Exam 3 eyes and ears Flashcards
Lens
The Lens is an approximately oval structure that is covered by a capsule of type IV collagen and a Subcapsular Epithelium.
Lens Fiber Cells are Elongated Cells which lose their nuclei and contain Crystalline proteins.
Lens Capsule is formed by subcapsular epithelial cells and corresponds to a thick, elastic basal lamina. Zonular Fibers insert into lens capsule.
Lens Capsule:
Consists of basement membrane
10 – 20 m thick
Composed mainly of Collagen Type IV, Glycoproteins, & Heparan Sulfate Proteoglycan
Subcapsular Epithelium:
Single layer of Cuboidal Epithelium
Present on anterior surface
At Lenticular Equator, begin to elongate into Fiber Cells
Lens Fiber Cells:
Elongated, highly differentiated cells derived from Epithelial Cells
Lose their nuclei and organelles
At maturation, 7-12 m in length and contain primarily proteins of the Crystallin Family. Hence the term, Crystalline Lens.
Cornea
Cornea is the Most Powerful Refractive Element within the eye.
Laterally, the corneal epithelium is continuous with the Conjunctiva.
Five layers of cornea from anterior to posterior aspect
- Corneal Epithelium & Basement Membrane:
Nonkeratinized stratified squamous 5-6 cells thick, contiguous with
bulbar conjunctival epithelium - Bowman’s Membrane or Layer:
Contains collagen fibrils, no cells - Corneal Stroma:
Collagen layers, rich in chondroitin sulfate, few stromal cells (keratinocytes) - Descemet’s Membrane:
Homogeneous structure of fine collagen fibrils, no cells - Corneal Endothelium:
Simple squamous epithelium
Both Corneal Epithelium and Corneal Endothelium are responsible for maintaining corneal transparency through their capability of transporting sodium (Na+) ions to their apical surface which keeps Stroma in relatively Dehydrated State.
Cornea is transparent due to its unique parallel fiber arrangement and continual active pumping out of interstitial fluid.
Limbus
Highly Vascularized Line of Demarcation Between
Transparent Cornea and Opaque Sclera
Sclera
The Sclera makes up the posterior 5/6 segment of the eyeball. Made primarily of Collagen Fibers.
This layer appears grayish-white and is site of attachment of Extra-Ocular (EO) Muscles.
Optic Nerve exits eye at Optic Nerve Head (ONH) through a specialized fenestrated structure called the Lamina Cribrosa
Ciliary body
Ciliary body is anterior expansion of choroid.
Ciliary processes are regions from which zonule fibers extend to anchor the lens.
Epithelial Cells (Pigmented and Non-Pigmented Ciliary Epithelial Cells) line ciliary processes.
Non-pigmented ciliary epithelial cells Actively Secrete Aqueous Humor into Posterior Chamber.
Secreted Aqueous Humor passes to Anterior Chamber via the Pupil and egresses into Episcleral Veins via:
Trabecular Meshwork
Schlemm’s Canal
External Collecting Channels (aka: Scleral Collector Channels)
Anterior Aqueous Veins.
Retina
The Layers from Outermost to the Vitreal Border:
- Retinal Pigment Epithelium (RPE)
- Outer (OS) and Inner (IS) Segments: Rod and Cone photoreceptors
- Outer Limiting Membrane: junctional complexes between photoreceptors and Müller cells
- Outer Nuclear Layer (ONL): Rod and Cone Cell Bodies
- Outer Plexiform Layer (OPL): Synapses of Photoreceptors and Bipolar Neurons
- Inner Nuclear Layer (INL): Cell Bodies of Bipolar Neurons (Cells), Müller, Horizontal and Amacrine Cells
- Inner Plexiform Layer (IPL): Synapses of Bipolar Neurons (Cells) and Ganglion Cells
- Ganglion Cell Layer (GCL)
- Nerve Fiber Layer: unmyelinated axons of GC
- Inner Limiting Membrane: Müller cell processes
Bipolar Cells
Bipolar Cells (soma in INL) interconnect Photoreceptors with Retinal Ganglion Cells. Serve as Interneurons
Rod Bipolars are different from Cone Bipolars based on synaptic inputs.
Each bipolar cell receives input from 20-50 photoreceptor cells.
Horizontal Cells
Horizontal Cells: Cell body is in INL. Receive glutaminergic input from multiple photoreceptors (in absence of light), and provide inhibitory feedback. Causes depolarization of horizontal cell which hyperpolarizes nearby photoreceptors.
Facilitate good vision in both bright and dim conditions, as this inhibitory feedback sharpens edge of receptive field.
Dendrites and Axons pass parallel to plane of retina to nearby and distant photoreceptors.
Amacrine Cells
Amacrine Cells: Cell body in INL. Work laterally (like horizontal cells), but affect output from bipolar cells (not photoreceptors).
This cell does not have an obvious axon but has a highly branched dendrite, which extends over a great distance.
33 different Amacrine Cells based on dendrite morphology and stratification.
These cells appear to modulate signals transiting bipolar cells from rods and cones.
