Physiology of vision Flashcards
Consider the anatomy of the eye.
What is the iris and what does it do?
Coloured part of the eye
Acts as a diaphragm, varying diameter by up to 4x and therefore retinal intensity by 16x
Consider the anatomy of the eye.
What is the cornea and what does it do?
How does it work with the lens?
The eyes clear, protective outer layer
It is responsible for 2/3 of ray bending
The lens is responsible for the other 1/3 of ray bending and together they produce a focused image on the brain
Consider the anatomy of the eye.
What is the retina and what does it do?
What is found behind the retina?
The sensory membrane that lines the inner surface of the back of the eyeball. Light is focused on the retina where photoreceptors interpret the signal.
Behind the retina is pigment layer which absorbs unwanted light
What is the optic disk?
Where the optic nerve leaves the eye and blood vessels enter and leave retina
What is meant by accomodation of the eye and which structures are responsible for it?
The process by which the eye changes optical power to maintain focus on an object as its distance varies.
Focus varied by changing the shape of the lens. Ciliary muscles contract which focus on short distance. This is controlled by the PNS
CLINICAL APPLICATION
There are two common focusing problems (refractive errors). Describe them and how they are corrected.
Hypermetropia (long sightedness)- eyeball too short or lens system too weak so light doesn’t converge onto retina but PAST it.
FIX: Converging/Convex lens
Myopia (short-sightedness)- eyeball too long or lens system too strong such that light converges BEFORE the retina
FIX- Diverging/Concave lens
What is refractive power measured in and how does it relate to focal length?
Refractive power is measured in dioptres. This is the reciprocal of focal length in metres
e.g. a 2D spectacle lens has a focal length of 1/2 =0.5m
State the relevant embryology of the retina
What is the ratio of rods to cones? What type of light are they sensitive to?
Evolved back to front: ganglion cells and blood vessels are in the light path to the photoreceptors (except in fovea)
We don’t see the retinal blood supply because it doesn’t change, and our eyes ignore unchanging images.
24 rods: 1 cone
Rods are sensitive to dim light
Cones are sensitive to bright light an colour
From the most anterior part of the retina to the most posterior edge of the eye state the processing layers
State the number of ganglion cells in each eye and its relevance
Receptors (rods/cones) Bipolar neurons Ganglion cells (3 direct layers) Horizontal cells Amacrine cells (2 Transverse layers: signal processing includes lateral inhibition) Muller cells -glial supportive cells
Only 1 million ganglion cells/eye
125:1 convergence into the optic nerve
Consider the various cell types within the retina.
What happens when a rod cell is stimulated?
Rods are 500nm
When hit by a photon RETINAL in rhodopsin flips from 11-cis to all-trans
Sets off series of biochemical events
Electrical change (hyperpolarisation) in cell membrane
Consider the various cell types within the retina.
What happens when a ganglion cell is stimulated?
Send APs down optic nerve: receptors and bipolars have only graded potentials
Respond weakily to changes in overall light intensity
Instead LOCAL CONTRAST: light on a dark background
General response is either ON CENTRE or OFF CENTRE (due to lateral inhibition)
What is lateral inhibition?
What is its relevance in vision?
lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction
Lateral inhibition increases the contrast and sharpness in visual response.
Consider the various cell types within the retina.
Which specific wavelengths of the electromagnetic spectrum are detected by the cone cells? What is the distribution ?
Cones provide coloured vision via trichromacy.
Red cones respond to 560nm
Green cones respond to 530nm
Blue cones respond to 420nm
There are R>G which in term are in much greater quantity than Blue cones
CLINICAL APPLICATION
Colour blindness results from loss or modification of one or more of the 3 cone cell types. Describe the 3 types of colour blindess
Red/green
-R&G Genes on X-chromosome –> damage to these genes causes blindness. More common in males (7% compared to 0.5%)
- B gene on chromosome 7, which is paired in both sexes. Much rarer than R/G blindness
- Central achromatopsia: rarer and caused by damage to cortical colour processing area (V4)
Outline the central visual pathway
Two visual fields : nasal and temporal
Optic nerve from each retina divide into left and right.
