Cranial nuclei and nerves, e.g. VII Figure 10-1. In the case of six cranial nerves (III, V, VII, VIII, IX, X), cells in multiple brainstem regions either send axons into or receive axons from a single cranial nerve. After exiting the skull, most of the cranial nerves split into smaller nerves that take divergent paths to reach peripheral targets. In the case of the facial nerve, illustrated here, four brainstem nuclei are connected to the nerve (more on this in Chapter 12). After exiting the skull, the facial nerve splits up into about a dozen smaller peripheral nerves to reach widespread target tissues. The major component of the facial nerve, motor output to the muscles of facial expression, is carried by five major nerves and a number of smaller nerves. Autonomic motor output to two parasympathetic ganglia is carried by two nerves, one of which also carries taste information back from the tongue into the brain. The facial nerve contains a small number of somatosensory afferents that innervate a region around the external auditory meatus of the ear. In sum, axons from several brainstem regions join together into the facial nerve and then diverge again to reach many distal targets. Consequently, injuries to the facial nerve pathway cause very different complements of symptoms, depending on the site of the lesion. A facial nerve lesion will impair all of the sensory, autonomic, and motor functions served by the facial nerve. Lesions that are progressively more distal, or far from the central nervous system, cause progressively more selective impairments of facial nerve function. Central lesions resemble distal lesions in that they often impair only one function of a cranial nerve. However, central lesions typically affect additional functions supported by areas neighboring the region involved in cranial nerve function.

Cranial nerve exit points Figure 10-2. This photograph shows the brain from the ventral surface. The pyramidal decussation, located at the site flanked by two red lines, marks the junction between the spinal cord (sc) and medulla (m) and the location of the foramen magnum. Rostral to the medulla is the pons, which has a bulbous base. Rostral to the pons is the midbrain (mb), which is marked by the black and yellow line (see detail in Fig. 10-3). The roots of the cranial nerves are labeled on the right side of the photomicrograph. On the left side of the photograph, the roots of cranial nerves II, III, V, VI, VII, VIII, and X are visible. Cranial nerve I, the olfactory nerve, consists of a large number of tiny axons that connect the nasal epithelium to the olfactory bulb (OB). These tiny axons are not visible, but the bulb is clearly seen at the base of the frontal lobes. The root of cranial nerve IV exits from the dorsal side of the midbrain and snakes around to the ventral surface of the brainstem near the junction of the pons and midbrain (see arrow marked IV). However, it is very small and very difficult to see in this photograph. The roots of cranial nerves IV and XI are also not visible on the left side of this photomicrograph. Photograph reprinted with permission from deArmond S., et al. Structure of the human brain: A photographic atlas. New York: Oxford University Press, 1989.

Optic pathway: retina to thalamus Figure 10-3. This photograph shows the ventral surface of the brain from the pons to the thalamus. Axons from retinal neurons travel through the optic nerve toward the optic chiasm. Some of the axons cross the optic chiasm to the contralateral optic tract and some retinal axons skirt the chiasm to travel in the ipsilateral optic tract (see Fig. 10-5). As the optic tract travels to the lateral geniculate nucleus (LGN) of the thalamus, it abuts the cerebral peduncles (cp), a major fiber tract at the base of the midbrain. Despite the different names, the optic nerve and the optic tract both contain the same retinal axons. Additional structures labeled for orientation include the pons, the oculomotor nerve (III), the infundibular stalk (is) and the mammillary bodies (MB). Photograph reprinted with permission from deArmond S., et al. Structure of the human brain: A photographic atlas. New York: Oxford University Press, 1989

Visual field: fixation straight ahead Figure 10-4A: Everything to the right of the fixation point (red dot) is in the right visual field, and everything to the left is in the left visual field. The visual fields are always relative to the point of fixation rather than to the body axis.

Visual field: fixation to the side Figure 10-4B: Thus, when a person fixates to the right, both the left and right visual fields are to the right of the body.

Binocular and monocular fields Figure 10-5A: The visual field covers almost 180 degrees, most of it binocular, centered at the fixation point (red dot). Each retina is divided in the horizontal axis between a nasal portion, closer to the nose, and a temporal portion, closer to the temple. Light (dashed lines) from the most peripheral 15 degrees or so of the visual field (dark blue) only reaches the ipsilateral nasal retina. It is blocked by the nose from reaching the contralateral temporal retina. This most peripheral portion of the visual field is the monocular portion. Light from the binocular portion of the left visual field (blue) hits the left nasal retina and the right temporal retina. The light hits both retinas at corresponding locations, located at the same eccentricity, or polar distance from the center (see inset).

The visual pathway Figure 10-5B: Visual pathways to the lateral geniculate nucleus (LGN) are illustrated. Light (dashed lines) from the right visual field (gray), the area right of the fixation point, hits the left (dashed lines) temporal retina and the right (solid lines) nasal retina. Light from the left visual field (blue), the area right of the fixation point, hits the right temporal retina and the corresponding point in the left nasal retina. Axons from the temporal retina project to the ipsilateral thalamus, whereas axons from the nasal retina cross in the optic chiasm to reach the contralateral thalamus. Because of the selective crossing of retinal axons, the right visual field is carried on the left side and the left visual field is carried on the right side beyond the optic chiasm. Therefore, retrochiasmatic lesions, lesions caudal to the optic chiasm, produce contralateral visual field deficits.

