Lecture 20 - Vision and the Pupils Flashcards
What is the job of the retina?
To transduce photons into electrical signals
Describe the pathway for vision once we cross the optic chiasm.
From the optic chiasm, information gets transmitted around the sides of the diencephalon up to the thalamus where almost all our sensory information is relayed (except oflactory info). The thalamus relays the infomration to the cortex, which is responsible for analyzing and doing somethign withthat information. In this case, the occiptial cortex (visual cortex receiving visual information)
Describe the pathway for conscious perception in a dog if a cotton ball is thrown to the right side of his head with respect to his left and right eyes.
- The light coming through the right eye will hit the retina on the medial aspect of the retina. The medial portion of the retina gives rise to the part of the optic nerve the decussates at the optic chiasm to the left side of the brain and sent to the left lateral geniculate nucleus. The lateral geniculate nucleus is the thalamic nucleus that releases the information to the left side of the brain, specifically the occipital cortex.
- The light coming through the left eye will hit will hit the retina on its lateral aspect. Theses fibers do not decussate at the optic chiasm, and stay on the ipsilateral side, and will travel to the left lateral geniculate nucleus which relays information to the occipital cortex on the left side.
What is stereoscopic vision?
The overlap of visual fieilds in the center (bilateral vision) that is responsible for the perception of depth. The right and left cortex can talk to eachother and percieve how far away an object is.
Clinically, how can we assess vision in animals via subjective evulations?
- Testing their conscious perception by creating an obstacle course to see if he can make it through to you without walking into things
- These are subjective evaluations
What is a more objective way of evaluating vission clinically in animals?
- Tracking - assessing fields
- Example: standing behind them and throwing a cotton ball on either side of their head to see if the dog reacts and tries to follow it
- We try to stay on either side to evaluate their visual field (right vs. left)
After performing subjective and more objective tets in assessing an animals vision, and we think the animal is blind but are unsure completely, what is the strongest test that can be done in assessing vision?
- Create a bright doorway from a dark room
- In the dark, pupils are dilated, so they recieve as much light to the retina as they can. Therefore, a bright doorway in a dark room is a strong stimulus and the animal should be able to go through it without bumping into the wall next to it if he can see
Why do we call the menace resposne a response and not a reflex?
Because it is a learned response. It is not something that is hard-wired that the dog is born with - they have to learn that if something is coming at their eye, they have to blink. This usually should be fully developed at around 16 weeks.
Describe the complex pathway involved in the menace response in a normal animal.
- Menacing gesture at the eye is percieved by the retina
- Information is transduced to an electrical stimulus to the otpic nerve
- Optic nerve decussates at the chiasm and goes the the lateral geniculate nucleus on the opposite side.
- Information at the lateral geniculate nucleus is then relayed to the occipital cortex
- Information is sent to the motor area in the frontal cortex to initiate movmement
- Cerebellum is involved because it is critical for motor learning
- From the cerebllum information gets sent down to the facial nerve (CN VII) and out to the eyelids where an animal should blink.
How is blindness defined?
- Loss of conscious perception
- Unilateral vs bilateral
- Central vs peripheral
Peripheral blindness involve problems with:
- Retina
- Optic nerves
- Chiasm
Central blindness involve problems with:
- Optic tracts
- Thalamus
- Cortex
How are pupils controlled and at what level of the CNS?
- At the level of the midbrain
- Controlled by the optic nerve (CN II) involving parasympathetic and sympathetic control
What reflex do we use to identify if the blindness is central or peripheral and why?
- Pupillary light reflex (PLR)
- Information that the brain needs to control the pupils (ie. amount of light falling on the retina) comes in through the optic nerve up to the optic chiasm. From the optic chiasm, the pathway for vision and pupil control is identical.
- Once we go past the optic chiasm, the pupillomotor fibers branch off from the rest of the visual pathway and head to the midbrain.
Why in a clinical setting, do we dim the lights and shine a very bright light to test PLR?
- In a clinical setting, we dim the lights to dilate the pupils, making it easier to see the response - the light is a stronger stimulus
- Animals are nervous, and we need a bright light to override sympathetic tone that wants to dilate the pupils
Describe the pathway for the pupillary light reflex when shining the light in the right eye.
- Shining light in right eye - activate the retina
- Information goes to optic nerve, through optic chiasm.
- Since we stimulate both the medial and lateral aspects of the retina, some information decussates and some stays ipsilateral.
- Past the optic chiasm, pupillo-motor fibers branch and run to both sides of the midbrain, to the left and right pre-tactal nuclei.
- From the pre-tactal nuclei we get another split that is ipsilateral and contralateral. So that from one pre-tactal nucleus, information goes to both the parasympathetic occulomoter nuclei.
- Pre-gangionic nerve fibers from both the occulomotor nuclei run to the ciliary ganglia on each side of the eye located in the retrobulbary ganglia.
- Post-ganglionic fibers run to smooth muscles of the pupil in both eyes, causing constriction.
What is the response to PLR in an animal with unilateral peripheral blindness when light is shown in the affected eye, and when light is shown in the unaffected eye?
- When light is shined in the affected eye, we do not see a direct response in the affected eye, nor do we see an indirect response in the unaffected eye.
- When light is shown in the unaffected eye, we see a normal PLR, with both direct response in the unaffected eye and an indirect responses in the affected eye.
What is the dopaminergic pathway and describe the pathway involved?
- It is a “rewarding” pathway
- Motivation, attnetion, and arrousal is mediated by the substantia nigra located in the brain stem nucleus
- Substantia nigra has dopaminic neurons that project out ipsilaterally (on the same side) to the caudate nucleus located in the forebrain
- Pretty much, substantia nigra sends dopamine neurons to cortex on that side and tells that side of the brain that was fun, lets do it again.
- Normally, both sides are doing the same thing at the same time.
Describe the pathway and how it is disrupted in unilateral lesion on the right dopaminergic pathway and the ultimate result.
- Ultimate result - loss of attention to contralateral side (left side in this case)
- With a lesion on one side of the brain, the animal can only pay attention to the sensory information coming from the ipsilateral side to that lesion (same side as the lesion) because sensory information decussates. So the right side sensory information in this case is sent to the dopaminergic receptors on the left side. The left side of the brain will control the right side of the body. And since all their attnetion is now directed toward the right side in this case, they circle to the right.
