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1. Cranial Nerves By Ruaa emad eldin 233136

2. Introduction Structure and Function Clinical Significane References 01 02 03 04 Table of contents

3. Introduction 01

4. Introduction The cranial nerves provide afferent and efferent innervation principally to the structures of the head and neck. Unlike spinal nerves, whose roots are neural fibers from the spinal grey matter, cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures. Unlike the spinal nerves, cranial nerve nuclei are functionally organized into distinct nuclei within the brainstem. Typically, the more posterior and lateral nuclei tend to be sensory, and the more anterior tend to be motor.

5. Introduction 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 purely efferent. The remaining cranial nerves, V (trigeminal), VII (facial), IX (glossopharyngeal), and X ( vagus ), are functionally mixed (sensory and motor ). [ 1]

6. Structure and Function 02

7. Structure and Function The cranial nerves can be considered both in terms of their anatomical numbering from I to XII , which describes their sequential origins from the caudal to the ventral brainstem, or in groups according to their developmental functions (i.e., sensory, motor, mixed). Here we will detail the 12 cranial nerves first broadly in their anatomical ordering before briefly outlining their functional groupings.

8. Cranial nerve I (olfactory nerve) Cranial nerve II (optic nerve) Special visceral afferent bipolar sensory neurons reside in the olfactory mucosa. [ 2 ] Some olfactory projections travel medially to the septal area and the contralateral bulb via the anterior commissure, while other fibers travel laterally to the amygdala and piriform cortex, also known as the primary olfactory cortex, where conscious odorant sense is processed . [ 3] Cranial nerve II, the optic nerve, conveys special somatic afferent (SSA) visual sensory information from the rods and cones retinal sensory receptors to the thalamus, especially the lateral geniculate nucleus (LGN) and the superior colliculus (SC). Ganglion cells, whose cell bodies are located deep in the retina, have central projections that form the optic nerve fibers, which traverse the optic canal to enter the cranium . [ 4]   Structure and Function

9. Cranial nerves III, IV, and VI ( oculomotor , trochlear, and abducens nerve) Structure and Function Cranial nerves III, IV, and VI are general somatic efferent (GSE) nerves responsible for innervating the extraocular muscles within the orbit.  The oculomotor nerve (CN III) travels through the common tendinous ring, the common attachment in the posterior orbit for the four extraocular recti muscles, along with the abducens nerve (cranial nerve VI).  The trochlear nerve (CN IV) travels into the orbit outside the common tendinous ring to innervate the superior oblique muscle of the eye.  The abducens nerve innervates the lateral rectus muscles only; thereby, this nerve can be tested by evaluating the abduction of the eye gaze . The eye movements test (abduction, adduction, infraduction , supraduction ) effectively assesses the viability of the GSE components of cranial nerves III, IV, and VI . [5]

10. Cranial nerve V (trigeminal nerve) Structure and Function C ranial Nerve V is the trigeminal nerve responsible for the general somatic sensory innervation (GSA) of the face through its three main branches, V1, V2, and V3 (ophthalmic, maxillary, and mandibular, respectively). This cranial nerve (via V3) is also responsible for motor innervation (SVE) of the muscles of mastication, the anterior belly of the digastric, mylohyoid , and two tiny tensor muscles: the tensor veli palatini and tensor tympani. While no autonomic fibers travel with the fifth cranial nerve as it exits the pons, parasympathetic fibers from the other mixed cranial nerves will join with peripheral branches of cranial nerve V to innervate their respective target structures, such as the lacrimal, parotid, submandibular and sublingual glands. . [6]

11. Cranial nerve VII (facial nerve) Structure and Function Cranial nerve VII (facial nerve) has both motor and autonomic fibers with minor somatosensory components. Special visceral efferent (SVE) motor innervation is to the muscles of facial expression and exits the skull through the stylomastoid foramen deep to the parotid gland. General visceral efferents (GVE) and special visceral afferents (SVA) fibers initially exit the brainstem as nervus intermedius , a separate nerve bundle that joins with the other components of the facial nerve within the facial canal. The GVE components from the superior salivary nucleus are responsible for parasympathetic innervation of the glands and mucosae of the face, with the exception of the parotid gland and the smaller buccal and labial glands. Taste fibers from the anterior two-thirds of the tongue travel centrally as the chorda tympani nerve to their cell body of origin in the geniculate ganglion before synapsing centrally in the solitary nucleus.[6]

12. Cranial nerve VIII ( vestibulocochlear nerve ) Structure and Function Cranial nerve VIII, the vestibulocochlear nerve, is responsible for the auditory sense and the vestibular sense of orientation of the head. This nerve conveys special sensory afferents (SSA) from the inner ear to the cochlear nuclei and the vestibular nuclei in the caudal medulla oblongata. Ganglionic neurons within the cochlea and the vestibular nerve receive this signal peripherally and transmit it centrally through the internal auditory meatus before entering the medulla . [6]

13. Cranial nerve IX (glossopharyngeal nerve) Structure and Function Cranial nerve IX (glossopharyngeal nerve) is responsible for motor (SVE) innervation of the stylopharyngeus and the pharyngeal constrictor muscles by the nucleus ambiguus . Inferior salivary nucleus fibers travel with cranial nerve IX to provide general visceral efferent (GVE) innervation to parotid, buccal and labial glands, while visceral afferents (GVA and SVA) receive sensory information from the carotid body and carotid sinus, and taste from the posterior third of the tongue to synapse on the solitary nucleus. The sensory afferents (GSA) receive information from the skin over the tongue, oropharynx, middle ear cavity, and auditory canal . [6]

