Retina: OPL Flashcards
What are the 2 synaptic layers in the retina?
- Outer plexiform layer
- Inner plexiform layer
What does the outer plexiform layer contain?
contains the synaptic endings of rods (spherules) and cones (pedicles)
-these synapse mainly with horizontal and bipolar cells
What is the rod synaptic ending?
- is smaller and spherical
- one invagination
- known as spherules
What is the cone synaptic ending?
- synaptic region of cone is quite big
- several invaginations
- known as pedicle
What is the main characterisitc of cone pedicle ?
-full of vesicles (vesicles of the neurotransmitter)
What does each pedicle contain?
- Each pedicle contains ca. 25 invaginations
- each of which is associated with a synaptic ribbon
- these synaptic ribbon are associated with the halo vesicle
- contains 3 synpases
- bigger than rods synaptic ending
What do the synaptic ribbon do?
that may serve to guide vesicles to their site of release
What is each invagination associated with?
-Each invagination is also associated with a ‘triad’
What is a triad?
3 post synaptic processes
What do the lateral elements of a triad are provided?
come from horizontal cells
What do the central elements of a triad are provided by?
the less deeply invaginating central element comes from a bipolar cell
What does a electron micrograph of a cone pedicle show?
invaginations (i.e2) and associated ‘triads’ (others are out of plane of section)
What does a cone pedicle make contact with?
- triads made up of 2 lateral elements from horizontal
cells and central element from bipolar cells
however not all post synaptic processes on pedicles are triads.
What are other post synaptic contacts that dont associate with cone pedicle?
There are about 500 superficial (non-invaginating) contacts which come from bipolar cells in each cone
What are the synapses in the triad of cone pedicles?
have horizontal cells and bipolar cells
and then superficial contact with bipolar cells
What is the final contact a cone pedicle makes?
- with neighbouring cone pedicles and rod spherules via ‘telodendria’. - where they contact adjacent photoreceptors (rods and cones) via electrical synapses
- cones are able to influence surrounding photoreceptors via telodendria
- These are electrical ‘gap’ junctions
What is the principal of uni-variance?
photoreceptor only responded to light falling directly on that photoreceptor
-however now know this is not so, and that receptors can interact and neighbouring cones can facilitate the response of their neighbours
What can Photoreceptors can influence ?
the electrophysiological response of neighbouring receptors.
How can we illustrate that receptors can interact and neighbouring cones can facilitate response of their neighbours ?
- cones
- put electrode in one cone
- shine small spot of light
- get a given hyper-polarisation
- if principle invariance was true in cones- if we stimulated retina with bigger spot- the size of response should be the same-would give the same response to a large and a small spot.
- however that is not true
- a big spot gives a much physiological/ larger response, and that can be explained by neighbouring cones must have an excitatory input (they facilitate response of photoreceptor).
How are excitatory interactions between photoreceptors ?
-between photoreceptors are mediated by telodendria
Summary of cone pedicle synaptic contacts?
- ca. 25 invaginations each associated with a ‘triad’ from horizontal and bipolar cells
- ca. 500 superficial contacts from bipolar cells
- with 6-15 neighbouring photoreceptors via telodendria
What are the rod spherules
- only have a single (1) invagination containing 2-7 postsynaptic processes so dont form triad like they do in cone pedicles.
- the lateral elements come from horizontal cells and the central processes from bipolar cells.
What do the horizontal cells synapse with ?
photoreceptors and bipolar cells in the OPL
What do Horizontal cells have?
a single long thin axon (which may be too thin to conduct)
