Visual pathways and perception Flashcards
where is the optic chains in relation to the pituitary
- The optic chiasm is immediately above the pituitary
what does a pituitary tube cause
It causes bilateral loss of peripheral receptive fields
where does the optic nerve synapse and become a 2nd order neurone
- in the optic tract in the lateral geniculate nucleus
what does the lateral geniculate nucleus project into
The lateral geniculate cells project into the optic radiation into the primary visual cortex(area 17) and then into the visual association cortex
what supplies the axon of the optic nerve
Retinal ganglion cells supply the axons that constitute the optic nerve (
what pathway mediates visual perception
- the lateral geniculate occipital cortex pathway
where do some other optic nerve fibres project into
some optic nerve fibres project to the superior colliculi in the midbrain
what do the superior colliculi do
- they are necessary from tracking an object
- they connect to the medial longitudinal fascicule which links and synchronises with the oculomotor nuclei
if you have damage to the superior colliculus pathway what happens
Damage to the superior colliculus pathway means that a patient has difficulty tracking a moving object
what is another name for the superior colliculi
- optic tectum
where do some fibres from the retina project into
Some fibres from the retina project to the pretectal nuclei and then bilaterally to the Edinger-Westphal nuclei
How is the pupillary light reflex caused
- Parasympathetic preganglionic fibres project from the Edinger-Westphal nuclei to the ciliary ganglia in the orbit behind each eye.
- From here postganglionic fibres enter the eye and act on sphincter muscles around the pupil to constrict it.
- This is the pupillary light reflex - this reflex is bilateral so both eyes are affected
where do the axons from the cells in the lateral geniculate go
- Axons from cells in the lateral geniculate nucleus project to the visual cortex via the optic radiation.
- The axons from the lateral geniculate cells project anteriorly and then posteriorly along the side of the posterior horn of the lateral ventricle eventually to reach the occipital cortex.
What is Meyers loop
- Fibres in the optic radiation mediating vision from the upper quadrants loop more anteriorly around the side of the lateral ventricle
- this is Meyers loop
- on the other hand fibres from the lower quadrants travel more directly back to the visual cortex
describe strokes and Meyer’s loop
Strokes or tumors can sometimes selectively damage Meyer’s loop on one or other sides of the brain
both eyes….
contribute to both visual fields
- the The left visual field projects to the right visual cortex and vice versa.
How can both eyes contribute to both visual fields
- The nasal hemiretina from the left eye projects to the right visual cortex
- the temporal hemiretina from the right eye projects to the same right visual cortex - this is because the temporal hemiretina do not cross over the chiasmata whereas the axons from the nasal hemiretina do
- Both hemiretinae view objects in the left visual field.
what the lesions in the visual system
1 Partial optic nerve lesion: Ipsilateral scotoma*
2 Complete optic nerve lesion Blindness in that eye
3 Optic chiasm lesions: Bitemporal hemianopia
4 Optic tract lesions: Homonymous hemianopia
5 Damage to of Meyer’s Loop:
Homonymous upper quadrant anopia (axons are arranged with upper part of visual field anterior)
6 Optic radiation lesion:
Homonymous hemianopia
7 Visual cortex lesion
Homonymous hemianopia
(Macular sparing)
what is a scotoma
- patch of blindness
once you get pas the chiasma the lesion is called …
homonymous
what is muscular sparing
The central parts of the eye (macula and fovea) have a bilateral projection to the visual cortex, so a lesion on one side will often not affect central vision; this is called macular sparing
what is Anton babinskin syndrome
- this happens when the visual cortical pathway mediates the visual recognition of an object
- if the pathway is damage the person is blind
- in some cases despite the loss of the visual cortex the person will insist that they can still see
- the visual pathways to the brainstem mediate visual reflexes and these can persist although visual perception does not exist
what are the key reflexes
- pupillary light reflex
- accommodation reflex
- vestibulo-ocular reflex
- the blink reflex
how do you block the pupillary light reflex
atropine
describe how the pupillary light reflex nerves
- goes in on optic nerve (CNII) AND COMES OUT ON CNIII oculomotor nerve
what is the accommodation reflex for
- it is to focus on a near object
- your frontal eye fields need to be working in order to generate this reflex
what are the three components of the accommodation reflex
a) Pupillary constriction.
b) Thickening of lens
c) Convergence
describe pupillary constriction
ACCOMODATION REFELX
- output pathway is the CN III oculomotor pathway
- Input to cause this constriction is from descending projections from frontal eye fields in premotor cortex.
