Special senses are…
confined to special sensory organs in the head
Eye and vision are the…
dominant sense in humans and most mammals
Visual system detects and interprets…
photic stimuli (visible light)
Visible light are electromagnetic waves between…
400 and 750 nm long
Eye is made up of…
- optical portion
- neural component
Optical portion focuses…
visual image onto receptor cells
Neural components:
transduce visual image into a pattern of neural discharges
Retina:
neural component and innermost tunic of wall in the eye
Retina has 5 types of neuronal cells:
- photoreceptors
- bipolar cells
- horizontal cells
- amacrine cells
- ganglion cells
Retina has a _____ of the visual signal from ______
convergence, photoreceptors to ganglion cells
Two types of photoreceptors:
- rods
- cones
Photoreceptors are attached to…
pigmented epithelium
Photoreceptors receive nutrients by…
diffusion from choroid
Function of photoreceptors:
transduce light to electrical signals
W/out light, photoreceptors…
release glutamate (excitatory neurotransmitters)
W/ light, photoreceptors…
undergo hyperpolarization and release of neurotransmitters are inhibited
Location of rods:
everywhere but optic disc and fovea
Rods:
- very sensitive
- best under reduced lighting
- responds to large range of light intensities
Type of images/vision that rods provide:
- night (scotopic) vision
- crude vision (low light discrimination of objects)
- black and white only
Rods have a ______ field
large receptive field that allow peripheral detection of movement
Loss of rod function leads to…
night blindness
Optic discs are…
axons of ganglion cells
Cones are only found in…
macula lutea and fovea centralis
Fovea centralis is the…
sharpest point of vision
Cones are only active at…
higher light levels
Cones provide…
- day (photopic) vision
- color discrimination
T/F: there are maximum convergence at cones
F, minimal convergence
- allows for sharp and detailed vision
Loss of all cone function leads to…
blindness
Color blindness is linked to…
- X gene deficit
- loss of one or more cone type
Functional anatomy of rods and cones:
- outer segment
- inner segment
- synaptic zone
Outer segment of rods and cones have:
- photo-sensitive transmembrane pigments absorb light rays
- leaky Na+ channels: intracellular messenger of cGMP gated channels
Outer segment of rods specifically have:
- rhodopsin (visual purple)
- 11 cis-retinal
- protein scotopsin
Outer segment of cones specifically have:
- blue, green, and red sensitive pigments
- 11 cis-retinal
- protein photopsin
Inner segment of rods and cones have:
- nucleus
- high [Na+/K+ ATPase pumps]
- lots of mitochondria
Synaptic zone of rods and cones have:
- VG Ca2+ channels
- vesicles w/ glutamate (excitatory neurotransmitter)
In absence of light, the retina…
- rhodopsin is intact (11 cis-retinal+opsin protein)
- membrane potential of photoreceptors: -40 mV
1. leaky Na+ channels open
2. VG Ca2+ channels open
3. steady release of glutamate
When exposed to light, the retina…
- rhodopsin absorbs photon
- conformational change initiated
- activates G-protein: G1 transducin
- Na+ channels close
- VG Ca2+ close
What happens when conformational change is initiated during the exposure to light?
- 11-cis-retinal changes to all trans retinal
- all trans retinal and opsin completely dissociates
Activation of G protein when retina is exposed to light leads to…
- the activation of cGMP phosphodiesterase
- cGMP gets converted to inactive 5’GMP
What happens when Na+ channels close during the exposure of light of retina?
