Ear and Ear (Vision, Hearing and Equilibrium)

Cherelene Pereira Anatomy 1

Ear and eye (Vision, hearing and equilibrium)

The Ear

The ear consists of three parts: external, middle and inner ear. The ear functions to hear and for equilibrium.

- External Ear

The external ear collects sound vibrations and protects the rest of the ear. It consists of the auricle and the external auditory meatus.

Auricle

The auricle is the elastic cartilage opposite from the external auditory meatus. The rim of the auricle is the helix and the inferior part is the lobe.

Ex ternal Auditory Meatus

The external meatus is about 25 mm long, extends from the concha to the tympanic membrane. Its lateral part is concaved anteriorly and made of cartilage. The longer medial part is bony. The meatus is lined by the skin of the auricle, which contains hairs, and sebaceous and ceruminous glands. In the exterior, there are sebaceous glands called ceruminous glands that secrete earwax (cerumen). The hairs and earwax prevent dust and foreign materials from entering the ear.

Tympanic Membrane

The tympanic membrane (ear drum), is about 1 cm in diameter and separates the external meatus from the tympanic cavity. Its base is attached to the tympanic plate of the temporal bone and is covered laterally by epidermis, and medially by the mucous membrane of the middle ear. The three layers of the ear drum, from superficial to deep, are (1) the epidermis, (2) dense connective tissue containing collagen, elastic fibers and fibroblasts, and (3)simple cuboidal epithelium. The tympanic membrane is positioned obliquely. Its lateral surface is concave, and its deepest point is called the umbo. The handle and lateral process of the malleus (the most superficial ear bone) are attached to the medial surface of the tympanic membrane. Tearing the tympanic membrane is called a perforated eardrum and can occur using a cotton swab or through trauma. Sensory Nerves and Blood Supply

The external ear is supplied by the vestibulocochlear (VIII) nerve and trigeminal (V) nerve (and potential contributions from cranial nerves VII, IX, and X). The posterior auricular and superficial temporal arteries of the external carotid artery provide the blood supply.

-Middle Ear The middle ear lies spacously in the petrous portion of the temporal bone and focuses the sound vibrations to the oval window. It contains the auditory ossicles and communicates with the mastoid air cells and mastoid antrum through the aditus and the nasopharynx through the auditory tube. Mucous membrane covers the structures in the tympanic cavity.

Boundaries

The tympanic membrane creates the lateral wall. However, the epitympanic recess forms a roof above the level of the tympanic membrane and communicates with the aditus. The epitympanic recess consists of the head of the malleus and the body and short crus of the incus.The roof is formed by the tegmen tympani of the temporal bone, which separates the middle ear from the middle cranial fossa. The floor is the jugular fossa, and contains the superior bulb of the internal jugular vein. The anterior wall presents the semicanal for the tensor tympani muscle, the opening of the auditory tube, and the carotid canal, where the internal carotid artery lies. The posterior wall contains the aditus (which leads to the mastoid antrum), and the pyramidal eminence, (which contains the stapedius muscle). The mastoid process, and mastoid portion of the temporal bone, is hollowed out by air cells. These air cells line the mucoperiosteum and communicate with each other and with the mastoid antrum.

The medial wall has many components: (a) the prominence of the lateral semicircular canal and the prominence of the facial nerve canal. (b) the oval window (fenestra vestibuli), closed by the base of the stapes, and the cochleariform process; (c) the promontory, formed by the basal turn of the cochlea; and (d) the round window (fenestra cochleae), closed by mucous membrane. The tympanic plexus lies on the promontory and is formed by the tympanic nerve (from cranial nerve IX), which gives sensory fibers to the middle ear and secretomotor fibers to the parotid gland.

Auditory Ossicles, Joints, and Muscles

The bones of the middle ear are the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus contains a handle (manubrium), lateral process (embedded in the tympanic membrane), and an anterior process (attached to the petrotympanic fissure). The incus has a body, short crus, and long crus. The stapes includes a head, anterior and posterior crura, and a base (footplate), which is attached by an annular ligament to the oval window (fenestra vestibuli). The incudomallear and incudostapedial joints are saddle and ball-and-socket synovial joints, respectively. The tensor tympani originates from the cartilaginous part of the auditory tube. It enters a semicanal, turns laterally around the cochleariform process, and inserts on the handle of the malleus. The tensor tympani is supplied by the mandibular nerve and tympanic plexus. It draws the handle of the malleus medially, and the tympanic membrane contracts. The stapedius is innervated by the facial (VII) nerve and begins in the pyramidal eminence and inserts on the neck of the stapes. Supplied by the facial nerve, the stapedius draws the stapes laterally. It protects the oval window and decreases the sensitivity of hearing. Both the tensor and the stapedius attenuate sound transmission through the middle ear.

