ABSTRACT
A. General Remarks 1. Twelve pairs of cranial nerves (CNs): CN I-CN XII 2. This chapter reviews CNs I, V, VII, VIII, IX, X, XI,
and XII; CNs II, III, IV, and VI are discussed in Chapter 3
B. Organization 1. Cranial nerves, like spinal nerves, contain sensory or
motor fibers or a combination of sensory and motor fibers
2. The fibers are classified by their embryologic origin or common structural and functional characteristics
3. Motor fibers a. General somatic efferent (GSE) fibers: innervate muscles
derived from somites (CNs III, IV, VI, and XII) b. Special visceral efferent (SVE) fibers: innervate muscles
derived from branchial arches (CNs V, VII, IX, X, and XI) c. General visceral efferent (GVE) fibers: innervate struc-
tures derived from mesoderm or endoderm; parasympathetic (CNs III, XII, IX, and X)
4. Sensory fibers a. General somatic afferent (GSA) fibers: sensory informa-
tion from body surface, joints, and mucosal membranes (CNs V, VII, IX, and X)
b. General visceral afferent (GVA) fibers: sensory information from pharynx and endodermally derived structures such as viscera (CNs IX and X)
c. Special visceral afferent (SVA) fibers: sensory information from taste and olfactory receptors (CNs I, VII, IX, and X)
d. Special somatic afferent (SSA) fibers: sensory information from visual, auditory, and vestibular receptors (CNs II and VIII)
5. Cranial nerves and their components and functions are summarized in Tables 4-1 and 4-2
6. Cranial nerves and their attachments to the brain are shown in Figure 4-1
7. Cranial nerves and their brainstem nuclei are shown in Figure 4-2
8. Foramina of the skull through which cranial nerves pass are shown in Figure 4-3
A. Olfactory Cranial Nerve 1. Type: SVA 2. Function: sense of smell 3. Receptor
a. Chemoreceptor (olfactory receptor cell) b. A bipolar neuron unlike other sensory systems in which
the receptor is distinct from the neuron 4. Pathway
a. Cells of origin: bipolar neurons (olfactory receptor cells) within the nasal cavity; axons extend through the cribriform plate to the olfactory bulb
b. Thalamic relay: none (unique feature) c. Course (Fig. 4-4)
1) Olfactory receptor cell axons synapse with mitral cells or tufted cells in olfactory bulb
2) Each mitral cell receives input from up to 1,000 olfactory nerve cells
3) Mitral cell axons project to primary olfactory cortex (in piriform cortex-uncus and entorhinal cortex) and amygdala via olfactory tract and lateral olfactory stria
4) Tufted and mitral cells may project to lateral, medial, and intermediate olfactory areas
5) Medial olfactory stria passes to medial frontal lobe
Olfactory pathway is the only sensory pathway that does not synapse in the thalamus
Table 4-1. Summary of Cranial Nerve Components
CN GSE SVE GVE GSA SVA SSA GVA
and is thought to be involved with emotional response to odors
6) Some collaterals from mitral and tufted cells synapse in anterior olfactory nucleus near where olfactory bulb becomes olfactory tract
7) Axons from some neurons in anterior olfactory nucleus cross in anterior commissure to synapse in contralateral olfactory areas
8) Olfactory cortex interconnects with various autonomic and visceral centers (thalamus, hypothalamus, amygdala), perhaps explaining why odors can cause nausea, salivation, or increased gut motility
B. Differential Diagnosis of CN I Disorders 1. Terminology
a. Anosmia/hyposmia: decreased or lack of smell/decreased sense of smell
b. Dysosmia: smell perception is distorted; can occur with seizure disorders of temporal lobe
c. Hyperosmia: increased sense of smell
d. Cacosmia: perception of a bad smell e. Parosmia: sensation of smell in absence of appropriate
stimulus 2. Disorders of CN I can occur anywhere along its path-
way (Table 4-3)
A. Overview 1. CN V has both sensory and motor components 2. Motor component: SVE, innervates muscles of
mastication 3. Sensory components
a. GSA: ipsilteral pain and temperature sensation of face and supratentorial dura mater
b. GSA: vibration, proprioception, tactile sensation of ipsilateral face
c. GSA: unconscious proprioception of jaw; reflexive chewing
B. Motor Component of CN V 1. Type: SVE 2. Function
a. Innervates muscles of mastication (temporalis, masseter, and medial and lateral pterygoid muscles); these muscles open (protract) the jaw, important in chewing
b. Also innervates anterior belly of digastric, mylohyoid, and tensor tympani muscles
3. Nucleus: motor nucleus of V (midpons) (Fig. 4-2 D) 4. Course: axons exit brainstem laterally and travel with
mandibular division (V3) of CN V through foramen ovale to innervate skeletal muscle
5. Corticobulbar: bilateral corticobulbar input to motor nucleus of V
6. Dysfunction a. Upper motor neuron lesion: no pronounced weakness
because of bilateral corticobulbar input
b. Lower motor neuron lesion: jaw deviates to weak side
C. Sensory Components of CN V 1. General: sensory nuclei of V receive information from
