The transmission of somatosensory sensations from the head, face, and oral cavity to the central nervous system (CNS) for conscious and unconscious processing follows comparable pathways to that of the body.
The anterolateral, dorsal column-medial lemniscus, and spinocerebellar pathways discussed in Chapter 12 mediate the somatosensory transmission from the body to the CNS. These paths are analogous to the trigeminal nerve pathways which convey pain, temperature, touch, and proprioception from the anterior head, face, and oral cavity.
The trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves convey general somatic afferent (GSA) sensations from cutaneous structures and mucous membranes of the head. Somatosensory input from the cranial region follows two principal pathways to the cortex for conscious processing: a ventral and dorsal trigeminothalamic tract (DTT) (trigeminal lemniscus), which comprise the trigeminal sensory system. In addition to somatosensory input, general visceral afferent (GVA) sensations from blood vessels, glands, and mucous membranes are carried by the facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves and mediate unconscious, reflexive activity. The present chapter summarizes the components of the trigeminal sensory system and reviews the general somatosensory and viscerosensory pathways followed by CNs V, VII, IX, and X.
Analogous to the organizational pattern of the somatosensory path of the body, the afferent fibers of the head utilize a three-neuron relay chain to transmit sensory input from peripheral receptors to the CNS ().
Afferent fibers follow distinct pathways and project to contralateral ventral posteromedial (VPM) nuclei of the thalamus and the somatosensory cortex for conscious perception and localization of stimulus.
Fig. 13.1 Principal ascending somatosensory paths of body and head. Protopathic and epicritic sensations for body and head follow a three-neuron relay chain. Primary afferent neurons reside in dorsal root or trigeminal sensory ganglia, and synapse on second-order neurons in the spinal cord or brainstem. Third-order neurons reside in specific thalamic nuclei and project to primary somatosensory cortex (S1). The ascending somatosensory pathways of the body and head decussate at different points, but travel in close proximity through the brainstem and may be susceptible to damage. (Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
The trigeminal nerve, which is the largest of the 12 cranial nerves, consists of a small motor root and a large sensory root. The motor and sensory roots of the trigeminal nerve enter and exit the brainstem at the level of the mid-pons ().
The small motor root of the trigeminal nerve provides efferent (motor) innervation to the muscles of mastication, and four smaller muscles. The function of these muscles and the nerve path are discussed in Chapter 20. The cell bodies for the motor neurons are found in the motor nucleus of V (trigeminal motor nucleus) within the pons.
The large trigeminal sensory root contains an aggregation of GSA fibers which is formed by the convergence of the three principal divisions of the trigeminal nerve. The three principal divisions are the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions ().
The cutaneous sensory innervation of the face follows the distribution pattern for CNs V1, V2, and V3, and results in a dermatome pattern that exhibits defined borders with little overlap of cutaneous sensations (a–c)
The dermatomes of the head, like that of the spinal nerves, have clinical relevance regarding the diagnosis of certain diseases. Symptoms such as rash or pain that follow a dermatome may indicate the involvement of the related nerve root (see Clinical Correlation Box 13.1).
The primary afferent fibers which comprise the ophthalmic (CN V1), maxillary (CN V2), and mandibular (CN V3) divisions pass from the periphery and enter the skull through three specific foramina: the superior orbital fissure, foramen rotundum, and foramen ovale, respectively ().
In the middle cranial fossa, the ophthalmic (CN V1) and maxillary (CN V2) divisions pass through the cavernous venous sinus, accompanied by the trochlear nerve (CN IV), the oculomotor nerve (CN III), the abducens nerve (CN VI) and the sympathetic fibers associated with the internal carotid artery (ICA). The proximity of the structures within the sinus puts the nerves at risk for compression injuries due to infections or thrombosis occurring within the sinus. The mandibular division which passes outside the sinus is usually spared (Clinical Correlation Box 13.2)(a, b).
