The Eye & Ear: Special Sense Organs
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Sclera (fibrous layer):
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Protects eyeball and provides sites for muscle insertion
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Consists mainly of dense connective tissue with bundles of type 1 collagen.
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Tendons of the extraocular muscles which move the eyes insert into the anterior region of the sclera
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The area where it surrounds the choroid, it contains suprachoroid lamina with less collagen.
Cornea (fibrous layer):
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Anterior 1/6​th
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Transparent and avascular
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Has 5 distinct layers:
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External stratified squamous epithelium (nonkeratinized)
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Anterior limiting membrane (Bowman’s membrane)
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Strength and stability of cornea
Thick stroma
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90% of the cornea
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Has keratocytes
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Posterior limiting membrane (Descemet’s membrane)
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Inner simple squamous epithelium
Limbus (fibrous layer):
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Encircles the cornea
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This is where cornea merges with sclera
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The bowman’s membrane here becomes more stratified as the conjunctiva that covers the anterior part of the
sclera.
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Has progenitor cells that move to the corneal epithelium
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Fluid (aqueous humor) moves through channels of trabecular meshwork into the canal of Schlemm (scleral
venous sinus) which encircles the eye. From there it drains into small blood vessels of sclera.
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Choroid (vascular layer):
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Consists of loose well-vascularized connective tissue
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Has many melanocytes which prevent light from entering the eye except through the pupil
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Has 2 layers:
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Inner choroido-capillary lamina:
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Bruch’s membrane:
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Rich microvasculature important for nutrition of outer retinal layers
Composed of collagen and elastic fibers surrounding the retina’s pigmented layer
Ciliary body (vascular layer)
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Encircles the lens, posterior to the limbus
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Rests on the sclera
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Important structures:
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Ciliary muscle:
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Most of the ciliary body’s stroma
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Has 3 groups of smooth muscles
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Their contraction change the shape of the lens for visual accommodation
Ciliary processes:
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Have Na+/K+ - ATPase activity and secrete aqueous humor
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This fluid is secreted into the posterior chamber (between lens and iris) and moves through
the pupil into the anterior chamber (between iris and cornea) and then drains at the angle
formed by the angle formed by the cornea and the iris into the canal of Schlemm.
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Ciliary zonule:
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Composed of fibrillin 1 and 2
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Hold the lens in place
Iris (vascular layer):
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Has a central pupil
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Its posterior surface is filled with melanin which blocks all light entering the eye except passing through the pupil.
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Myoepithelial cells for a partially pigmented epithelial layer and extend contractile processes called dilator
pupillae muscle (has sympathetic innervation to enlarge pupil)
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Smooth muscle fibers form a circular bundle near the pupil known as the sphincter pupillae muscle (has
parasympathetic innervation to constrict pupil)
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Lens:
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Biconvex structure behind the iris which focuses light on the retina
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Derived from the embryonic surface ectoderm
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Avascular and elastic
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Has 3 components:
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Homogenous lens capsule composed of type 4 collagen and surrounds lens. This is where ciliary zonule
attaches.
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Subscapular lens epithelium that consists of single layer of cuboidal cells and are only on the anterior surface
of lens.
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Lens fibers with a cytoplasm that has proteins called crystallins and are for refraction.
Accommodation: permits focusing on near and far objects by changing the curvature of the lens.
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When at rest, ciliary muscles relax and lens is flat
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When focusing, ciliary muscles contract, thus relieving tension on zonule thus lens becomes more rounde.
Presbyopia: happens in fourth decade of life. The lens loses elasticity and ability to undergo accommodation.
Viterous body:
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Occupies the vitreous chamber behind the lens
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Is 99% water (vitreous humor) with collagen fibrils and hyaluronate.
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The only cells in it are called hyalocytes which synthesize hyaluronate and collagen and a few macrophages.
Retina:
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Innermost tunic of the eye
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Has 2 layers from the inner and outer layers of embryonic optic cup:
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Outer pigmented layer:
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Simple cuboidal epithelium attached to Bruch’s membrane on choroido-capillary lamina
Inner retinal region (neural layer)
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They allow growth of lens
Has various neurons and photoreceptors
The Retina pigmented epithelium:
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Cuboidal or low columnar cells
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Have gap junctions
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Melanin granules are numerous
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Numerous phagocytic vacuoles and secondary lysosomes and SER specialized for retinal isomerization (vitamin-A)
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Has different functions:
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Absorbs scattered light that passes through the neural layer
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Protective blood-retina barrier, isolating retina photoreceptors from highly vascular choroid and regulating
ion transport between these compartments.
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Responsible for retina regenerating, having enzymes that isomerize all-trans-retinal released from
photoreceptors and produce 11-cis-retinal that is transferred back to photoreceptors.
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Phagocytosis of shed components from adjacent photoreceptors
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Remove free radicals by antioxidant activities and support neural retina by secretion of ATP.
Neural retina:
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Has 9 layers:
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Near pigmented epithelium, there is the ​outer nuclear layer ​ which contains cell bodies of photoreceptors
(rods and cones)
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Inner nuclear layer ​contains nuclei of various neurons notably the bipolar cells
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Ganglionic layer ​near the vitreous body has neurons (ganglion cells) with large axons.
