Eye and visual pathways

Question 1

Basic anatomy of eye

Answer 1

• 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)

Question 2

What is the structure like around the retina?

Answer 2

• choroid (outermost), RPE, neural retina

Question 3

Cones vs Rods

Answer 3

Cone = day vision
Rods = night vision

Question 4

What connects the photoreceptors and ganglion cells?

Answer 4

Big gap between is filled with interneurons

- one ganglion gets its input from several photoreceptors

Question 5

What is the receptive field?

Answer 5

• 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

Question 6

Why is the peripheral retina only capable of coarse vision?

Answer 6

• 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

Question 7

What is the fovea?

Answer 7

• 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

Question 8

How do we capture and refract light onto our fovea?

Answer 8

• 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.

Question 9

What are the 3 eye muscles that deal with vision?

Answer 9

• sphincter pupillae
• ciliary muscle
• dilator pupillae

Question 10

Sphincter pupillae

Answer 10

• 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.

Question 11

What is the pathway for the light reflex?

Answer 11

• light > photoreceptors > ganglia > pretectal nucleus (control centre) > Edinger-Westphal nucleus > preganglionic fibres of parasymp > ciliary ganglion > ciliary nerves > contraction of sphincter pupillae

Question 12

Ciliary muscle

Answer 12

• 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)

Question 13

Dilator pupillae

Answer 13

• radial muscle fibres
• contraction causes them to drag the pupil open
• emotional inputs
• symp (NA)

Question 14

Give 4 problems that can affect the eye

Answer 14

• myopia (short-sighted)
• hypermetropia (long-sighted)
• Presbyopia (lens becomes fixed and stiff)
• Cataracts (opaque lens)

Question 15

Myopia

Answer 15

• 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

Question 16

Hypermetropia

Answer 16

• optics are too weak and so focus behind retina
• can focus on far objects by increasing refractive power of lens
• cannot fatten any more to see close objects
• need convex lens to increase refraction

Question 17

Primary visual pathway

Answer 17

• axons running through the optic tract go through superior colliculus, pretectal nucleus and accessory optic nuclei (subconscious functions)
• also to lateral geniculate nucleus - project backwards via optic radiation to primary visual cortex
• fovea has much larger mapping in cortex respectively to rest of retina - right at back of cortex

Question 18

MAIN RULES OF DEFECTS

Answer 18

• Left part of visual world -> right part of retina -> right side of brain
• right part of visual world -> left part of retina -> left side of brain
• If damage is in front of chiasm, defect will be assymetrical
• if damage is behind chiasm, defect is bifocal and symmetrical