Chapter 7B - Trigeminal sensory system
The trigeminal nerve is the principal sensory nerve of the head. It supplies most of the face (to near the vertex of the skull), but does not extend to the angle of the jaw and only slightly onto the auricle. The trigeminal nerve also is the somatic afferent innervation of the nose, mouth and eyes. It provides sensory innervation of the dura and most of the cranial blood vessels, where its peripheral terminals can release a variety of vasoactive compounds that may be involved in the generation of headaches.
Most sensory fibers in the trigeminal nerve have their cell bodies in the trigeminal (semilunar, Gasserian) ganglion. This is located in the lateral wall of the cavernous sinus, immediately lateral to the pituitary gland. There is one very unusual exception, sensory cell bodies of propriceptive nerve fibers are located inside the CNS, in the mesencephalic nucleus of the trigeminal.
The trigeminal nerve enters the brain stem through the middle cerebellar peduncle of the pons (figure 15). There is a large sensory and a small motor root for this nerve.
Once inside the pons, sensory fibers segregate into three bands. The largest, most heavily myelinated fibers (proprioceptive fibers), extend into the mesencephalon to reach their sensory ganglion cells, located in the mesencephalic nucleus of the trigeminal. Sensory axons conveying touch, pressure and vibration terminate in the chief or principal sensory nucleus of the trigeminal, located in the pons immediately lateral to the entry site of the nerve.
The small diameter sensory nerve fibers conveying temperature and pain sensation run caudally along the lateral side of the brain stem through the pons and medulla (figure 16). They comprise a tract of fibers called the spinal tract of the trigeminal, and they terminate in a long nucleus called the spinal nucleus of the trigeminal. This nucleus extends into the upper cervical spinal cord. The spinal nucleus of the trigeminal has a similar cytoarchitecture and neurochemistry to that found in the substantia gelatinosa of the spinal cord, which is also a site of processing of pain and temperature information.
From the spinal nucleus of the trigeminal, second order sensory axons decussate in the brain stem to lie in close proximity to the spinothalamic tract. These nerve fibers have been called the trigeminal lemniscus or the ventral trigeminothalamic tract. This tract terminates in the contralateral ventral posteromedial nucleus (VPM) of the thalamus.
Second order sensory fibers from the chief sensory nucleus of the trigeminal (conveying touch and pressure sensation) also decussate to follow the ventral trigeminothalmic tract (figure 16). However some fibers from this nucleus also follow the dorsal trigeminal thalamic tract to the ipsilateral VPM of the thalamus. Therefore, touch and pressure touch, pressure and vibration sensory fibers terminate bilaterally in the VPM. For this reason, it is very unusual to lose all sensations on one side of the face from lesions of the brain.
The third order neuron from the VPM of the thalamus projects to the face area of the somatosensory cortex on the inferior portion of the postcentral gyrus where initial cortical processing takes place.
Many cranial nerves have some GSA nerve fibers and these nerve fibers will terminate in the spinal nucleus of the trigeminal regardless of the nerve that they follow in the head. For example there are GSA nerve fibers in the facial, glossopharyngeal and vagus nerves (from the ear, pharynx and tongue and larynx, respectively). Additionally, the spinal trigeminal nucleus extends well into the neck. Upper cervical nerve roots terminate on trigeminal sensory cells, providing an important substrate for referred pain in the head and neck. The spinal trigeminal nucleus is also important on the sensory side of many cranial reflex pathways, such as the corneal reflex, sneezing, coughing and gag responses.
Some of these reflexes are easily tested, such as the corneal reflex (consensual reflex eye closure to corneal stimulation). Some are not as readily tested. For example, tearing is a reflex that involves activation of general visceral efferent fibers in the facial nerve following corneal stimulation. Also, some reflexes have very complex responses, such as coughing and sneezing in response to irritation of the larynx and nose, respectively. The oculo-cardiac reflex involves a slowing of the heart following pressure on the eyeball, with a reflex loop involving the trigeminal nerve (input) and the vagus nerve (output).
The trigeminal motor root is relatively small. It distributes only with the mandibular division of the trigeminal and goes to muscles of mastication. When there is unilateral weakness, the jaw deviates towards the weak side when the mouth is forcibly opened due to imbalance in strength in the lateral pterygoid muscles. The jaw jerk (pontine) reflex is a monosynaptic muscle stretch reflex which is mediated by the muscle stretch (afferent) fibers associated with the mesencephalic nucleus of the trigeminal, while the motor responses is mediated via activation of neurons in the motor nucleus of the trigeminal, located in the pons.