Ophthalmology - Physiology Flashcards
What is the anterior chamber of the eye?
Anterior to the iris
What is the posterior chamber of the eye?
Posterior to the iris
Where does aqueous humor originate from?
Ciliary epithelium
Where does aqueous humor flow?
Flows from ciliary epithelium anteriorly –> in between iris and lens to anterior chamber.
Where does aqueous humor flow from anterior chamber?
Through trabecular meshwork –> Canal of Schlemm.
What happens when ciliary bodies contract?
Zonular fibers LOOSEN. Lens thickens. (Accomodation)
What happens when ciliary bodies relax?
Zonular fibers are pulled tight. Lens thins.
What happens to light entering the eye?
Focuses on point on the retina.
What happens if the eye is too short for the refractive power of cornea and lens?
Light is focused behind retina. Hyperopia. Blurry images up close.
What happens if the eye is too long for the refractive power of cornea and lens?
Light is focused in front of retina. Myopia. Blurry images far away.
Myopia is called?
Nearsighted - can see NEAR things.
Hyperopia is called?
Farsighted - can see FEAR things.
Astigmatism is caused by?
Abnormal curvature of the cornea. Different refractive power at different axes.
What is the primary defect in presbyopia?
Lens is less elastic due to age related changes - unable to thicken. Accomodation is impaired (harder to focus on near objects). –> reading glasses.
What is miosis?
Constriction.
Is constriction of pupils sympathetic or parasympathetic?
Parasympathetic.
Which neurons are responsible for constriction of pupils?
1st order neuron: From edinger-westphal nucelus to ciliary ganglion via CN III.
2nd order neuron: From short ciliary nerves to pupillary sphincter muscles
What are the steps of the pupillary light reflex?
- Light in either retina sends a signal via CN II to pretectal nuclei in midbrain
- Pretectal nuclei activates bilateral edinger-westphal nucleus
- Pupils constrict bilaterally
What is mydriasis?
Dilation
Is mydriasis sympathetic or parasympathetic?
Sympathetic.
What is the first neuron involved in pupillary dilation?
Hypothalamus to ciliospinal center of Budge (c8-T2)
What is the second neuron involved in pupillary dilation?
Exits at T1 to superior cervical ganglion (travels along cervical sympathetic chain near lung apex, subclavian vessels).
What is the 3rd neuron involved in pupillary dilation?
Plexus along internal carotid, through cavernous sinus; enters orbit as long ciliary nerve to pupillary dilator muscles.
What else do the sympathetic fibers that innervate pupillary dilators innervate?
Smooth muscle of eyelids (minor retractors) and sweat glands of forehead and face.
What is a Marcus Gunn pupil?
Afferent pupillary defect. Defect of bilateral pupil constriction.
- when light shines in affected eye, bad constriction
- when light shines in unaffected eye, good constriction
(swinging flashlight test).
What lesion causes Marcus Gunn pupil?
Optic nerve damage or severe retinal injury.
What muscle does CN VI innervate?
Lateral Rectus: Abduction.
What muscle does CN IV innervate?
Superior Oblique: Inward and Downward.
What muscles does CN III innervate?
Superior Rectus, Inferior Oblique, Superior Rectus
Medial Rectus Medial Rectus
Inferior Rectus Inferior rectus
What does CN VI damage cause?
Medially directed eye that cannot abduct
What does CN IV damage cause?
Eye moves upward, particularly w/ contralateral gaze, and head tilt towards size of lesion. (problems going down stairs). May present w/ head tilt in opposite direction.
When CN III is damaged, what motor defects arise?
Ptosis, down and out gaze.
What causes damage to motor output of CN III?
Primarily vascular disease; eg, DM (glucose/sorbitol). Decreased diffusion of oxygen and nutrients.
When parasympathetic output of CN III is damaged, what are the defects?
Diminished or absent pupillary light reflex, blown-pupil, often w/ down and out gaze.
What causes damage to parasympathetic output of CN III?
Fibers are on the periphery and are first affected by compression: posterior communicating artery aneurysm, uncal herniation.
What coordinates conjugate horizontal gaze?
MLF.
What nerves must cross-talk for conjugate horizontal gaze?
CN VI and CN III.
Eg when you look to the RIGHT, Right eye moves laterally (CN6), and left eye moves medially (CN3
Why is the MLF highly myelinated?
Must communicate quickly so eyes move at the same time.
What happens if there is a lesion in the MLF?
internuclear opthalmoplegia (INO), conjugate horizontal gaze palsy.
If a person w/ INO tries to look right, what happens?
Right eye looks right (CN 6abducts)
Left eye doesn’t. (No communication to CN 3)
Right eye gets nystagmus because CN 6 overfires to stimulate CN 3.
Does “right INO” mean right eye has nystagmus, or right eye isn’t moving?
Right eye isn’t moving. Refers to which eye isn’t moving upon contralateral gaze.
In what disease is INO classically seen?
MS.
What is the effect of a1 receptors on the eye?
Pupillary dilator muscle –> mydriasis
What is the effect of a2 receptors on the eye?
decrease aqueous humor production
What is the effect of B2 receptors on the eye?
Ciliary muscle relaxation
Decrease aqueous humor
What is the effect of M3 receptors on the eye?
pupillary sphincter muscle contraction –> miosis
Ciliary muscle contraction –> accomodation
