Week 5: Cranial Nerves Flashcards
Where do cranial nerves originate? Where do the exit?
CN I and II - forebrain, exit directly
CN III - XII - mid/hindbrain, exit via brainstem
What do the cranial nerves innervate?
Mostly the head and neck, except for the vagus and a few others, which innervate visceral organs
What kind of nerves are cranial nerves? Parasympathetic or sympathetic?
Parasympathetic, NOT sympathetic. This includes somatic and visceral innervation, as well as special sensory innervation for vision, smell, taste, hearing and equilibrium.
How do we remember the 12 nerves?
I: Oh - Olfactory
II: Oh - Optic
III: Oh - Oculomotor
IV: To - Trochlear
V: Touch - Trigeminal
VI: And - Abducens
VII: Feel - Facial
VIII: Virgin - Vestibulocochlear
IX: Girls’ - Glossopharyngeal
X: Vagina - Vagus
XI: Ah - Accessory
XII: Heaven - Hypoglossal
What does the olfactory nerve do, and which CN is it? Why is it distinct?
CN I, functions in smell via senses detected from many olfactory receptor neurons. The olfactory “nerve” is unique because it is a collection of nerve fibers, not one large, singular nerve that can be identified on its own.
What does the optic nerve do, and which CN is it?
CN II, the optic nerve functions in vision. The nerve itself has axons stemming from the uppermost ganglion cell layer of the retina, which project towards the basement sclera where the rod and cone layer lies.
How does the optic chiasm function in processing vision as it relates to objects in the center of the visual field versus in the periphery?
Light from the center of the visual field hits the lateral parts of the eye, which contains nerves that do not cross the optic chiasm (red). However, peripheral objects hit the medial portion of the eye, which contains nerves that DO cross at the optic chiasm (blue).
What do the right and left optic tracts carry?
The right optic tract carries information from the central visual field, which hits the far right side of the right eye, as well as information from the left periphery, which hits the far right side of the left eye, and decussates at (crosses over at) the optic chiasm.
The left optic tract carries information from the central visual field, which hits the far left side of the left eye, as well as information from the right periphery, which hits the far left side of the right eye, and decussates at the optic chiasm.
What do optic chiasm lesions cause?
Bi-temporal hemianopsia, or loss of peripheral vision on both sides
Bi = two
temporal = temporal/peripheral sides
hemi = half
anopsia = unable to see
What CNs are involved in eye movement?
CN III: Oculomotor
CN IV: Trochlear
CN VI: Abducens
What are the functions of the oculomotor nerve, and which CN is it?
CN III functions to move the eye and give parasympathetic input to the pupil. It provides somatic motor input to:
levator palpebrae superioris (elevates eyelid) and 4/6 extraocular muscles that move the eyeball
It provides visceral motor input to:
sphincter pupillae muscle (pupil constriction) and the ciliary muscle which accommodates (maintains focus as distance from an object changes) the lens (contraction = near vision)
What are the 6 extraocular muscles innervated by? What movements do they control?
CN III: Oculomotor innervates 4/6, which move the eyeball in all other directions other than abduction and depression
CN IV: Trochlear (somatic efferent) innervates 1/6 of the extraocular muscle which abducts (outwards) and depresses (down) the eyeball
CN VI: Abducens (somatic efferent) innervates the final 1/6 of the extraocular muscle, which also abducts (outwards) the eyeball
What are the intrinsic and extrinsic eye muscles and what are they innervated by? What kinds of nerves are these?
Intrinsic:
ciliary muscle (parasymp, CN III)
sphincter pupillae (parasymp, CN III)
dilator pupillae (symp)
Extrinsic:
movement of eyeball and eyelid, somatic motor innervation via CN III, IV and VI
What do the sphincter pupillae and dilator pupillae do, and how do they accomplish this?
The sphincter pupillae (parasymp, CN III) constricts the pupil by tightening in a circular pattern
The dilator pupillae (sympathetic) dilates the pupil by pulling outwards in a radiating pattern.
What does the trigeminal nerve do, and what CN is it? What are it’s three main branches?
CN V functions as the major sensory nerve of the face, and contains some skeletal muscle/motor nerve innervations to the head and neck. It is divided into three branches:
V1: opthalmic (eye region)
V2: maxillary (upper cheek and nose)
V3: mandibular (upper and lower jaw), ALSO contains somatic motor innervation to mastication muscles like the temporalis, masseter and pterygoid (lateral/medial) muscles
How do we remember the innervation type of the cranial nerves (sensory, motor or both)?
Some Say Marry Money But My Brother Says Big Brains Matter More
What does the facial nerve do, and which CN is it?
CN VII provides somatic motor innervation to muscles of facial expression, provides (special) taste sensation to anterior 2/3 of the tongue, and parasympathetic innervation to facial glands (lacrimal, submandibular, sublingual) EXCEPT the parotid gland
Why would surgery of the parotid gland be a delicate/careful situation?
