Exam 3 Week 14 ppt 3&4 optics & projections Flashcards
How does light refract in convex lens?
Convex lens will bend light at different angles along its curved surface & will focus light coming from a distant image at fixed point some distance from the convex lens
focal length of a convex lens
-point at which the light is focused -dependent upon the shape of the convex lens
focal length in normal eye
matches the distance from lens of eye and retina so image is focused on the retina
What is myopia?
–Nearsightedness
–Overly curved lens & focal length falls short of retina
–Corrected with the use of a concave lens to lengthen the focal distance
What is Hyperopia?
–Farsightedness
–Lens not curved enough so focal length falls behind the retina
–Corrected with the use of a convex lens to shorten the focal distance
How does lens accomidate for near and far vision?
–Lens is pliable & changes shape
–The lens is held by suspensory ligaments whose tightness is controlled by the ciliary muscles.
How do ciliary muscles respond For vision > 20 ft
relaxed & suspensory ligaments taut
How is lens affected For vision > 20 ft?
Lessened curvature of lens to adapt to far vision
How do ciliary muscles accommodate for close vision?
contract & suspensory ligaments loose
How does lens accomidate for close vision?
Greater curvature of lens to adapt to close vision
How does accommodation affect pupils? (4)
- Change in pupillary diameter
- Pupils constrict for close vision
- Constricted pupils produce a greater depth of field so objects at different distances will still stay in focus
- Depth of field need for reaching of close objects
How do parasympathetics to eye come into play during accomidation?
- Oculomotor nerve (CN III) from Edinger-Westphal nucleus via ciliary ganglion
- Pupils constrict & ciliary muscle contracts
What is presbyopia?
–Lens hardens with age
–Age related loss of power accommodation even though ciliary muscles can still contract
–Results in inability to focus on near objects
Visual field
portion of space that can be viewed from the retina when the eye is fixated straight ahead
Retinal field
the portion of retina that alters its firing rate in response to a stimulus
Action of lens with regards to visual and retinal field
•reverses and inverts image so reversed & inverted relationship between the visual and retinal fields
1° afferent pathway
bipolar cells
pathway of 2° neuron with ganglion cell
–Axons form optic nerve
–Axons from nasal retinal field decussate in optic chiasm but temporal retinal fields stay ipsilateral
–So right visual field goes to left brain & left visual field goes to right brain via the optic tract
Pathway of 3° neurons - Lateral geniculate nucleus of thalamus (6 points to know)
–Retinotopic pattern with half of LGN devoted to fovea
–Respond similar to ganglion cells with “on” or “off” center and opposing surround
–Only 10-20% synapses from optic tract
–Most input from visual cortex & brainstem reticular formation
- Cortical inputs provide feedback regulation of flow of information to cortex and is related to visual attentiveness
- Brainstem input is part of switch of attentiveness between sensory inputs
What do neurons of the LGN project as?
optic radiation or geniculocalcarine tract to the occipital lobe
Where do dorsal axons from the inferior visual field project to?
almost directly to the occipital lobe
Geniculocalcerine tract: Describe the two pathways, where their axons are from in the visual field, and where they end
Superior visual field axons:
- –Ventral axons (from the superior visual field) project anteriorly and downward as Meyer’s loop
- –Inferior retinal (superior visual) field goes to inferior aspect of calcarine sulcus
Inferior visual field axons:
- –Dorsal axons (from the inferior visual field) project almost directly to the occipital lobe
- –Superior retinal (inferior visual) field goes to superior aspect of calcarine sulcus
Describe the path of the superior visual field axons in the geniculocalcarine tract:
Superior visual field axons:
- –Ventral axons (from the superior visual field) project anteriorly and downward as Meyer’s loop
- –Inferior retinal (superior visual) field goes to inferior aspect of calcarine sulcus
Describe the path of the inferior visual field axons in the geniculocalcarine tract:
Inferior visual field axons:
- –Dorsal axons (from the inferior visual field) project almost directly to the occipital lobe
- –Superior retinal (inferior visual) field goes to superior aspect of calcarine sulcus
Geniculocalcarine tract (4)
- Ventral axons (from the superior visual field) project anteriorly and downward as Meyer’s loop
- Inferior retinal (superior visual) field goes to inferior aspect of calcarine sulcus
- Dorsal axons (from the inferior visual field) project almost directly to the occipital lobe
- Superior retinal (inferior visual) field goes to superior aspect of calcarine sulcus
What is direct retinohypothalamic projection?
Small contingent of optic tract axons terminate in the hypothalamus – suprachiasmatic nucleus
Role of direct retinohypothalamic projection
>
–entraining circadian rhythms in day–night cycle
Where do some optic tract axons go specifically that bypass the LGN and project to in midbrain? (2)
and what is the function of projections to those areas?
- Superior colliculus for visual reflexes
- Accessory optic & pretectal area for visual light reflexes (pupillary constriction)
Pulvinar- 3 points to know
- Largest thalamic nucleus
- Receives input from the superior colliculus, visual cortex & pretectum
- Sends info to visual association areas in the posterior parietal lobe to supplement dorsal stream
what specific nucleus do the retinohypothalamic projections go to?
(and what is it part of?)
the suprachiasmic nucleus of the hypthalamus
