Visual Senses Flashcards
what is the role of the lacrimal duct
secrete saline tear
- cleans and lubracates front of the eye during a blink
- prevents cornea from drying out and frost injury
- tears are emptied into the nasal cavity to maintain mucous membrane function
- lysozyme + immunoglobulin A to protect against infections
what are the axes of the eye
optical axis gives most optically clear image (crosses cornea, lens dead centre)
visual axis to fovea gives best colour vision
what is the refraction of an image
light bends when passing from across the cornea, lens and vitreous humour
light information through a convergent lens are refracted and focused at a focal point
what is accomodation
ability of lens to change power through the ciliary muscles
focal length is fixed but increased power increases ability of lens to refract divergent light (near objects) and achieve focus
what is the species variation of accomodation
horses lack the full ability to accomodate
shape of retina means it can see near and far by moving head/eyes
what are the subcortical reflexes
- pupillary light reflex
- dazzle reflex
what is the clinical relevance of the PLR
indicate functional state of the afferent and efferent pathways that control the pupil
ex. if a light stimulus directed to the left eye elicits a consensual constriction in the right eye, but not a direct one in the left eye –> afferent limb of reflex (optic nerve) is intact but the efferent limb to the left eye is damaged (damage to the oculomotor nerve)
what is the dazzle reflex and what does it indicate
subcortical mediated brainstem response
palpebral fissure closes in response (blink, head movement) in response to sudden intense illumination of the eye
ipsilateral response is greater than the contralateral response
absence of response indicates blindness
what is the PLR pathway (7)
- light penetrates the globe of the eye to the retina
- electrochemical receptors in the retina are excited and the generated impulses are proected into the optic nerve (CN II)
- light is projected from retina through the optic nerve to the optic chiasm where the optic nerve fibres cross over to enter the contralateral optic tract
- after crossing at the chiasm the impuse is proejcted through the optic tract to synapse on the pretectal nucleus
- pretectal neurons synapse on the opposite oculomotor nuclei
- stimulation is related along axons of parasympathetic portion of CN III out of brainstem and to the ciliary ganglion
- post ganglionic fibres in the short ciliary nerves innverate the ciliary muscles and the pupillary constrictor muscles
what is the components of the PLR pathway
what are the cells in the neural response of vision
what are the five major cell types in the retina
- photoreceptor cells
- bipolar cells
- horizontal cells
- amacrine cells
- ganglion cells
what occurs when light strikes rhodopsin/opsin
- in dark, many gated Na+ and Ca++ channels remain open –> allows leakage of Na+ and Ca++ ions into the rod –> keeps membrane at depolarized state
- when photons of light strike rhodopsin/opsin –> isomerization of retinal
- activation of alpha subunit of transducin
- activation of photodiesterase (PDE)
- decreases cGMP levels
- closes ion channels
- hyperpolarizes the cell –> decreases the transmitter released at synapse with bipolar cell
what is the distribution of photoreceptors in the retina
ratio of receptors to nerves also change across retina
at fovea nerve number = receptors
at periphery ~300 receptor/afferent nerve
what is melanospin
signals light control for daily rhythms
ganglion cells show intrinsic photoresponsiveness
melanopsin is a photoreceptor that responds to irradiance info –> mutations have detrimental effects on daily rhythms
what is the neural circuit from the retina to brain
goes to the lateral geniculate nucleus located in the thalamus
what does the LGN maintain
a retinotopic (topography)
what are the cells in the LGN
6 layers of the retinal map consists of parvocellular (1-4) and magnocellular (5,6) cells
parvocellular cells are small, integrate signals from cones and necessary for colour and form
magnocellular cells are large, integrate signals from rods, involved in movement, depth and irradiance
what integrates LGN cells
primary visual cortex
how does the visual pathway work
- the right environmental visual field projects to the left half of each retina and the left environment visual field projects to the right half of each retina
- the right half of each retina sends info to the right lateral geniculate and from there to the right primary visual cortex
- the left half of each retina sends info to the left lateral geniculate and to the left primary visual cortex
information from the left visual field arives in the right primary visual cortex and information from the right visual field arrives in the left primary visual cortex
how is motor movement controled
dorsal stream flows from V1 to V5/V3A
guides movements such as the hand posture for grasping mug or pen
damage to ventral stream prevents identification of objects; damage to dorsal stream results in optic ataxia
reaching movement directed toward individual body (tactile or proprioceptive feedback) remains intact
what are the four pathways of central visual and non-visual pathways
- lateral geniculate nucleus –> visual cortex
- rostral colliculi –> dazzle reflex
- pretectum, pons –> pupillary light reflex
- suprachiasmatic nucleus –> hypothalamic pineal gland
what are the additional visual pathways for proprioception and posture
visual proprioceptive system –> linked to vestibular apparatus
provides information by which head movement is sensed
what are the theories of colour vision
- trichromatic theory: colour vision is based on coding of 3 basic colours –> red, green, blue
- opponent-process theory –> colour vision that emphasizes the importance of the opposition of pairs of colours –> red vs. green and blue vs. yellow
where does colour vision start
in ganglion cells
multiple colour opsins information is integrated by ganglionic cells then transmit info to LGN parvocellular cells
what is colour vision represented by in the visual cortex
blobs
complex neurons in the visual temporal cortex (V4) respond to objects in the visual field that have colour that is dependent on orientation, texture and shape
what is special about photoreception in birds
photoreceptors are expressed in multiple tissues
brain photoreceptors control many physiological processes –> rapid effects on reproduciton
