Head and Neck 4: Cranial and Spinal Nerves

• CNI: olfactory nerve
• Olfactory nerve exits the cranial cavity through the cribriform plate to reach the roof of the nasal cavity; carries special sensory fibers for olfaction
• Damage to CNI leads to the loss of the sense of smell
• CNII: optic nerve
• Optic nerve exits the cranial cavity through the optic canal to reach the orbit and supply the eye; it is connected directly to the back of the eye; carries special sensory fibers for vision
• Damage to the CNII leads to blindness
• CNIII: oculomotor nerve
• Oculomotor nerve emerges from the brain stem and transverses the superior orbital fissure to reach the orbit; most branches supply the extraocular muscles to move the upper eyelid and eye
• Oculomotor nerve transmits motor somatic fibers to the extraocular muscles, as well as motor parasympathetic fibers to the smooth muscles inside the eye (intrinsic muscles of the eye)
• Damage to CNIII leads to diplopia (double vision), strabismus (eyes not pointing in the same direction), and the pupil always remaining dilate
• CNIV: trochlear nerve
• Trochlear nerve exits the cranial cavity and enters the orbit through the superior orbital fissure, supplies motor somatic fibers to the superior oblique
• Damage of CNIV leads to diplopia and strabismus
• CNVI: abducens nerve
• Abducens nerve exits the cranial cavity and enters the orbit through the superior orbital fissure, supplies motor somatic fibers to the lateral rectus muscle
• Damage to CNVI leads to the inability to adbuct the eye, diplopia, and strabismus
• CNIII, CNIV, and CNVI supply the muscles of the orbit, and all enter through the superior orbital fissure (as well as opthalmic nerve of the trigeminal)
• CNV: trigeminal nerve
• Before the trigeminal nerve exits the skull, it forms the trigeminal ganglion where three divisions emerge (opthalmic, maxillary, mandibular), and each division exits the skull through different openings to reach different parts of the head
• Opthalmic nerve exits through the superior orbital fissure and enters the orbit, carries general sensory fibers to provide general sensory information from the eyeball
• Maxillary nerve exits through the foramen rotundum and enters the pterygopalatine fossa before further branching out; carries general sensory fibers to transmit sensation from the midface, upper lip, inside the nasal cavity, palate, and upper teeth
• Mandibular nerve exits through the foramen ovale and enters the infratemporal fossa, carries general sensory and motor somatic fibers to transmit sensation from the floor of the mouth, anterior part of the tongue, and lower teeth; motor somatic fibers supplies the muscles of mastication
• CNVII: facial nerve
• Facial nerve exits the cranial cavity through the internal acousti meatus and enters the petrous part of the temporal bone, where it travels close to the ear and forms three branches (greater petrosal, chorda tympani, main trunk)
• The main trunk of the facial nerve exits the temporal bone through the stylomastoid foramen and pierces the parotid gland, then branches out into five branches that supply the muscles of facial expression (temporal, zygomatic, buccal, mandibular, cervical)
• Damage to CNVII leads to facial paralysis
• Facial nerve also carries motor parasympathetic fibers that supply the glands of the face, general sensory fibers, and special sensory fibers to receive taste from the anterior 2/3 of the tongue
• CNVIII: vestibulocochlear nerve
• Vestibulocochlear nerve exits the cranial cavity through the internal acoustic meatus and enters the internal ear to supply the receptors for hearing and balance with special sensory fibers
• Damage to CNVIII leads to deafness and vertigo
• CNIX: glossopharyngeal nerve
• Glossopharyngeal nerve exits through the jugular foramen and enters the carotid sheath in the neck and reaches the floor of the mouth; carries general sensory fibers (e.g. triggering gag reflex), special sensory fibers (transmits taste from the posterior 1/3 of the tongue), motor somatic fibers (supplies stylopharyngeal muscle), and motor parasymapthetic fibers (supplies parotid gland)
• CNX: vagus nerve
• Vagus nerve exits the skull through the jugular foramen and descends through the carotid sheath (the only cranial nerve that exits the neck and reaches the rest of the body)
• The mucosa of the larynx, pharynx etc. are supplied by the vagus nerve, it also carries special sensory fibers (taste from epiglottis), motor somatic fibers (muscles of soft palate, larynx, pharynx), and motor parasympathetic fibers (not to the head and neck, but to organs of the chest and abdomen)
• CNXI: accessory nerve
• Accessory nerve exits the cranial cavity through the jugular foramen and exits the carotid sheath to supply the two superficial muscles of the neck, the trapezius and SCM
• Damage to CNXI leads to the inability to shrug the shoulder and it will be difficult to turn the head
• CNXII: hypoglossal nerve
• Hypoglossal nerve exits through the hypoglossal canal and exits the carotid sheath to travel to the front of the head, enters the floor of the mouth by travelling superficially/laterally to the hyoglossus muscle