Cranial nerves are a set of 12 paired peripheral nerves that arise directly from the brain and innervate the structures of the head and neck. Each cranial nerve is named according to its structure or function and sequentially numbered with Roman numerals (CNs I–XII). The numerical classification reflects the rostral to caudal position of the cranial nerves as each nerve enters or exits the cerebrum and brainstem ().
Cranial nerves carry motor, sensory, and autonomic fibers, along with several unique nerve fibers, which reflect the special functional attributes of the head. The types of fibers carried by the nerves serve as the basis for classifying the neurons of the cranial nerves into functional components/modalities ().
In comparison to spinal nerves that pass through the intervertebral foramina in an orderly arrangement, the cranial nerves arise from various levels of the brain and travel through numerous fissures and foramina in the cranium (skull) (). Among the 12 cranial nerves, the CN I (olfactory) and CN II (optic) nerves develop as outgrowths of the forebrain directly and are considered extensions of the central nervous system (CNS). The remaining cranial nerves, CNs III to XII, develop in association with the brainstem and enter and exit from distinct regions of the midbrain, pons, and medulla.
The following sections discuss the functional components of the cranial nerves, the location of the motor and sensory nuclei, and the general distribution path of each cranial nerve. The specific ascending pathways of the special senses are covered in Chapter 15, while more detailed descriptions for the trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), vagus (CN X), and hypoglossal (CN XII) nerves are provided in Chapters 20 and 21.
Fig. 9.1 Cranial nerves. Inferior (basal) view. The 12 pairs of cranial nerves (CNs) are numbered according to the order of their emergence from the brainstem. Note: The sensory and motor fibers of the cranial nerves enter and exit the brainstem at the same sites. In comparison, the sensory and motor fibers of spinal nerves enter and leave through posterior and anterior roots, respectively. (Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Cranial nerves transmit motor, sensory, or autonomic input and function in a manner analogous to spinal nerves. However, cranial nerves exhibit a more complex innervation pattern due to the functional and structural diversity of the head and neck region.
The seven fiber types include four types of motor (efferent) fibers and three sensory (afferent) fiber types. Motor and sensory fibers are classified as general or special and further designated as somatic (body-related) or visceral components.
The second letter is S = somatic or V = visceral. Somatic refers to the types of neurons that innervate somite-derived structures, such as skeletal muscle, connective tissue, and skin of the body. Visceral refers to neurons that supply the internal organs, mucosa, glands, blood vessels, and structures derived from the pharyngeal arches.
The third letter is either A = afferent or E = efferent. Afferent refers to sensory neurons and associated fibers that convey somatic and visceral sensations. Efferent refers to motor neurons that supply skeletal and smooth muscle, along with providing glandular secretion.
General functional modalities found in the cranial nerves are analogous to those carried by the spinal nerves and include general somatic efferent (GSE), general somatic afferent (GSA), general visceral efferent (GVE), and general visceral afferent (GVA).
Special functional components that are unique to the head and found only in the cranial nerves include special visceral afferent (SVA), special somatic afferent (SSA), and special visceral efferent (SVE).
CNs III, IV, VI, and XII carry GSE fibers and provide voluntary motor function to the extrinsic skeletal musculature of the eye and tongue. GSE fibers found in the head are homologous to fibers carried by spinal nerves.
CNs I, II, and VIII carry only special sensory fibers. Special sensory modalities include vision, auditory, balance (vestibular), and taste. These special sensory sensations are unique attributes of cranial nerves and include SSA and SVA, based on the type of receptor stimulation. SSA receptors respond to light and mechanical stimulation, while SVA fibers utilize chemoreceptors and respond to chemical stimuli.
The optic (CN II) and vestibulocochlear (CN VIII) carry SSA fibers. For the eye, specialized neuroepithelial cells, known as rods and cones, reside within the retinal epithelium and serve as sensory receptors or photoreceptors. The sensory receptors for the vestibulocochlear apparatus are neuroepithelial cells known as hair cells, which are specific to the organ of Corti, the semicircular canals, the utricle, and the saccule of the inner ear. The sensory neurons of the vestibulocochlear apparatus convey information about hearing and balance.
The olfactory nerve (CN I) carries SVA fibers associated with the sense of smell. Olfactory receptors are bipolar neurons found within the olfactory epithelium lining the roof of the nasal cavity and act as chemoreceptors to mediate the sense of smell (olfaction).
It is important to note that SVA fibers are also associated with chemoreceptors that detect the special sensation of taste. Specialized neuroepithelial cells found within taste buds synapse with the SVA fibers of CNs VII, IX, and X to mediate the sensation of taste (gustation).
