Ganglion Cranial nerve association association Trigeminal Function(s)

Ciliary Oculomotor (III) Ophthalmic division Constricts pupil; lens accommodation Pterygopalatine Facial (VII) Maxillary division Lacrimation; nasal gland, and minor salivary gland secretion Submandibular Facial (VII) Mandibular division Salivation (parotid gland) Otic Glossopharyngeal (IX) Mandibular division Salivation (submandibular and sublingual glands) Intramural Vagus (X) Gland secretion;

Table 15.2 = Parasympathetic ganglia of peristalsis the cranial nerves. control eye movements, whereas the hypoglossal nerve supplies motor innervation to the muscles of the tongue, mediating movement of the tongue.

 

3 General visceral afferent (GVA). General sensation from the viscera is transmitted by the facial, glossopharyngeal, and vagus nerves.

4 General visceral efferent (GVE). These fibers provide visceral motor (parasympathetic) innervation to the viscera.

The only cranial nerves that transmit parasympathetic fibers are the oculomotor, facial, glossopharyngeal, and vagus nerves.

 

5 Special somatic afferent (SSA). These fibers carry special sensory input from the eye (retina), for vision, and from the ear (vestibular apparatus for equilibrium, and cochlea for hearing). The only nerves transmitting this component are the optic and vestibulocochlear nerves.

 

6 Special visceral afferent (SVA). These are special sensory fibers from the viscera. These fibers convey the special sense of smell transmitted by the olfactory nerve and the special sense of taste transmitted by the facial, glossopharyngeal, and vagus nerves.

7 Special visceral efferent (SVE). These motor fibers are special because they supply motor innervation to skeletal muscles of branchial arch origin. These fibers are carried by the nerves of the branchial arches, which are the trigeminal, facial, glossopharyngeal, and vagus nerves. Table 15.4 summarizes the modalities, nuclei, ganglia, and functions of the cranial nerves.

 

 

OLFACTORY NERVE (CN I)

The bipolar olfactory receptor cells (first ordersensory neurons) of theolfactory apparatus residenot in a sensory ganglion,but instead in the olfactory epithelium (neuroepithelium) of the modified nasal mucosa lining the roof and adjacent upper walls of the nasal cavities (see Fig. 19.1).

 

 

The axons of these bipolar neurons are SVA fibers transmitting olfactory sensation.

These axons assemble to form bundles, the olfactory fila (L., “threads”), which collectively form cranial nerve I. Theolfactory fila traverse the fenestrations of the cribriform plateof the ethmoid bone to terminate in the olfactory bulb wherethey synapse with second order relay neurons and interneurons.

OPTIC NERVE (CN II)

The optic nerve mediates the special sense of vision via its SSA fibers. Light entering the eye activates cells known as rods and cones, the photoreceptors of the retina. Electrical signals generated by the photoreceptors are transmitted to other cells of the retina that process and integrate sensory input. The first order sensory bipolar neurons of the visual pathway reside in the retina and transmit electrical signals of visual sensory input to the multipolar second order ganglion cells of the retina. The ganglion cells give rise to unmyelinated axons that converge at the optic disc and traverse the lamina cribrosa, a sieve-like perforated area of the sclera, to emerge from the back of the eyebulb. At this point, the ganglion cell axons acquire a myelin sheath and assemble to form the optic nerve. This nerve, an outgrowth of the diencephalon, leaves the orbit via the optic canal to enter the middle cranial fossa.

There, the optic nerves of the right and left sides join each other to form the optic chiasma (G., “optic crossing”) where partial decussation of the optic nerve fibers of the two sides takes place. All ganglion cell axons arising from the nasal half of the retina decussate (through the central region of the chiasma) to the opposite optic tract. All ganglion cell axons arising from the temporal half of the retina proceed (through the lateral aspect of the chiasma) without decussating and join the optic tract of the same side. The ganglion cell axons coursing in each optic tract curve around the cerebral peduncle to terminate and relay visual input in one of the following four regions of the brain: the lateral geniculate nucleus, a thalamic relay station for vision; the superior colliculus, a mesencephalic relay station for vision associated with somatic reflexes; the pretectal area, a mesencephalic region associated with autonomic reflexes; and the hypothalamus (see Figs 16.5, 16.7, 16.9).

