What is the structure of the retina?
Laminated layers of cells with photoreceptors at the bottom
What is the structure of the fly retina?
- Many different lenses with similar laminated retina
- Leads to a lower spatial resolution (pixilated)
What can the larvae visual system detect?
Detect colour and light intenisty but not images
What are the structural components of rods and cones?
- Cell body
- Projection
- Mass filled with pigments
Describe the effect of light on a photoreceptor
Induces second messenger cascade
- Rhodopsin changes confirmation
- G protein activates and cleaves cGMP from Na channel
- channel closes and cell hyperpolarises
What are the two types of bipolar cell?
Sign conserving (off) and sign inverting (on)
How do bipolar cells transmit an off signal?
Ionotropic
Decreased glutamate transmission results in glutamate gated Na channels closing
How do bipolar cells transmit an on signal?
Metabotropic
- Less glutamate means less activation of mGluR
- Then mGluR cannot maintain low cGMP levels allowing it to open cGMP dependent Na channels
- Cell depolarises
What are the characteristics of receptor ganglion cells?
- Spiking
- Have receptive fields
What spiking activity is seen in a RGC with an on-centre off-surround receptive field when:
a) no light
b) light at on-centre
c) light on entire receptive field
d) light at off-surround only
a) baseline firing
b) maximum firing
c) baseline firing
d) no firing
What are the characteristic of horizontal cells?
- Connect to surrounding cones of a receptive field
- Receive excitatory inputs and emits inhibitory inputs
What happens when the on-centre is iluminated?
- Centre cone is hyperpolarised and surrounding cones are depolarised
- Strong depolarisation from surrounding cells means that horizontal cells receive a strong input and so send strong inhibition
- This means even less neurotransmitter is released from the centre cone and so the biopolar cell becomes very depolarised
What happens when the entire receptive field is illuminated?
- All of the cones are hyperpolarised so horizontal cells do not provide much inhibition
- Bipolar cell only moderately depolarised
What happens when the off-surround of the receptive field is illuminated?
- Surround cones are hyperpolarised while centre cone is depolarised, therefore there is little input to the horizontal cell and little inhibitory input
- Depolarised centre does not depolarise bipolar cell
What is the difference between an on centre and an off centre receptive field?
Off centre - off bipolar cell (sign conserving)
On centre - on bipolar cell (sign inverting)
Why are the non-spiking inputs converted into spiking?
In order to travel long distances without decay
What pathway does input to the retina take in the brain?
Via LGN to primary visual cortex
What are the characteristics of LGN neurons?
Also have field properties with several different types of ganglion cell
What are the characteritics of the primary visual cortex?
- Where initial visual processing occurs
- Left and right eye are processed by alternating Hubel and Weisels columns
- Other columns reflect processing channels
What are the 3 different processing channels of the primary visual cortex? What do they process?
- M channel (magnocellular) - motion
- P-IB channel (Parvocellularinterblob) - shape
- Blob channel (Parvo and koniocellular) - colour
Describe the cells of the M channel?
- IVC alpha contains ‘simple’ cells that respond to slits of light and dark with on/off fields that are selective to orientation
- IVB cells are similar but can respond to both eyes
Describe the cells of the P-IB channel
- Layer III has ON/OFF field, highly responsive to orientation and small fields for object shape
- Neighbouring cells respond to different orientations
What are the cells of the blob channel responsive to?
Different wavelengths
What are the mechanisms behind direction sensitivity?
- When moving in one direction the response of the first cells is delayed and so arrives at the same time as the rest causing a summation of depolarisation
- When going in reverse responses arrive at different times and so do not reach threshold