The facial nerve (CN VII) passes through (but does NOT innervate) the parotid gland, so surgeons must be careful not to cut this nerve when operating on the parotid gland!
What do the intracranial branches of the facial nerve innervate?
Branches include the parasympathetic innervation of lacrimal, palatine and nasal glands
The chorda tympani nerve acts to parasympathetically innervate the sublingual and submandibular glands. It allows for special sensory taste from the anterior 2/3rds of the tongue.
What are the extracranial nerves of the facial nerve, and what do they innervate?
The five extracranial branches of the facial nerve include the temporal, zygomatic, buccal, mandibular, and cervical branches, and supply the muscles involved in facial expression
What two issues can cause facial weakness?
Either an issue with CN VII (for example, Bell’s palsy) or an issue with the facial nerve connections in the brain (i.e. a stroke).
Why is an upper motor neuron lesion different from a lower motor neuron lesion in the facial nerve?
The upper motor neuron lesion would still allow the upper part of the face to work, since the upper face is innervated by both hemispheres (contralateral signals usually predominate). Thus, a leisure in just one side would not make much of an impact, since the ipsilateral (same side) hemisphere can compensate.
However, an upper motor neuron lesion would affect the entire face, since it would damage the nerves traveling to both the upper and lower face
What does the vestibulocochlear nerve do, and which CN is it?
CN VIII functions in hearing and balance (special sensory)
Vestibular = balance
Cochlear = hearing
What does the glossopharyngeal nerve do, and which CN is it?
CN IX carries sensory information from the back 1/3 of tongue, pharynx and the back of the palate. It has visceral sensory innervation at the carotid baro/chemoreceptors.
It also has some (minor) visceral motor innervation (parasympathetic) to the parotid gland
What does the vagus nerve do, and which CN is it?
CN X has both sensory and motor function.
Sensory: lower pharynx/larynx, visceral sensory in thoracic and abdominal cavities, and chemo/baroreceptors in the aorta/aortic arch
Motor: parasympathetic innervation to thoracic and abdominal (fore/midgut) smooth muscle and glands. Somatic motor innervation to one extrinsic tongue muscle, and most muscles of the soft palate and larynx for phonation
What does the spinal accessory nerve do, and which CN is it? Why is it unique?
CN XI innervates the trapezius and sternocleidomastoid (neck) muscles, and supports the turning and balance of the head. It is much more like a spinal nerve than a cranial nerve. It goes up into the skull and exits the skull with the other cranial nerves, although it contains many more nerves innervating the spinal cord.
What does the hypoglossal nerve do, and which CN is it?
CN XII provides somatic innervation to the intrinsic and extrinsic tongue muscles
Intrinsic = muscles that originate and insert inside of the tongue
Extrinsic = muscles that originate outside the tongue, but insert on the tongue
How is the tongue innervated?
General sensory: anterior 2/3 = V3 of CN V, posterior 1/3 = CN IX
Special (taste) sensory: anterior 2/3 = CN VII, posterior 1/3 = CN IX
Somatic motor: most tongue muscles = CN XII, palatoglossus = CN X
What are the four parasympathetic (autonomic) ganglia that relate to the CNs, and what are their pre/postganglionic innervations?
Otic: preganglionic = CN IX, innervates the parotid gland
Ciliary: preganglionic = CN III, innervates the intrinsic eye muscles
Submandibular: preganglionic = CN VII (chorda tympani), innervates the submandibular and sublingual glands
Pterygopalatine: preganglionic = CN VII, innervates the lacrimal, nasal and palatine glands
All of these have postganglionic nerves that travel with branches of CN V, which distribute to target organs for general sensory innervation. However, CN V DOES NOT control these nerves.
How is the head innervated by sympathetic nerves?
They exit the upper thoracic spinal segments, ascending the sympathetic chain to the cervical ganglia and synapse at the very top of that chain. Post-ganglionics distribute to gray rami communicans to skin or along arteries and/or nerves to internal structures.
How do we test the cranial nerves with an assessment? II-XII each have their own options!
Optic (II): cover the eyes to test for sight, shine light to see if pupils dilate
Oculomotor, trochlear, abducens (III, IV, VI) see if pt is able to follow finger with eyes
Trigeminal (V): Touching periphery of face from top to bottom, sharp or dull with point/curved edge of tool
Facial (VII): smile, puff out cheeks (push on them to see if they rebound), clench jaw and palpate temples
Vestibulocochlear (VIII): snap in ears, check to see if sound fades when moving hand away, use a tuning fork to determine if pt can assess volume
Glossopharyngeal (IX): see if pt can stick tongue out, wiggle tongue
Vagus (X): check to see if pt can speak properly
Accessory (XI): see if pt can shrug and turn head
Hypoglossal (XII): see if pt can push tongue from side to side in mouth