T he motor fibers carried by CNs V, VII, IX, and X are classified as SVE fibers because the SVE neurons provide motor innervation to a unique group of skeletal muscles that develop from pharyngeal arch (branchiomeric) mesoderm. These voluntary skeletal muscles function in visceral tasks such as eating, swallowing, phonation, and speaking, which are distinct functions to the head. Pharyngeal arch derived muscles are morphologically identical to skeletal muscles in the body; however, the neurons of these efferent fibers have a distinct location in the brainstem. The accessory nerve, CN XI, also carries motor fibers to skeletal muscle; however, there is controversy in the literature concerning the classification of the fibers as GSE or SVE. The basis for the controversy of the accessory nerve stems from the location of the neuron cell bodies in the spinal cord and the origin of the neck skeletal muscles. The motor fibers of the accessory nerve innervate the trapezius and sternocleidomastoid muscles.
The GSA fibers transmit pain, temperature, crude and discriminative touch, as well as stereognostic, proprioceptive (positional), and kinesthetic (movement) inputs from the skin, temporomandibular joint (TMJ), muscles, and mucous membranes of the orofacial and pharyngeal regions.
CN V transmits the majority of GSA input from the head, face, and oral cavity, with minor contributions provided from the region of the ear by CNs VII, IX, and X. CNs IX and X also convey GSA information from the mucosa of the ear, auditory tube, oropharynx, and laryngeal region.
CNs III, VII, IX, and X carry autonomic (GVE) fibers from the cranial portion of the parasympathetic nervous system. The parasympathetic fibers are analogous to those carried by the sacral spinal nerves.
During development, motor and sensory neurons of CNs III to XII become organized within the alar and basal plates of the brainstem into seven functionally similar groups of neuronal cell bodies known as nuclei.
In a manner analogous to spinal nerves, the transmission of motor and sensory input from cranial nerves occurs through distinct multisynaptic nerve pathways in which the signal passes between a chain of several sequentially arranged neurons.
In general, the first neuron in the relay chain is the first to propagate information and synapses on the second neuron in the chain. The location of the first neuron varies between the sensory and motor pathways. Sensory, first-order neurons are found in the peripheral nervous system (PNS), while motor neurons are in the CNS. The second-order neuron usually resides in cranial nerve nuclei.
Sensory pathways, also known as ascending pathways, are groups of axons that form ascending tracts and function to transmit information from peripheral receptors to cranial sensory nuclei and then to higher regions of the brain for conscious and unconscious processing.
Motor pathways, also known as descending pathways, consist of groups of axons (tracts) that descend from neurons in the cortex, hypothalamus, and brainstem and function to modulate efferent neurons situated within cranial nerve motor nuclei of the brainstem or within the spinal cord.
For afferent sensory paths, the primary afferent (first-order) neuron typically resides in the PNS within cranial sensory ganglia. Cranial sensory ganglia are homologous structures to the dorsal root (spinal) ganglia.
The peripheral axons of first-order neurons receive input from various types of sensory receptors, while the central axonal processes transmit the information to the integrative second-order neurons found within cranial sensory nuclei of the gray matter of the brainstem. A synapse does not occur in the sensory ganglion.
Axons of second-order sensory neurons, which may decussate (cross the midline), form ascending tracts that synapse on third-order integrative (relay) neurons in the thalamus. Relay fibers from the thalamus project to the primary somatosensory cortex for conscious processing.
Alternatively, axons of ascending tract may terminate in the cerebellum or on integrative neurons in visceral autonomic reflex centers of the hypothalamus and the reticular formation of the brainstem to modulate autonomic functions.
The efferent motor pathways involved in mediating voluntary motor activity consist of two principal neurons: the upper motor neurons (UMNs), also known as supranuclear neurons, and the lower motor neurons (LMNs), which comprise cranial motor nuclei (a).
For voluntary motor activity, UMNs reside in the primary motor cortex, the premotor motor cortex (PMC), and supplementary motor areas (SMAs). Supranuclear (UMN) neurons represent first-order motor neurons and function in the execution and planning of voluntary movements as well as maintaining posture and balance.
For involuntary visceromotor activity associated with the autonomic nervous system, supranuclear (preautonomic) neurons reside within hypothalamic nuclei and integrative nuclei of the brainstem and reticular formation.
Axonal tracts descend from preautonomic neurons to synapse on preganglionic parasympathetic neurons in cranial visceral motor (GVE) nuclei or synapse on preganglionic sympathetic neurons of the intermediolateral column (IMLC) of the spinal cord at T1–L2 vertebral level.
(b) GVE neurons of visceral efferent motor pathway mediate autonomic function. Postganglionic autonomic fibers originate in autonomic ganglia and terminate on their target structures. In the head and neck, theses fibers travel with cranial nerves or blood vessels. GSE, general somatic efferent; GVE, general visceral efferent; SVE, special visceral efferent. (Fig. 9.5a: Reproduced 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.)
