5 CHAPTER VSHORT REVIEW OF CRANIAL NERVES.SIGNS OF LESIONS
Figure 5.1. Cranial nerves (A.A. Skoromets, 1995)
The twelve pairs of cranial nerves are named so because they pass through various apertures in the cranial bones. Like the 31 pairs of spinal nerves, they are the part of the peripheral nervous system (PNS). Each cranial nerve has both a number, designated by a roman numeral, and a name. The numbers indicate the order, from anterior to posterior, in which the nerves arise from the brain. The names designate a nerve's distribution or function.
Cranial nerves emerge from the nose (cranial nerve I), the eyes (cranial nerve II), the inner ear (cranial nerve VIII), the brainstem (cranial nerves III-XII), and the spinal cord (part of cranial nerve XI). Two cranial nerves, I (olfactory) and II (optic), contain only sensory axons and thus are called sensory nerves. The rest are mixed nerves because they contain axons of both sensory and motor neurons. The cell bodies of sensory neurons are located in ganglia outside the brain, whereas the cell bodies of motor neurons lie in nuclei within the brain. Cranial nerves III (oculomotor), IV (trochlear), VI (abducens), XI (accessory), and XII (hypoglossal) are mainly motor. While a few of their axons are sensory axons from muscle proprioceptors, most of their axons are motor neurons that innervate skeletal muscles. Cranial nerves III, VII, IX, and X include both somatic and autonomic motor axons. The somatic axons innervate skeletal muscles whereas the autonomic axons, which are the part of parasympathetic division, innervate glands, nonstriated muscles, and cardiac muscle.
I. Olfactory nerve (sensory)
Origin: Bipolar olfactory neurons in olfactory mucosa.
Site where nerve leaves the brain or enters it: Olfactory bulb;
Site of exit from the cranium: Cribriform plate ethmoid bone;
Arises in olfactory mucosa, passes through foramina in the cribriform plate of the ethmoid bone, and ends in the olfactory bulb. The olfactory tract extends via two pathways to olfactory areas of cerebral cortex;
Function: Smell;
Clinical application: Loss of the sense of smell, called anosmia (partial loss of sense of smell called hemianosmia), may result from head injuries in which the cribriform plate of the ethmoid bone is fractured and from lesions along the olfactory pathway. Diseases involving the anterior temporal lobe and its base (intrinsic or extrinsic tumors), which may produce olfactory hallucinations (signs of irritation). Temporal lobe seizures may start with an olfactory aura.
II. Optic nerve (sensory) (Fig. 5.2).
Origin: Ganglion of the retina;
Site where nerve leaves the brain or enters it: Optic chiasma on base of brain;
Site of exit from the cranium: Canalis opticus;
Arises in the retina of the eye, passes through the optic foramen, forms the optic chiasm and then the optic tracts, and terminates in the lateral geniculate nuclei of thalamus. From the thalamus, axons extend to the primary visual area of the cerebral cortex;
Function: Vision;
Clinical application: Fractures in the orbit damage along the visual pathway, of the NS may result in visual field defects and loss of visual acuity. Amaurosis. It is partial or complete blindness. Hemianopia-absence of half of the normal field of vision. The commonest type is homonymous hemianopia, in which the same half (right or left) is lost in both eyes. Sometimes the inner halves of the visual field are lost in both eyes, producing a binasal hemianopia, while in others the outer halves are lost, producing a bitemporal hemianopia (Oxford Medical Dictionary, 2003). Result of irritation of visual cortex is visual hallucination.
Figure 5.2. Anatomy & function of optic nerve (J.D. Fix, 1994).Visual pathway from the retina to the visual cortex showing visual field defects: (1) Total blindness in left eye; (2) Binasal hemianopia; (3) Bitemporal hemianopia; (4) Right hemianopia; (5) Right upper quadrantanopia; (6) Right lower quadrantanopia; (7) Right hemianopia with macular sparing.
