Cranial Nerves

Brain Anatomy Review

  • Reviewing brain anatomy with Complete Anatomy.
  • Identifying the longitudinal fissure separating the hemispheres.
  • Locating the superior sagittal sinus.
  • Understanding that the superior sagittal sinus drains cerebrospinal fluid (CSF) and is part of the venous system.
  • Tracing blood flow from the superior sagittal sinus to the transverse sinus, sigmoid sinus, and then the jugular vein.
  • The jugular vein exits the skull through the jugular foramen.
  • Identifying the carotid canal and other foramina.
  • Locating the optic canal within the sphenoid bone (eyes of the bat).
  • The optic canal transmits the optic nerve.

Cranial Nerves Overview

  • Theme: Cranial nerves exiting the cranium.

  • There are 12 cranial nerves on each side of the brain.

  • Roman numerals are used to identify the cranial nerves.

  • The meninges separate the cranium from the brain.

  • The meninges consist of three layers: dura mater, arachnoid mater, and pia mater.

  • The dura mater has two layers: periosteal and meningeal.

  • The falx cerebri is formed by the dura mater.

  • Beneath the dura is the arachnoid mater.

  • Cerebrospinal fluid (CSF) and blood vessels are located within the subarachnoid space.

  • The pia mater is the thinnest layer of the meninges.

Cranial Nerves - Origins and Functions

  • Cranial nerves exit from the midbrain, pons, and medulla of the brainstem.

Cranial Nerve I: Olfactory Nerve

  • Exits above the nose and goes into the nasal mucosa.
  • Function: Sensory - enables the sense of smell.
  • Passes through the cribriform plate of the ethmoid bone.
  • Olfactory foramina allow extensions of the olfactory nerve to pass through.
  • There are approximately 25,000,000 olfactory neurons, each living about three months.
  • There are about 400 different receptors that are spread across many times over all of those different neurons, and an individual scent is perceived by a subset of those receptors in different amounts.
  • We lose about 1% of our sense of smell each year.

Cranial Nerve II: Optic Nerve

  • Exits through the optic foramen of the sphenoid bone.
  • Function: Sensory - vision.
  • The optic chiasm allows for the crossing of nerve fibers, where visual information from the lateral sides of the visual field crosses over.
  • What you are seeing to your left is perceived on your right. What you're seeing to your right is perceived on the left in terms of of where that's processed in the visual centers of the of the occipital cortex.

Cranial Nerve III: Oculomotor Nerve

  • Controls most of the extraocular muscles.
  • Provides parasympathetic fibers to intraocular muscles.
  • Controls the sphincter pupillary muscle and the ciliary muscle to dilate the pupil.

Extraocular Muscles

  • Superior rectus, inferior rectus, medial rectus, lateral rectus. Have a rectus muscle on the top, the bottom, and each side (medial and lateral).
  • Inferior oblique muscle - rotates the eyeball.
  • Superior oblique muscle - has a trochlea (pulley-like structure) to rotate the eye.

Clinical Assessment

  • Pupillary dilation test with light.
  • Tracking finger movements in cardinal directions (H-pattern) to assess extraocular muscle function.

Cranial Nerve IV: Trochlear Nerve

  • Innervates only one muscle: the superior oblique muscle.
  • Smallest of the cranial nerves.
  • Runs through the superior orbital fissure.

Cranial Nerve V: Trigeminal Nerve

  • Largest cranial nerve with three divisions.

Divisions:

  • Ophthalmic division (V1): Sensory, exits through the superior orbital fissure.
  • Maxillary division (V2): Sensory, exits through the foramen rotundum.
  • Mandibular division (V3): Mixed (sensory and motor), exits through the foramen ovale.
  • Ophthalmic division (V1) - tested by touching the forehead.
  • Maxillary division (V2) - tested by touching the cheeks.
  • Mandibular division (V3) - controls muscles of mastication (masseter, temporalis, pterygoids).
  • Mandibular division passes through the mandibular foramen and mental foramen.
  • Maxillary division exits the infraorbital foramen.

Cranial Nerve VI: Abducens Nerve

  • Innervates the lateral rectus muscle.
  • Responsible for abduction (lateral rotation) of the eye.
  • Clinical evaluation: Following a finger to assess lateral eye movement.

Cranial Nerve VII: Facial Nerve

  • Has autonomic fibers (parasympathetic) to the lacrimal, submandibular, and sublingual glands.
  • Sensory for taste on the anterior two-thirds of the tongue.
  • Controls muscles of facial expression.
  • Viruses, such as those causing COVID-19 or Bell's palsy, can affect this nerve.

Branches of the Facial Nerve

  • Cervical, mandibular, buccal, zygomatic, temporal.
  • These branches innervate different muscles of facial expression.
Path of the Facial Nerve:
  • Goes through the internal acoustic meatus (internal ear hole).
  • Exits the skull through the stylomastoid foramen (between the mastoid and styloid process).

Cranial Nerve VIII: Vestibulocochlear Nerve

  • Two nerves in one: vestibular and cochlear nerves.
  • Vestibular nerve: Sensory - sense of equilibrium and balance.
  • Cochlear nerve: Sensory - sensation of hearing.
  • Passes through the internal acoustic meatus.
  • Testing: Hearing and balance tests.

Cranial Nerve IX: Glossopharyngeal Nerve

  • Sensory component of taste on the posterior one-third of the tongue.
  • Responsible for gag reflex.
  • Three cranial nerves pass through the jugular foramen.
  • Responsible for secretion of the parotid gland.
  • Relays feedback from baroreceptors in the carotid sinuses (sensing blood pressure).
  • Glossopharyngeal nerve is critical in relaying the variations in blood pressure back to the brain.
  • When we have high blood pressure, those baroreceptors are compressed. Being stretched receptors, that means that they're being stretched

Cranial Nerve X: Vagus Nerve

  • Sensory and motor nerve with autonomic functions.
  • Major nerve of the parasympathetic nervous system (craniosacral).
  • Branches go to the heart (lowering heart rate) and many other organs in the thoracic and abdominal regions.

Cranial Nerve XI: Accessory Nerve

  • Motor nerve that innervates muscles of the neck.
  • Innervates the sternocleidomastoid and trapezius muscles.
  • Passes through the jugular foramen.
  • Rootlets communicate up, then forms the the nerve, goes up and up and up into the foramen magnum (the big hole - right there). Before turning around and heading out of the jugular foramen.
  • If you’re thinking about what muscle is innervated by the accessory nerve, you can just shrug, and you’d be correct.

Cranial Nerve XII: Hypoglossal Nerve

  • Below the tongue.
  • Controls muscles of the tongue (extrinsic and intrinsic).
  • Tested by sticking out the tongue and doing different motions with it.
  • Exits through the hypoglossal canal in the occipital bone.

Mnemonics for Memorizing Cranial Nerves

  • Names: "On occasion, our trusty truck acts funny. Very good vehicle anyhow."
  • Type (sensory, motor, or both): "Some say marry money, but my brother says big brains matter more."