Physiology Of Sight Flashcards
What information do the rods in the eyes pass to the CNS?
They tell the CNS about the absence or presence of photons without regard to wavelength
Where does the transduction of light energy into a receptor potential occur in the photoreceptors?
The outer segments of both rods and cones
Describe the plasma membrane in the outer segment of the cones.
Folds back and forth in a pleated fashion
Describe the plasma membrane in the outer segment of the rods.
The pleats formed pinch off from the plasma membrane to form discs - around 1000 stacked discs piled in each outer segment
How are the outer segments of the rods renewed?
One to three new discs are added to the base each hour, while old discs are court off the tip and phagocytosed by pigment epithelial cells
What is contained in the inner segment of the photoreceptors?
Mitochondria
Golgi complex
Nucleus
Describe the proximal end of the photoreceptors.
Expands into a bulblike synaptic terminal filled with synaptic vesicles
What is the first step in signal transduction?
Absorption of light by a photopigment
What is a photopigment?
A coloured protein that undergoes structural changes when it absorbs light in the outer segment
What is the photopigment found in rods?
Rhodopsin
What are the photopigments in the outer section of the cones?
Three different types - one for each colour
- colour vision comes from selective activation of these pigments
What do all photopigments in the eye have in common?
Their two constituent parts
- Opsin (glycoprotein)
- retinal (pigment)
Out of retinal and Opsin, which part of photopigments varies depending on cone colour or rod?
Opsin (4 different types)
- small variations in amino acid sequence of the different opsins permit the rods and cones to absorb different wavelengths of light
Describe retinal.
Vitamin A derivative formed from carotene
This is the light absorbing party of all visual photopigments
What is the shape of retinal in darkness?
Retinal has a bent (cis-retinal) shape which fits into the opsin portion of the photoreceptor
What happens to cis-retinal when it absorbs a photon of light?
It undergoes isomerisation - where it straightens out to become trans-retinal
What happens after isomerisation of trans-retinal?
Several unstable chemical intermediates form and disappear - leading to the production of a receptor potential
What happens to the retinal after it has been straightened.
It detaches from the opsin in a method known as bleaching (because the final products look colourless)
What is the function of retinal isomerase?
It converts trans-retinal back into cis-retinal, so it can bind to opsin (reforming a functional photopigment) in a process called regeneration
How does the pigmented area contribute to regeneration?
It stores a large quantity of vitamin A for the retinal in the rods
Describe the release of neurotransmitters by the photoreceptors in darkness.
1) cGMP gated sodium channels open
2) sodium influx
3) membrane potential increases (partial depolarisation)
4) causes constant glutamate release at the synaptic terminals and inhibition of the bipolar cells
Describe neurotransmitter release during light transduction.
1) isomerisation of retinal activates an enzyme that breaks down cGMP
2) cGMP gated sodium channels close
3) sodium inflow slows down
4) hyperpolarisation induces a receptor potential
5) this turns off neurotransmitter release, exciting the bipolar cell
What happens to glutamate release in dim light?
Small and brief receptor potentials are activated that only partially turn off glutamate release
How many rods synapses with a single bipolar cell?
6-600 depending on the area
- this increases the light sensitivity of rod vision, but slightly blurs the perceived image
How many bipolar cells do cones synapse to?
Normally just one
- this is less sensitive to light, but yields a sharper image
What happens to the cone bipolar cells when light enters the eye?
They can either be excited or inhibited depending on the cone they are attached to
What is the function of the lateral inhibitions by horizontal cells?
It sends inhibitory signal a to bipolar cells in the area lateral to the excited rods and cones
It does this because it enhances contrast of the visual scene between the areas that are strongly stimulate and adjacent areas that are more weakly stimulated
What do amacrine cells do?
These are excited by bipolar cells, synapse with ganglion cells and transmit information to them that signals a change in the level of illumination of the retina
This (and excitement of the bipolar cells) depolarises the ganglion cells and initiate nerve impulses
Where do the axons of the optic nerve terminate?
In the lateral geniculate nucleus of the thalamus
What does the optic nerve axons synapse with in the lateral geniculate nucleus?
Neurons whose axons form the optic radiations
Where do the optic radiation a project to?
The primary visual areas in the occipital lobes and visual perception begins
What are some of the other fibres in the optic tract that synapse with the optic nerve in the lateral geniculate nucleus of the thalamus?
Some project to the superior colliculus - control extrinsic eye muscles
Other projects to the pretectal nucleus - controls pupillary and accommodation reflexes
What is a visual field?
Everything that can be seen by one eye
- the two visual fields overlap considerably
What are the two regions the visual field can be divided into?
Nasal and temporal halves
A.k.a. Central and peripheral halves
Where do light rays from the nasal and temporal parts of the visual field fall in the retina?
Light from the nasal part of the visual field falls on the temporal part of the retina
Vice versa
Which side of the brain does visual information from the right half of each visual field travel?
The left
Which side of the brain does visual information from the left half of the visual field travel to?
The right side
At the optic chiasm, what happens to axons from the temporal half of the retina?
They do not cross
At the optic chiasm, what happens to axons from the nasal half of the retina?
They cross the optic chiasm and travel to the opposite side of the brain for interpretation
What are the things processed by different systems to produce visual signals?
Shape of objects
Colour of objects
Movement, location and spatial organisation