III. Oculomotor nerve (mixed, mainly motor)
Nuclei: a) nucleus of oculomotor nerve and nucleus (Yakubovich) and unaired median, which locates in a tegmentum pedunculi cerebri;
Site where nerve leaves the brain or enters it: Median sulcus, cerebral peduncle, interpeduncular fossa;
Site of exit from the cranium: Superior orbital fissure;
Sensory portion: Consists of axons from proprioceptors in eyeball muscles that pass through the superior orbital fissure and terminate in the midbrain;
Motor portion: Originates in the midbrain and passes through the superior orbital fissure. Axons of somatic motor neurons innervate the levator palpebrae superioris muscle of the upper eyelid and four extrinsic eyeball muscles (superior rectus, medial rectus, inferior rectus, and inferior oblique);
Parasympathetic axons innervate the ciliary muscle of eyeball and the circular muscles (sphincter pupillae) of the iris;
Function: Proprioception. Somatic motor function: Movement of upper eyelid and eyeball. Autonomic motor function (parasympathetic): accommodation of lens for near vision and constriction of pupil;
Clinical application: Nerve damage causes strabismus divergent (a deviation of the eye in which both eyes do not fix on the same object), ptosis (drooping) of the upper eyelid, dilation of the pupil, movement of the eyeball downward and outward on the damaged side, loss of accommodation for near vision, and diplopia (double vision).
IV. Trochlear nerve. (mixed, mainly motor)
Nuclei: Nucleus of trochlear nerve, which locates in tegmentum pedunculi cerebri;
Site where nerve leaves the brain or enters it: Dorsally, behind lamina quadrigemina, from superior medullary velum;
Site of exit from the cranium: Superior orbital fissure;
Sensory portion: Consists of axons from proprioceptors in the superior oblique muscles, which pass through the superior orbital fissure and terminate in the midbrain;
Motor portion: Originates in the midbrain and passes through the superior orbital fissure. Innervates the superior oblique muscle, an extrinsic eyeball muscle;
Functions: Proprioception. Somatic motor function: Movement of the eyeball;
Clinical application: In trochlear nerve paralysis diplopia and gaze downward strabismus occur.
V. Trigeminal nerve (mixed). (Fig. 5.3, 5.4.).
Nuclei: a) motor nucleus of trigeminal nerve (locates in upper part of pars dorsalis pontis; b) superior sensory nucleus of trigeminal nerve (same as that of previous nucleus, laterally of it); c) nucleus of spinal tract of trigeminal nerve (continuation of previous one along the length of medulla oblongata); d) mesencephalic nucleus of trigeminal nerve (tegmentum of cerebral peduncle lateral of aqueduct of midbrain);
Site where nerve leaves the brain or enters in; Anteriad of middle cerebellar peduncle;
Site of exit from the cranium: Ophthalmic nerve, superior orbital fissure, maxillary nerve – foramen rotundum, mandibular nerve – foramen ovale;
Sensory portion: Consists of three branches, all of which end in the pons. The ophthalamic nerve contains axons from the skin over the upper eyelid, eyeball, lacrimal glands, nasal cavity, side of nose, forehead, and anterior half of scalp that pass through superior orbital fissure. The maxillary nerve contains axons from the mucosa of the nose, palate, parts of the pharynx, upper teeth, upper lip, and lower eyelid that pass through the foramen rotundum. The mandibullar nerve contains axons from the anterior two-thirds of the tongue (somatic sensory axons but not axons for the special sense of taste), the lower teeth, skin over mandible, cheek and mucosa deep to it, and side of head in front of ear that pass through the foramen ovale;
Motor portion: It’s part of the mandibular branch, which originates in the pons, passes through the foramen ovale, and innervates muscles of mastication (masseter, temporal is, medial pterygoid, lateral pterygoid, anterior belly of digastric, and mylohyoid muscles);
Functioins: Sensory: Conveys impulses for touch, pain, and temperature sensations and proprioception. Motor: Chewing;
Clinical application: Neuralgia (pain) of one or more branches of the trigeminal nerve is called trigeminal neuralgia (tic douloureux). Injury of the mandibullar nerve may cause paralysis of the chewing muscles and a loss of the sensations of touch, temperature, and proprioception in the lower part of the face. “Trismus” is a tonic spasm of the masticatory muscles caused by acute encephalitic lesions in the pons, by rabies, by tetanus, or by other conditions. Because of the strong abnormal tension in these muscles, the patient is not able to open his mouth.
