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You are watching: Which cranial nerve is generally thought of as a mixed nerve?

StatPearls . Endowment Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The cranial nerves carry out afferent and also efferent (sensory, motor, and autonomic) innervation to the frameworks of the head and neck. Uneven spinal nerves whose roots are neural fibers from the spinal grey matter, cranial nerves are written of the neural processes connected with unique brainstem nuclei and also cortical structures. When the spinal grey issue is organized into a posterior sensory horn, an autonomic and interneuron intermediary grey, and an anterior engine horn, this cranial nerve nuclei are functionally arranged into unique nuclei in ~ the brainstem. Typically, the much more posterior and also lateral nuclei often tend to be sensory, and the much more anterior of often tend to it is in the motor. Cranial nerves I (olfactory), II (optic), and VIII (vestibulocochlear) are considered purely afferent. Cranial nerves III (oculomotor), IV (trochlear), vi (abducens), XI (spinal accessory), and XII (hypoglossal) are completely efferent. The remaining cranial nerves, V (trigeminal), VII (facial), IX (glossopharyngeal), and also X (vagus), space functionally mixed (sensory and also motor) (Figure 1A). Damages to the cranial nerves, your tracts, or nuclei outcomes in stereotypical dysfunctions. When this is the classical way of organizing and also indexing cranial nerves, the scientific truth is more complicated and are still disputed in the scholastic realm, including the classification and also identification of the courses of distinct cranial nerves fibers and also the visibility or absence of other less known structures, such together the terminal nerve, additionally called nerve nulla or cranial nerve zero. The latter might be potentially classified as a pure afferent nerve complying with the traditional cranial nerves" organization.


Nerves

Afferent Cranial Nerves and also Clinical Implications

Cranial nerves I, II, and also VIII are thought about purely afferent nerves due to the fact that they command sensory information from the olfactory region, the retina the the eye, and also the inside ear structures, respectively. Cranial nerve I, the olfactory nerve, is created of one-of-a-kind visceral afferents (SVA). Chemo-sensory receptors in the olfactory mucosal lining tie to odorant molecules and conduct a signal v the nerves traveling through the cribriform plate of the ethmoid bone come synapse on the neurons of the olfactory pear within the cranial vault. The central processes of this olfactory pear neurons job through the olfactory trigone medially to the septal area and the contralateral pear via the anterior commissure, while other fibers travel laterally come the amygdala and piriform cortex, likewise known together the primary olfactory cortex where conscious odorant sense is processed. Traumatic injury, specifically “whiplash” from automobile collisions, can sever the olfactory projections v the cribriform plate, leading to anosmia, which has been linked with the breakthrough of depression.<1><2><3> The sense of olfaction additionally appears to have actually a non-conscious function in activating the limbic system, which may account for such an effect.<4>

Cranial nerve II, the optic nerve, conveys unique somatic afferent (SSA) intuitive sensory information from the rods and cones retinal sensory receptors to the thalamus, especially the lateral geniculate nucleus (LGN), and the premium colliculus (SC). Ganglion cells, whose cell bodies are situated deep in the retina, have central projections that form the optic nerve fibers, i beg your pardon traverse the optic canal to get in the cranium. From there, fibers representing the medial visual fields travel posteriorly there is no crossing in ~ the optic chiasm, while yarn of the lateral visual areas do cross within the chiasm. Therefore, areas of the visual ar are retinotopically organized within the optic nerve and also at their synapse in the LGN. Collaterals are likewise given off centrally come innervate the SC, responsible for the pupillary light reflex; and also connections within the pulvinar of the thalamus,<5> providing unconscious optic intake responsible for the phenomenon that blindsight.<6> In cortically remote individuals, these pulvinar collaterals from the optic nerve allow for unconscious eye movements in an answer to the detection of light and a weak directional feeling of the place of the light within the visual field.<7><8><9>

Cranial nerve VIII, the vestibulocochlear nerve, is responsible for the hear sense and the vestibular feeling of orientation the the head. This nerve conveys one-of-a-kind sensory afferents (SSA) indigenous the inside ear to the cochlear nuclei and the vestibular nuclei in the caudal medulla oblongata. Hair cell within the cochlear duct, semicircular canals, utricle, and also saccule are polarized sensory receptor cells v apical ciliary expansions that transduce an electrochemical signal upon mechanically deformation. Ganglionic neurons in ~ the cochlea and the vestibular nerve receive this signal peripherally and transmit the centrally v the internal auditory meatus before entering the medulla. Damage to the vestibular component of this nerve causes dizziness, while damages to the cochlear component causes peripheral, or sensorineural, listening loss. The internal auditory meatus is a small canal the the temporal bone with which this nerves course and also schwannoma the the vestibular or cochlear nerves in this meatus conveniently compresses and also impinges these nerves. At an early stage signs and symptoms are a progressive worsening hearing loss through tinnitus, imbalance, resulting in a feeling of press in the ear and also facial weak or paralysis.<10> Vestibular schwannomas have an incidence price of 6 to nine brand-new cases every year every million people and also are readily treatable with surgical procedure or radiation.<11> However, if the condition is left untreated, the can an outcome in potentially large and life-threatening tumors.