 

OCULOMOTOR NERVE (CN III)

The oculomotor nerve supplies skeletal motor (somatomotor) innervationto the superior rectus, medialrectus, inferior rectus, andinferior oblique muscles(which move the bulb of theeye) and the levator palpebrae superioris muscle (whichelevates the upper eyelid). It also provides parasympathetic (visceromotor) innervation to the ciliary and sphincter pupillaemuscles, two intrinsic smooth muscles of the eye.

The triangular-shaped oculomotor nuclear complex islocated in the mesencephalon. It is situated ventral to theperiaqueductal gray, adjacent to the midline at the levelof the superior colliculus. The oculomotor nucleus consistsof several subnuclei representing each of the extraocularmuscles. These subnuclei are composed of groups of nervecell bodies of the GSE neurons that innervate the listedextraocular muscles and the levator palpebrae superiorismuscle. The cell group innervating the levator palpebraesuperioris is located in the midline, sending motor fibers tothis muscle bilaterally (both right and left upper eyelids). The cell group innervating the superior rectus sends projections to the opposite side; whereas the cell group innervating the medial rectus, inferior oblique, and inferior rectus sends projections to the same side. The Edinger–Westphal nucleus, a subnucleus of the oculomotor nuclear complex is located dorsally, medially, and rostral to the GSE nuclear complex. It contains the cell bodies of GVEpreganglionic parasympathetic neurons whose axons join the GSE fibers as they converge and pass ventrally in the midbrain to emerge from the ventral aspect of the brainstem in the interpeduncular fossa as the oculomotor nerve. The oculomotor nerve proceeds anteriorly within the cranial vault, travels within the cavernous sinus, and by passing through the superior orbital fissure, enters the ipsilateral orbit. Within the orbit, the oculomotor nerve gives rise to branches carrying the GSE fibers that innervate the levator palpebrae superioris muscle and all but two of the extraocular muscles. The preganglionic parasympathetic fibers of the oculomotor nerve terminate in the ciliary ganglion where they synapse with postganglionic parasympathetic nerve cell bodies. Postganglionic parasympathetic fibers exit the ganglion and reach the sphincter pupillae and ciliary muscles via the short ciliary nerves to provide them with parasympathetic innervation. The parasympathetic fibers, when stimulated, cause contraction of the sphincter pupillae muscle, which results in constriction of the pupil. Pupillary constriction reduces the amount of light that impinges on the retina. Stimulation of the parasympathetic nervous system causes pupillary constriction (whereas stimulation of the sympathetic nervous system, which innervates the dilator pupillae muscle, causes pupillary dilation). Ciliary muscle contraction releases the tension on the suspensory ligaments of the lens, changing its thickness to become more convex. This accommodates the lens for near vision. GSA pseudounipolar neurons, whose cell bodies reside within the mesencephalic nucleus of the trigeminal nerve, send their peripheral processes to terminate in the muscle spindles of the extraocular muscles. These fibers travel via the branches of the ophthalmic division of the trigeminal nerve. GSA (GP) sensory input is transmitted from the muscle spindles via the spindle afferents centrally to the trigeminal nuclear complex, mediating coordinated and synchronized eye movements by reflex and voluntary control of muscles.

