vision Flashcards
Review: tongue
taste cells (roughly 100) are composed of a taste bud. taste buds are on papillae. more taste buds = stronger discernment of taste
Review: taste cells
NON-NEURONAL but can transduce molecules in food to electrical signals that synapse onto gustatory aferent axons via chemical synapses.
Neurotransmitter mechanism is unknown.
Most taste cells respond to >tastants
Review: olfactory neurons
cilia protrude into mucus layer that senses volatile molecules and transduce that signals to the olfactory bulb (through specialized epithelium, and pores in the cribiform plate). Utilizes special GPCR that promotes Na+ and Ca2+ influx as well as Cl- efflux
Review: olfactory sensory neurons
Neural encoding of odorants, the role of olfactory interneurons and the transduction of smells by the olfactory receptor cells -> olfactory bulb -> many brain regions (thalamus/hypothalamus/amydala) -> olfactory cortex
Properties of light
reflection, absorption, and refraction
Anatomy of eye
pupil, sclera, iris, cornea, and optic nerve
Pupil
opening where light enters the eye (the black center)
Iris
gives color to eye
sclera
white of the eye
cornea
glassy transparent external surface of the eye (the part you put your contacts on)
optic nerve
bundle of axons from the retina (behind the eye)
refraction of light by the cornea
incoming light is refracted, focused on the retina, and transduced into an image
fovea
a small depression in the retina of the eye where visual acuity is highest. Center of field of vision is focused in this region, where retinal cones are particularly concentrated
does sharp central vision (useful for things like reading and driving)
Direct (vertical) pathway of retinal signal transduction
photoreceptors -> bipolar cells -> ganglion cells
Retinal signal transduction
retinal processing influenced by: lateral connections, ie: horizontal cells and amacrine cells
- the only light-sensitive cells in retina are photo receptors EXCEPT for intrinsically photosensitive retinal ganglion cells
- the ganglion cells are the only source of output from the retina
Horizontal cells
receives input from photoreceptors and projects laterally to influence surrounding bipolar cells and photoreceptors
amacrine cells
receives input from bipolar cells and projects to ganglion cells, bipolar cells, and other amacrine cells
photoreceptor structure
converts light to neural structure via the four main regions: outer segment inner segment cell body synaptic terminal
types of photoreceptors
rods and cones (“classic” retinal ganglion cells)
intrinsically photosensitive retinal ganglion cells (ipRGCs)
rods
100 million in the human retina
1 type
exquisitely sensitive to a photon of light
more discs = more sensitivity
responsible for night vision (scotopic conditions)
contains rhodopsin (retinal+opsin)
cones
6 million in the human retina
3 types (L = long wave; M = medium waves; and S = short wave)
less sensitive to light, but faster to respond
responsible for daytime color vision (photopic conditions)
contains photopsin
photopsin
retinal and an opsin variant (distinguished by its absorption spectrum, rhodopsin is red when there is no light, reason why our eyes get redder)
phototransduction
photons leading to closing of constitutively active cation channels in photoreceptor outer segments
this causes hyperpolarizing potential and decreasing neurotransmitter release
single photon amplification
a rhodopsin molecule that absorbs a photon has 67% chance of being photoisomerized, which triggers electrical response with >80% reliability
each R molecule activates 100 molecule of transducin
2 T per one phosphodiesterase
1 phosphodiesterase capable of hydrolyzing >1000 molecules of cGMP per second
cGMP sink closes hundreds of the ten thousand channels that are open; this causes 10 MILLION na+ ions to be prevented from entering outer segment
**TL;DR: tight control of amplification process, amplifies v big
mechanism of phototransduction
light absorbing retinal (aldehyde form of vit A undergoes conformatoinal change from absorbing photon of light from 11-cis retinal to all-trans retinal)
retinal is covalently attached to opsin and because all-trans isomer fits differently inside opsin (specifically not in 11-cis binding site) opsin undergoes conformational change
this change triggers visual transduction by way of GPCR
regional differences in retinal structure
cones found primarily in central retina near fovea
central retina has lower ratio of photoreceptors to ganglion cells = better resolution
peripheral retina has higher ratio of photoreceptors to ganglion cells = better at detecting dim light
dark adaptation
dark:
takes roughly 20 minutes to stabilize, pupils dilate - more light enters eye and hits retina; regeneration of unbleached rhodopsin
can increase sensitivity to light by >1,000,000x
light adaptation
~5 minutes to stabilize
pubils constrict - less light reaches retina
cones transition from maximally hyperpolarized to gradually depolarized
Na+/Ca2+ channels gradually reopen, establishes new baseline; absolute light level is lost but relative changes can now be sensed
ON and OFF bipolar cells
ON bipolar cells: GPCR, responds to glutamate by hyperpolarizing and releasing less glutamate
OFF bipolar cells: ionotropic (glutamate gated Na+ channel); responds to glutamate by depolarizing and releasing more glutamate
retinofugal projection
neuronal pathway that projects away from retina
image forming vision
right and left hemifields
small number of optic tract axons connect to hypothalamus
~10% form connections in the midbrain
most ~88% innervate lateral geniculate nucleus (LGN) of the dorsal thalamus
neurons in LGN project to primary visual cortex via optic radiation
macular degeneration
degeneration of macula, including fovea and surrounding region
loss of central region
retinitis pigmentosa
heretidary disease, initially affects rods and peripheral vision
progresses to cones and then blind
lateral geniculate nucleus (LGN)
little or no information process beyond this structure done in retina
two hypothesis as to why this does information processing:
- brings retinotopic maps from both eyes into register to make easy for the cortex to combine inputs from both eyes
- only 10% of inputscome from the retina, and the other 90% are modulatory inputs from the cortex and brainstem - if instead all of these modulatory inputs fed back to the retina you’d have a blind spot 10x bigger due to huge increase in neuronal matter
retinal input into lgn
structure in each hemisphere receives input from temporal retina of ipsilateral eye and the nasal retina of contralateral eye
dorsal pathway
visually guided movement
ventral pathway
object recognition
what is the role of the inferior temporal cortex
facial recognition
