Retina and RPE Part 2 Flashcards
1
Q
How many bipolar cells are in the retina?
A
35.7 million
2
Q
What is the primary function of bipolar cells?
A
transmit signals from photoreceptors to ganglion cells, between which they are interposed
3
Q
Within which layer of the neurosensory retina can bipolar cells be found?
A
cell bodies are in the inner nuclear layer
oriented in a radial fashion parallel to the photoreceptors
4
Q
What is the arrangement of bipolar clels dendrites?
A
single or multiple dendrites pass outwards to synapse principally with photoreceptors (but also horizontal cells)
5
Q
What are the 2 types of cells which bipolar cell dendrites synapse with?
A
- photoreceptors (principally)
- horizontal cells
6
Q
What is the arrangement of bipolar cells’ axons?
A
passes inwards and synapses with ganglion and amacrine cells
7
Q
What are the 2 types of cells which bipolar cells’ axons synapse with?
A
- ganglion cells
- amacrine cells
8
Q
What is the ratio of cones : bipolar cells : ganglion cells in the foveal region of the central retina?
A
1 : 1: 1
9
Q
What is the ratio of bipolar cells : rods cells in the peripheral retina?
A
1 bipolar cell receives stimuli from 50-100 rods
10
Q
What does the ratio of bipolar cells : photoreceptors correspond to in the retina?
A
corresponds to decreasing visual acuity present in the peripheral retina
the summation of stimuli is a crucial factor in the sensitivity of the rod system to low levels of stimulation
11
Q
How many subtypes of bipolar cells exist in humans?
A
9 morphological subtypes
- 1 rod bipolar type
- 8 cone bipolar types
- 5 types of diffuse cone bipolars
- 3 types of midget bipolars
12
Q
What are the 3 types of midget bipolar cells?
A
- invaginating midget bipolar cells
- flat midget bipolar cells
- blue cone-specific bipolar cells (blue S-cone)
13
Q
What proportion of all bipolar cells is accounted for by rod bipolar cells?
A
20%
14
Q
What is the distribution of rod bipolar cells in the retina?
A
- have a small receptive field or dendritic tree in the central retina (15 μm wide, 10–20 rods)
- larger in the peripheral retina (30 μm and 30–50 rods)
15
Q
Where are rod bipolar cells most dense?
A
around the fovea
16
Q
How many rods are contacted by rod bipolar cells in the periphery?
A
up to 50
17
Q
What type of cells do rod bipolar cells synapse with?
A
AII amacrine cells
&, only rarely, with diffuse ganglion cells
18
Q
What is the role of diffuse cone bipolar cells?
A
concerned with converging information from many cones
19
Q
What is the arrangement of diffuse cone bipolar cells?
A
Their dendrites fan out (up to 70–100 μm) to end in clusters of between five and seven cone pedicles (can be as high as 15–20)
20
Q
In what region is overlap of diffuse cone bipolar cells extensive?
A
the perifoveal region
21
Q
Which of the types of bipolar cells is smallest?
A
invaginaing midget bipolar cells
22
Q
What are the connections formed by invaginating midget bipolar cells?
A
dendrites penetrate the base of a single cone pedicle (occasionally two) to form a central element in triads
23
Q
What is the ratio of midge bipolar cells to cones like?
A
near 1:1 midget bipolar cells to cones, decreases peripherally
24
Q
What are 2 types of cells that dendrites of invaginating midget bipolar cells may synapse with?
A
- amacrine cells
- midget ganglion cells
25
What is the arrangement of **flat** midget bipolar cells? What type of cells do they connect?
connect single cones with single **midget** **ganglion** cells or more rarely may connect several cones
26
How do flat midget bipolar cells differ from invaginaing midget bipolar cells?
they are similar except flat midget bipolar cell dendrites do not invaginate deeply into the cone pedicle
27
What type of bipolar cells can most cones be in contact with?
can be in contact with flat and invaginating midget bipolar cells, as well as diffuse cone bipolar cells
28
What type of contacts with other cells are made by blue cone-specific bipolar cells (blue S-cone)?
only a **limited** **number** of **cones** in their territory - specifically **blue** **cones** (it is suggested)
29
Where are the cell bodies of most ganglion cells located?
innermost nucleated layer of the retina (**ganglion cell layer)** situated between the nerve fibre layer and the inner plexiform layer
30
In addition to the ganglion cell layer, where are other displaced ganglion cells sometimes found?
inner nuclear layer
31
What do ganglion cells represent in the visual pathway?
they are the last neuronal link in the retinal component of the visual pathway
32
What layer are formed by ganglion cell axons?
nerve fibre layer on the innermost surface of the retina
33
Where do ganglion cell axons synapse?
on cells in the lateral geniculate nucleus of the thalamus
34
What separates and sheaths bundles formed by ganglion cell axons?
glial cells
35
What structure is formed by bundles of ganglion cell axons?
optic nerve
36
Via which structure do ganglion cells exit from the gloe and enter the optic nerve?
lamina cribrosa
37
What changes about the ganglion cell axons once they exit via the lamina cribrosa?
the axons become myelinated with oligodendrocytes
38
How many layers of ganglion cell bodies exist in the central retina/ fovea vs the peripheral retina?