The three divisions, which carry the peripheral axonal processes of primary afferent (GSA) neurons, converge in the middle cranial fossa and enter a large peripheral sensory ganglion known as the trigeminal ganglion.
Fig. 13.3 Somatosensory nerve distribution pattern of trigeminal nerve (CN V). Right lateral view of trigeminal divisions. The trigeminal nerve is the major sensory nerve of the face. It has three major divisions that convey general somatic afferent (GSA) sensation of touch, pain, and proprioception from the face (a) and select mucosa (b, c). Each division exhibits a distinct dermatome distribution pattern with little cutaneous overlap and is of clinical relevance regarding diagnosis. Mandibular nerve shown in yellow; maxillary division in green; ophthalmic division in blue. Additional shading for GSA distribution from CN VII, CN IX, and CN X shown in region of ear. (a: Modified with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b,c: Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Fig. 13.4 Peripheral course of the trigeminal nerve divisions. Right lateral views. (a) Partially open right orbit showing ophthalmic division (CN V1). (b) Partially opened right maxillary sinus showing maxillary division (CN V2). (c) Partially opened mandible with middle cranial fossa windowed showing mandibular division (CN V3). (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Third Edition, Vol 3. © Thieme 2020. Illustrations by Markus Voll and Karl Wesker.)
Fig. 13.5 (a) Intracavernous course of cranial nerves that enter the orbit. Anterior and middle cranial fossa on right side, superior view. The lateral and superior wall opened. The trigeminal ganglion has been retracted laterally; the orbit roof removed. The oculomotor (CN III), trochlear (CN IV), and abducens (CN IX), which innervate the ocular muscles, enter the cavernous sinus and lie in close proximity with ophthalmic (CN V1) and maxillary (CN V2) divisions of the trigeminal nerve. (b) Cavernous sinus, coronal section through middle cranial fossa. Anterior view, coronal section, showing the relationship of the cranial nerves within the cavernous sinus to the hypophysis, optic nerve (CN II), and internal carotid artery. Sympathetic fibers travel with the internal carotid artery as they pass through the sinus. (a: Modified with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy. Second Edition. © Thieme 2012. Illustrations by Markus Voll and Karl Wesker.)
The peripheral sensory nervous system is the primary target of the herpes simplex virus (HSV) infections. During a primary infection, the virus may undergo retrograde axonal transport along nerves to the neuronal cell bodies in the sensory ganglia and establish chronic latency. The trigeminal nerve and ganglia are principal targets of HSV and reactivation causes recurrent HSV infections. Outbreaks are often associated with cold sores, facial herpetic lesions, or corneal infections. The pattern of recurrence tends to follow the dermatome of the primary infection; however, because of the involvement of the sensory ganglia, a recurring infection may become associated with another dermatome.
The varicella-zoster virus that causes chickenpox is another example of a herpes virus that may remain dormant in nerve ganglia. Reactivation of the virus leads to shingles also known as herpes zoster, which is associated with a painful rash and blisters along a defined dermatome. Although often associated with the trunk, it can also develop on the face, mouth, and around the eyes. The complication associated with shingles developing in the orofacial region is an ipsilateral loss of sensation on the affected side or the development of postherpetic neuralgia, which is hallmarked by sharp, burning pain along the affected dermatome.
Tumors, aneurysms of the ICA, venous thrombosis, or the spread of infection in the cavernous sinus, may lead to compression and subsequent trigeminal neuropathy of the sensory nerve branches of ophthalmic and maxillary divisions, as they pass through the cavernous sinus to exit the skull. The resulting neuropathy will follow the trigeminal dermatome and may manifest as cutaneous sensory loss, or in some cases, as sharp intermittent pain associated with trigeminal neuralgia. Compression of the oculomotor, trochlear, and abducens nerve may also occur due to their shared route through the cavernous sinus, leading to weakness or paralysis of extraocular eye muscles (ophthalmoplegia). Horner’s syndrome associated with damage to sympathetic fibers may also be manifested. Typical signs include decreased pupil size, a drooping eyelid (partial ptosis), and decreased sweating of the face on the affected side.