Müller Cells (Müller Glia)
Müller Cells (Müller Glia): Cells Span Entire Neural Retina
Maintain stability of retinal extracellular environment via:
Regulation of K+
Uptake/degradation of neurotransmitters (ACh, GABA)
Removal of debris
Storage of glycogen
Electrical insulation of receptors and other neurones
Mechanical support of neural retina
Synaptogenesis
Fundamental to transmission of light due to unique funnel shape, spatial orientation, favorable physical properties
Ganglion Cells
Retinal Ganglion Cells (RGCs) are the output cells of the retina. Axons form wiring that connects retina to brain (thalamus, hypothalamus, mesencephalon).
Receives input from Bipolar Cells and Amacrine Cells
RGC soma in Ganglion Cell Layer (GCL). Axons extend into Nerve Fiber Layer (NFL) and form Optic Nerve.
Vary in terms of size, connections, responses to visual stimulation. But all have long axon extending to brain.
Small % of RGC contribute nothing to vision but are photosensitive (circadian rhythm; pupillary light reflex).
Unmyelinated axons of ganglion cells exit retina at optic nerve head (a structurally weak region in posterior pole of eye). Axons become myelinated in optic nerve.
3 Groups of RGC:
W-Ganglion Cell: Small, ~40% of total, broad fields in retina, excitation from rods, detect direction movement.
X Ganglion Cell: Medium diameter, ~55% of total, involved in color vision.
Y Ganglion Cell: Largest, ~5%, respond to rapid eye movement or rapid change in light intensity. Transient response.
The ear regions
Outer ear – collects sound waves, and conducts sound to the tympanic membrane
Middle ear – (tympanic cavity)
That contains auditory ossicles, which plays an important role in transforming sound waves into mechanical vibrations that stimulates the inner ear (fluid medium).
Inner ear – encloses:
Cochlea which contains receptor for hearing
(Organ of corti),
Vestibular apparatus and semicircular canals which contains the receptors (maculae and cristae respectively) associated with maintenance of equilibrium.
External Ear
Composed of:
Auricle or pina,
External acoustic meatus /External auditory canal
Tympanic membrane/Ear drum.
External acoustic meatus /External auditory canal:.
The lateral 1/3 -cartilaginous
The medial 2/3 - bony
The skin contains Ceruminous glands (modified apocrine sweat glands) which produce cerumen = ear wax (pigmented lipid).
Tympanic membrane/Ear drum
Separating the external acoustic meatus from the tympanic cavity (middle ear).
Vibrates in response to sound.
Concave towards the external acoustic meatus (lateral surface) and bears a central depression, the umbo.
The handle of the malleus (ear ossicles) is firmly attached to the medial surface of the umbo.
When the membrane is illuminated through an otoscope, the concavity produces a “Cone of Light” which radiates anteroinferiorly.
Chorda tympani nerve crosses over the medial surface of the tympanic membrane; it has no function in the ear.
Innervation of the external ear
Auricle or pina-
Posterior and anterior aspect- Great auricular and lesser occipital nerve ( ventral rami of C2, C3- from cervical plexus)
External acoustic meatus /External auditory canal- Auriculotempral, a branch of CNV3 and a branch of Vagus nerve CN X.
Tympanic membrane/Ear drum-
External or lateral surface- Auriculotempral, a branch of CNV3, a branch of Vagus nerve CN X.
Internal or medial surface- a branch of glossopharyngeal nerve CN IX.
Middle Ear/ the tympanic cavity.
Middle ear connects :
Anteromedially- with the nasopharynx via pharyngotympanic/Auditory tube Posterosuperiorly- with the mastoid air cells via the mastoid antrum.
Contents:
Auditory ossicles/ bones (three) (lateral to medial):
1. Malleus (Latin. a hammer)
2. Incus ( L. an anvil) 3. Stapes (L. a stirrup)
2 muscles:
1. Tensor tympani
2. Stapedius
Roof (tegmental wall):
A thin plate of bone, called tegmen tympani
It separates the tympanic cavity from the dura matter of the middle cranial fossa.
Floor (jugular wall):
It separates the tympanic cavity from the superior bulb of the internal jugular vein
Carotid Wall (Anterior Wall):
Opening for the tensor tympani muscle tendon above
Opening of the Auditory or pharyngotympanic tube lies in the middle
The inferior part is separated from the internal carotid artery
Mastoid Wall (Posterior Wall):
Opening or aditus through which middle ear communicates with mastoid or tympanic antrum.
The tendon of the stapedius muscle passes through the junction of the posterior and medial walls.
Canal for CN VII facial n. descends between wall and antrum.
Lateral wall (membranous wall):
The tympanic membrane.
The handle of the malleus is attached to the tympanic membrane.
Medial Wall (Labyrinthine Wall): It Separate the tympanic cavity from the inner ear.
Features –
The promontory- a bulging produced by the first turn of the cochlea.
The oval window - closed by the foot plates /base of the stapes. Scala Vestibule of cochlea begins at oval window.
The round window- is inferior to the promontory. It is closed by a secondary tympanic membrane. Scala Tympani of cochlea terminates at round window.
The prominence of the facial canal just above the oval window, it transmits CN VII (Facial nerve).
