Role of superior colliculus
- Receives 10% of visual input
- Integration with other sensory inputs
- Controls saccadic eye movement
- Descending motor output controlling neck muscles
Role of pretectal nucleus in the midbrain
Bilateral input to Edinger-Westphal nucleus providing parasympathetic input to pupil- control of light level
Where does the optic nerve synapse
- The Lateral geniculate nucleus
- Information from ipsilateral eye synapse in layers 2,3&5
- Contralateral layers 1,4&6
- Rods terminate in 1&2
- Cones in 2,3&6
Which portion of the retinas decussate
Nasal portion (at optic chiasma)
Role of pupillary light reflex
Change size of pupil in response to changing light levels
What muscles control the size of the pupil
Expand - dilator pupillae
(Sympathetic)
Contract- sphincter pupillae
(Parasympathetic)
Anatomical pathway of pupillary light reflex
- Photoreceptors
- Optic nerve
- Skips out LGN but synapses in the pretectal area in the midbrain
- This info splits to both sides of the brain and synapses in the Edinger-Westphal nucleus
- Info travels back via the occulo-motor nerve to the ciliary ganglion
- Eye muscles
What is the function of the accommodation reflex
To focus light onto the Fovea - making the image in focus
What are the three processes that occur?
- Ciliary muscles contract reflexively and lens becomes convex (+ refractive power)
- Convergence of two eyes
- Pupillary constriction improves focus by reducing aberration and increasing depth of focus
Pathway for accommodation reflex
- Visual cortex sends signal to pretectal area that image if out of focus
- From pretectal to EW nucleus
- From EW via O-M nerve to ciliary ganglion
- Then to muscles in eye
How do photoreceptors transducer light stimulus
Absorption of light protons by photopigments or photopsin (photo proteins)
What substance do rods use to transducer light
- Rhodopsin
2. 2- component protein that triggers signalling pathway
What substance do cones use to transducer light
- Several different photopigments with different absorption maxima
- Therefore sensitive to different wavelengths of light (colour vision)
Describe the structure of a photoreceptor
Outer segment
- Na+ channels gated by cGMP
- cGMP binds to cytoplasmic side of channel which allows Na+ influx
Inner segment
- non-gated K+ channel
- steady intracellular concentrations of Na+ and K+ maintained by Na/K pumps
In the dark potential sits at -30mV
Describe a photoreceptor in the dark
- cGMP gated Na+ channel is open
- Na+ enters cell
- Cell depolarised
- Glutamate release
- Hyperpoalrises ‘on’ bipolar cell
- Depolarises ‘off’ bipolar cell
Define a receptive field
Area of the retina that will evoke a response from a bipolar cell
Describe the input to bipolar cells
- Direct input from photoreceptors at centre of receptive field
- Indirect input from surrounding photoreceptors via horizontal cell
- Light hitting centre = opposite of light hitting surround