In addition, the terminal nerve or cranial nerve zero<12> has been identified as a different cranial nerve in the human mind dating earlier to 1914 yet is overlooked by most existing anatomical textbooks. Also dubbed nervus terminals because that its proximity come the lamina terminalis and also nerve nulla (i.e., nothing, zero), cranial nerve zero is consisted of of one independent central plexus of tiny unmyelinated (possibly special visceral afferent ) fibers located medially and in very close proximity to the olfactory street by the olfactory trigone (Figure 1B). The discrete positioning may explain its negative identification throughout standard dissection techniques. The cranial nerve zero fibers travel centrally to subcortical structures, it sends out projections to the medial pre-commissural septum and also the medial septal nucleus, among others. It appears to have actually a well-off bundle the well-vascularized yarn ascending indigenous the nasal submucosa and also projecting to crucial limbic structures (e.g., amygdala, hypothalamic nuclei). Functionally it has actually been concerned as pheromones processing unconsciously by regulating autonomic responses through hypothalamic gonadotropin-releasing hormone (GnRH) perhaps via the kisspeptin neuronal network. Clinically, a disruption of the regular embryological migratory pathway of GnRH neural crest cell of the olfactory placode and also basal forebrain<13> can an outcome in Kallman syndrome, a hereditary condition identified by hypogonadotropic hypogonadism through partial or complete anosmia, additionally resulting in abnormal sexual advancement in both sexes.

Efferent Cranial Nerves and also Clinical Implications

Cranial nerves III, IV, VI, XI, and XII are thought about purely efferent due to their motor output to the orbit, the neck, and the tongue. Cranial nerve III, IV, and also VI (oculomotor, trochlear, and abducens nerves, respectively) are basic somatic efferent (GSE) nerves responsible for innervating the extraocular muscles in ~ the orbit. This nerves travel unilaterally from the brainstem to the calvarium through the exceptional orbital fissure from identified brainstem nuclei. The oculomotor nerve (CN III) travels with the common tendinous ring, the typical attachment in the posterior orbit for the four extra-ocular recti muscles, in addition to abducens nerve (cranial nerve VI). Trochlear nerve (CN IV) travels right into the orbit exterior of the usual tendinous ring come innervate the superior oblique muscle of the eye. The abducens nerve innervates the lateral rectus muscles only; in order to this nerve can be experiment by examining the abduction of the eye gaze. Cranial nerve III innervates many of the eye muscles, by splitting into a superior and an inferior branch to innervate the staying three recti muscles, the worse oblique, and also the bones muscle component of levator palpebrae superioris. However, cranial nerve III likewise has a general visceral efferent (GVE) component originating in the Edinger-Westphal cell core (also dubbed accessory oculomotor cell nucleus or visceral oculomotor nucleus). This fibers travel with cranial nerve III to synapse in the ciliary ganglion within the orbit. The post-ganglionic sympathetic yarn of the ciliary ganglion pierce the sclera the the eye come innervate the pupillary sphincter and also ciliary smooth muscle responsible for pupillary constriction and also lens accommodation. Pupillary constriction deserve to be tested because that by method of the pupillary light reflex<14> with the efferents indigenous the remarkable colliculus come the accessory oculomotor nucleus. The eye activities test (abduction, adduction, infraduction, supraduction) is one effective technique to assess the viability the the GSE contents of cranial nerves III, IV, and VI.

Cranial nerve XI, the spinal accessory nerve, is responsible because that the basic somatic efferent (GSE) engine innervation of the trapezius and also sternocleidomastoid muscles by method of the spinal nucleus of the accessory nerve. The spinal cell nucleus of the accessory nerve is located within the cervical spinal cord from the level of C1 through roughly C5/6. The fibers emerge as independent roots, different from the anterior or dorsal spinal roots of the main spinal grey matter, and also ascend with the foramen magnum to go into the cranial cavity.  this fibers then departure via the jugular foramen follow me with cranial nerve IX and X. Central root or nuclear damage to spinal accessory nerve outcomes in ipsilateral flaccid paralysis that the sternocleidomastoid (with difficulty in turning the head versus force) and also partial ipsilateral trapezius paralysis resulting in shoulder drop. The trapezius is innervated through anterior horn grey issue from cervical spinal regions C3 v C4/5 in addition to the spinal accessory nerve. Therefore a complete paralysis that trapezius muscle will certainly not occur adhering to a simple focal lesion.<15>