The trigeminal ganglion, also known as the semilunar ganglion or Gasserian ganglion, is housed in a small depression ( Meckel’s cave) in the petrous temporal bone, just lateral to cavernous sinus (see a).
The trigeminal ganglion represents a functional equivalent to the dorsal root ganglion and contains most of the first-order pseudounipolar neuron cell bodies that convey GSA fibers for the head, face, and intraoral region.
The ganglion contains the pseudounipolar cell bodies of the first-order (GSA) neurons associated with protopathic (pain, temperature) and epicritic (discriminative tactile, pressure, vibration) sensations.
There is one notable exception to the types of primary sensory neurons located within the trigeminal ganglia. The primary afferent neurons that transmit proprioceptive information from the masticatory jaw-closing muscles, tongue, periodontal ligament, and extraocular eye muscles lie within the mesencephalic trigeminal nucleus of the midbrain.
The presence of first-order sensory neurons in the CNS rather than the peripheral nervous system (PNS) is unique to the mesencephalic nucleus. All other first-order sensory neurons of the head and body reside within the PNS in cranial sensory ganglia (CNs V, VII, IX, and X) or the dorsal root ganglia.
The central axonal processes of GSA neurons enter the brainstem at the level of the pons as the sensory root and project to the trigeminal nuclear (sensory) complex—a cluster of three sensory nuclei that extends from the midbrain to upper cervical region of the spinal cord (, ).
Fig. 13.6 Trigeminal nerve nuclei. Anterior view of brainstem (a). Superior view; cross section through the pons (b). Primary afferent neurons of the trigeminal nerve divisions convey general somatic afferent (GSA) sensations (touch, pain, proprioception) to the central nervous system (CNS). The central axons of the primary afferent neurons enter the mid-pons as the sensory root and synapse in three brainstem nuclei, which comprise the trigeminal nuclear complex. The nuclei include the spinal trigeminal nucleus, which conveys nociceptive and thermal sensations; the main sensory nucleus, which transmits discriminative touch and conscious proprioception; and the mesencephalic, nucleus which carries unconscious proprioception from the jaw. Special visceral efferent fibers carried by the mandibular division (CN V3) arise from lower motor neurons in the trigeminal motor nucleus and exit the pons as a small motor root. (a: Modified with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
The spinal trigeminal nucleus is the largest nuclei of the trigeminal system and extends from the caudal region of the pons to the upper cervical levels (C2) of the spinal cord, where it merges with the Rexed lamina (marginal zone, substantia gelatinosa) in the dorsal horn gray matter.
The spinal trigeminal nucleus contains second-order neurons and is the only cranial nerve nucleus to process thermal and nociceptive input from the ipsilateral part of the face. It receives crude touch, thermal, and noxious stimuli from all three trigeminal divisions, as well as the facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves.
Nociceptive and temperature sensations conveyed by central axons of primary afferent neurons descend from the trigeminal ganglia to the ipsilateral spinal trigeminal nucleus via the spinal (descending) trigeminal tract. Protopathic sensation carried by primary afferent fibers of CNs VII, IX, and X enter at different levels of the brainstem and follow the spinal trigeminal tract to the spinal trigeminal nucleus.
The somatotopic distribution reflects the segmental organization of the trigeminal spinal nucleus and corresponds to a concentric semicircular pattern (Sölder lines) of sensory distribution on the face. Therefore, the axons from the central orofacial (perioral) region synapse in the rostral portion of the nucleus, whereas peripheral axons from the lateral facial region project to the caudal portion of the spinal trigeminal nucleus (a, b) (Clinical Correlation Box 13.3).
The spinal trigeminal nucleus is similar in function to Rexed laminae I, II, and IV, which process protopathic input for the anterolateral (spinothalamic) system (Clinical Correlation Box 13.4).