Sensory Portion

RECEPTORS

Cutaneous receptors for the trigeminal nerve are primarily mechano-, thermo-, and nociceptive endings. These include pacinian corpuscles, free nerve endings, Merkel's discs, and Ruffini's endings. Although these are often associated with facial hairs, areas of particularly dense innervation include the lips, buccal and gingival surfaces, and the cornea. Some sensory nociceptive afferents are not myelinated, whereas others, and tactile and mechanical afferents, are heavily so. In addition, muscle spindles and Golgi tendon organs exist within facial muscles and extend via type la and Ib fibers to the pons.

FIRST ORDER NEURONS

Primary sensory neuron cell bodies for the afferent trigeminal fibers (V1-V3) are within the trigeminal (gasserian or semilunar) ganglion, which is embedded in the petrous portion of the temporal bone in the middle cranial fossa. y , y In this area is Meckel's cave, a dural-lined cavity in the middle cranial fossa adjacent to the temporal lobe, which surrounds the gasserian ganglion and its branches as they exit into the petrous temporal bone. This region may be viewed by magnetic resonance imaging and is important clinically. A single large trigeminal sensorimotor root passes into the gasserian ganglion and then emits the three trigeminal divisions that exit the skull base via distinct foramina: V1 exits via the superior orbital foramen, V2 via the foramen rotundum, and V3 via the foramen ovale.

The ophthalmic division of the trigeminal nerve is entirely sensory (all modalities) and its branches innervate the orbit and eye (lacrimal and nasociliary branches), upper eyelid, forehead, and nose (frontal branches), nasal cavity, and nasal sinuses (nasociliary branches). V1 passes within the cavernous sinus where it lies inferolateral to the oculomotor, trochlear, and abducens nerves. From the cavernous sinus, V1 extends through the superior orbital fissure again in association with cranial nerves Ill, IV, and VI, before dividing into the lacrimal, frontal, and nasociliary nerves. Cutaneous fibers reach the skin via the supraorbital foramen along the ridge of the brow. V1 sensory afferents from the cornea provide afferents for the corneal reflex (see later). Smaller tentorial and dural branches innervate the tentorium cerebelli and dura mater.[4 Vasomotor fibers also branch from the trigeminal system (trigeminovascular innervation) to provide autonomic inputs to intracranial blood vessels. The majority of these fibers are believed to use calcitonin gene-related peptide, substance P, and vasoactive intestinal peptide as transmitters.^ Stimulation of the trigeminal ganglion in experimental animals and humans increases brain blood flow. M , y Autonomic fibers to the lacrimal gland extend from the facial nerve and pterygopalatine ganglion to run with the lacrimal branch of V1, but they originate from the superior salivatory nucleus.

The maxillary division (V2) is also completely sensory; it innervates the skin of the cheek, nose, lower eyelid, upper lip, nasopharynx, soft and hard palate, maxillary sinus, and upper teeth. A small meningeal branch follows the middle meningeal artery and supplies the dura. [4 V2 fibers leave the gasserian ganglion, exit the foramen rotundum, and run inferiorly within the cavernous sinus. V2 axons pass through the pterygopalatine fossa, to exit the infraorbital foramen. These cutaneous branches include the zygomaticotemporal, zygomaticofacial, and infraorbital nerves, whereas branches innervating the nasopharynx and maxillary sinuses include the greater and lesser palatine nerves, nasopalatine nerve, and pharyngeal nerve. V2 innervation of the upper teeth, maxillary sinuses, and palate is via the anterior, middle, and posterior superior alveolar nerves, respectively. Autonomic fibers that originate from the facial nerve (superior salivatory) nuclei accompany V2 branches and comprise the superficial petrosal nerve. These fibers synapse within the pterygopalatine ganglion and provide parasympathetic input to the lacrimal, nasal, and palatine glands. Fibers of the superficial petrosal nerve unite with the deep petrosal nerve to form the nerve of the pterygoid canal. The deep petrosal nerve is a branch of the internal carotid plexus and carries postganglionic sympathetic fibers from the superior cervical ganglion to the lacrimal glands.

The mandibular division (V3) carries both sensory and motor fibers (see earlier) to the lower face. V3 axons leave the gasserian ganglion to exit the skull base via the foramen ovale. The fibers ramify through the deep face lateral to the medial pterygoid muscles and then divide into branches that provide sensation to the skin around the mandible, chin, and ear (lingual, auriculotemporal, and mental branches); mucosa around the inner cheek (buccal branch), lower teeth (inferior alveolar nerves), and dura (meningeal branch). Parasympathetic fibers synapse within the otic and submandibular ganglia and project to the submandibular and parotid glands, respectively.

