Case 3 Flashcards
What bones form the lateral wall of the orbit?
Frontal process of zygomatic and greater wing of sphenoid.
What bones form the medial wall of the orbit?
Orbital plate of ethmoid, contributions from frontal process of maxilla, lacrimal and sphenoid.
What bones form the superior wall of the orbit?
Mainly orbital part of frontal bone and lesser wing of sphenoid.
What bones make up the inferior wall of the orbit?
Mainly maxiall, partly by zygomatic and palatine bones.
Where are the lacrimal glands located?
Superior, outer portion of each orbit in the lacrimal fossa.
What is the general structure of the Lacrimal gland?
Size and shape of an almond, divided into two parts, a larger orbital part and a smaller palpebral part, these are seperated by the levator palpebrae superioris muscle.
What is the path of lacrimal secretions?
Glands drain into 6-12 excretory lacrimal ducts, which empty into lacrimal canals. it moves medially over the eyeball and accumulates at the medial aspect of the eye in a lacrimal lake where it passes into a superior or inferiorlacrimal punctum to enter the superior or inferior lacrimal canaliculus. These unite to form the lacrimal sac, which is compressed during blinking to empty into the nasolacrimal duct. This opens into the nasal cavity at the inferior nasal meatus.
What is spherical aberration?
the rays do not focus after the lens in one focal point, Peripheral rays are more strongly refracted than paraxial, may be due to peculiar curvature of cornea or lens or the iris blocking peripheral rays.
What is chromatic aberration
Failure of lens to focus all colours into one focal point.
What are the four steps of phototransduction?
1) Activation of rhodopsin to its active form metarhodopsin II 2) Metarhodopsin activates transducin causing GDP to be replaced by GTP and the alpha-subunits seperate 3) Subunits activate phosphodiesterase which converts cGMP to GMP 4) Closes Na+ and some Ca2+ channels leading to hyperpolarisation.
What is the resting membrane potential in the dark?
-40mV
What potential is reached when photoreceptor cells are excited?
-70/80mV
Why does glutamate release decrease with hyperpolarization of the photoreceptor cells?
Calcium levels drop and calicum is required for glutamate-containing vesicles to fuse with cell membrane and release their contents.
In the dark what does glutamate bind to once released by photoreceptor cells and what effect does it have?
metabotropic glutamate receptors (mGluR6) on bipolar cells. Causes non-specific cation channels to close leading to hyperpolarization so no nerve impulse is produced in bipolar cells.
What is macular sparing?
When the central 5-10 degrees of the visual field in an otherwise hemianopic defect, usually associated with damage to the cortex.