CLINICAL CONSIDERATIONS

Unilateral damage to the oculomotor nerve results in deficits in the ipsilateral eye. The following ipsilateral muscles will be paralyzed: the levator palpebraesuperioris, resulting in ptosis (G., “drooping”) of the upper eyelid; the superiorand inferior recti, resulting in an inability to move the eye vertically; and themedial rectus, resulting in an inability to move the eye medially. The eyedeviates laterally (due to the unopposed lateral rectus), resulting in lateralstrabismus. This causes the eyes to become misaligned as one eye deviatesfrom the midline, resulting in horizontal diplopia (double vision). The inferioroblique is also paralyzed. Since the innervation to the lateral rectus (CN VI)and superior oblique (CN IV) muscles is intact and these two muscles arefunctional, the eye ipsilateral to the lesion deviates inferiorly and laterally(Fig. 15.3). The sphincter pupillae muscle becomes nonfunctional due to interruptionof its parasympathetic innervation. The pupil ipsilateral to the lesion willremain dilated (mydriasis) and does not respond (constrict) to a flash of light.This may be the first clinical sign of intracranial pressure on the GVE fibers ofthe oculomotor nerve. The ciliary muscle is also nonfunctional due to interruptionof its parasympathetic innervation, and cannot accommodate the lensfor near vision (that is, cannot focus on near objects).

TROCHLEAR NERVE (CN IV)

The trochlear nerve is the smallest (thinnest) cranial nerve and the only one whose fibers originate totally from the contralateral nucleus.

The trochlear nerve provides motor innervation to only one of the extraocular muscles of the eye, the superior oblique muscle (acommon mnemonic is SO4). The nerve cell bodies of GSE neurons reside in the trochlear nucleus, which lies adjacent to the midline in the tegmentum of the caudal midbrain. Fibers arising from this nucleus initially descend for a short distance in the brainstem and then course dorsally in the periaqueductal gray matter. The fibers decussate posteriorly and emerge from the brainstem at the junction of the pons and midbrain, just below the inferior colliculus. The trochlear nerve is unique because it is the only cranial nerve whose fibers originate totally from the contralateral nucleus, it surfaces on the dorsal aspect of the brainstem, and it is the smallest (thinnest) of the cranial nerves. As the trochlear nerve emerges from the brainstem, it curves around the cerebral peduncle and proceeds anteriorly within the cavernous sinus to pass into the orbit via the superior orbital fissure. Consequently, this cranial nerve has the longest intracranial course and is highly susceptible to increased intracranial pressure.

CLINICAL CONSIDERATIONS

Damage to the trochlear nucleus results in paralysis or paresis of the contralateral superior oblique muscle, whereas damage to the trochlear nerve results in the same deficits but in the ipsilateral muscle.Normally, contraction of the superior oblique muscle causes the eye to

intort (rotate inward) accompanied by simultaneous depression (downward) and lateral (outward) movement of the bulb of the eye. This is sometimes referred to as the “Salvation Army muscle” (“down and out”). Intorsion of the eyeball is the turning of the eyeball around its axis, so that the superior pole of the eyeball turns inward. Imagine that extreme intorsion (which we really cannot do) will bring the superior pole of the eye facing the medial wall of the orbit. When the superior oblique muscle is paralyzed, the ipsilateral eye will extort (rotate outward) accompanied by simultaneous upward and outward movement of the eye (Fig. 15.4B). This is caused by the unopposed inferior oblique muscle and results in external strabismus. Since the eyes become misaligned following such a lesion, an individual with trochlear nerve palsy experiences vertical diplopia (double vision), accompanied by weakness of downward movement of the eye, most notably in an effort to adduct the eye (turn medially). The diplopia is most apparent to the individual when descending stairs or while reading (looking down and inward). To counteract the diplopia and to restore proper eye alignment, the individual realizes that the diplopia is reduced as he tilts his head towards the side of the unaffected eye (Fig. 15.4B). Normally, tilting of the head to one side elicits a reflex rotation about the anteroposterior axis of the eyes in the opposite direction (Fig. 15.4A), so that the image of an object will remain fixed on the retina. Tilting of the head toward the unaffected side causes the unaffected eye to rotate inward and become aligned with the affected eye which is rotated outward. Also, pointing the chin downward (“chin tuck”) rolls the normal eye upward.