Proximal connections of the primary sensory neurons synapse in the brain stem in three sensory subnuclei that extend from the cervical spine to the pontomesencephalic junction. These include the spinal (descending) trigeminal tract and nucleus, the principal sensory nucleus, and the mesencephalic nucleus, each of which subserve a distinct trigeminal function. Sensory axons from V1- to V3-innervated regions of the face reach the trigeminal ganglion where cell bodies of these peripheral axons send central axons in a solitary sensory root into the midpons. Within the brain stem, these axon bundles bifurcate into fascicles that terminate rostrally in the principal sensory nucleus or caudally in the spinal trigeminal tract and nucleus. y

Fibers entering the spinal trigeminal tract pass caudally to the spinal nucleus in an inverted somatotopic organization (V1 ventrally, V2 medially, and V3 dorsally) and convey most of the nociceptive and cutaneous inputs mediated by the trigeminal system. The spinal trigeminal tract extends into the upper cervical spinal cord, and afferents synapse on immediately adjacent cells within the nucleus of the spinal trigeminal tract that lie medial to the tract for its entire length. The nucleus of the spinal trigeminal tract blends at its most rostral extent with the principal sensory nucleus (see later) and caudally with the substantia gelatinosa

in the cervical spinal cord (see Chapters 19 and 20 on pain).

The spinal trigeminal nucleus may be divided cytoarchitecturally into a pars oralis, which receives sensory inputs from the oral and nasal regions, the pars interpolaris and pars caudalis, which receive afferents from cutaneous portions of the face. Within the pars caudalis, four somatotopically organized laminae (I-IV) similar to the central gray area of the spinal cord parcel sensory inputs into pain and tactile stimuli. Two somatotopic homunculi for facial representation of pain are proposed within the spinal trigeminal nucleus. First, a rostrocaudal representation of facial innervation such that mandibular regions terminate more rostrally, followed by maxillary and ophthalmic regions more caudally extending into the cervical cord, has been proposed. This facial representation is distinct from the second homuncular pattern proposed, the so-called "onion skin" pattern, in which the mouth and nose (central regions) are represented rostrally in the brain stem, whereas the cheeks, eyes, and ears (more peripheral facial areas) are represented more caudally.

Impulses carrying tactile and pressure sense enter the midpons and extend rostrally to terminate in the principal trigeminal sensory nucleus. These inputs are somatotopically organized similar to the spinal trigeminal tract. Cells within the principal sensory nucleus have large receptive fields; they respond to various tactile and pressure stimuli applied to the skin, mucous membranes, palate, orbit, and teeth.

The third brain stem nucleus, the mesencephalic nucleus, is located dorsolaterally above the middle cerebellar peduncle near the pontomesencephalic junction and adjacent to the fourth ventricle. Afferent fibers to the mesencephalic nucleus travel within the motor root of the trigeminal nerve and convey primarily kinesthetic sensation from the teeth, oropharynx, and jaws. These afferents synapse on unipolar-shaped neurons. Stretch receptors in masticatory muscles send information regarding bite force to the mesencephalic nucleus and function as the afferent portion of the jaw jerk reflex (see later).

SECOND AND THIRD ORDER SENSORY NEURONS

The second order sensory neurons located within the spinal trigeminal, main sensory, and mesencephalic nuclei send projections rostrally via the trigeminothalamic tract.[10i , [11] Neurons within the pars oralis and caudalis of the spinal trigeminal nucleus and cell bodies from the mesencephalic and principal nuclei project medially into the pontine reticular formation, cross within the median raphe, and ascend contralaterally within the trigeminothalamic tract closely adjacent to the medial lemniscus.

Axons from the second order neurons terminate somatotopically within the venteroposteromedial nucleus (VPM) of the thalamus contralateral to their nucleus. [1 , [10] , [11] A smaller proportion of fibers terminate within the intralaminar nuclei. In contrast, a small fascicle of ipsilateral projections extend rostrally via the dorsal trigeminal tract to terminate in the VPM. Trigeminocerebellar fibers project bilaterally from the mesencephalic and motor trigeminal nuclei.

Third order thalamocortical projections from the VPM travel in the anterior limb of the internal capsule and terminate primarily in sensory cortical areas 3, 1, and 2. [10] Less dense projections reach the parietal lobule and the precentral gyrus. The presence of some sensory afferents in the cortical motor region explains why sensory abnormalities may be reported by patients with lesions affecting only the precentral gyrus.

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