Combine in optic chiasm
Optic tracts relay in the lateral geniculate nuclei of thalamus
Part of each tract goes to superior colliculus in the midbrain
Optic radiation (occurs in visual association cortex: Brodmann areas 18 and 19)
Output almost exclusively to the striate cortex in the occipital lobe (CV1). Here the image of 1/2 of each combined visual field is represented in 1/2 of V1
The cortical input passes on to areas that process depth, motion, colour etc
How does the primary visual cortex respond to stimulation?
Simple responses are constructed from rows of ganglion cell (or Lateral geniculate nuclei) on and off centre fields
Describe the columnar organisation of the visual cortex (Brodmann 17)
What is a hypercolumn?
- Ocular dominance columns
- Smaller orientational columns in which the orientation of optimal stimuli varies, systemically across the surface
- Colour blobs: colour information is kept seperate from 2, and passed on to other regions such as V4)
A hypercolumn contains 1 set of everything
CLINICAL APPLICATION
Consider the causes of loss of vision. Which pathology gives rise to the following
- Left -eye blindess (Left anopia)
- Bitemporal hemianopia
- Homonymous hemianopia
- Scotoma
- Lesion on left optic nerve
- Lesion on optic chiasm (pituitary gland neoplasm). Light reflex remains
- Lesion just before lateral geniculate nucleus
- Retinal damage, lesions in visual cortex or by pressure from tumours restricting optic nerve, chiasm, tract or radiation
What are the dorsal and ventral streams in the cortex?
The dorsal stream: occiptal –> parietal cortex , concerned with location, motion, and action
The ventral stream: occipital –> temporal, concerned with object (and face) identity, and with conscious perception
CLINICAL APPLICATION
Consider the pathologies that may arise from the dorsal and ventral streams. What is visual agnosia?
How does it relate to visual ataxia?
A condition in which a person can see but cannot recognize or interpret visual information, due to a disorder in the parietal lobes.
No difficulty in ‘posting’ card into slot with correct orientation but cannot perceptually identify orientation of the card.
In visual ataxia, the patient can describe how but cannot act on it
Site of DF lesion in ventral stream
CLINICAL APPLICATION
Consider the pathologies that may arise from the dorsal and ventral streams. What is Prosopagnosia?
Inability to recognise familiar faces
Associated with damage to specific parts of temporal lobe (fusiform gyrus on underside of temporal lobe)
CLINICAL APPLICATION
What is blindsight?
Outline possible blindsight pathways
Destruction of the striate cortex (V1) –> blindness in part of the visual field corresponding to damaged area
Visual information bypasses LGN and goes directly to striate cortex
Same as above but from SC, goes to LGN
Same as above but from LGN goes to temporal lobe
From eye to ear, outline the pathway the neurons take and where they synapse
First order neuron synapses at oculomotor nuclei
Second order neuron synapses at vestibular nucleus
Third-order neuron synapses at semi-circular canals
What is the vestibulo-ocular reflex (VOR)?
Stabilises gaze by countering movement of head
What is the optokinetic reflex (OKR)?
Stabilises the image of a moving object on the retina
First order neuron synapses at oculomotor nuclei, second order neuron synapses at nucleus of optic tract, third order neuron destined for retina
CLINICAL APPLICATION
Consider the pupillary reflex.
- Explain the direct and consensual response
- What happens when there is damage to 1 optic nerve?
- What happens when there is damage to 1 oculomotor nerve?
- Constriction of the pupil in response to light directed in that eye is referred to as the direct response. Constriction of the contralateral pupil is referred to as the consensual or indirect response.
- Damage to 1 optic nerve causes loss of pupillary reflex
- Damage to 1 oculomotor nerve means that the direct light reflex is lost (Pupil contraction is prevented in that eye) but the consensual one is maintained (Stimulation of another eye will cause a contraction in the pupil in the 2nd eye)
Consider the light (pupillary) reflex
- Outline the origin of the oculomotor nerve and its path
- Why do both pupils contract if only one eye is illuminated?
- The oculomotor nerve originates from the nucleus of Edinger-Westphal. It travels with the PNS fibers.
It travels to the ciliary ganglion where it will synapse with fibers (short ciliary nerves) destined for the ciliary muscles.
- Both pretectal nuclei and Edinger-Westphal nuclei receive signals from both eyes