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these axons make the ​nerve fiber layer ​and converge to form the optic nerve
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outer plexiform layer: ​includes axons of the photoreceptors and dendrites
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inner plexiform layer: ​consists of axons and dendrites connecting neurons of the INL with the ganglion cells.
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Rod and cone layer ​ where rods and cones with polarized neurons are found
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All neurons of the retina are supported physically by glial cells called ​Muller cells ​ which have their
nuclei in the INL.
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These cells organize 2 layers:
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Outer limiting layer:
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Inner limiting layer
The layers of the retina by order:
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Viterous body
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Inner limiting layer
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Nerve fiber layer
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Ganglionic layer
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Inner plexiform layer
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Inner nuclear layer
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Outer plexiform layer
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Outer nuclear layer
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Outer limiting layer
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Rod and cone layer
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Non-neural pigmented layer of retia
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Choroid
Rod cells:
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Around 92 million
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Very sensitive to light, responding to a single photon
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Have 2 segments:
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Outer segment: modified primary cilium and photosensitive
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Inner segment: has glycogen and polyribosomes for cell’s biosynthetic activity
The rod-shaped segment (outer) has 600 to 1000 membranous discs
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Proteins on the cytoplasmic surface of each disc include rhodopsin (visual purple) which initiates visual
stimulus.
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Connecting stalk is between outer and inner segment which is part of modified primary cilium
Cone cells:
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Less numerous (around 4.6 million) and less sensitive to light
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Produce color vision in bright light
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3 classes of cones each containing 1 type of visual pigment iodopsin (photopsin)
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Each iodopsin has sensitivity to different wavelength (red, blue or green)
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Have inner and outer segments and connecting stalk.
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Discs in cones are shed less frequently than in rods.
Phototransduction:
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Each visual pigment has opsin protein with a small light-sensitive chromophore bound to it.
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Vitamin A derivative called retinal acts as the chromophore of rhodopsin in rods.
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In darkness, rhodopsin is not active and cation channels in cell membrane are open
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Cell is depolarized and releases neurotransmitters at synapse with bipolar neurons
When retinal on rhodopsin absorbs a photon of light, it isomerizes from 11-cis-retinal to all-trans-retinal ​→
configuration change in opsin ​→​activates membrane-associated protein transducin (heterotrimeric G protein to
which opsin is coupled) ​→​transducing closes cGMP-gated Na+ channels ​→​hyperpolarization →
​ ​reduces synaptic
release of neurotransmitters ​→​depolarizes sets of bipolar neurons which send action potentials to ganglion cells
of optic nerve
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Bleaching : when chromophore dissociates from opsin (induced by light)
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All-trans-retinal is transported from the rod to adjacent pigmented epithelial cell where it is converted back to
11-cis-retinal then transported back into a photoreceptor for reuse.
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Specialized areas of the retina:
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Optic disc (blind spot) has no photoreceptors and conducting neurons
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Produces optic nerve which leaves the retina
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Also has central artery and vein of retina
Fovea centralis is where visual acuity is maximal.
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Has only cone cells at its center with ganglion cells and neurons at its periphery
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Has no blood vessels
Macula lutea surrounds to fovea
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Its plexiform layers have carotenoids which give it the yellowish color
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Carotenoids have antioxidant properties and filter damaging short-wavelength light
Nonvisual receptors contain 11-cis-retinal bound to protein melanopsin and serve to detect changes in light
quantity and quality during each dawn/dusk cycle.
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Their axons are in the retinohypothalamic tract to the suprachiasmatic nuclei and pineal gland for the
circadian rhythm.
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Accessory structures of the eye:
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Conjunctiva:
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Transparent mucosa that covers anterior portion of sclera and continues as inner surface of eyelids
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Has numerous goblet cells
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Mucous secretions are added to the tear film that protects cornea
Lacrimal glands:
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Produce fluid for tear film that lubricates cornea and conjunctiva and supplies oxygen to the corneal
epithelial cells
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Has lysozyme
● EARS
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External ear:
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Auricle (pinna) is funnel-shaped plate of elastic cartilage covered by skin and directs sound waves into the ear
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Waves enter external acoustic meatus (stratified squamous epithelium).
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Near its opening there are hair follicles, sebaceous glands and modified apocrine sweat glands called
ceruminous glands.
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Cerumen is the waxy material is for protection
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The wall of the external auditory meatus has elastic cartilage
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Tympanic membrane (eardrum) has fibroelastic connective tissue and it is vibrated by waves to transmit energy
to middle ear.
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Middle ear:
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Contains air-filled tympanic cavity (between tympanic membrane and bony surface of internal ear)
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Communicates with the pharynx through auditory tube (estachian tube)
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In the medial bony wall of the middle ear are 2 small bones called oval and round windows with the internal ear
behind them.
