VIsual system- pathways Flashcards
what is the visual pathway?
neurological pathway whereby vision is converted to neurological impulses to be transmitted from eye to visual cortex in posterior part of brain
how is the eye connected to the brain?
optic nerve (second cranial nerve)
what is the optic nerve made up of?
- Optic nerve made up of myelinated ganglion nerve fibres w. cell bodies originating within retina
how to signals travel from the eye?
from eye to optic nerve
- Optic nerves from both eyes converge at optic chiasm
- Above + in front brain stem
- Half ganglion nerve fibres cross at optic chiasm and exit along contra-lateral optic tract
- Remaining ganglion nerve fibres exit along optic tract on same side
- via optic tract to lateral geniculate nucleus
- via optic radiation to primary visual cortex
where do ganglion fibres originate?
within retina
- Synapse upton next order neruons
- At lateral geniculate nucleus
- Relay centre situated within thalamus
what forms the 4th order neuron?
optic radiation
what does the optic radiation do?
- Relaying signsals from lateral geniculate ganglion to primary visual cortex within occipital lobe for lower visual processing
what does the primary visual cortex do?
- Relays visual information to the extra-striate cortex
- Region adjacent to primary visual cortex for further higher visual processing
what is the visual pathway within retina?
- 1st order neurons= photoreceptors detect light
- 2nd order neurons= synapse upon bipolar cells
- 3rd order nerons= retinal gangion nerve cells
- half retinal ganglion nerve cells cross to opposite side at optic chiasm= partial decussaion
- retinal ganglion fibres terminate at lateral geniculate nucleus in thalamus and synapse on 4th order neurons
- 4th order neurons= optic ratiation
- optic radiation relays visual information to visual cortex
what do retinal ganglion nerve fibres do?
- Relay visual information out of eye to brain along optic nerve (CNII)
- To improve signal transmission-retinal ganglion nerve fibres become myelinated after entering optic nerve
what is the effect of lesions anterior to optic chiasm?
affect visual field in one eye only
what is the effect of lesions posterior to optic chiasm?
affect visual field in both eyes
due to ganglion fibre crossing in chiasm to exit along contralateral optic tract (53%)
contralateral homonymous hemianopia in both eyes
where do crossed fibres predominantly originate?
nasal retina- responsible for temporal half of visual field in each eye
where do uncrossed fibres originate?
temporal retina- responsible for nasal half of visual field in each eye
what happens in lesions at optic chiasm?
- Damages crossed ganglion fibres from nasal retina in both eyes but spares uncrossed ones originating from temporal retina
- Temporal field loss in both eyes simultaneously
- Bitemporal hemianopia
what happens in a right sided lesion posterior to optic chiasm?
left-sided homonymous hemianopia in both eyes and vice versa
overall what are the effects of lesions posterior and anterior to opic chiasm?
what are the effects of each of these disorder locations?
what is a bitemporal hemianopia usually caused by?
enlargement pituitary gland tumour (tumour sits under optic chiasm)
what is a homonymous hemianopia usually caused by?
stroke of CVA in brain
how does homonymous hemianopia with macular sparing occur?
- Damage primary visual cortex
- Often due to stroke
- Area within primary visual cortex representing macular is well protected as received dual blood supply from both right and left posterior cerebral arteries
what happens to the pupil in light?
- Pupil constriction
- Iris circular muscle contracts and constricts pupillary aperture
- Decreases spherical aberrations and glare
- Increases field of depth
- Reduces bleaching of photo-pigments
- Pupillary constriction mediated by parasympathetic nerve (within CN III)
what happens to pupil in light?
- Pupil dilation
- Increases light sensitivity in dark by allowing more light in to eye
- Pupillary dilation mediated by sympathetic nerve activating iris radial muscle
- radial muscles contract
what is the afferent pathway of signals from eye?
- Rod and cone photoreceptors synapsing on bipolar cells synapsing on retinal ganglion cells
- Pupil-specific ganglion cells exit at posterior 3rd of optic tract before entering the lateral geniculate nucleus
- Afferent pathways from each eye synapses on Edinger-Westphal nuclei on both sides in brain stem
what is the efferent pathway of signals from the eye?
- Provided by parasympathetic nerve arising from brainstem synapsing at ciliary ganglion upon short posterior ciliary nerve
- Short posterior ciliary nerve innervates directly on iris pupillary sphincter
- Edinger Westphal nucleus -> ocular motor nerve efferent -> synapse ciliary ganglion -> short posterior ciliary nerve -> pupillary sphincter
- Note: afferent pathway from either eye stimulates efferent pathway on both eyes
- Only 1 eye needs to be stimulated with light to elicit pupillary constriction response in both eyes
what is the direct light reflex?
- constriction of pupil of the light stimulated eye
what is the consensual light reflex?
- constriction pupil of other eye
what is the neurological basis of direct and consensual reflex?
- Afferent pathway on either side alone with stimulate efferent pathway on both sides
what happens on a right afferent defect?
- E.g damage optic nerve
- No pupil constriction in both eyes when right eye is stimulated with light
- Normal pupil constriction in both eyes when left eye is stimulated with light
what happens on a right efferent defect?
- E.g damage right 3rd nerve
- No right pupil constriction whether right of left eye stimulated with light
- Left pupil constricts normally whether right or left eye is stimulated with light
what is the swinging torch test?
Relative efferent pupillary defect:
- Partial pupillary response still present when damaged eye stimulated
- Elicited by swinging torch test- alternating stimulation right and left eye with light
- Both pupils constrict when light swings to left undamaged side
- Both pupils dilate when light swings to right damaged side (reduced drive for pupillary constriction in both eyes)
