Excitable cells 19: sensory integration Flashcards
Describe the travel of information in the visual pathway
Retina -> LGN -> V1
V1 to MT via dorsal (parietal) pathway
V1 to V4 via ventral (temporal) pathway
3 junction between outer nuclear layer and ganglion cell layer where signal processing occurs
Outer plexiform layer
inner nuclear layer
inner plexiform layer
Where are these cells located and what is their function:
Bipolar cells
horizontal cells
Amacrine cells
Bipolar cells: transfer signals from photoreceptors to retinal ganglion cell (RGC) outputs
- allows 2 regions for signal processing (plexiform layers = dendritic processes)
Horizontal cells: inhibitory cells functioning at the photoreceptor / bipolar cell interface (in the outer plexiform layer)
- helps form receptive field of RGCs for detecting contrast
Amacrine cells: inhibitory cells functioning at the bipolar / retinal ganglion interface (in the inner plexiform layer)
- help form receptive field of RGCs for detecting motion
2 main types of retinal ganglion cells? Characteristics?
Parvocellular cells (neurons)
- small receptive field
- detect fine features
- colour sensitive
- low luminance sensitivity
- low motion sensitivity
Magnocellular cells
- large receptive fields
- not colour sensitive
- high luminance sentitivity
- high motion sensitivity
Characteristics of action potentials in
- on-centre cells
- off-centre cells
on-centre
- High freq. AP fired when light shines only on centre
- no AP if only surround is hit by light
- no AP if no light
- low freq. AP if light hits everywhere
off-centre
- High freq. AP fired when light shines only on surround
- no AP if only centre hit
- low freq. AP if light hits everywhere
- no light no AP
How are inhibitory surround cells mediated - what neurotransmitter is used?
Inhibitory surround mediated by horizontal cells
via GABA
Ligand and receptor in on and off-centre bipolar cell synapses?
Ligand = glutamate in both cases
mGluRs (metabotropic) in on-centre
AMPR receptors (ionotropic) in off-centre
What is special about the direction of inhibition in starburst amacrine cells?
Inhibition is produced in the direction of motion
Where do retinal ganglion cell fibres cross contralaterally? What are the implications?
55% in each eye cross at the optic chiasm
Means each cortical hemisphere sees the opposite visual field
Each cortical hemisphere receives inputs from right and left eye
Why do we need an optic chiasm?
1) Improve signal to noise
- dissimilar information between eyes - noise
- similar information - signal
2) So we see our hands on the same side as the part of the brain that controls it i.e the contralateral side
- improves speed of control
3) To provide retinal disparity through the separation of the 2 eyes for 3D vision
What is area V1? Characteristics?
The primary visual cortex
Retina is spacially mapped onto primary visual cortex
Cortical magnification of foveal region -> another reason we move our eyes
What are occular dominance colums in V1?
Alternating stripes across the visual cortex from left and right eyes
Equivalent representations from each eye brought together in V1
