Trigeminal Mechanisms of Nociception: Peripheral and Brainstem Organization

The trigeminal nerve (Vn) is the largest and most complex of the 12 cranial nerves. Although the V and spinal systems share the general function of relaying somatic sensory information from the head and lower body, respectively, to the central nervous system there exist significant organizational differences of both a qualitative and quantitative nature that argue against the notion of the V system as simply a rostral extension of the spinal cord. The peripheral and central components of the sensory V system are organized to warn the organism against changing environmental conditions. In addition to supplying sensory fibers to all craniofacial tissues, the Vn also innervates all specialized sense organs of the head such as the retina, cochlea, taste papillas, and olfactory epithelium. The diversity arrangement of Vn endings in craniofacial tissues range from penetrating to the outermost layers of the corneal epithelium capable of responding to the slightest changes in temperature, environmental chemicals, or foreign body intrusion to endings in the dental pulp that are housed within a tough enamel coat and become sensitive to external stimuli only after injury. The central organization of Vn afferents is quite unlike that at spinal levels in that a somatotopic map of the head and oral cavity is preserved at multiple rostrocaudal levels of the trigeminal sensory brainstem nuclear complex (TSNC). The most caudal portion of the TSNC, trigeminal subnucleus caudalis (Vc), receives extensive convergent input from cranial nerves V, VII, IX, and X as well as from upper cervical rootlets, and is the preferential terminal zone for the majority of small unmyelinated sensory fibers that convey mainly nociceptive information. The Vn system is tightly coupled to the autonomic nervous system, especially the parasympathetic outflow. Unlike spinal nerves, some Vn afferents project without interruption to autonomic control regions of the lower brainstem such as the nucleus tractus solitarius (NTS), while in the periphery blood flow to the face is under a robust vasodilatory control system. Many chronic pain conditions involving the Vn system such as migraine headache and temporomandibular joint disorders are distinguished by a higher prevalence in women than men and a strong linkage to autonomic and endocrine controls. Trigeminal mechanisms of nociception: peripheral and brainstem organization.