Eye and visual pathways Flashcards
1
Q
Basic anatomy of eye
A
- outside is made of tough collagen and elastic fibres
- cornea at front - collagen fibrils glued together by proteoglycans (makes it transparent)
- shape is maintained by pressure inside
- within outer layer is vascular layer - choroid gives blood flow to photoreceptors
- ciliary body has muscle that controls shape of lens via susp. ligaments
- Optic nerve enters eye at back - covered by 3 meninges (outgrowth of brain)
- inner layer is neural layer (2 layers) - inner = neural retina, outer = retinal pigment epithelium - RPE pumps fluid out of gap between giving suction to hold together)
2
Q
What is the structure like around the retina?
A
- choroid (outermost), RPE, neural retina
3
Q
Cones vs Rods
A
Cone = day vision Rods = night vision
4
Q
What connects the photoreceptors and ganglion cells?
A
Big gap between is filled with interneurons
- one ganglion gets its input from several photoreceptors
5
Q
What is the receptive field?
A
- The specific patch on the retina that the photoreceptors for a specific ganglion will represent.
- the smaller the RF, the better the fine detail it can pick up
6
Q
Why is the peripheral retina only capable of coarse vision?
A
- Cone receptors are largely and widely spaced (separated by rods)
- signals from many cones converge onto single ganglion cells
- light has to pas through a lot of the retina before its detected by photoreceptors
7
Q
What is the fovea?
A
- very small spot in the centre where the most accurate of images focuses
- no overlying layers or blood vessels
- only has thin, specialised cones which can be packed closely (no rods)
- no convergence onto ganglia - 1:1
8
Q
How do we capture and refract light onto our fovea?
A
- iris controls how much light enters via pupil
- cornea has strongest refracting power
- lens provides additional variable focus
- smaller the aperture the light has to go through, the easier it is to focus
- iris makes hole as small as poss - in low light it dilates to allow more light in, sacrificing focus.
9
Q
What are the 3 eye muscles that deal with vision?
A
- sphincter pupillae
- ciliary muscle
- dilator pupillae
10
Q
Sphincter pupillae
A
- controlled by light
- circular muscle, when contracts it gets smaller
- Parasymp control (ACh) - muscarinic agonists -> dilation
- innervated by short ciliary nerves from ciliary ganglion - innervated by CNIII.
11
Q
What is the pathway for the light reflex?
A
- light > photoreceptors > ganglia > pretectal nucleus (control centre) > Edinger-Westphal nucleus > preganglionic fibres of parasymp > ciliary ganglion > ciliary nerves > contraction of sphincter pupillae
12
Q
Ciliary muscle
A
- Circular muscle - contraction = shorter
- when it is relaxed, suspensory ligaments are taut = flat lens
- when contracted, susp. ligaments are slack = fat lens (more refraction)
- Parasymp (ACh)
13
Q
Dilator pupillae
A
- radial muscle fibres
- contraction causes them to drag the pupil open
- emotional inputs
- symp (NA)
14
Q
Give 4 problems that can affect the eye
A
- myopia (short-sighted)
- hypermetropia (long-sighted)
- Presbyopia (lens becomes fixed and stiff)
- Cataracts (opaque lens)
15
Q
Myopia
A
- optics are too strong and so focus in front of retina
- can reduce refractive power of lens to see close objects
- cannot flatten further to see long distances
- need concave lens to weaken refraction