- Na+/K+ ATPase pumps continues
- hyperpolarizes cell to -70 to -80 mV
What happens when VG Ca2+ channels close during the exposure of light of retina?
glutamate is prevented from being released
Recovery process of the retina:
- all trans retinal reconverted to 11-cis-retinal by isomerase enzyme
- reconverted 11-cis-retinal spontaneously recombines w/ opsin protein
- rhodopsin kinase
Rhodopsin kinase:
inactivates the activated rhodopsin
Vitamin A (all-trans retinol):
storage form for 11-cis-retinol
Night blindness is a result of…
a severe deficiency of vitamin A
Types of pathways of signal w/in the retina:
- direct pathway
- indirect pathway
Direct pathway:
- photoreceptor -> bipolar cell -> ganglion cell
- cone -> bipolar -> ganglion cell (fovea)
Indirect pathway:
- signal is modified along the pathway
- horizontal and amacrine cells
Types of bipolar cells:
excitatory or inhibitory
- depends on neurotransmitters released
Excitatory bipolar cells:
- depolarizes in response to glutamate
- direct excitation from rods/cones
- increases the frequency of ganglionic cell signals
- opens cation channels
Inhibitory bipolar cells:
- hyperpolarize in response to glutamate
- receives signal indirectly through horizontal cell
- decreases the frequency of ganglionic cell signals
- closes cation channels
Horizontal cells:
- part of indirect pathway
- output is always inhibitory (hyperpolarizing)
- provides lateral inhibition
- prevents spill-over effects of neurotransmitters
Amacrine cells:
- part of indirect pathway
- starts integration of visual signals
- starts to analyze signals
- depolarizing
Ganglion cells:
- only cells in retina that exhibit true all or nothing potential
- continuously fires AP’s
- rates are modified by photoreceptor inputs
- convergence determines the response of ganglion cells
Ganglion cells throughout the retina differ in:
- size of receptive fields
- proportion of rods/cones
- speed of conduction (axon diameter)
Ganglion cells have 2 basic patterns of receptive fields:
- on center/off surround
- off center/on surround
The 2 basic patterns of receptive fields are classified based on…
rate of AP output in response to light in center of field
Ganglion has three cell types:
- W
- X
- Y
W ganglion cells transmit…
rod vision
X ganglion cells transmit…
- visual image and color
- color analysis occurs in the retina
When x ganglion cells are stimulated by red, green, and blue cones…
white is seen
When x ganglion cells are stimulated by one cone color…
the other cone color inhibits it
Y ganglion cells transmit…
instantaneous changes in visual image
Visual pathway from retina to brain:
- myelinated axons of ganglion cells leave eye as optic nerve
- optic nerves meet at optic chiasma, but don’t synapse
- ganglion cell axons leave optic chiasma through optic tracts
- travels to primary visual cortex in occipital lobe
- travels to secondary visual cortex
Optic disc is where the ____ passes through the retina
optic nerve
- “blind spot” in visual field
At the optic chiasma, fibers from the nasal halves of the retina…
cross over
Fibers from the nasal halves of the retina carry info from…
temporal portions of visual fields
At the optic chiasma, fibers from the temporal halves of the retina…
stay on the side of origin
Fibers from the temporal halves of the retina carry info from…
nasal portions of visual fields
Optic tracts have two types of ganglion cells:
- magnocellular ganglion cells
- parvocellular ganglion cells
Magnocellular ganglion cells control…
movement
Parvocellular ganglion cells control…
fine detail
Ganglion cell axons in the optic tracts synapse in the…
lateral geniculate nucleus
Optic radiations synapse in the…
primary visual cortex
In the primary visual cortex, there is a large representation for…
fovea
Primary visual cortex is organized into…
columns
- specific to right/left eye
Lateral geniculate nucleus:
specific area for visual information
- highly mapped
- gates transmission
Output from lateral geniculate nucleus goes through…
optic radiations
Three types of columns in the primary visual cortex:
- simple cells
- complex cells
- rando ones: provide color information
Columns with simple cells respond to…
- bars of light
- lines
- edges in specific orientation
Columns with complex cells respond to…
linear/angular objects moving laterally across visual field
Secondary visual cortex:
analyzes information from primary visual cortex
- sends to association areas for identification
Secondary visual cortex is made up of…
stereopsis, which helps with binocular depth perception
Damage to any area in the visual pathway can lead to…
visual field defects
Pituitary tumor can compress fibers at optic chiasm, which causes…
tunnel vision
Lesions in secondary visual cortices causes…
visual agnosia: inability to recognize faces of friends, familiar objects, or colors