Special senses Flashcards
What percentage of our sensory receptors does the eye contain?
70%
How much of the cerebral cortex is involved in processing visual information?
1/2
What is the function of the eyebrows, eyelids, and eyelashes?
to protect the eyes
What do the eyebrows do to protect the eyes?
- SHADE eyes from bright light
- prevent sweat from trickling in to eyes
How do the eyelashes act to protect the eyes?
they are innervated with many sensory nerve endings that trigger reflex response to stimuli
How do the eyelids act to protect the eyes?
- muscles are activated by reflex to cause blinking
- spread fluids across the eye surface to prevent from drying out
- lubricants are produced by glands on the edge of the eyelid and between the eyelash folicles
What muscle closes the eyelids?
contraction of orbicularis oculi
What muscle opens the eyelids?
contraction of levator palpebrae superioris
What is the conjunctiva of the eye?
the transparent mucous membrane that covers the eye
- produces lubricating mucous to prevent drying of the eyes
What are the two parts of the conjunctiva?
PALPEBRAL conjunctiva: lines the eyelids and folds back over the anterior of eyeball
BULBAR conjunctiva covers the white of the eye (but not the cornea)
Where do contact lenses sit in the eye?
in the conjunctival sac
What makes up the lacrimal apparatus? What do each of these do?
Lacrimal GLANDS: continuously produce lacrimal fluid (tears)
Lacrimal FLUID: cleans, protects, disinfects the eye
Lacrimal DUCTS: drain tears into the nasal cavity (under normal flow rates)
What are some substances that the lacrimal fluid contains?
mucus
antibodies
lysozymes
How many extrinsic eye muscles are there?
6
What cranial nerves control the external eye muscles?
VI abducens
III Oculomotor
IV trochlear
What are the two types of movement of the eye balls that the extrinsic eye muscles allow?
SACCADES: quick jerky movements that allow eye to quickly see whole visual field
SCANNING MOVEMENTS: tracking or following of an object
What is diplopia?
homeostatic imbalance of extrinsic eye muscles:
it is double vision
Results when the extrinsic eye muscles are not properly coordinated
What are the two types of strabismus.
homeostatic imbalance of extrinsic eye muscles: Internal strabismus (cross-eyed)
external strabismus (wall eyes): eyes turn away from nose
What causes internal strabismus?
damage to the lateral rectus muscle or the abducens nerve
What causes external strabismus?
damage to the medial rectus muscle or oculomotor nerve
What are the three layers (or tunics) of the eyeball?
Fibrous (outer)
Vascular
Sensory
What are the two components of the fibrous tunic of the eye?
Schlera (white)
Cornea (transparent)
Describe the structure of the schlera of the eye
1) made up of opague, dense, tough CT
2) attaches to the dura mater posteriorly around the optic nerve
What is the function of the schlera of the eye?
protects and shapes the eyeball, provides strong anchoring site for the extrinsic eye muscles
Describe the structure of the cornea.
Forms the anterior portion of the fibrous tunic
- composed of transparent, closely packed collagen fibers
- many nerve endings; trigger blinking and tear reflexes
- NO blood supply
- vulnerable to damage but regenerates quickly
What is an important aspect of the cornea regarding implants?
makes it easy to find suitable donors because it has no blood supply (won’t be rejected)
What are the four components of the vascular tunic of the eye?
Choroid
Ciliary body
Iris
Lens
Describe the structure and function of the choroid of the vascular tunic of the eye.
- pigmented layer at back off eyeball
- MELANIN absorbs scattered light to prevent interference with vision
- VESSELS supply nutrients to all the tunics
What is the ciliary body of the vascular tunic of the eye?
ring of tissue surrounding the lens.
consists of three parts
What are the three parts of the ciliary body? What do they do?
ciliary PROCESSES: produce the aqueous humour
ciliary ZONULE (suspensory ligaments): - extends from ciliary process to lens, hold lens upright
ciliary MUSCLES: bundles of smooth muscle which control lens shape by changing tension in the ciliary zonule
Describe how the lens flattens and bulges to accommodate for near and close objects
When objects are far away, light is more direct therefore require less refraction in the lens to absorb the light so lens flattens. When the object is close, need more refraction to absorb so lens bulges
Ciliary muscles are circular 0 and lens is in the middle attached by ciliary zonule
ciliary muscles contract to dec tension -> bulging
ciliary muscles relax to inc tension -> flattening
Describe the Iris of the eye
it is the coloured portion of the frontal eye
- anterior portion of the choroid
located between the cornea and the lens
contains two layers of smooth intrinsic muscles to control pupil size
How are the intrinsic muscles of the iris of the smooth muscle of the eye innervated? what do each of these cause
dual innervation by the ANS
Parasympathetic triggers constriction
Sympathetic triggers dilation
What are the two layers of smooth muscle in the iris of the eye? How do these two layers interact to control constriction/dilation of the pupil?
