateral rectus)	Pontomedullary junction (V)
VII. Facial	Facial expression, taste	Pontomedullary junction (V/L)
VIII. Vestibulocochlear	Hearing, equilibrium	Pontomedullary junction (V/L)
IX. Glossopharyngeal	Taste, swallowing	Rostral medulla (L)
X. Vagus	Visceral sensory Speaking, swallowing Preganglionic parasympathetic	Rostral medulla (L)
XI. Accessory	Head and shoulder movement	Upper cervical spinal cord (L)
XII. Hypoglossal	Tongue movement	Rostral medulla (V/L)

D, L, and V indicate dorsal, lateral, and ventral attachment points.

The wiring principles discussed in Chapter 3 and applied to the spinal cord in Chapter 10 also apply, for the most part, to cranial nerves and their connections. At its attachment point, each brainstem cranial nerve is carrying sensory information from ipsilateral receptors or motor output to ipsilateral muscles (except in the case of afferents and efferents for places like abdominal viscera, where the ipsilateral-contralateral concept loses much of its meaning). For example, the right vestibulocochlear nerve (VIII) carries information from the right cochlea and the left oculomotor nerve ( III ) innervates muscles of the left eye. Similarly, the fibers of these cranial nerves mostly terminate in or originate from the ipsilateral side of the CNS. The principal exceptions are the trochlear ( IV ) and part of the oculomotor nerve. All trochlear fibers and some oculomotor fibers to the superior rectus muscle of the eye originate from motor neurons in the contralateral half of the CNS.

Some fairly simple rules can be used to predict the approximate locations of cranial nerve nuclei—collections of second-order sensory neurons, lower motor neurons, and preganglionic parasympathetic neurons. The medial-lateral location of a nucleus is predicted by the sulcus limitans , and the longitudinal location by the level of attachment of the cranial nerve associated with it ( Fig. 12.2 ). Sensory nuclei are typically located lateral to the sulcus limitans and motor nuclei medial to it because of the way the neural tube opens up at the level of the fourth ventricle. Visceral nuclei are located closer to the sulcus limitans, so for example, second-order taste neurons are just lateral to the sulcus limitans and visceral motor (= preganglionic parasympathetic) neurons just medial to it.

Cranial Nerves III, IV, VI, and XII Contain Somatic Motor Fibers

Key Concepts

The oculomotor nerve (III) innervates four of the six extraocular muscles.
The trochlear nerve (IV) innervates the superior oblique.
The abducens nerve (VI) innervates the lateral rectus.

The oculomotor (III), trochlear (IV), abducens ( VI ), and hypoglossal ( XII ) nerves are the simplest cranial nerves, in the sense that they just contain motor axons to ordinary skeletal muscle (except for some clinically important parasympathetics in the oculomotor nerve).

The trochlear nerve (IV) innervates the superior oblique muscle and the abducens nerve (VI) innervates the lateral rectus. The oculomotor nerve (III) innervates the remaining extraocular muscles (medial, superior, and inferior recti, inferior oblique) and the elevator of the eyelid (levator palpebrae); it also contains the preganglionic parasympathetic fibers for the pupillary sphincter and the ciliary muscle. All three of these nerves, once they leave the brainstem, proceed to the ipsilateral eye (although trochlear and some oculomotor fibers for superior rectus cross before leaving the brainstem). The hypoglossal nerve (XII) innervates the muscles of the ipsilateral half of the tongue.

As expected from their embryological development, the oculomotor, trochlear, abducens, and hypoglossal nuclei are located near the midline in the floor of the ventricular system. The oculomotor nucleus is in the rostral midbrain, the trochlear nucleus in the caudal midbrain, the abducens nucleus in the caudal pons, and the hypoglossal nucleus in the rostral medulla.

The Abducens Nucleus Also Contains Interneurons That Project to the Contralateral Oculomotor Nucleus

Any time we look to the left or the right, we need to contract the lateral rectus of one eye and the medial rectus of the other eye simultaneously. Theoretically, this could be accomplished by having separate, parallel inputs to both sets of motor neurons. However, we have evolved a different mechanism to achieve the same end. The abducens nucleus contains not only the motor neurons for the ipsilateral lateral rectus but also an equal number of interneurons, whose axons cross the midline, join the medial longitudinal fasciculus ( MLF ), and ascend to medial rectus motor neurons in the contralateral oculomotor nucleus ( Fig. 12.3 ). Anything that stimulates lateral rectus motor neurons during attempted horizontal gaze also stimulates the MLF interneurons, so coordinated gaze is achieved when we try to contract one lateral rectus.