* up to 7 layers in the central retina/fovea (ganglion cell layer is 60–80 μm thick)
* as few as 1 layer in the peripheral retina (10–20 μm thick)
39
How many ganglion cells are there per retina?
1.2 million ganglion cells
40
How many a) rods and b) cones exist per ganglion cell?
* a) rods: 100
* b) cones: 4-6
41
What are 3 features that characterise ganglion cells morphologically?
1. large cell body
2. abundant **Nissl** substance (arrays of **rough endoplasmic reticulum**)
3. Large **Golgi** apparatus
42
What are 4 things which are used to classify different types of ganglion cells?
1. cell body size
2. dendritic tree spread
3. branching pattern
4. branching level in the 5 strata of the inner plexiform layer
43
What differentiates some ganglion cells in the macular area from others morphologically?
some may contain yellow pigment (xanthophyll carotenoid) in the cytoplasm, although cone axons and Müller cells thought to also contain these pigments in the macular region
44
What are 2 cells that ganglion cells primarily receive impulses from?
1. bipolar cells
2. amacrine cells
45
What are 2 modes via which ganglion cells receive impulses from bipolar and amacrine cells?
1. axodendritic synapses
2. axosomatic synapses
46
Within which layer do axosomatic synapses between ganglion cells and amacrine/bipolar cells predominantly occur?
inner plexiform layer
47
What happens at axosomatic synapses between ganglion cells and amacrine/bipolar cells in the inner plexiform layer?
dendrites repeatedly branch to form the 'dendritic tree' whose form and size vary considerably and may be correlated with location in the retina and therefore function (receptive field size)
48
Up to how many subtypes of ganglion cell exist in humans?
18
49
What are 3 broad types of ganglion cells referred to as?
α, β and γ, or X, Y and W types
50
What is thought to be the role of a recently discovered non-rod, non-cone photoreceptive pathway arising from a population of retinal ganglion cells?
the ganglion cells express photopigment **melanopsin** and by signalling gross changes in light intensity they serve the subconscious non-imaging forming functions of **circadian photoentrainment** and pupil constriction via the **suprachiasmatic** nucleus
51
How do imaging forming and non-image forming retinal pathways merge?
the retina is strongly activated by rods and cones to signal irradiance over the full dynamic range of human vision, and **melanopsin-based signals** may contribute to conscious visual **perception**
52
What are 2 key types of ganglion cells?
1. **midget** ganglion cells
2. **diffuse** (parasol) ganglion cells
53
Which cells do midget ganglion cells synapse with?
**exclusively with amacrine cells** and **one midget bipolar cell** (and thus usually one cone)
54
What is dendritic spread of midget ganglion cells in the retina?
Dendritic spread is around 5–10 μm in diameter in central retina
increases 10-fold in a zone of 2–6 mm eccentricity and attains a maximum of over 100 μm
55
How are neighbouring midget ganglion cell dendritic fields related?
they do not overlap but form ***mosaics***
56
What are midget ganglion cells in humans also known as and why?
**P-cells** - they project to the *parvocellular* layer of the lateral geniculate nucleus (LGN)
57
Which types of bipolar cells do diffuse (parasol) ganglion cells synapse with?
comprise large synaptic field with **all types of bipolar cells** *except* midget bipolar cells
58
Where do diffuse (parasol) ganglion cells occur in the retina and what is the arrangement of their cell bodies (soma)?
occur in the **central retina** and their cell bodies (soma) are 8–16 μm in diameter with 30–70 μm dendritic fields, these being smaller nearer the fovea than the periphery
59
How does the size of the **dendritic fields** of **diffuse** (parasol) ganglion cells differ throughout the retina?
they are **smaller** nearer the **fovea** than the periphery
60
What are diffuse (parasol) ganglion cells also known as and why?
**M-cells,** because they porject to the *magnocellular* layer of the LGN
61
What is the relevance of the finding that **midget ganglion cells** synapse exclusively with **midget bipolar cells**, and that both are common near the fovea?
provides the anatomical basis for the observation of **small receptive fields** and **high visual acuity** in this region
62
How many types of **diffuse** ganglion cell exist? How are they classified?
5 - classified on basis of morphology
63
What is the arrangement of fields surrounding receptive fields and what is the size like?
anatomical basis of **antagonistic** fields surrounding receptive fields is complex, although they do not appear to vary much in size from within an 8 mm radius of the fovea
64
What may form the basis of the antagonistic field surrounding receptive fields in the retina?
lateral extensions of the **amacrine** **cell**, with its extensive interconnections with ganglion cell dendrites and bipolar cells as well as fellow amacrine cells
65
What are the 2 key **association** neurones?
