TBL 29: Oribt, eye, extraocular muscles, nerves & vessels, & ear Flashcards
What occupies the anterior and posterior portions of the orbit?
Anterior = Eye ball Posterior = Optic nerve (CN II) + extraoccular muscles surounded by white fat
What are the boundaries of the orbit? (lateral wall, medial wall, floor, & roof)
Lateral wall = zygomatic bone + flat great wing of sphenoid bone + ethmoidal air cells occupying the ethmoidal bone
Medial wall = mainly ethmoidal bone
Floor = maxilla (also happens to be the roof of the maxillary sinus that is right below the orbit)
Roof = frontal bone (also happens to be the floor of the frontal cranial fossa that is right above the orbit)
What demarcates the floor from the lateral wall of the orbit?
The inferior orbital fissure
What is the tarus?
A band on connective tissue found in the eyelid that provides extra support
What’s the role of lacrimal gland? How is the fluid spread and drained?
The lacrimal gland is responsible for secreting lacrimal fluid across you eye. The gland is found superolateral to the orbit. Thus, the fluid runs toward the medial angle of the eye.
It’s drained by the puncta in the lower eyelid that allow the fluid to leak through the nasolacrimal ducts into the inferior meatus in the nasal cavity
What are the sensory and motor innervations of the lacrimal gland?
Sensory: Somatic sensory form the lacrimal nerve (CN V1)
Motor: Lacrimal nerve transports post-synaptic parasympathetic motor fibers from the zygomatic nerve. These originally came from the greater petrosal nerve after it synapsed at the pterygopalatine fossa.
Describe the embryological formation of the eye. (include optic vesicles, optic cups, lens placode, lens vesicles, choroid fissure & hyaloid artery)
In the pharyngeal forgut, you have bilateral projections from the neuroectoderm of the diencephalon that produce optic vesicles. These optic vesicles continue to project outward (held to the diencephalon via optic stalks), until they reach the surface ectoderm.
At this point, the surface ectoderm begins to thicken, forming a lens placode. Meanwhile, the optic vesicles continue to invaginate forming double-layered optic cups. The lens placode continues to invaginate, forming the lens vesicles.
The lens cup and the optic stalk both have invaginations that form the choroid fissure. The hyaloid artery runs through this fissure to go to the lens placode.
Describe the important of mesenchyme in the optic cups and it’s role in the vasculature of the optic vesicle and the formation of the vitreous body
The mesenchyme infiltrates the optic cup via the choroid fissure and brings branches of the hyaloid artery to the developing lens. Eventually, these branches disappear and the space left is replaced by a gelatinous substance (eventually becomes the vitreous body)
What does the mesenchyme differentiate into in the optic cups? How is the optic nerve formed?
The outer most portion becomes a thin pigmented layer, while the inner portion starts to develop neurons and is thus known as the thick neural layer
The neural layer has neurons whose axons go to form the optic nerve. The nerve travels through the choroid fissure and encloses the hyaloid artery. Only w/ the closure of the choroid fissure is the optic stalk converted into the optic nerve.
After complete maturation, the optic nerve fully encloses the central artery and vein of the retina
What is the remnant of the hyaloid artery?
Central retinal artery
What is the choroid and sclera? How are they formed?
The choroid is a thin vascularized layer that is formed from mesenchyme directly external to the optic cups
The sclera is a fibrous layer that is found external to the choroid
Name the continuations of the sclera & the choroid
Sclera is continuous posteriorly w/ the cranial dura that surrounds the optic nerve and continuous anteriorly w/ the cornea
Choroid is terminates anteriorly at the ciliary body
What is the double epithelium of the ciliary body and the iris?
When the neural layer of the optic cup reaches the ciliary body, it barely produces any neurons and is turned into an inner non-pigmented retinal layer. This + the outer pigmented retinal layer form the double epithelial layer of the ciliary body & the iris
What forms the pupils?
The closure of the choroid fissure
What are the layers of the photoreceptors in the neural retinal layer? Describe how we see images can be viewed
From retinal surface of neural layer to the retinal pigmented layer
Ganglion –> bipolar cells –> photoreceptors (rods or cones)
Light impulses travel past the ganglion and bipolar cell layers to create impulses w/ the photoreceptors of the neural retinal layers. Then the impulses go to the bipolar cells and the ganglion cells, which send them to the brain through their axons that comprise of the optic nerve
Differentiate b/w rods & cones
Rods are more plentiful than Cones
Rods —> show you visual images in shades of gray from a dim light perspective
Cones —> show COLOR because they can be sensitive for wavelengths of red, blue, or green light
What is the optic disc?
Marks the exit of the optic nerve from the posterior eyeball
What is the fovea centralis?
This is in the posterior aspect of the retina and is the SITE OF HIGHEST VISUAL AQUITY
b/c the photoreceptors are almost all cones and their ganglion cells and bipolar neurons are all compressed to the periphery
What helps to hold the lens in a fixed position?
Zonular fibers and the vitreous body
How can lens thickness be adjusted?
Via smooth muscles in the ciliary body that can contract and pull on the zonular fibers attached to the lens
By regulating the tension of the zonular fibers, the ciliary muscles regulate lens thickness
Describe the formation and drainage of aqueous humor in the eye
Aqueous humor is formed by the nonpigmented retinal layer (recall this was the previous neural layer but in the iris and ciliary body it has no neurons)
The fluid goes to the posterior chamber first, before going to the anterior chamber via pupillary apertures. From the anterior chamber, the fluid goes to through the Canal of Schlemm and empties into the venules of the sclera
What determines eye color?
The relative number of melanocytes in the stroma of the eye
Where are the pupillary constrictor and dilator muscles located?