CONSTRICTION of pupil: circular layer (sphincter) contracts, longitudinal (dilator) layer relaxes
DILATION of pupil circular layer (sphincter relaxes, longitudinal layer (dilator) constricts
Describe the structure of the Lens.
Biconvex disc composed of transparent fibrous PROTEIN
- changes shape to focus image on the retina
- held in place behind iris by ciliary zonule
- has NO blood supply
Describe what happens to the lens of the eyes as we age.
new fibers are added over time so it becomes thicker and less flexible.
Describe what cataracts are.
homeostatic imbalance of the lens:
inadequate nutrient supply to inner lens -> change in chemical structure of protein fibers -> lens becomes opaque
- most cases associated with age
risk factors: genetics, UV light exposure, chemicals, smoking, and diabetes
Describe what presbyopia is.
homeostatic imbalance of the lens:
also age related
lens becomes thicker and less flexible with age -> cannot accommodate for CLOSE vision
- close objects are blurred. ex reading
Describe the sensory tunic
Also called the retina.
Outer pigmented layer: absorbs stray light and prevents scattering
inner neural layer: contains photo receptors and conducting cells
3 layers of neurons in the neural layer
What are the three layers of neurons in the inner neural layer of the sensory tunic (or retina) of the eye?
deepest -> superficial with superficial being the closest to the surface of the inner eye
PHOTORECEPTORS: absorb and transduce light
BIPOLAR cells: process and conduct signals
GANGLION cells: process and conduct signals. Axons run along the inner surface of the retina and join to form the optic nerve
What are the two kinds of photoreceptors? explain the function and location of each.
RODS: low light and peripheral vision
- densest in periphery and none in fovea
- pathways converge resulting in fuzzy and indistinct images
CONES: high light and high acuity colour vision
- concentrated in fovea, none at periphery
- pathways do not converge resulting in detailed, high resolution images
Explain what causes the blind spot in our eye.
It is the site where the axons of the ganglion join to form the optic nerve
there are no photoreceptors present here
Describe what the macula lutea is.
a oval spot at the posterior pole of the eye.
It is an area concentrated with mainly cones
light is focused onto this spot, passes directly to cones and there are no overlying neurons
Describe what the fovea centralis is.
a depression in the macula lutea that contains ONLY cones
- area of greatest visual acuity
What are the two segments of the eye? What structures divide the eye into these two segments?
Posterior segment
Anterior segment
seperated by the lens and suspensory ligaments
What is the function of the posterior segment?
- transmit light
- reinforce the eyeball, support lens
- holds the retina in place against the pigmented layer
- contributes to intraocular pressure
What type of fluid does the posterior segment contain and when is this fluid formed?
Vitreous humor (gel-like) - formed prior to birth, lasts for lifetime
what type of fluid is in the anterior chamber? what is its function
Aqueous humor (watery): - provides O2 and nutrients to lens and cornea
Describe how the aqueous humor is formed and how it drains.
Continuously formed by ciliary processes from the capillary blood
DRAINS through the scleral venous sinus (canal of Schlemm) into the blood
What are the two chambers of the anterior segment and what divides it.
Anterior chamber: between iris and cornea
Posterior chamber: between iris and lens
IRIS
What wavelengths of light can the eye detect?
400 - 700 nm
What is the purpose of the lens in the eye?
to control the degree to which the light is refracted in order to focus it on a focal point of the retina
focal point is the macula lutea
Describe how the image is formed on the retina
Upside down and reversed right to left
Describe the pathway of light as it enters the eye.
cornea, aqueous fluid, pupil, lens, vitreous humor, neural layer of the retina, photoreceptors
What is the normal preset of the eyes regarding focus.
normal preset is for distance vision
- ciliary muscles are relaxed, ciliary zonule is tense, lens is flat
dec refraction
Describe the three ways that the eyes accommodate for close vision.