TRIGEMINAL NERVE (CN V)

The trigeminal system consists of the trigeminal nerve, ganglion, nuclei, tracts, and central pathways. The trigeminal sensory pathway, which transmits touch, nociception, and thermal sensation, consists of a three neuron sequence (first, second, and third order neurons) from the periphery to the cerebral cortex respectively (Figs 15.5, 15.6). The peripheral processes of the first order neurons radiating from the trigeminal ganglion gather to form three separate nerves, the three divisions of the trigeminal nerve whose peripheral endings terminate in sensory receptors of the orofacial region. Their cell bodies are housed in the trigeminal ganglion. The central processes of these neurons enter the pons, join the spinal tract of the trigeminal, and terminate in the trigeminal nuclei where they establish synaptic contacts with second order neurons housed in these nuclei. The trigeminal nuclei, with the exception of the mesencephalic nucleus, contain second order neurons as well as interneurons. The second order neurons give rise to fibers that may or may not decussate in the brainstem and join the ventral or dorsal trigeminal lemnisci. These lemnisci ascend to relay trigeminal sensory input to the ventral posterior medial (VPM) nucleus of the thalamus, where they synapse with third order neurons. The third order neurons then relay sensory information to the postcentral gyrus (somesthetic cortex) of the cerebral cortex for further processing. The trigeminal nerve is the largest cranial nerve. It provides the major GSA innervation (touch, pressure, nociception, and thermal sense) to part of the scalp, most of the dura mater, the conjuctiva and cornea of the eye, the face, nasal cavities, paranasal sinuses, palate, temporomandibular joint, lower jaw, oral cavity, and teeth. It also provides SVE (branchiomotor) innervation to the muscles of mastication (temporalis, masseter, medial pterygoid, lateral pterygoid), and the mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini muscles. The trigeminal nerve is the only cranial nerve whose sensory root enters and motor root exits at the ventrolateral aspect of the pons (see Fig. 15.1). The larger, sensory root consists of the central processes (axons) of the pseudounipolar ensory neurons of the trigeminal ganglion. These axons enter the pons to terminate in the trigeminal sensory nuclear complex of the brainstem. The motor root is smaller and consists of the axons of motor (branchiomotor) neurons exiting the pons (Fig. 15.7). The motor root joins the sensory portion of the mandibular division of the trigeminal nerve just outside the skull, to form the mandibular trunk. Before the motor root joins it, the trigeminal nerve displays a swelling, the trigeminal ganglion, which lies in a bony depression of the petrous temporal bone on the floor of the middle cranial fossa. Since this is a sensory ganglion there are no synapses occurring here. As the peripheral processes of the pseudounipolar neurons exit the ganglion, they form three divisions (hence “trigeminal,” meaning the “three twins”). These divisions traverse the foramina of the skull to exit the cranial vault on their way to reach the structures they innervate. The ophthalmic division is purely sensory and innervates the upper part of the face;

 

the maxillary division is also purely sensory (although there may be some exceptions) and innervates the middle part of the face.

 

 

 

 

The mandibular division is mixed, that is it carries sensory innervation to the lower face and branchiomotor innervation to the muscles listed above.

 

 

Trigeminal nuclei

The trigeminal system includes four nuclei: one motor nucleus, the motor nucleus of the trigeminal; and three sensory nuclei, the main (chief, principal) sensory nucleus of the trigeminal, the mesencephalic nucleus of the trigeminal, and the spinal nucleus of the trigeminal (see Fig. 15.2; Table 15.5)

 

Motor nucleus

The motor nucleus of the trigeminal nerve contains the cell bodies whose axons form the motor root of the trigeminal nerve, which provides motor innervation to the muscles of mastication

The motor nucleus of the trigeminal is located at the midpontine levels, medial to the main sensory nucleus. It contains interneurons and the cell bodies of multipolar alpha and gamma motor (branchiomotor) neurons whose axons form the motor root of the trigeminal nerve as they exit the pons. The branchiomotor fibers join the mandibular division of the trigeminal nerve and are distributed to the muscles of mastication as well as to the mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini muscles.

Sensory nuclei

The sensory nuclei of the trigeminal nerve transmit sensory information from the orofacial structures to the thalamus The sensory nuclei consist of a long cylinder of cells, which extends from the mesencephalon to the first few cervical spinal cord levels. Two of these nuclei—the main sensory nucleus and the spinal nucleus of the trigeminal—receive the first order afferent terminals of pseudounipolar neurons whose cell bodies are housed in the trigeminal ganglion. These nuclei serve as the first sensory relay station of the trigeminal system.