Cranial nerve XII, the hypoglossal nerve, is responsible because that the basic somatic efferent (GSE) innervation that the intrinsic and also extrinsic muscles of the tongue, other than palatoglossus muscle, from the nerve’s identified nucleus. This includes the genioglossus, geniohyoid, hyoglossus, and also styloglossus muscles. Fibers from the hypoglossal nucleus exit the medulla from the sulcus between the pyramids and the olives as a collection of fibers the coalesce before enter the hypoglossal canal to leave the cranium. Damages to the cell nucleus or nerve fibers results in tongue deviation towards the next of the lesion, together the ipsilateral genioglossus muscle becomes weak or flaccid reducing its ability to protrude the tongue.

Mixed Cranial Nerves and Clinical Implications

Cranial nerves V, VII, IX, and also X are taken into consideration mixed cranial nerves because of the visibility of afferent and also efferent fibers v both sensory and also motor components. Cranial Nerve V is the trigeminal nerve responsible because that the general somatic sensory innervation (GSA) the the face through that is three main branches, V1, V2, and V3 (ophthalmic, maxillary, and also mandibular, respectively). This cranial nerve (via V3) is additionally responsible because that motor innervation (SVE) the the muscles of mastication, the anterior belly of the digastric, mylohyoid, and two small tensor muscles: the tensor veli palatini and also tensor tympani. While no autonomic yarn travel with the fifth cranial nerve as it exits the pons, parasympathetic yarn from the other blended cranial nerves will sign up with with peripheral branches of cranial nerve V come innervate their particular target structures, such together the lacrimal, parotid, submandibular and also sublingual glands. In this sense, main nuclear or supranuclear lesions may result in ipsilateral sensory or motor deficits, but parasympathetic features will just be impaired by peripheral nerve damages to the particular branches.<16>

Cranial nerve VII (facial nerve), has actually both motor and also autonomic fibers v minor somatosensory components. Unique visceral efferent (SVE) motor innervation is to the muscle of facial expression and exit the skull v the stylomastoid foramen deep to the parotid gland. Damage to this fibers results in ipsilateral face paralysis (facial palsy). General visceral efferents (GVE) and also special visceral afferents (SVA) fibers initially exit the brainstem as nervus intermedius, a separate nerve bundle that joins v the other components of the face nerve in ~ the face canal. The GVE components from the exceptional salivary nucleus space responsible for parasympathetic innervation that the glands and also mucosae the the face, v the exception of the parotid gland and also the smaller buccal and labial glands. Taste yarn from the anterior two-thirds that the tongue travel centrally together the chorda tympani nerve to their cell body of beginning in the geniculate ganglion prior to synapsing centrally in the solitary nucleus. Relying on the ar of the lesion, these visceral contents may also be affected in lesions the the facial nerve. Somatic afferents (GSA) carry out sensory innervation from the auricle and a little external part of the hear canal.

Cranial nerve IX (glossopharyngeal nerve), is responsible because that motor (SVE) innervation that the stylopharyngeus and also the pharyngeal constrictor muscles by the nucleus ambiguus. Worse salivary cell core fibers take trip with cranial nerve IX to carry out general visceral efferent (GVE) innervation come parotid, buccal and also labial glands while visceral afferents (GVA and SVA) obtain sensory info from the carotid body and also carotid sinus, and also taste native the posterior 3rd of the tongue come synapse ~ above the solitary nucleus. Whereas, the sensory afferents (GSA) receive information from the skin end the tongue, oropharynx, center ear cavity, and also auditory canal.

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Cranial nerve X is the vagus nerve. The parasympathetic efferents (GVE) fibers from the dorsal vagal nucleus come the thoracic and abdominal muscle viscera to the splenic flexure the the colon represent its significant neural component. These fibers type a substantial plexus the travels along the esophageal serosa to the viscerae. It likewise has a substantial motor output (SVE) from the nucleus ambiguous come the pharyngeal and soft palate muscles, and the intrinsic laryngeal muscles via the superior and also recurrent laryngeal nerves. Somatic afferents (GSA) supply the posterior cranial dura and also a section of the ear and external auditory canal epithelium. Visceral afferents (GVA) native the pharynx, larynx, aorta, thoracic and abdominal muscle viscerae and taste buds from the source of the tongue and also epiglottis (SVA) synapse on the solitary nucleus as well. Damage to the recurrent laryngeal branch of the vagus nerve can result in vocal hoarseness or acute dyspnea with bilateral avulsion.