Eye, Oculomotor Nerve, Extraocular Muscle Testing Flashcards
what are the 3 layers of the eyeball?
- sclera: tough, white fibrous layer into which extraocular muscles insert + cornea (transparent anterior portion)
- uvea: choroid (vascular layer) + ciliary body + iris
- retina: contains rods and cones
in what layer of the eyeball is the vasculature contained?
choroid - part of the middle uvea layer
which smooth muscle controls accommodation of the eye lens?
ciliary muscle - contained in the middle uvea layer, controls shape of the lens
contraction —> decreases radius of ciliary body, allowing slackening of suspensory ligaments and elasticity of the lens to allow it to become thicker and more refractive - near objects/ accommodation reflex
relaxation —> increase radius of ciliary body, tightening suspensory ligaments and stretching the lens to make it thinner/ less refractive - distant objects
what secretes aqueous humor in the eyeball
ciliary process - part of the middle uvea layer, also supports suspensory ligaments
which 2 smooth muscles control size of the pupil?
- dilator papillae
- sphincter papillae
contained within the iris
what is contained in the blind spot vs fovea?
blind spot = optic disk, where all axons of retina form optic nerve, no receptor cells here
fovea contains greatest density of cones and is located in the center of the yellow spot macula lutea just lateral to the optic disk
how does the ciliary muscle of the eye respond to near vs distant objects?
near/ accommodation reflex: contraction —> decreases radius of ciliary body, allowing slackening of suspensory ligaments and elasticity of the lens to allow it to become thicker and more refractive
distant objects: relaxation —> increase radius of ciliary body, tightening suspensory ligaments and stretching the lens to make it thinner/ less refractive
why does presbyopia occur?
lens loses elasticity with aging, such that even when ciliary muscle of the eye contracts, the lens does not spring back and become thicker, which is needed to create more refraction for near objects
presbyopia = age-related compromise of near vision (why many people need reading glasses)
where is the vitreous vs aqueous humor found in the eye?
region Anterior to the lens contains Aqueous humor
region posterior to the lens contains vitreous humor
how is aqueous humor circulated through the anterior chamber and posterior chamber of the eye?
anterior chamber (between cornea and iris) and posterior chamber (between iris and lens) communicate via pupil
ciliary process secretes aqueous humor into posterior chamber —> passes through pupil —> enters venous channels (canal of Schlemm) in the anterior chamber
which cranial nerve is actually part of the CNS, and therefore affected by demyelination disease (such as multiple sclerosis)?
optic nerve (CN II) - myelinated by oligodendrocytes
also surrounded by meninges (“optic nerve sheath”)
what make up the 4 walls of the orbit bone?
roof = frontal bone
floor = maxilla
medial wall = ethmoid and sphenoid bones
lateral wall = zygoma and sphenoid bones
what are the 3 posterior openings of the orbit? what do they contain?
- optic canal: contains optic nerve (CN II) + ophthalmic artery
- superior orbital fissure: contains superior ophthalmic vein (to cavernous sinus) and CN III, CN IV, CN V1, CN VI
- inferior orbital fissure: contains inferior ophthalmic vein and CN V2 (infraorbital nerve + zygomatic nerve)
what kind of veins are the superior and inferior ophthalmic veins, and why is this clinically important?
ophthalmic veins communicate with veins of face, cavernous sinus, and pterygoid venous plexus
thus, they are emissary veins - can be important for spread of infection from face to deeper intracranial or extra-cranial regions
explain how the supraorbital artery creates an anastomosis between the internal and external carotid arteries
supraorbital artery is a branch of ophthalmic artery, which is itself a branch of the internal carotid artery
when it exits the supraorbital foramen, it anastomoses with the transverse facial artery, which is a branch of the superficial temporal artery, which itself is a branch of the external carotid artery
normally, pressure gradient causes flow from ICA to ECA, but if there is an occlusion of ICA, pressure gradient will reverse from ECA to ICA (can be detected via Doppler)
describe how lacrimal fluid circulates in the eye
- secreted by lacrimal gland in superolateral wall of orbit into conjunctival sac
- lacrimal fluid flows from lateral to medial to drain into lacrimal canaliculi, leading into nasolacrimal duct
where do the rectus vs oblique muscles of the eye insert?
rectus muscles (superior, inferior, medial, lateral) have origins at common ring tendon and insert on front of eyeball
oblique muscles (superior, inferior) have independent origins from the wall of the orbit and insert on the back of eyeball
which extraocular muscle utilizes a pulley system?
tendon of the superior oblique passes through the trochlea (fibrocartilage pulley) before turning back to insert on the back of the eyeball
(this makes sense because it also has its origin near the back of the orbit)
what 2 extraocular muscles must work together to create pure elevation vs depression?
superior rectus + inferior oblique = pure elevation
inferior rectus + superior oblique = pure depression
explain why either a CN III lesion or sympathetic lesion can result in ptosis
the levator palpebrae superioris is composed of skeletal muscle innervated by CN III + smooth muscle (aka superior tarsal muscle) innervated by sympathetic fibers
what type of autonomic innervation do the following structures in the orbit receive?
a. dilator pupillae muscle
b. constrictor pupillae muscle
c. ciliary muscle
d. levator palpebrae (smooth muscle)
e. lacrimal gland
a. dilator pupillae muscle - SNS
b. constrictor pupillae muscle - PNS
c. ciliary muscle - PNS
d. levator palpebrae (smooth muscle) - SNS
e. lacrimal gland - SNS + PNS
from where is the parasympathetic innervation for each of the following? state pre and postganglionic
a. constrictor pupillae and ciliary muscles
b. lacrimal gland
a. constrictor pupillae and ciliary muscles - CN III (preganglionic) + ciliary ganglion (postganglionic)
b. lacrimal gland - CN VII (preganglionic) + pterygopalatine ganglion (postganglionic)
which branches of the ophthalmic division of the trigeminal (V1) provide general sensation to the eyeball?
long and short ciliary nerves
[recall all sensory nerve fibers of the trigeminal have their cell bodies in the trigeminal ganglion in the middle cranial fossa]
prior to entering the orbit, CN V1 (ophthalmic) divides into… (3)
- lacrimal nerve: innervates superolateral to the orbit, note PNS fibers hitch a ride to lacrimal gland
- frontal nerve: divides into supraorbital and supratrochlear nerves
- nasociliary nerve: divides into long/short ciliary nerves + ethmoidal nerves + infratrochlear nerve