Sensory physiology- photoreception Flashcards
What are the 4 major types of animal eyes?
Flat sheet
Convex
Cup-shaped
Vesicular
Describe the flat sheet eye
Contain a layer of photoreceptor cells that form a primitive retina
Provide some sense of light direction and contrast
The pigment layer contains shading pigment that helps provide directional information by shading light coming from some directions
Describe the cup-shaped eye
Retina is folded to form a narrow aperture
Similar to pinhole camera where resolution is poor and image is dim
Describe the convex eye
Photoreceptors radiate outwards from the base forming a convex light-gathering surface
Describe the vesicular eye
A lens is inserted into the pinhole aperture
Lens collects light from multiple sources and retracts it thus focusing it onto the retina
The lens is able to change shape allowing it to focus on objects
Describe the structure of the human eye
Light entering the eye passes through the cornea, the aqueous humour, the pupil, the lens, and the vitreous humour before striking the retina
Describe the parts of the retina
The fovea and the choroid
What is the fovea?
A pit in the retina which provides the clearest vision
What is the choroid?
The vascular layer of the eye which supports the retina with blood
Describe the process of accommodation
The lens is connected to the round ciliary body (muscle) by fibres called zonules
When the ciliary muscle is relaxed, the zonules are stretched thereby flattening the lens
When accommodated to a nearby object, the ciliary muscle contracts thereby relaxing the zonules and the lens will have a rounder shape
What is visual acuity?
Clarity of eyesight
What are the 2 main factors that affect visual acuity?
Optical and retinal factors
Give an example of an optical variable of visual acuity
The diameter of the pupil (smaller=higher acuity)
Give an example of a retinal variable of visual acuity
The density of photoreceptors and retinal ganglion cells and their ratio
Where is acuity at its max?
In the foveal centre and decreases rapidly towards the periphery
Describe the structure of eagles eyes
The eyes occupy 50% of the skull
The eagle can voluntarily alter the curvature of its lens and cornea thus changing focus
Oxygen is supplied through a structure called the pecten which increases the visual acuity by allowing more rods and cones to be packed in
Have 4-5x the visual acuity of humans
They have 2 foveae per eye to allow for monocular vision
A trench of cones between these foveae allow for horizon scanning without moving the head
They can spot an ant from a ten-story building
What is the purpose of the pigmented epithelium in the retina in vertebrates?
To absorb the light to prevent photo-oxidative stress
Where are the photoreceptors located?
In the deepest layer of the retina
What are the photoreceptors connected to?
Bipolar cells
What are the bipolar cells connected to?
Ganglion cells
What are the 4 important components for signal processing in the retina?
Horizontal cells and amacrine cells
Bipolar cells and ganglion cells
What does the retinal pigment epithelium do?
Provides metabolic and supportive functions to the photoreceptors
What does the outer nuclear layer contain?
The photoreceptor cell bodies
What does the outer plexiform layer contain?
It’s where the photoreceptor, horizontal, and bipolar cells synapse
What does the inner nuclear layer contain?
It’s where the bipolar, amacrine and retinal ganglion cells synapse
What does the retinal ganglion cell layer contain?
The retinal ganglion cell bodies
What does the optic nerve layer contain?
The ganglion cell axons travelling to the optic disc
Describe the organisation of the retina is cephalopods
A single layer of photoreceptor cells and supporting cells
Light strikes the photoreceptors directly without passing through multiple layers
No interneurons thus no signal processing in the retina
Did the cephalopod and vertebrate eye evolve independently?
Yes
Describe rods
Contain many photopigments and are very sensitive
Describe the conditions and results of light hitting rods
Active under dim light and can’t discriminate colours
Describe cones
Less photopigment
Describe the conditions and results of light hitting cones
Can detect colours and active under stronger light
What is the photopigment in rods called?
Rhodopsin
What is the light receptor in rods called?
11-cis retinal
How does 11-cis retinal work?
It absorbs a photon of light and rotates to form all-trans-retinal
What is rhodopsin in its constituent parts?
11-cis retinal and the protein opsin
Where is rhodopsin located?
In the membrane disks of the rod photoreceptors
Describe the state of vertebrate photoreceptor cells when unstimulated
Vertebrate photoreceptor cells are depolarised when unstimulated
Describe the state of vertebrate photoreceptor cells when stimulated with light
They are hyperpolarised
What does a stronger light stimulus do to the membrane potential in vertebrate photoreceptor cells?
A stronger light stimulus leads to a more negative membrane potential
When do photoreceptor cells release transmitter?
In the dark
What is the transmitter photoreceptor cells release?
Glutamate
Describe the process of transduction
In the absence of light Na+ channels are open and create a depolarising dark current
Rhodopsin absorbs light energy
This activates a G protein (transducin) that activates PDE
Activated PDE hydrolyses cGMP causing Na+ channels to close and the cell to hyperpolarise
What is a receptive field?
Where 1 bipolar cell is connected to more than one photoreceptor in the retina
What is the benefit of receptive fields in the retina?
There are many more photoreceptors than neurons in the retina
What are the 2 types of receptive fields?
OFF and ON centre
How are OFF and ON centre receptive fields categorised?
By the type of retinal ganglion cells beneath the photoreceptor layer
Describe ON centre, OFF surround RGCs
They respond to light stimuli in the photoreceptors surrounding the centre of the receptive field
Describe OFF centre, ON surround RGCs
They respond to light stimuli in the photoreceptors at the centre of the receptive field
How are ON and OFF centre RGCs physiologically distinct?
They respond to light falling on their receptive field differently
How are ON and OFF centre RGCs anatomically different?
The dendrites branch out in separate sub-layers of the retina and they receive synaptic inputs from different subclasses of bipolar cells
Where do the signals from the RGCs go?
They remain segregated in the retina and thalamus and then merge in the primary visual cortex
Describe lateral inhibition in an ON centre RGC
The action potentials arrive at the lateral geniculate nucleus (LGN) in the thalamus
Light hitting a photoreceptor in a receptive field leads to hyperpolarisation of all photoreceptors that receive a photon
In an ON centre RGC, if there is a concentration of light at the centre of the receptive field, negative feedback signals in the horizontal cells depolarise the surrounding photoreceptor cells and further hyperpolarise the centre cell
Combining lots of these RGCs emphasises edges over block colour
What is the mach band effect?
The combined effect of lateral inhibition across multiple receptive fields emphasises edges
The contrast at the edges of each neighbouring band is increased
The illusion makes the bands appear slightly concave
Why do we need multiple cone receptors?
Multiple cone receptors are an evolutionary advantage to allow better discrimination between objects
What wavelengths of EM radiation can humans detect?
Between 350nm and 750nm
How many colours can humans detect?
10 million
What is trichromatic colour vision
Only have 3 specialised cones- blue, green and red