The main (chief, principal) sensory nucleus of the trigeminal nerve is located in the midpons. Based on its anatomicaland functional characteristics, it is homologous to the nucleus gracilis and nucleus cuneatus. It is associated with the transmission of mechanoreceptor information for discriminatory (fine) tactile and pressure sense. The mesencephalic nucleus of the trigeminal is unique, since it is a true “sensory ganglion” (and not a nucleus), containing cells that are both structurally and functionally ganglion cells. During development, neural crest cells are believed to become embedded within the CNS, instead of becoming part of the peripheral nervous system, as othersensory ganglia. This nucleus houses the cell bodies of sensory (first order) pseudounipolar neurons, thus there are no synapses in the mesencephalic nucleus. The peripheral large-diameter myelinated processes of these neurons convey GP input from the muscles innervated by the trigeminal nerve and the extraocular muscles, as well as from the periodontal ligament of the teeth.

The spinal nucleus of the trigeminal is the largest nucleus of the three nuclei. It extends from the midpontine region to level C3 of the spinal cord, and is continuous inferiorly with the dorsal-most laminae (substantia gelatinosa) of the dorsal horn of the spinal cord. This nucleus consists of three subnuclei: the rostral-most subnucleus oralis (pars oralis), the caudal-most subnucleus caudalis (pars caudalis), and the intermediate subnucleus interpolaris (pars interpolaris). The subnucleus oralis merges with the main sensory nucleus superiorly and extends to the pontomedullary junction inferiorly. It is associated with the transmission of discriminative (fine) tactile sense from the orofacial region. The subnucleus interpolaris is also associated with the transmission of tactile sense, as well as dental pain, whereas the subnucleus caudalis is associated with the transmission of nociception and thermal sensations from the head. The subnucleus caudalis extends from the level of the obex (medulla) to the C3 level of the spinal cord. It is the homologue of the substantia gelatinosa since their neurons have similar cellular morphology, synaptic connections, and functions. Since the subnucleus caudalis lies immediately superior to the substantia gelatinosa of the cervical spinal cord levels, it is also referred to as the “ medullary dorsal horn. ”

The trigeminal nerve does not have any parasympathetic nuclei in the CNS, or parasympathetic ganglia in the peripheral nervous system. However, it is anatomically associated with the parasympathetic ganglia of other cranial nerves (oculomotor, facial, and glossopharyngeal) and carries their autonomic “hitchhikers” to their destination.

Trigeminal tracts

The spinal tract of the trigeminal nerve consists ofipsilateral first order afferentfibers of sensory trigeminalganglion neurons and mediates tactile, thermal, and nociceptive sensibility from theorofacial region to the spinal nucleus of the trigeminal. Thespinal tract of the trigeminal also carries first order sensory axons of the facial, glossopharyngeal, and vagus nerves. These nerves terminate in the spinal trigeminal nucleus, conveying GVA or GSA sensory input from their respective areas of innervation to be processed by the trigeminal system. The spinal tract descends lateral to the spinal nucleus of the trigeminal, its fibers synapsing with neurons at various levels along the extent of this nucleus. Inferiorly this tract overlaps the dorsolateral fasciculus of Lissauer at upper cervical spinal cord levels.

The ventral trigeminal lemniscus (ventral trigeminothalamic tract) consists of mainly crossed nerve fibers from themain sensory and spinal nuclei of the trigeminal. This tract relays mechanoreceptor input for discriminatory tactile and pressure sense (from the main nucleus) as well as sharp, well localized pain and temperature and nondiscriminatory (crude) touch sensation (from the spinal nucleus) to the contralateral ventral posterior medial (VPM) nucleus of the thalamus.

The dorsal trigeminal lemniscus (dorsal trigeminothalamic tract) carries uncrossed nerve fibers from the mainsensory nucleus of the trigeminal, relaying discriminatorytactile and pressure sense information to the ipsilateral VPMucleus of the thalamus. Th e thalamus also receives indirect trigeminal nociceptive d ull, aching pain) input via the reticular formation (reticulothalamic pr ojections).

