UNIT 5- The special senses eye and vision Flashcards
Special senses
-Sight
- Smell
- Taste
- Hearing
- Equilibrium
Functions of especial senses
To provide us with information about our internal and external environment.
Sensation
an awarenes of a change in our internal and/ or external environment ( but might not be on a conscious level)
Perception
our conscious interpretation of those stimuli
conditions requiered for perception
stimulus
receptor
conduction
interpretation
Orbit
Bone socket around the eye protecting the porterior 80%
Eyebrows
-Help shade the eyes from sunlight
-Prevent sweat from trickling into the eyes
Eyelids= Palpebrae
▪ Protect the front of the eye.
▪ Eyelashes offer added protection by preventing entry of dust and other particles.
Levator palpebrae supeioris muscle
Muscle that raises the eyelid
Conjuctiva
▪ Mucous membrane that lines inside surface of the eyelid and portion of the anterior surface of the eye (it covers the sclera but not the cornea)
Function:
Produces a lubricating mucus that prevents the eye from drying out.
Lacrimal apparatus
▪ Consists of ductwork and the lacrimal gland which makes lacrimal fluid (tears).
▪ Lacrimal fluid contains mucus, antibodies, and lysozyme, an enzyme that destroys bacteria.
▪ Overall, lacrimal fluid cleanses and protects eye as well as lubricates and moistens the eye.
Pathway of lacrimal fluid flow
- Blinking spreads tears from lacrimal gland across to medial corner of eye.
- Tears flow through lacrimal puncta into lacrimal canaliculi.
- Tears next enter lacrimal sac.
- Then, the nasolacrimal duct.
- Finally, lacrimal fluid empties
into nasal cavity below
inferior nasal concha.
Extrinsic eye muscle
●Each eye has six associated extrinsic eye muscles that insert on the sclera and cause movement of the eyeball.
Innervation of the extrinsic eye muscles
▪ Lateral rectus is innervated by the abducens nerve (VI)
▪ Superior oblique is innervated by the trochlear nerve (IV)
▪ Other four are innervated by the oculomotor nerve (III)
Instrinsic eye muscles
▪ Ciliary muscles – control focusing of the lens.
▪ Iris – controls the diameter of the pupil; continuous with the ciliary body posteriorly.
Tunics of the eye - Fibrous tunic , vascular tunic , nervous tunic) - layers that form the outer part of the eye.
Fibrous tunic , vascular tunic , nervous tunic
Fibrous tunic - characteristics
▪ outermost
▪ tough fibrous connective tissue
▪ gives shape to eyeball
▪ protects inner structures
Firbous tunic - main parts
Sclera - white of the eye
Cornea - clear window over anterior portion of iris and pupil; permits entry of light
Vascular Tunic - characteristics
▪ middle layer
▪ has many blood vessels
Includes :
-Iris
-Ciliary body
- Choroid
Vascular tunic - includes
▪ Iris – anterior portion of vascular tunic that forms a ring of muscles around the pupil.
◦ Circular muscle fibers – when these contract, the pupil constricts
◦ Radial muscles fibers – when these contract, the pupil dilates.
- Pupil – opening that light passes through.*
Vascular tunic- includes 2
▪ Ciliary body – thickened portion of tunic located behind the iris; forms a muscular ring around the lens.
Function of ciliary body:
▪ Controls shape of lens, and thus the focus.
suspensory ligaments – c.t. fibers that connect ciliary body to lens.
Vascular Tunic - includes
▪ Choroid – darkly pigmented layer that surrounds most of the inside of the eye.
Function of choroid:
▪ Helps nourish eye.
▪ Absorbs light & prevents it from scattering/reflecting in eye.
Nervous Tunic - Retina
▪ innermost layer composed of nervous tissue
▪ begins slightly behind the ciliary body and covers the inner surface of the eye
Function of Retina:
▪ To convert light into an action potential
(nerve impulse)
A closer look at the retina - componets
▪ outer pigmented epithelial layer which abuts the choroid
▪ three layers of neurons
Neuron Layers of retina ( Photoreceptors, bipolar neurons, ganglion naurons)
- Photoreceptors – actual receptors that sense light and initiate a nerve impulse
- outermost nerve cells of retina (located next to pigmented epithelium).
Two types: ◦ Rods◦ Cones
Bipolar neurons
middle layer of neurons
- connect photoreceptors to ganglion
neurons
Ganglion neurons
innermost layer of nerve cells
- synapse with the bipolar neurons
→ their axons combine to form the optic nerve.
Optic Disc
The optic disc is the site where fibers of optic nerve come together to penetrate the choroid and sclera to exit through the back of the eye.