– the smaller the pupil the better and clearer the focusing on anything this is called improving the depth of field, therefore pupillary constriction improves focusing on close objects
the smaller the pupil…
the better and clearer the focusing on anything this is called improving the depth of filed
accommodation is an act of..
Accommodation is an act of will and can only occur in a conscious person
what is the accommodation reflex controlled by
It is controlled by the Frontal Eye Fields (FEF) which are specialised parts of the premotor area that are dedicated to motor control of the extraocular eye muscles
what happens if there is damage to the frontal eye fields
Damage to FEFs - inability to direct gaze from one object to another. (Also loss of fast phase of nystagmus)
Describe how the thickening of the lens works (ACCOMODATION REFLEX)
- When viewing a distant object the ciliary muscle sphincter is relaxed and the lens is under tension from the suspensory ligaments which pull on it and stretch and flatten it
- in the accommodation reflex the ciliary muscle contracts and opposes the suspensory ligaments, this causes the lens to thicken and become Wider
- this wider lens enables the eye to focus on nearby objects
what do suspensory ligaments do
pull on the lens and stretch and flatten it
describe how convergence of the eye takes place (ACCOMMODATION REFLEX )
- Convergence is controlled from frontal eye fields.
- It involves simultaneous activity in the medial rectus muscles on each side.
What mediates the convergence part of the accommodation reflex
- This convergence is mediated by the oculomotor nerve (CN III) bilaterally.
- Because it is mediated by the visual cortex,
- Convergence cannot be triggered in an unconscious patient.
how can the convergence part of the accommodation reflex be distributed
- The convergence part of the accommodation reflex is very delicate and can be disturbed by fatigue, trauma, alcohol, drugs, etc
what happens if the convergence part of the accommodation reflex goes wrong
If it occurs incorrectly or not at all we see double; this is diplopia
describe the vestibulocular reflex
- If you turn your head to the side while looking at a distant object
your eyes rotate in the opposite direction to the head to keep the direction of gaze constant and the object kept in view. - This is the vestibulo-ocular reflex (VOR).
- It is important to realise that the VOR involves the oculomotor nerve in one eye (the one moving inwards) and the abducens nerve in the other eye (moving outwards). .
- Meidal rectus contracts to pull the eye in, lateral recuts is driven by the abducens nerve therefore there is cooperation
]
what is the input and output for the vestibulocular reflex
- The input (afferent) arc of this reflex is the vestibulo-cochlear nerve (CN VIII) which receives signals from the semicircular canals.
- The output is the abducens nerve (CN VI) and the oculomotor nerve (CN III).
what is the vestibuloocular reflex sometimes called
dolls eye reflex
How can you test the vestibulocular reflex
It is tested by holding someone’s eyes open and gently rotating the head from side to side; in a normal person the eyes should rotate in the head to keep the gaze direction constant. With the reflex absent (as below) the eyes stay fixed in the head.
Caloric stimulation test
- If the water is warm (44°C or above) this mimics a head turn to the ipsilateral side.
- Both eyes will turn slowly away from the irrigated ear toward the contralateral ear, followed by horizontal fast flick (nystagmus) towards the irrigated ear.
- If the water is cold (30°C or below), this mimics a head turn to the contralateral side.
- The eyes then turn slowly toward the ipsilateral ear, with horizontal fast flicks towards the non-irrigated ear
how are the vestibulocohlear, Abducens and oculomotor nerve all linked
- The nerves are all linked by the MLF
what is Nystagmus
Nystagmus is a form of the VOR caused by continuing rotation of fluid in the semicircular canals.
- It shows as an initial slow rotation followed by a fast flick back.
what is the direction of the Nystagmus always given by
The direction of the nystagmus is always given by the direction of the fast flick.
what is the blink reflex for
Protects eyes from foreign bodies (grit etc)
what is the input and output of the blink reflex
- .Input: sensory nerve endings in cornea or conjunctiva; these are branches of the ophthalmic branch (V1) of the trigeminal (V) nerve.
- Output: motor fibres in the facial (VII) cranial nerve to the obicularis oculi muscle which pushes the eyelids together