three separate divisions-ophthalmic (V1), maxillary (V2), and mandibular (V3) (Fig. 4-5 and Table 4-4)
2. Cell bodies of V1, V2, and V3 are in trigeminal ganglion (also called gasserian ganglion, semilunar ganglion) which lies in Meckel’s cave in the middle cranial fossa
3. Spinal tract and nucleus of V (Fig. 4-6) a. Type: GSA b. Function: pain and temperature sensation of face, oral
cavity, dorsum of head, including tempormandibular joint, supratentorial meninges, and teeth
c. Receptors: cutaneous pain and temperature receptors (C fibers)
d. Ganglion (first-order neuron): CN V (trigeminal
Table 4-2. Summary of Cranial Nerves
Cranial nerve Type Ganglion/nucleus Function
ganglion), CN VII (geniculate ganglion), CN IX (superior ganglion), and CN X (superior ganglion)
e. Nucleus (second-order neuron): spinal nucleus of V (Fig. 4-2 B)
f. Thalamic relay (third-order neuron): ventral posteromedial nucleus
g. Course 1) Sensory fibers enter the midpons and descend as
spinal trigeminal tract in lateral brainstem 2) Axons may terminate at low pons or medulla or
descend as far as C2-3 (near zone of Lissauer) before synapsing
3) Axons terminate in spinal trigeminal nucleus, which is medial to the tract
4) Axons of second-order trigeminothalamic neurons decussate and form anterior trigeminothalamic tract
5) This tract ascends through brainstem just posterior to medial lemniscus
6) Tract synapses in ventral posteromedial thalamic nucleus (third-order neurons)
7) Axons of third-order neurons terminate in ipsilteral primary somatosensory cortex (SI, Brodmann areas 3, 1, 2)
8) Some collaterals of trigeminothalamic tract terminate in reticular formation and other thalamic nuclei; others are involved in local oral reflexes
h. Dysfunction: ipsilateral loss of pain and temperature of the face
i. Corneal reflex: bilateral blink in response to corneal stimulus 1) Afferent arm: V1
2) Interneuron: spinal nucleus of V 3) Efferent arm: CN VII (Fig. 4-7)
4. Chief sensory nucleus of V (Fig. 4-6) a. Type: GSA b. Function: vibration, proprioception, and light
touch/tactile discrimination sensation of face c. Receptor: Pacinian corpuscle, Merkel cells, Meissner
corpuscles d. Ganglion (first-order neuron): trigeminal ganglion e. Nucleus (second-order neuron): chief sensory nucleus
(Fig. 4-2 D) f. Thalamic relay nucleus (third-order neuron): ventral
posteromedial nucleus g. Course
1) Axons from trigeminal ganglion enter midpons to synapse in chief sensory nucleus in pontine tegmentum
2) Second-order axons cross midline and ascend in lateral brainstem to ventral posteromedial thalamic nucleus
3) Axons from this nucleus terminate in ipsilteral primary somatosensory cortex
h. Dysfunction: ipsilateral loss of tactile discrmination 5. Mesencephalic nucleus of V
a. Type: GSA b. Function
1) Unconscous proprioception 2) Muscle spindles in pterygoid, masseter, and tempo-
ralis muscles supply information about force of bite and/or stretch of muscle to mesencephalic nucleus of V, which forms a reflex circuit with motor nucleus of V to adjust and control force of bite
Table 4-2. (continued)
Cranial nerve Type Ganglion/nucleus Function
c. Nucleus: mesencephalic nucleus of V-unique because it has no ganglion; unipolar neuron with cell body in midbrain
d. Course 1) Peripheral axon innervates muscle spindles 2) Central axon terminates in the motor nucleus of V
e. Jaw jerk 1) Contraction of masseter and temporalis muscles
when patient’s lower jaw is tapped 2) Afferent arm: peripheral axons of mesencephalic
nucleus of V traveling with V3 3) Efferent arm: central axons of mesencephalic nucleus
of V synapse in motor nucleus of V and axons from this nucleus (traveling with V3) innervate muscles of mastication
4) Lesions anywhere along this reflex arc cause depression of ipsilateral jaw reflex, and bilateral supranuclear lesions produce accentuated response
D. Differential Diagnosis of CN V Disorders 1. Disorders of CN V are listed in Table 4-5
E. Selected Disorders of CN V 1. Trigeminal neuralgia (see Chapter 18)
2. Idiopathic trigeminal sensory neuropathy a. Clinical findings
1) Usually gradual onset 2) Loss of sensation in a trigeminal distribution (general-
ly V2 and V3, but can involve all three divisions) 3) Pain is absent
b. Pathology 1) Thought to be inflammatory process primarily affect-
ing trigeminal ganglion 2) Pathology examination typically shows cell loss, lym-
phocyte infiltration, and fibrosis in trigeminal ganglion c. Diagnosis
1) Rule out other causes of trigeminal nerve dysfunction (Table 4-5)
2) Because trigeminal sensory neuropathy commonly associated with connective tissue disease, appropriate rheumatologic and serologic testing is important
3) Nerve conduction studies may help establish involve-
The mesencephalic nucleus of V is unique. It is the only general somatic sensory pathway with a firstorder neuron cell body in the central nervous system.