This is the “blind spot” because:
No photoreceptors are located here.
Macula lutea - Retina special features
- oval spot in the retina at the point directly behind the pupil. This area has a high proportion of cone cells.
Fovea centralis
small depression in center of macula lutea which contains only cones.
- This is our point of clearest vision; anything we view in detail must be focused here.
Cones
Cones – densely concentrated in fovea centralis and progressively decline in density towards the edges of the retina.
Rods
Rods – densely concentrated at edges of retina and decrease in density as you move towards the fovea centralis.
Cavities & Humors of the eye
Anterior cavity, aqueous humor, scleral venous sinus, posterior cavity, vitreous humor, lens)
Anterior Cavity (segment)
▪ Space between lens and the cornea
▪ Filled with aqueous humor
Aqueous Humor
▪ Thin watery fluid produced by specialized capillaries in the processes of the ciliary body.
▪ Secreted into anterior cavity between lens & iris.
▪ Supplies O2 and nutrients to cornea & lens and also removes wastes.
Scleral venous sinus
(Canal of Schlemm)
▪ Drains aqueous humor into the venous blood
▪ Located at cornea– scleral junction
What disorder results from blockage of this canal?
Glaucoma
Posterior Cavity (Segment)
▪ Space between lens and retina
▪ Filled with vitreous humor
Vitreous Humor
▪ A transparent, jelly-like fluid.
▪ Maintains shape of eye & holds retina in place.
▪ Vitreous humor is formed prenatally; none is formed after birth.
Lens
▪ Biconvex disc composed mainly of folded proteins called crystallins.
▪ It is anchored to the ciliary body by suspensory ligaments.
Function:
▪ To focus light on the retina.
What is the condition called when “clouding” of the lens occurs?
Cataract
Physiology vision
Formation of an image on the retina
Transmission of light
Light must pass from anterior of eye through the eye to reach the retina on the posterior surface.
Refraction
▪ Refraction is the bending of light rays.
→ it occurs as light moves from one medium to a medium of a different density.
→ it can also occur as light strikes a curved object.
▪ Most refraction occurs
at the cornea and lens.
Refraction
▪ Since the curvature of an object affects refraction, the curvature of the lens affects the amount of refraction light will experience as it strikes and passes through.
▪ Thus, by adjusting the shape of the lens, we are able to provide fine control for focusing light directly on the retina.
Accomodation of the lens
Lens changes shape in response to contractions of the ciliary muscles to focus images on the retina.
Specifically:
▪ When ciliary muscles contract, the lens becomes more rounded to focus on near images.
▪ When ciliary muscles relax, the lens becomes flattened to focus on distant images.
Converge of the eyes
▪ Both eyes swivel medially to point at the object viewed, especially when viewing close objects.
▪ Allows both eyes to focus on the same thing, preventing double vision.
Constriction of the pupil
▪ Allows only the amount of light necessary for clear vision to enter the eye and keeps out rays that cannot be focused.
For close objects or bright light conditions:
- circular muscles of iris contract to constrict pupil.
For distant objects or dim light conditions:
- radial muscles of the iris contract to dilate pupil.
Notes on image formation
▪ The brain can sense the degree of accommodation and convergence and use this to judge the distance of an object.
▪ The image formed on retina is upside-down & reversed from right to left.
- It’s the brain’s job to interpret this
Retinas’s initiation of nerve impulses - steps 1
- Light passes through ganglion & bipolar neurons and then strikes the photoreceptors.
Retina’s initiation of nerve impulses - step 2
- Photoreceptors send signal to the bipolar neurons.
Retinas’s initiation of nerve impulses - steps 3
- The bipolar neurons transmit the signal to the ganglion neurons.
Retin’as initiation of nerve impulses - step 4 & 5
- The ganglion neurons, whose axons contribute to the optic nerve, generate an action potential.
- The optic nerves will convey this visual sensory information to the CNS.
Conduction pathway
Optic nerve → optic chiasma →
optic tract → thalamus →
visual cortex of occipital lobe
Stimulation of Rods for Black & White Vision
▪ Rods exhibit much neuron convergence.
(Many rods synapse with one bipolar neuron and many bipolar neurons synapse with one ganglion neuron.)
→ this produces fuzzy vision
Stimulation of cones for color vision
▪ Cones exhibit little neuron convergence.
(An impulse from one cone cell travels to only one ganglion neuron.)
→ this produces sharper vision.
▪ Three types of cones:
▪ Blue ▪ Green ▪ Red
◦ Each responds to different wavelengths of light.
◦ Perception of color is determined by the number of each type of cone cell stimulated.
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