Function Of the Middle Ear

Sound waves vibrate the tympanic membrane and these vibrations convert into intensified movements of the stapes by the lever-like action of the auditory bones. Then the oval window moves, causing movements at the round window. Sound vibrations are transmitted to the inner ear by (1) the auditory ossicles and the oval window, (2) air in the tympanic cavity and the round window, and (3) bone conduction through the skull.

Facial nerve

The facial nerve (VII) has a close relationship to the middle ear. The facial nerve has two components: the larger part, which supplies the muscles for facial expression; and a smaller part (nervus intermedius) which contains taste fibers for the anterior 2/3rds of the tongue, secretomotor fibers for the lacrimal and salivary glands, and some pain fibers. The two parts leave the brain at the caudal border of the pons (cerebellopontine angle) and enter the internal acoustic meatus. The facial nerve passes through the facial canal in the temporal bone. The facial nerve is superior to the promontory on the medial wall of the middle ear, and expands to form the geniculate ganglion, which contains the taste sensory ganglion cells. The nerve turns sharply posteriorward and then sweeps down immediatly posterior to the middle ear. It exits through the stylomastoid foramen. Finally, the facial nerve enters the parotid gland, forms the parotid plexus, and creates terminal branches for the facial muscles. The facial nerve first passes through the posterior cranial fossa, then the internal acoustic meatus, facial canal, and finally parotid gland and face.

In the facial canal, the geniculate ganglion connects to the greater petrosal nerve. This branch passes anteromedially to join the deep petrosal nerve (from the carotid sympathetic plexus) and form the nerve of the pterygoid canal, which reaches the pterygopalatine ganglion. The greater petrosal nerve has secretomotor fibers for the lacrimal and nasal glands and many afferent fibers of uncertain distribution and function.

Continuing in the facial canal, the facial nerve connects a nerve to the stapedius muscle and the chorda tympani. The chorda tympani enters the tympanic cavity, passes medial to the tympanic membrane and the handle of the malleus, and reenters the temporal bone. It leaves the skull through the petrotympanic fissure to reach the infratemporal fossa where it joins the lingual nerve, where it distributes to the anterior two thirds of the dorsum and sides of the tongue. The chorda contains (1) taste fibers from the anterior two thirds of the tongue and soft palate and (2) preganglionic secretory fibers, which synapse in the submandibular ganglion, and the postganglionic fibers which supply the submandibular, sublingual, and lingual glands. Immediately inferior to the base of the skull, the facial nerve gives off muscular branches to the stylohyoid muscle and to the posterior belly of the digastric muscle. It also leads to the posterior auricular nerve, which supplies motor fibers to the auricular and occipitalis muscles, and sensory fibers to the auricle.

Within the parotid gland, the facial nerve forms the parotid plexus. The parotid plexus is where terminal branches spread out to provide the muscles for facial expression. The branches are usually classified as temporal, zygomatic, buccal, marginal, and cervical. They contain afferent as well as motor fibers.

- Internal ear

The internal ear is in the petrous part of the temporal bone and consists of the inner membranous labyrinth and the outer bony (osseous) labyrinth.

The cochlea is the essential organ used for hearing. The vestibular apparatus is used for maintaining equilibrium and contains the utricle and semicircular ducts.

Osseous Labyrinth

The osseous labyrinth comprises of a layer of dense bone (otic capsule) and the enclosed perilymphatic space, which contains a fluid called the perilymph. The fluid also surrounds the membranous labyrinth. The perilymphatic space is comprised of continuous cavities: semicircular canals, vestibule, and cochlea.

Semicircular canals

The anterior, posterior, and lateral semicircular canals all lie at right angles one to another. The anterior and posterior semicircular canals are placed in a vertical position while the lateral one is in a horizontal position.

Vestibule

The vestibule is in the middle part of the bony labyrinth, immediately medial to the tympanic cavity. It contains the utricle and saccule of the membranous labyrinth. The oval window, placed between the vestibule and the tympanic cavity, is closed by the footplate (base) of the stapes.

Cochlea

The cochlea looks like a snail’s shell. It has is a helical tube of about 2.5 turns. Its base lies against the lateral end of the internal acoustic meatus. The apex is directed anterolaterally and the basal coil forms the promontory of the middle ear. The bony core is called a modiolus and transmits the cochlear nerve, and contains the spiral sensory ganglion. The osseous spiral lamina projects from the modiolus. The cochlear duct extends from the lamina to the wall of the cochlea so the space in the cochlea gets divided into three ducts: the scala vestibuli (anterior), cochlear duct (medial) and the scala tympani (posterior). The scala vestibuli and tympani contain perilymph while the cochlear duct contains endolymph.The scala vestibuli begins in the vestibule and passes to the apex of the cochlea (helicotrema), where the two scalae communicate with each other. The scala tympani returns to end near the round window, which is closed by the secondary tympanic membrane.