Stroma of the eye
What’s the function of the aqueous humor?
It can help to nourish the corneal epithelia & the avascular stroma of dense connective tissue
What is the epithelial makeup of the posterior & anterior surfaces of the cornea?
Anterior: Non-keratinized stratified squamous epithelium
Posterior: Simple, squamous epithelium
What is the lens capsule?
It’s a layer that surrounds the outer surface of the lens and acts as a site of attachment for the zonular fibers
Describe the epithelium of the lens
Single layer of epithelium that is found only on the anterior portion
Generates lens fibers that are elongated, anucleate epithelial cells w/ crystallins
What is the embryonal zone?
This is the earliest formed lens fiber that developed and all future lens fibers were formed as successive waves from the embryonal zone outwards
What extraocular muscles are used to look straight up? straight down?
Straight up = Inferior oblique + superior rectus
Straight down = Superior oblique + inferior rectus
What are the three ossicles in the tympanic cavity?
Malleus, incus, & stapes
Describe the connections between the different portions of the ear
external ear (external acoustic meatus to tympanic membrane) then to the middle ear (whose’s lateral walls are made via the tympanic membrane)
The middle ear has the auditory tube (along the anterior wall) that goes into the nasopharynx. The middle ear is separated from the inner ear via the oval window
what are the boundaries of the tympanic membrane?
Anterior wall: has pharyngotympanic (auditory) tube
Posterior wall: Contains the facial canal that houses the facial nerve
Lateral wall: Has the tympanic membrane
Medial wall: Has the oval window, lesser petrosal nerve, tympanic plexus, & tympanic nerve
Inferior wall: Chorda tympani pierces this (to leave) & tympanic nerve pierces it to enter the tympanic cavity
What is the oval window?
It’s the membrane that the stapes attaches to. Also, separates the tympanic cavity from the vestibule of the bony labyrinth. Provides a way to pass sound waves into pressurized waves that go through the cochlea to produce auditory stimuli in the cochlear nerve.
What are the functions of the tensor & levator palatine in the auditory tube?
The auditory tube can allow for air to pass from the nasal cavity into the tympanic cavity to help equalize the pressure difference on both sides of the tympanic membrane, allowing it move more easily
But to keep the auditory tube open, you need to have the cartilagenous portion held open. This can be done w/ the use of the these muscles.
Describe the formation of the membranous labyrinth. What’s the difference b/w the membranous and bony labyrinth? What fluid do they both contain?
There is an otic vesicle at the level of the hindbrain. This vesicle goes on to form the membranous labyrinth.
The bony labyrinth is just a bunch of cavities and such in the petrous part of the temporal bone that envelops & contains the membranous labyrinth
They both contain CSF-like fluid. The membranous has endolymph and the bony has perilymph
What is the utricle? saccule?
Utricle - this is part of the vesicle part of the membranous labyrinth. It forms three ring like structures known as the semi-circular ducts.
Saccule - Also part of the vesicle part of the membranous labyrinth and goes on to form the cochlear duct that occupies the cochlea in the bony labyrinth
What forms the vestibular nerve portion of the vestibulocochlear nerve?
The utricle, saccule, and one end of the semi-circular duct have sensory epithelium
Peripheral fibers of bipolar nerves in the vestibular ganglion synapse w/ these sensory epithelium and their central axons form the vestibular nerve portion of CN VIII
When are the sensory epithelium of the vestibular nerve innervated?
When you have circular motion/angular acceleration you activate the sensory epithelium in the semi-circular canals
When you have linear acceleration you activate the sensory epithelium in the utricle & saccule
Describe the derivation of the cochlear nerve
In the cochlea, you have a cochlear duct that separates it into scala vestibuli & scala tymphani. In the membrane between these two chambers you have a spiral organ that has hair cells and sensory epithelium. The movement of the hair cells sends impulses vaia the sensory epithelium to the peripheral fibers of the spiral ganglion that synapse at those sensory cells. The impulse then goes through the central fibers of the spiral ganglion to the cochlear nerve.
NOTE: the central fibers of the spiral ganglion form the cochlear nerve
What is the helicotrema?
It’s at the apex of the cochlea and it’s what allows for the scala vestibuli and the scala tympani to communicate w/ one another
How is sound converted into impulses that run through your cochelar nerve and eventually CN VIII?
Sound waves go through the external acoustic meatus into the tympanic membrane. The ensuing vibrations cause the ossicles of the middle ear to also vibrate. These vibrations are conveyed into the oval window via the stapes. The movement of the stapes causes pressurized sound waves that move through the perilymph of the scala vestibuli of the cochlea and then to the scala tympani at the helicotrema.
During this journey, the pressurized waves move the spiral organ, stimulating the sensory epithelium at different rates. This causes it to activate and pass impulses through the cochlear nerve
What is the role of the round window?
Found at the end of the scala tympani, the round window allows for the pressurized waves to dissipate into the air
What’s the role of the stapedius & the tensor tympani?
Tensor tympani: Attaches to the malleus & helps to tense the tympanic membrane and reduces the amplitude of ossilations….PROTECTS THE INTERNAL EAR FROM DAMAGING, LOUD SOUNDS
Stapedius: inserts into the stapedius & helps to pull the stapes posteriorly & tilt the base of the oval window. This contracts/tightens the anular ligament and PREVENTS EXCESSIVE MOVEMENT OF THE STAPES
BOTH muscles dampen the oscillatory range of the tympanic membrane and the oval window
What are the tensor tympani & stapedius innervated by?
Tensor tympani = CN V3/Mandibular nerve
Stapedius = Facial nerve/CN VII
