lecture 15- visual system sensory III Flashcards
Optic disk (blind spot)
area where optic nerve and blood vessels leave the eye
lens
bends light to focus it on the retina
Zonules
attach lens to ciliary muscle
pupil
changes amount of light entering the eye
retina
layer that contains photoreceptors
sclera
is connective tissue
how does light enter the eye
the cornea and lens focus light rays onto the retina
specialized cells in the retina (photoreceptors) transduce light energy into an electrical signal
a network of neurons collect electrical signals to be transmitted along the optic nerve to the brain
how is light refracted when it enters the eye?
refracted twice
2/3 cornea (large difference in refractive index)
1/3 by lens (large curvature)
accomodation
the lens can change its shape to focus onto the retina
(due to contraction and relaxation of the ciliary muscle
ciliary muscle
a right of smooth muscle surrounding the lens
presbyopia
loss of accommodation due to loss of elasticity
what happens when ciliary muscle is relaxed
the lens is flattened
–> more distant objects are focused on the retina
when ciliary muscle contracts
the lens is rounded
–> closer objects are focused on the retina
concave lens
scatters light rays (diverges)
convex lens
light rays diverge
Light from center of field of view is focused on the
fovea
area of most acute vision and center of visual field
fovea and macula
does the fovea have neurons or blood vessels?
no
Rods and cones
sensory cells (photoreceptors)
- transduce light energy into electrical signal
- only produce graded potentials
Bipolar cells, amacrine cells, horizontal cells
- connect sensory cells to transmitting cells, process and integrate information, converge signals from several photoreceptors
Ganglion cells
transmitting cells (output to CNS)
- carry information via the optic nerve to the brain
- produce action potentials
info from —- rods and cones converges on —– ganglion cells
100 million rods and cones
1 million ganglion cells
the size of the receptive field depends on…
the location on the retina
compare ganglion cell receptive fields in the periphery vs in the fovea
ganglion cells in the periphery have large receptive fields
ganglion cells in the fovea have very small receptive fields
Rods are responsible for
low light/night vision
rhodopsin
is a photopigment that is the signal transducer
Cones are responsible for
sharp vision and colour vision
3 types of cones
each has a different photopigment (related to rhodopsin) and responds to specific colour (red, blue, green)
what does light do to rod and cone cells?
it hyperpolarizes them
What is rhodopsin? 2 components
opsin + retinal
opsin is the GPCR
retinal is the visual pigment
when opsin and retinal are tightly bound, rhodopsin is
inactive
disks are the location of —- in rods
signal transduction
rods and cones transduce
light photons into electrical currents
rods and cones project to
bipolar cells
bipolar cells synapse with
ganglion cells
ganglion cell axons from the
optic nerve and project to the CNS
Phototransduction in rods
(in darkness)
7 steps
- retinal and opsin are bound
- rhodopsin is inactive - cGMP levels inside the cell are high
- CNG channels are open
- rod cell is depolarized (-40mV)
- V-gated Ca2+ channels are open
- Ca2+ goes into the cell
- NT (glutamate) is released
phototransduction in rods
(in the presence of light)
9 steps
-retinal phtotoisomerizes (11-cis –> trans) causes a conformation change in the intracellular C-terminus of rhodopsin
- rhosopsin is activated (retinal is released)
- activates associated G protein: transducin
- transducin activates phosphodiesterase
- phosphodiesterase breaks down cGMP
- decreased [cGMP] –> CNG channels close
- cell hyperpolarizes
- voltage gated Ca2+ channels close
(less Ca2+ into cell) - Decreased NT (glutamate release)
- adjacent bipolar cell and ganglion cell are excited