14. Cranial nerve X ( vagus nerve) Structure and Function Cranial nerve X is the vagus nerve. The parasympathetic efferents (GVE) fibers from the dorsal vagal nucleus to the thoracic and abdominal viscera to the splenic flexure of the colon represent its major neural component. These fibers form a comprehensive plexus that travels along the esophageal serosa to the viscera. It also has a considerable motor output (SVE) from the nucleus ambiguous to the pharyngeal and soft palate muscles and the intrinsic laryngeal muscles via the superior and recurrent laryngeal nerves. Somatic afferents (GSA) supply the posterior cranial dura and a portion of the ear and external auditory canal epithelium. Visceral afferents (GVA) from the pharynx, larynx, aorta, thoracic and abdominal viscera, and taste buds from the root of the tongue and 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 .[6]

15. Cranial nerve XI (Accessory nerve) Structure and Function Cranial nerve XI, the spinal accessory nerve, is responsible for the general somatic efferent (GSE) motor innervation of the trapezius and sternocleidomastoid muscles by way of the spinal nucleus of the accessory nerve. The fibers emerge as independent roots, separate from the anterior or dorsal spinal roots of the central spinal grey matter, and ascend through the foramen magnum to enter the cranial cavity. These fibers then exit via the jugular foramen along with cranial nerves IX and X .[6]

16. Cranial nerve XII (Hypoglossal nerve) Structure and Function Cranial nerve XII, the hypoglossal nerve, is responsible for the general somatic efferent (GSE) innervation of the intrinsic and extrinsic muscles of the tongue, except the palatoglossus muscle, from the nerve’s synonymous nucleus. This includes the genioglossus , geniohyoid , hyoglossus , and styloglossus muscles. [6]

17. Clinical Significane 03

18. Clinical Significane CN I Traumatic injury, especially “whiplash” from automobile collisions, can sever the olfactory projections through the cribriform plate, resulting in anosmia, which has been associated with the development of depression . The sense of olfaction also appears to have a non-conscious role in activating the limbic system, which may account for such an effect . [7]

19. Clinical Significane CV VIII Damage to the vestibular component of this nerve causes dizziness, while damage to the cochlear part causes peripheral or sensorineural hearing loss. The internal auditory meatus is a narrow canal of the temporal bone through which these nerves course and schwannoma of the vestibular or cochlear nerves in this meatus easily compresses and impinges these nerves. Early signs and symptoms are progressively worsening hearing loss with tinnitus, and imbalance, leading to a sense of pressure in the ear and facial weakness or paralysis . [8]  

20. Clinical Significane CN XI Central root or nuclear damage to the spinal accessory nerve results in ipsilateral flaccid paralysis of the sternocleidomastoid (with difficulty turning the head against force) and partial ipsilateral trapezius paralysis leading to shoulder drop. [9]

21. Clinical Significane CN XII Damage to the nucleus or nerve fibers results in tongue deviation toward the side of the lesion, as the ipsilateral genioglossus muscle becomes weak or flaccid, reducing its ability to protrude the tongue .[9]

22. References 04

23. References Traylor KS, Branstetter BF. Cranial Nerve Anatomy. Neuroimaging Clin N Am. 2022 Aug;32(3):565-576. [ PubMed ] López-Elizalde R, Campero A, Sánchez-Delgadillo T, Lemus-Rodríguez Y, López -González MI, Godínez-Rubí M. Anatomy of the olfactory nerve: A comprehensive review with cadaveric dissection.  Clin Anat. 2018 Jan;31(1):109-117. [ PubMed ] Milardi D, Cacciola A, Calamuneri A, Ghilardi MF, Caminiti F, Cascio F, Andronaco V, Anastasi G, Mormina E, Arrigo A, Bruschetta D, Quartarone A. The Olfactory System Revealed: Non-Invasive Mapping by using Constrained Spherical Deconvolution Tractography in Healthy Humans. Front Neuroanat . 2017;11:32. [ PMC free article ] [ PubMed ] Maleki N, Becerra L, Upadhyay J, Burstein R, Borsook D. Direct optic nerve pulvinar connections defined by diffusion MR tractography in humans: implications for photophobia. Hum Brain Mapp . 2012 Jan;33(1):75-88. [ PMC free article ] [ PubMed ] Yoo YJ, Hwang JM, Yang HK. Differences in pupillary light reflex between optic neuritis and ischemic optic neuropathy.  PLoS One. 2017;12(10):e0186741. [ PMC free article ] [ PubMed ]

24. References Patel NM, Jozsa F, M Das J.  StatPearls [Internet].  StatPearls Publishing; Treasure Island (FL): Oct 18, 2022. Neuroanatomy , Spinal Trigeminal Nucleus. [ PubMed ] Mutic S, Brünner YF, Rodriguez- Raecke R, Wiesmann M, Freiherr J. Chemosensory danger detection in the human brain: Body odor communicating aggression modulates limbic system activation.  Neuropsychologia . 2017 May;99:187-198. [ PubMed ] Kentala E, Pyykkö I. Clinical picture of vestibular schwannoma .  Auris Nasus Larynx. 2001 Jan;28(1):15-22. [ PubMed ] Wiater JM, Bigliani LU. Spinal accessory nerve injury.  Clin Orthop Relat Res. 1999 Nov;(368):5-16. [ PubMed ]

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