Trigeminal pathways

Touch and pressure sense

Nearly half of the sensory fibers in the trigeminal nerve are Aelinated discriminatory touch fibers. As the central processes of pseudounipolar (first order) neurons enter the pons, they bifurcate into short ascending fibers, which synapses in the main sensory nucleus, and long descending fibers, which terminate and synapse mainly in the subnucleus oralis and less frequently in the subnucleus interpolaris of the spinal nucleus of the trigeminal. These fibers descend in the spinal trigeminal tract to reach their target subnuclei. Some second order fibers from the main sensory nucleus cross the midline and join the ventral trigeminal lemniscus to ascend and terminate in the contralateral VPM nucleus of the thalamus.

Other second order fibers from the main sensory nucleus do not cross. They form the dorsal trigeminal lemniscus, and then ascend and terminate in the ipsilateral VPM nucleus of the thalamus. Descending fibers terminating in the subnucleus oralis or interpolaris synapse with second order neurons whose fibers cross the midline and ascend in the ventral trigeminal lemniscus to the contralateral VPM nucleus of the thalamus. The VPM nucleus of the thalamus houses third order neurons that give rise to fibers relaying touch and pressure information to the postcentral gyrus of the cerebral cortex.

Pain and thermal sense

The subnucleus caudalis is involved in the transmission of pain and thermal sensation from orofacial structures The remaining half of the sensory fibers in the trigeminal nerve are similar to the Aand C nociceptive and temperature fibers of the spinal nerves. As the central processes of pseudounipolar neurons enter the pons, they descend in the spinal tract of the trigeminal and most of them synapse in the subnucleus caudalis of the spinal nucleus of the trigeminal. Nociceptive sensory input relayed in the subnucleus caudalis is modified, filtered, and integrated prior to its transmission to higher brain centers.

Interneurons located in the subnucleus caudalis project superiorly to the subnucleus oralis and interpolaris of the spinal nucleus and to the main sensory nucleus of the trigeminal, where they modulate the synaptic activity and relay of sensory input from all of these nuclei to higher brain centers. Furthermore, interneurons residing in the subnucleus oralis and interpolaris project to the subnucleus caudalis where they may in turn modulate the neural activity there. Most of the second order fibers from the subnucleus caudalis cross the midline and join the contralateral ventral trigeminal lemniscus, whereas others join the ipsilateral ventral trigeminal lemniscus. All the fibers ascend to the VPM nucleus of the thalamus where they synapse with third order neurons in that nucleus. The fibers of third order neurons ascend in the posterior limb of the internal capsule to relay

somatosensory information from the trigeminal system to the postcentral gyrus of the somatosensory cortex for further processing. Electrophysiological observations have indicated that electrical stimulation of the midbrain periaqueductal gray matter, the medullary raphe nuclei, or the reticular nuclei, has an inhibitory effect on the nociceptive neurons of the subnucleus caudalis. Substance P, a peptide in the axon terminals of smalldiameter first order neurons, has been associated with the transmission of nociceptive impulses. A large number of substance P axon terminals have been located in the subnucleus caudalis. Opiate receptors have also been found in the subnucleus caudalis, which can be blocked by opiate antagonists. These findings indicate that there may be an endogenous opiate analgesic system that could modulate the transmission of nociceptive input from the subnucleus caudalis to higher brain centers.

Motor pathway

The motor root fibers of the trigeminal nerve innervate the muscles of mastication Branchiomotor neurons housed in the motor nucleus of the trigeminal give rise to fibers which, upon exiting the pons, form the motor root of the trigeminal nerve (see Fig. 15.7).

This short root joins the sensory fibers of the mandibular division of the trigeminal nerve outside the skull. Motor fibers are distributed peripherally via the motor branches of the mandibular division, providing motor innervation to the muscles of mastication (temporalis, masseter, medial pterygoid, lateral pterygoid) and the mylohyoid, anterior belly of the digastric, tensor tympani, and tensor veli palatini muscles.