1. Horizontal cells
2. Amacrine cells
66
From what do horizontal cells derive their name?
extensive horizontal extension of their cell processes
67
What are the 2 distinct morphological varieties of horizontal cells?
1. **Type A:** large, *sturdy*, axonless cell with stout _dendrites_ that contact *only cones*
2. **Type B:** smaller, *bushier* _dendritic_ tree that contacts *cones exclusively* but, in addition, has _axon_ up to 300 μm in length that ends in extensive arborisation that is postsynaptic only to **rods**
68
What is the morphology like of type A horizontal cells?
large sturdy axonless cell with stout dendrites that contact only cones
69
Which type of cells do type A horizontal cells form synapses with?
only cones
70
What is the morphology like of Type B horizontal cells?
smaller bushier dendritic tree that contacts cones exclusively but, in addition, has an axon up to 300 μm in length that ends in extensive arborization that is postsynaptic only to rods
71
Which cells do type B horizontal cells form synapses with?
* _dendritic tree_ contacts **cones** exclusively
* long _axon_ with extensive arborisation is postsynaptic only to **rods**
72
How do the receptive fields of type A vs type B horizontal cells compare?
type **A** have much **larger receptive fields** than type **B**
73
How many connections do a) rods and b) cones have with horizontal cells?
* a) RODS: at least *2* horizontal cells
* b) CONES: 3-4 cells **of each type** *(6-8)*
74
How many cones are contacted by type B horizontal cell **soma processes**? How does this vary across the retina?
* around **7 cones** near the **fovea** (dendritic tree covering 15 μm)
* increases to as many as **18 cones** further from the fovea (dendritic tree covering 80–100 μm).
75
What is the pathway of the **axon** of type B horizontal cells?
passes laterally and terminates up to 1mm away in a thickened axon terminal bearing a fan-shaped protrusion of lollipop-like endings in **rod spherules** (up to 100)
76
How do the dendritic trees of Type A horizontal cells compare with type B and how many cones do they contact?
type A are generally more spidery and **contact twice as many cones** as type B
77
How long are the **axons** of type A horizontal equivalent cells in primates and what cells do they contact?
axons are generally shorter (100–200 μm) and contact **cone** **pedicles** by small wispy terminals
78
Which retinal layer are horizontal cell bodies located in?
outer part of the inner nuclear layer
79
What is the only distinctive cytoplasmic organelle in horizontal cells?
few except for **crystalliods**, a series of densely stacked **tubules** with associated **ribosomes**
80
Where do the processes of the horizontal cells ramify?
in the **outer** plexiform layer close to the **cone pedicles**
81
What is the overlap of horizontal cells like?
overlap between horizontal cells is **considerable**
any one area of retina may be served by up to 20 horizontal cells
82
What are the roles of horizontal cells in the retina? 3 key things
1. **integrative** role in retinal process
2. release **inhibitory** neurotransmitters, mainly γ-aminobutyric acid (**GABA**)
3. some **colour-specific wiring** for the three types of horizontal cells in the human retina
83
Do amacrine cells have axons?
previously thought to lack axons, but recent studies show some do have an axon
84
In which layer are amacrine cells located in the retina?
in the vitread or inner aspect of the **inner nuclear layer** (bipolar cell layer)
85
What 2 things make amacrine cells distinguishable?
* **larger** size - 12μm
* **oval** shape
86
How many types of amacrine cells exist in humans?
at least 25 - show remarkable degree of diversity
87
What is the shape of the cell body of amacrine cells?
flask-shaped
88
Where do the numerous dendritic processes of amacrine cells terminate?
ramify and terminate predominantly in the synaptic complexes formed by the **bipolar and ganglion cell processes**, namely the **inner plexiform layer**
89
What is the shape of the amacrine cells' dendritic fields?
highly variable - see some examples in the image:
90
What are 3 ways in which amacrine cells can be divided into subtypes?
1. based on **stratification** **of their dendrites** in the inner plexiform layer
2. or their **shape**, e.g. diffuse, starburst and stratified
3. on basis of their **neurotransmitters**
91
What are 3 types of shapes of amacrine cell dendrites?
diffuse, starburst, stratified
92
What is the dendritic field like of a diffuse amacrine cell?
can cover narrow fields (approximately 25 μm wide), their fibres being cone-shaped
93
What are 4 types of neurotransmitters that can be released by amacrine cells and what does this indicate?
1. **GABAergic**
2. **dopaminergic**
3. **acetylcholine** releasing
4. peptide **somatostatin** - neuroactive peptide in the retina
play a role in *modulation* (most probably _inhibitory_) of signals reaching ganglion cells
94
What is thought to be the function of amacrine cells?
play a role in modulation (most probably inhibitory) of signals reaching ganglion cells
95
What are 3 things that somatostatin, released by some amacrine cells, can function as?
1. neurotransmitter
2. neuromodulator
3. trophic factor
