Block 3 Flashcards
sclera
- tough, outer layer of the eye
- Gr. skleros, hard
cornea
- transparent, focusing apparatus of the eye
- performs 2/3 of the light bending
- is innervated by many nerve endings
lens
- performs the final 1/3 of light bending for proper focus
- shape controlled by the ciliary muscles
ciliary muscles
- control the shape of the lens
- by contracting they cause the lens to become more convex which sharpens the visual image on the retina
accommodation
The change in the shape of the lens is controlled by the ciliary muscles inside the eye. Changes in contraction of the ciliary muscles alter the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation.
what controls how much light enters the eye by adjusting the size of the pupil?
-the smooth muscles of the iris
aqueous humor
-fills in the space in front of the lens and between the iris and the lens
vitreous humor
fills in the space between the lens and the retina
what muscles control eye position in the socket?
-extraocular skeletal muscles
retina
- is part of the CNS and derived from neural tube
- laminated and 250 microns thick
- 3 primary cell layers arranged inside out
the pigmented epithelium behind the retina
- absorbs light to prevent backscatter
- recycles retinal as part of the transduction process
- possesses tight junctions and forms a blood-retina barrier
name the principle cells of the retina
- photoreceptors: rods and cones
- local circuit neurons: bipolar, horizontal, and amacrine
- projection neurons: ganglion cells
fovea
- point of the retina with the highest visual acuity
- it lies in the center of the retina (aka the macula )
- there is a higher density of photoreceptors there
papilla
aka optic disk
- where axons of ganglion cells are bundled to form optic nerve
- since no photoreceptors are there, there is a blind spot
direct vertical pathway
-from photoreceptor to bipolar to ganglion cell
indirect lateral pathway
-involves horizontal spread of information through local circuit neurons, the horizontal and amacrine cells
what is the overall convergence ratio of rods and cones to ganglion cells
100 to 1
what photoreceptor dominates at the fovea?
cones
name the ways to increase visual acuity
- absence of intervening cells
- increased density of photoreceptors
- high concentration of cones
- low degree of convergence from photorecetors to ganglion cells
visual field
-part of the environment impinging upon the whole retina at any one time
binocular field
-subset of the visual field that impinges upon both eyes at the same time
monocular field
-parts of the visual field outside of the overlap of the binocular field
retinal field
- reversed image of the visual field that is projected onto the retina
- medial part: nasal field
- lateral: temporal field
receptive field
-part of the retina that when stimulated by light alters the electrical activity of a neuron
what is the photopigment in rods
-rhodopsin
what is the photopigment in cones?
-photopsin: either blue, red, or green
rhodopsin
- an aldehyde derivative of vitamin A
- is sensitive to electromagnetic radiation from 400-700 nm
photoreceptors release transmitter from their ____ onto bipolar cells
-synaptic endings
what photoreceptor is responsible for night vision
-rods due to large outer segments, and more photopigment
in the dark, rhodopsin is ____, ___ is high, and ion channels are open
-inactive, cGMP is high, ion channels are open
in the light, what happens in the outer segment of the photoreceptor
- light bleaches rhodopsin
- opsin decreases cGMP
- Na+ channels close and the cell hyperpolarizes
on center ganglion cells
- increase activity when the center of its receptive field is illuminated and decrease activity when its surround is illuminated
- responds best to rapid increases in illumination intensity and to objects brighter than the background
ganglion cells show ___ as a result of ____ in the circuit
-center-surround antagonism as a result of lateral inhibition
on-center ganglion cell
-increases activity when the center of its receptive field is illuminated and decreases activity when the surround is illuminated
off center ganglion cells
-decrease activity when the center of its receptive field is illuminated and increases activity when its surround is illuminated
responds best to: rapid decreases in illumination intenisty and to objects that are darker than the background
in the dark, glutamate released from the photoreceptor _____ the off-center bipolar cell and _____ the on-center bipolar cell
- depolarizes the off-center bipolar cell
- hyperpolarizes the on-center bipolar cell
how is surround antagonism for cones achieved?
-lateral inhibition from horizontal cells
in the dark horizontal cells are ____ by ____ which is released by photoreceptors
depolarized by glutamate
depolarized horizontal cells release ___ onto the __-synaptic terminals of _____
release GABA onto the presynaptic terminals of photoreceptors
retinal ganglion cells: M cells
- magnocellular
- large cell bodies, dendritic trees and axons
- large receptive field sizes and are slowly adapting
- sensitive to: contrast illumination, large objects, and movement
retinal ganglion cells: P cells
- parvocellular
- small cell bodies, dendritic trees, and axons
- more numerous with small receptive fields
- rapidly adapting
- specially sensitive to: fine spatial resolution, form, and color for slow events
what thalamic subdivision is dedicated to relaying visual information to the cortex?
-lateral geniculate nucleus