Cranial Nerves Med Sci 531 Spring 2008 D. M. Conley
What to do about all these cranial nerves???!!!! Here’s the important stuff NOW: Names Attachments to brain Exit/enter skull Functions Regions they supply Don’t sweat the details of topography and branches yet.
Names of cranial nerves • I = Olfactory • II = Optic • III = Oculomotor • IV = Trochlear • V = Trigeminal • VI = Abducens • VII = Facial • VIII = Vestibulocochlear • IX = Glossopharyngeal • X = Vagus • XI = Spinal accessory • XII = Hypoglossal Mnemonics can help!
Anatomy of a nerve Nerve fibers Nerve fascicle Nerve
The 12 pairs of cranial nerves are part of the peripheral nervous system PNS NERVES GANGLIA SPINAL NERVES CRANIAL NERVES
The PNS has two divisions Spinal part – consists of spinal nerves. Cranial part – consists of cranial nerves.
Differences: Cranial nn. & spinal nn. Cranial nerves attach to brain, not spinal cord. Cranial nerves not formed by union of anterior & posterior rami Cranial nerves do not form plexuses
Most cranial nerves attach to the brainstem Midbrain , pons & medulla
All of the cranial nerves emerge from or attach to the ventral (anterior) brain or brainstem. Exception = Trochlear nerve (CN IV).
Differences: Cranial nn. & spinal nn. Cranial nerves exit cranial cavity of skull to supply regions of the head and neck. Canals, meatuses, fissures, and foramina.
Similarities: Cranial nn. & spinal nn. Cranial nerves, like spinal nerves, are associated with sensory ganglia. Trigeminal ganglion (Cranial nerve V)
Similarities: Cranial nn. & spinal nn. Cranial nerves, like spinal nerves, are associated with autonomic ganglia. However, the cranial nerves are most closely associated with parasympathetic ganglia. Remember the term “craniosacral outflow”? Submandibular ganglion
Otic ganglion Lateral view looking from midline of the head
Similarities: Cranial nn. & spinal nn. Most cranial nerves carry postganglionic sympathetic fibers to smooth muscle and/or glands. Cell bodies of neurons are in superior cervical ganglion. The pathway to target organ is different compared to that for spinal nerves.
Functional components of mixed cranial nerves • Recall that nerves are bundles of nerve fibers (= axons of multipolar neurons & peripheral processes of pseudo-unipolar neurons). • In cranial nerves (especially the mixed ones), the nerve fibers carry specific types of information (somatic, visceral, general & special). The “make up” of the nerves (i.e. – what types of info they carry) are categorized as the functional components.
Purely sensory cranial nerves CN I = Olfactory (smell) CN II = Optic (vision) CN VIII = Vestibulocochlear (hearing and balance) Functional components: Special sensory fibers
Special sensory = smell CN I = Olfactory nerve Olfactory bulb Olfactory tract Nasal cavity
Olfactory nerves & pathway Olfactory cortex of cerebrum Olfactory tract Olfactory bulb Cribriform plate NOTE the difference between olfactory NERVE & olfactory TRACT Nasal cavity Olfactory nerves with cell bodies in the mucous membrane of the nasal cavity
Special sensory = vision CN II = Optic nerve Axons of retinal ganglion cells (neurons) project from the eye to the diencephalon – these axons make up the optic “nerves” and optic tracts. Axons from medial retina cross-over at optic chiasma– axons from lateral retina do not.
Optic nerves • Optic chiasma • Optic tracts
CN VIII = Vestibulocochlear nerve Special sensory = Hearing and balance
Two parts of Vestibulocochlear nerve Vestibular nerve: Receptors in semicircular ducts, utricle & saccule Cochlear nerve: Receptors in spiral organ (organ of Corti)
Mainly motor cranial nerves CN III = Oculomotor CN IV = Trochlear CN VI = Abducens CN XI = Spinal accessory CN XII = Hypoglossal Functional components: Skeletal motor fibers
CN III = Oculomotor nerve Components: Skeletal motor = to 4 extrinsic eye muscles + levator palpebrae superioris muscle. Visceral motor = parasympathetic. Preganglionic fibers synapse in ciliary ganglion. Innervate ciliary muscle and sphincter pupillae m. in eye.
CN III = Oculomotor nerve Superior division Inferior division
Dissection Tip: Note the relationship of CN III to the posterior cerebral and superior cerebellar arteries.
CN IV= Trochlear nerve Components: Skeletal motor = innervates superior oblique muscle (an extrinsic eye muscle). Arises from the dorsal side of the midbrain. CN IV is the only motor nerve – cranial or spinal – that arises from the dorsal aspect of the CNS.
CN VI = Abducens nerve Components: Skeletal motor = innervates lateral rectus muscle (an extrinsic eye muscle). Lateral rectus causes the eye to turn away from the midline so the gaze is directly laterally = thus the muscle produces abduction of the eyeball.
CN XI = Spinal accessory nerve Components: Skeletal motor = innervates sternocleidomastoid and trapezius muscles. Does NOT arise from brainstem – instead the motor nuclei are in upper six cervical spinal cord segments. Ascends into skull via foramen magnum.
CN XII Posterior view – skull bones and cerebellum removed
CN XI = Spinal accessory nerve CN XI crosses the posterior triangle of the neck and is vulnerable to injury here.
CN XII = Hypoglossal nerve Components: Skeletal motor = innervates all muscles of the tongue – both intrinsic and extrinsic – except one (the palatoglossus muscle). Arises as many rootlets from the medulla – between the pyramid and the olive.
CN XII = Hypoglossal nerve Note that C-1 motor fibers “hitch-a-ride” on CN XII. It courses “under” the tongue to supply tongue muscles.
The mixed cranial nerves • V – Trigeminal • VII – Facial • IX – Glossopharyngeal • X – Vagus Carry motor and sensory fibers. Carry somatic and visceral fibers.
CN V = Trigeminal nerve • Trigeminal is the largest cranial nerve. It arises from the pons in two parts: • Sensory root (large) • Motor root (small) Trigeminal is the major sensory nerve of the face.
Functional Components: V - Trigeminal Somatic sensory – skin of the face. Visceral sensory – mucous membranes of the nose & mouth. Skeletal motor– Mastication (chewing) muscles and others.
CN V has three divisions • Ophthalmic (V1) • Maxillary (V2) • Mandibular (V3)
CN VII = Facial nerve • The facial nerve arises from the lower pons in two parts • Motor root of VII (larger) • Nervus intermedius (smaller) • Nervus intermedius carries general sensory fibers, taste fibers, and visceral motor (parasympathetic) fibers).
Functional Components: Somatic sensory – skin of external ear (small). Visceral sensory – Taste buds on anterior 2/3 of tongue. Visceral motor (parasympathetic) – salivary and lacrimal glands. Skeletal motor – Muscles of facial expression and others.