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The tympanic membrane is connected to the oval window by 3 small bones (auditory ossicles) which transmit
mechanical vibrations of the tympanic membrane to the inner ear
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Malleus, incus and stapes
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Mallues is attached to the tympanic membrane and the stapes to the membrane across the oval window
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The 3 bones articulate at synovial joints
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Tensor tympani and stapedius muscles insert into the malleus and stapes respectively, restricting ossicle
movements and protecting the oval window and inner ear from loud sound
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Internal ear:
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Found in temporal bone and has bony labyrinth and membranous labyrinth.
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The embryonic otic vesicle (otocyst) forms membranous labyrinth with its major divisions:
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2 connected sacs (utricle and saccule)
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Semicircular ducts
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Cochlear duct
Sensory regions that have hair cells:
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The 2 maculae of the utricle and saccule
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3 cristae ampullares in semicircular ducts
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Spiral organ of Corti in cochlear duct
The membranous labyrinth is in the bony labyrinth which contains:
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Vestibule which houses saccule and utricle
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Osseous semicircular canals that enclose semicircular ducts
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Cochlea which has cochlear duct
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It makes 2.75 turns around a bony core called modiolus
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Modiolus contains blood vessels and surrounds cell bodies of acoustic branch of CN8 in the
large spiral (cochlear) region
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Fluids:
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Perilymph:
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Fills all regions of the bony labyrinth
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Similar to CSF
Endolymph:
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Fills membranous labyrinth
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Has high K+ and low Na+
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Similar to intracellular fluid
Utricle and saccule:
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Maculae in both is innervated by branches of the vestibular nerve
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Hair cells in maculae act as mechanoelectrical transducers, converting mechanical energy into electrical
energy of nerve action potentials
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Each has an apical hair bundle consisting of 1 rigid cilium (kinocilium) and a bundle of 30-50 sterocilia.
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Tips of stereocilia and kinocilium are in the otolithic membrane (gelatinous layer of
proteoglycans)
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This membrane has CaCO3 and protein called otoliths
2 types of hair cells:
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Type 1 hair cells: completely surrounded by afferent terminal calyx
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Type 2 hair cells: are more numerous with bouton endings from afferent nerves
Sensory information from utricle and saccule allows the brain to monitor static position and linear
acceleration of the head (for equilibrium)
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Bending of stereocilia changes the hair cells’ resting potential
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When the hair bundle Is bended toward the kinocilium, protein fibrils called tip links connecting the
stereocilia and pulled and mechanically gated channels open allowing influx of potassium ions thus
depolarizing hair cells which opens voltage-gated Calcium ion channels which stimulates release of
neurotransmitter and generates impulse
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When head stops moving, stereocilia straighten and hair cells quickly repolarize ​→​resting potential
Semicircular ducts:
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Extend from and return to utricle
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Each duct has ampulla end containing hair cells and supporting cells on a crest of the wall called crista ampullaris.
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Hair cells here act as mechanoelectrical transducers like the ones in the maculae
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These mechanoreceptors detect rotational movements of the head as they are bent by endolymph movement in
the semicircular ducts
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Neurons of vestibular nuclei in the CNS receive input from semicircular ducts to interpret head rotation
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Inputs from semicircular ducts travel together with those from utricle and saccule along the eighth cranial nerve
to vestibular nuclei of CNS.
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Cochlear duct:
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Part of membranous labyrinth
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Contains hair cells and other structures for auditory function
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Scalae:
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The cochlear duct itself forms the middle compartment or ​scala media ​ filled with endolymph. It is
continuous with the saccule and ends at apex of cochlea.
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Scala vestibule ​contains perilymph and is separated from scala media by vestibular membrane (Ressiner’s
membrane)
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has tight junctions to block ion diffusion between perilymph and endolymph
scala tympani ​also contains perilymph and is separated from scala media by basilar membrane
Scala tympani and vestibuli communicate with each other at the apex of cochlea via an opening called
helicoterma.
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Stria vascularis is located on the lateral wall of the cochlear duct(scala media) and produces endolymph with high
levels of potassium ions
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Organ of Corti (spiral organ) is where sound vibrations of different frequencies are detected and consists of hair
cells.
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The sensory hair cells have V-shaped bundles of rigid stereocilia (no kinocilium)
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2 major types of hair cells present:
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Outer hair cells: about 12,000 in total and occur in 3 rows near the saccule, increasing to 5 rows
near apex of cochlea.
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Inner hair cells: shorter and form a single row of about 3,500 cells.
2 major types of columnar supporting cells are attached to basilar membrane in the organ of Corti:
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Inner and outer ​phalangeal cells
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Pillar cells ​have bundles of keratin
On the outer hair cells, the tips of the tallest stereocilia are embedded in tectorial membrane (acellular layer) that
extends over the organ of Corti.
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Tectorial membrane consists of bundles of collagen (type 2, 5, 9, 11)
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Hair cells are also mechanoelectrical transducers and mediate sense of hearing
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High-frequency sounds displace the basilar membrane maximally near the oval window
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Low-frequency sounds are farther along the scala vestibule and displace the organ of Corti at points farther from
the oval window.
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Depolarization of outer hair cells causes columnar cells to shorten very rapidly, an effect mediated by
transmembrane protein called prestin.