Diplopia Figure 10-5C: Diplopia results from the two eyes fixating on different sites. In the example illustrated, the left eye is fixated on a spot (L) to the right of where the right eye is fixated (R). As a result, two different spots in the visual field are represented in corresponding retinal locations (dashed lines). Conversely, as illustrated on the right, light from any one spot in the visual field hits noncorresponding spots on the retina.

Neutral eye position Figure 10-6D. The neutral eye position is straight ahead.

Vertical & torsional eye movements Figure 10-6. There are three axes about which the eyes move—vertical, torsional, and horizontal. Vertical (A) and torsional (B) movements are always conjugate, meaning that the two eyes move together in the same direction. A: Moving the eyes up and down is termed elevation and depression. B: Rotating the eyes around the axis going through the pupil is called torsion, with rotating the eyes toward the nose termed intorsion and away from the nose, extorsion.

Horizontal eye movements Figure 10-6. Horizontal movements can either be conjugate (C) or disconjugate, in the case of vergence movements (D). C: Moving an eye in the horizontal plane toward the nose is termed adduction, and moving it toward the temple is abduction. D: When viewing near objects, the two eyes converge. This convergence is disconjugate with both eyes adducting. In addition, the pupil narrows during near viewing. For viewing far objects straight ahead, the eyes adopt the neutral position.

Extraocular muscle insertions Figure 10-7A-B: The eye approximates a globe in shape and sits within the bony orbit. Therefore, we can consider the circumference halfway between the cornea in the front and the optic nerve in the back as the equator. Four of the extraocular muscles attach to the globe at a point anterior to the equator. These four muscles—medial, lateral, inferior, and superior recti—all pull the eye back (red arrow in B), so that the pupil rotates toward the muscle. In contrast, the remaining extraocular muscles, the inferior and superior oblique muscles attach to the globe posterior to the equator. When the oblique muscles contract, the eye rotates toward the front (blue arrow in B). Modified with permission of MacKinnon, P., and Morris, J. Oxford textbook of functional anatomy, volume 3, head and neck. New York: Oxford University Press, 2005.

Action of rectus muscles Figure 10-7C: The lateral and medial recti (LR, MR) pull the eye laterally and medially, respectively. D: The superior rectus elevates the eye from the neutral position. The inferior rectus depresses the eye in an analogous fashion (not shown). Modified with permission of MacKinnon, P., and Morris, J. Oxford textbook of functional anatomy, volume 3, head and neck. New York: Oxford University Press, 2005.

Action of oblique muscles Figure 10-7E-F: E: The superior oblique pulls the eye toward the nose, a movement that is termed intorsion, when the eye starts in the neutral position. F: The pulling directions of the extraocular muscles change according to the initial position of the eye in the orbit. For example, when the eye is adducted, the action of the superior oblique is primarily to depress the eye. Modified with permission of MacKinnon, P., and Morris, J. Oxford textbook of functional anatomy, volume 3, head and neck. New York: Oxford University Press, 2005.

Trochlear nerve exit Figure 10-8. The trochlear nerve (IV n) exits from the dorsal surface of the brainstem, the only cranial nerve to do so. A dorsal view of the brainstem, with the cerebellum removed is shown with the medulla, pons, midbrain, thalamus, and fourth ventricle (IV) labeled for orientation. A magnified view of the region between the dashed lines, shown at right, shows the exit point of the trochlear nerve, near the junction between the pons and midbrain and just caudal to the inferior colliculi (IC). Photograph reprinted with permission from deArmond S., et al. Structure of the human brain: A photographic atlas. New York: Oxford University Press, 1989

Sensory innervation of the head Figure 10-9. Fibers exit the trigeminal ganglion in three bundles, or branches, destined for three different regions of the face. Each region of the face is innervated by one branch of the trigeminal nerve. The external regions supplied by the ophthalmic (also termed V1), maxillary (V2), and mandibular (V3) branches of the trigeminal nerve are shown. Unlike the case with spinal dermatomes, there is little overlap between the regions supplied by the three branches of the trigeminal nerve. The skin of the ear is innervated by spinal nerve C2 and cranial nerves V, VII, IX, and X. The back of the head and neck is innervated by spinal nerves C2 and C3.

Bell’s palsy Figure 10-10. A person with Bell’s palsy cannot contract muscles on the affected side of the face. All facial expressions, even wrinkles, depend on activity in the facial nerve. Therefore, patients have far fewer wrinkles on the affected half of his face than on the unaffected half. Particularly notable are the absences of a nasolabial fold (arrow), the wrinkle between the lateral edges of the nose and mouth, and forehead wrinkles (in older patients) on the affected side. A young woman with a left sided Bell’s palsy (A) and a young man with a right sided Bell’s palsy (B) are shown as they attempt to smile, demonstrating the asymmetric smile of patients with Bell’s palsy. Modified from Harrison, D.H. Surgical correction of unilateral and bilateral facial palsy. Postgrad Med J 81:562-567, 2005, with permission of the publisher, BMJ Publishing Group Ltd.

Cerebellopontine angle Figure 10-11. The photograph of the ventrum of the brainstem shows the cerebellopontine angle (cpa), which is located at the lateral edge of the pontomedullary junction (white line labeled p-m j) where cranial nerves VII and VIII emerge. The cartoon of the base of the brain illustrates the location of the cerebellopontine angle at the convergence of the cerebellum, pons, and medulla. The smallest tumors at the cerebellopontine angle cause hearing and balance problems. With progressively larger tumors, symptoms attributable to impairment of the facial, trigeminal and abducens nerves can also occur. Photograph is reprinted with permission from deArmond S., et al. Structure of the human brain: A photographic atlas. New York: Oxford University Press, 1989.