1) ACCOMODATION of lens:
Parasympathetic stimulation -> ciliary muscles contract -> dec tension in ciliary zonule -> lens bulges -> inc refraction
2) CONSTRICTION of pupil:
Parasympathetic stimulation -> circular muscles contract
- eliminates divergent light rays
3) CONVERGENCE of the eyeballs:
- extrinsic muscles cooperate to rotate eyes medially
- image focused on fovea in both eyes
Describe what myopia is.
Short sightedness
focus in front of retina because eyeball is too long
TREATMENT: concave lenses (or laser surgery)
Describe what hyperopia is.
Far sightedness
focuses behind the retina, eyeball is too short
Treatment: convex lenses
Describe what astigmatism is.
Unequal curvature in lens or cornea
corrected with lenses, corneal implants, or laser procedures
Describe how the photo receptors function.
Outer segments contain pigments that change shape as they absorb light.
Inner segments attached to cell body
change in shape of the pigments -> change in permeability of the membrane -> receptors become hyperpolarized and stop release of neurotransmitter
info is transmitted by shutting off of stimulus, rather than turning it on.
How do rods perceive light?
as grey tones only
What is the pigment for rods?
Rhodopsin
Describe how rhodopsin functions for rod vision.
Rhodopsin accumulates in the dark by regeneration from bleached retinal and formed from Vit A
light is absorbed -> rhodopsin bleaches -> retinal and opsin separate -> membrane potential becomes hyperpolarized
Describe how colour is determined
Different types of cones have one of these cone pigments:
Red, green, or blue
colour is perceived by combining these stimuli
pigment absorbs light -> bleaches -> retinal and opsin separate -> membrane becomes hyperpolarized
What causes colour blindness?
a deficiency in one of the cone pigments (red, blue, green)
Describe regeneration of photoreceptors
terminal segments of both rods and cones are damaged by absorbing light
- rod terminal discs replaced at end of night
- cone terminal segments are replaced at the end of the day
Describe how the signal is transmitted in the retina of the eye
photo receptors and bipolar cells generate graded potentials
light hyperpolarizes rods and cones -> stop releasing inhibitory neurotransmitter glutamate -> bipolar cells then release their neurotransmitter onto ganglion cells -> ganglion cells generate action potentials and transmit along optic nerve
Describe how the eye adapts to light
- temporary blind because rod pigments are rapidly bleached
- pupils constrict -> directs light to macula lutea
- sensitivity decreases as it switches from rods to cones
Describe how the eye adapts to dark.
cones stop functioning because light intensity is too low
pupils dilate -> rhodopsin accumulates in the rods and they take over
takes 20-30 minutes to reach peak sensitivity
Describe the visual pathway of transmission from photoreceptors to the brain.
photoreceptors -> bipolar cells -> ganglion cells -> axons of ganglions form optic nerves -> medial fibers cross at optic chiasma, later fibers do not cross -> most fibers continue through the lateral geniculate body of the thalamus -> to visual areas of cortex
Where are the three places that optic fibers can go to?
visual cortex
hypothalamus: to coordinate circadian rhythms
midbrain: visual reflex centers
Where does visual processing occur?
RETINA: activation of photoreceptors (sensation)
THALAMUS: preliminary sorting and organizing before sending to cortex
CEREBRAL CORTEX: primary and association cortex. Conscious perception
What do olfactory neurons respond to
chemicals in aqueous solutions
Where is the olfactory epithelium located?
below the cribriform plate of the ethmoid bone
Describe the structure of olfactory neurons
bipolar with a long apical dendrite that terminates in a knob with cilia that extend into the mucous
What type of cells embed the olfactory neurons? what is there function?
columnar epithelial cells
- cushion and support
What is the life span of the olfactory receptors?
60 days
How many chemicals can be detected by olfactory neurons, how many receptor types are there, and what does this suggest about smell?
10000 smells and 1000 receptor types
suggests there is an interaction between them
how many type of receptors does each olfactory neuron have?
just one type
describe the process of smell
substance dissolves in fluid, binds to receptor protein on olfactory cilia -> g protein mech is activated -> produces cAMP as secondary messenger -> opens cation channels -> depolarization -> receptor potential
