HTHSCI 1H06 Lecture Notes - Lecture 12: Scarpa'S Ganglion, Elastic Cartilage, Semicircular Canals
Course CodeHTHSCI 1H06
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CNS 5 (Eye, Ear & Hearing)
Vision uses 6 out of the 12 cranial nerves (3 sensory and 3 motor). CN II is optic
light detection; CN V is trigeminal cornea sensations.
Somatic motor neurons control movements of the eyeball and upper eyelid.
Motor aspects of vision
Oculomotor (III) nerve- has its motor nucleus in the anterior part of the
midbrain. It extends anteriorly and divides into superior and inferior branches,
both of which pass through the superior orbital $ssure into the orbit. This nerve
controls the levator palpebrae.
Facial (VII) nerve- a mixed cranial nerve. Its sensory axons extend from the
taste buds of the anterior two thirds of the tongue, which enter the temporal bone
to join the facial nerve.
Saccadic eye movements- this is necessary to expose the entire fovea to the
scene. The fovea retina is where we actually see from and the rest of the eye
helps with eye coordination.
Extraocular eye muscles- 4 rectus muscles and 2 oblique muscles. The oblique
eye muscles help with movement and rotation of the eyes. The superior oblique
muscles are innervated by CN 4. The superior and inferior rectus muscles
help us look up and down. Muscles come from the nose so when you look up you
look up and in.
Superior rectus- CN III help you look up
Inferior oblique- CN III help you look up
Lateral rectus- CN VI help you abduct your eyes
Inferior rectus- CN III help you look down
Superior rectus- CN IV help you look down
Medial rectus- CN III help you adduct your eyes
Diplopia- double vision. This is caused by problems with conjugate eye
movements. CN III oculomotor palsy, CN IV trochlear palsy, CN VI abducens palsy.
Conjugate eye movements- CN are separated by the brain stem so they must have a
way to connect. This is how we move our eyes. The brain stem circuits are tying the two
eyes together for saccades. When looking right the
lateral rectus muscle has to contract to abduct the eye. To look medially the
medial rectus muscle (CN VI) (CN III) ties cranial nerves together.
Convergent gaze- there is convergence for near objects and for far objects. To
see near objects: contraction of both medial rectus muscles (CN III), contraction of
ciliary body muscle to focus lens (CN III), contraction of papillary sphincter muscle
Accommodation- can focus on something near your face. To look at something
close up we look down and the ciliary muscles contract. When looking near the
lens is relaxed and rounded.
Miosis- when the eyes pupil contracts to block out the light when reading a book.
Astigmatism- when either the cornea or the lens has an irregular curvature. As a
result, parts of the image are out of focus and the vision is blurred or distorted.
Pupil- the hole in the center of the iris. The pupil is black because as you look
through the lens you see the heavy pigmented black of the eye. Pupil
constriction: (miosis) parasympathetic reaction to bright light. Accompanied by
convergence and accommodation. CN III. How is this accomplished? Pupil
dilation: (mydriasis) sympathetic reaction to dim light. Accompanied by reduced
blinking and sweating. CN III. How is this accomplished?
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