Among the four sensory modalities carried by the cranial nerves, there are three corresponding columns of sensory nuclei found in the brainstem. Each column of sensory nuclei contains second-order interneurons and receives input from central afferent fibers of the first-order neurons which lie within cranial sensory ganglia in the PNS ().
The eight cranial sensory ganglia include the trigeminal (semilunar) ganglion (CN V), geniculate ganglion (CN VII), vestibular and spiral ganglion (CN VIII), superior and inferior (petrous) ganglion of CN IX, and the superior (jugular) and inferior (nodose) ganglion of CN X.
The first-order (primary afferent) neurons found within the ganglia are primarily pseudounipolar neurons. A notable exception to this pattern is that of the vestibular and cochlear (spiral) ganglia, which contain bipolar sensory neurons.
CNs I (olfactory) and II (optic) deviate from this pattern and do not have sensory ganglia. The primary afferent neurons are specialized neuroepithelial cells found in the olfactory mucosa and retinal epithelium, respectively.
GSA nuclei receive primary afferent fibers carrying pain, temperature, touch, and proprioceptive input from receptors in the skin, muscle, and joints. The trigeminal (CN V) nerve transmits most of the GSA information from the head, along with smaller contributions from facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves.
GVA and SVA fibers terminate on the same nuclei, the nucleus solitarius. The nucleus, also known as the nucleus tractus solitarius (NTS) or the solitary nucleus, consists of a rostral and causal division. The rostral part of the nucleus solitarius receives taste fibers carried by CNs VII, IX, and X, while the caudal portion of the nucleus receives visceral afferent input from blood vessels, internal organs, and the mucous membranes lining the oropharynx, and larynx via CNs IX and X.
The nucleus solitarius, which lies in the medulla, serves as the principal relay station for first-order visceral and gustatory neurons. The nucleus solitarius functions in the integration of visceral and autonomic responses.
The cell bodies of first-order neurons for the olfactory (CN I) and optic (CN II) nerves reside in the olfactory mucosa of the nose and retinal sensory epithelium of the eye, respectively. The primary afferent fibers then synapse on second-order neurons located within the olfactory bulb and retinal epithelium.
Somatic motor nuclei are the most medially positioned column in the brainstem and contain alpha and gamma multipolar LMNs. GSE fibers project from LMNs to skeletal muscles derived from somites. CNs III, IV, IX, and XII each have a separate motor nucleus that is named for the cranial nerve. GSE nuclei include the oculomotor (CN III), trochlear (CN IV), abducens (CN VI), and hypoglossal (CN XII) motor nuclei.
Special visceral efferent nuclei associated with CNs V, VII, IX, and X contain LMNs that innervate skeletal muscles derived from pharyngeal arch mesoderm. These muscles mediate chewing, swallowing, phonation, and articulation. The pharyngeal arch muscles include the muscles of mastication, facial expression, as well as the palatal, pharyngeal, and laryngeal musculature.
It should be noted that the accessory nerve (CN XI) originates in the accessory nucleus that lies outside the brainstem and is situated in the ventral horn of the upper cervical levels (C1–C6) of the spinal cord. The nucleus contains LMNs which innervate the trapezius and sternocleidomastoid muscles. There is controversy in the literature on whether the nucleus is GSE or SVE in origin.
Cortical UMNs reside in the primary motor, the premotor, and supplementary motor cortices of the frontal lobe, and function to control the planning and execution of movement through the modulation of LMNs found within cranial GSE and SVE motor nuclei.
Many of the cranial motor nuclei involved in oropharyngeal functions also receive direct input from the reticular formation. These neurons act as central pattern generators (CPGs) and mediate rhythmic orofacial movements.
The nuclei which comprise the GVE column lie between the GSE and SVE columns in the brainstem and correspond to multipolar preganglionic neurons from the cranial portion of the parasympathetic nervous system.
Preganglionic parasympathetic neurons of GVE motor nuclei receive central input, primarily from integrative (preautonomic) neurons found in the hypothalamic nuclei and visceral reflex control centers of the brainstem.
The parasympathetic postganglionic cell bodies associated with the vagus (CN X) lie outside the head, near or in the wall (intramural ganglion) of their target organ as indistinct, unnamed groups of neurons.
Axons of cranial postganglionic parasympathetic neurons synapse on target organs such as salivary, nasal, and lacrimal glands, and the smooth muscle associated with the eye. The postganglionic fibers that terminate on structures within the head accompany branches of the trigeminal division (V1–V3) to reach their target organs. Postganglionic parasympathetic nerve fibers from the vagus target muscles of the heart, respiratory, and gastrointestinal tract.