Figure 5.3. Anatomy of nucleus of III, IV, VI cranial nerves (A.A. Skoromets, 1995)
Figure 5.4. Inervation of facial skin and head skin (A.A. Skoromets, 1995)
VI. Abducens nerve (mixed, mainly motor)
Nuclei: Nucleus of abducens nerve (locates in dorsal portion of pons, in region of facial colliculus);
Site where nerve leaves the brain or enters in: posterior edge of pons in sulcus between pons and pyramid;
Site of exit from the cranium: superior orbital fissure;
Sensory portion: Consists of axons from proprioceptors in the lateral rectus muscle, which passes through the superior orbital fissure and ends in the pons;
Motor portion: Originates in the pons, passes through the superior orbital fissure, and innervates the lateral rectus muscle, an extrinsic eyeball muscle;
Function: Proprioception. Movement of the eyeball;
Clinical application: With damage to this nerve, the affected eyeball cannot move laterally beyond the midpoint, and the eye usually is directed medially. All three motor nerves of one eye are interrupted, the eye looks straight ahead and cannot be moved in any direction, and its pupil is wide and does not react to light (ophthalmoplegia totalis). Bilateral paralysis of the eye muscles is usually the result of the nuclear damage. The most frequent causes of nuclear paralysis are encephalitis, neurosyphilis, multiple sclerosis, circulatory condition, hemorrhages, and tumors. The most frequent causes of peripheral eye muscle palsies are meningitis, sinusitis, cavernous sinus thrombosis, aneurysm of internal carotid artery or posterior communicating artery, fractures, tumors of the cranial base as well as of the orbit, diphtheria, and botulism.
VII. Facial nerve (mixed). (Fig. 5.5, 5.6).
Nuclei: a) nucleus of tractus solitaries; b) superior salivary nucleus (situate in dorsal portion of pons, reticular formation);
Site where nerve leaves the brain or enters in: At the back of middle cerebellar peduncle;
Site of exit from the cranium;
Sensory portion: Arises from taste buds on the anterior two-thirds of the tongue, passes through the stylomastoid foramen and geniculate ganglion (located beside the facial nerve), and ends in the pons. From there, axons extend to the thalamus, and then to the gustatory areas of the cerebral cortex. Also contains axons from proprioceptors in muscles of the face and scalp;
Motor portion: Originates in the pons and passes through the stylomastoid foramen. Axons of somatic motor neurons innervate facial, scalp, and neck muscles. Parasympathetic axons innervate lacrimal, sublingual, submandibular, nasal, and palatine glands. Sensory function: proprioception and taste;
Somatic motor function: Facial expression;
Autonomic motor function(parasympathetic): Secretion of saliva and tears;
Clinical application: Injury produces Bell?s palsy (paralysis of the facial muscles), loss of taste, decreased salivation, and loss of ability to close the eyes.
Figure 5.5. Left peripheral facial palsy (P.Duus, 1989): a) Central facial palsy: the forehead muscles are not affected. b) Peripheral facial palsy: the forehead muscles are involved along with the rest of the face on the affected side.
Figure 5.6. Anatomy of facial nerve (J.D. Fix, 1994)
VIII. Auditory nerve (mixed, mainly sensory)
Nuclei: Ventral and dorsal cochlear nuclei medial, lateral superior and inferior vestibular nuclei (In region of lateral angle of rhomboid fossa (area vestibularis);
Site where nerve leaves the brain or enters in: Lateral of facial nerve, on posterior edge of pons laterally of olive;
Site of exit from the cranium: Porus acusticus internus.
Vestibular nerve
Sensory portion: Arises in the semicircular canals, saccule, and utricle and forms the vestibular ganglion. Axons end in the pons and cerebellum;
Motor portion: Originates in the pons and terminates on hair cells of the semicircular canals, saccule, and utricle. Cochlear nerve, sensory portion: Arises in the spiral organ (organ of Corti), forms the spiral ganglion, passes through nuclei in the medulla, and ends in the thalamus. Axons synapse with thalamic neurons that relay impulses to the primary auditory area (areas 41 and 42) of the cerebral cortex.