Perilymphatic duct

The perilymphatic duct (aqueduct of the cochlea) lies in the cochlear canaliculus and connects the scala tympani with the subarachnoid space.

Mechanism of Equilibrium

There are two types of equilibrium: (1) static equilibrium is used to maintain the position of the body relative to the force of gravity; (2) dynamic equilibrium is for maintaining body position in response to movements like rotation, acceleration and deceleration. The vestibular apparatus (saccule, utricle, and semicircular ducts) is used to detect equilibrium.

Membranous Labyrinth

The membranous labyrinth lies within the bony labyrinth and contains endolymph. The membranous labyrinth consists of a series of continuous cavities: semicircular ducts, utricle, saccule, and cochlear duct.

Semicircular ducts

The anterior, posterior, and lateral semicircular ducts are situated in the X, Y, and Z planes. At the end of each duct is a bulge called an ampulla, which contains a neuroepithelial ampullary crest. The crest is stimulated by relative movement of the endolymph during dynamic head movements (i.e., spinning, pitching or tumbling). The crest contain hair cells and supporting cells covered by a gel layer called the cupula. The semicircular ducts detect cceleration or deceleration. When the head moves the semicircular ducts and hair cells move too. The endolymph in the ampulla isn’t attached and lags behind. As the moving hair cells drag along the stationary fluid, the hair bundles bend. The hair bundles bending causes a response that leads to nerve impulses passing along the ampullary nerve, a branch of the vestibular branch of the (XIII)th nerve.

Utricle and saccule

The utricle and saccule lie in the vestibule and communicate with each other by utricular and saccular ducts. The utricle has five openings for the semicircular ducts. The saccule connects to the cochlear duct by the ductus reuniens. The endolymphatic duct derives from the utricular and saccular ducts and is transmitted by the aqueduct of the vestibule. The duct ends in the endolymphatic sac. Maculae

The walls of the utricle and saccule create a neuro-epithelial macula, which is stimulated by gravity and linear acceleration. The maculae are receptors for static equilibrium but also participate in dynamic equilibrium. In static equilibrium, the maculae provide sensory information on the position of the head to maintain posture and balance. The maculae have two cells: the sensory receptors called hair cells, and supporting cells. The hair cells have stereocilia and kinocilium and together it’s referred to as a hair bundle. There are columnar supporting cells that secrete a thick gelatinous layer referred to as the otolithic membrane, that rests over the hair cells. Otoliths are a layer of calcium carbonate crystals over the otolithic membrane. When the head is tilted forward, the otolithic membrane and otoliths are pulled by gravity and slide downhill over the hair cells in the direction of the movement. The hair cells synapse with the sensory neurons in the vestibular branch of the (XIII)th cranial nerve.

Cochlear duct

The cochlear duct comes from the saccule to the apex of the cochlea, where it ends and extends from the osseous spiral lamina to the wall of the cochlea. Its anterior and posterior walls are the vestibular and basilar membranes. The spiral organ or organ of Corti (used for hearing) lies against the basilar membrane and includes neuro-epithelial hair cells attached to the tectorial membrane. The spiral ligament creates the third side of the triangular-shaped duct. Furthermore, at the tip of each hair cell is stereocilia and kinocilium that reach into the endolymph of the cochlear duct. Blood vessels, such as the stria vascularis, produce endolymph.

Functions of the Internal Ear

The functional details of the internal ear aren’t completely understood. Movement of the stapes in the oval window creates movement in the secondary tympanic membrane in the fenestra cochleae. Vibrations made in the perilymph from the scala vestibuli and scala tympani will move the membranes of the cochlear duct and progress this movement to the basilar membrane. These vibrations will displace the hair cells in the spiral organ and send the impulse to the tectorial membrane. This causes the stereocilia to bend and generate a nerve impulse to the cochlear nerve fibers. The movement of the fluid depends on the frequency of the sound. Low- frequency sounds cause maximum displacement in the basilar membrane, where high-frequency sounds maximally vibrate the membranes of the basal portion of the cochlea.

Vestibulocochlear nerve

The vestibulocochlear (XIII) nerve contains afferent fibers from the internal ear. It leaves the brain at the lower border of the pons and enters the internal acoustic meatus. The vestibular branch is responsible for equilibrium and is distributed to the maculae of the utricle and saccule, and zmpullary crests of the semicircular ducts. The vestibular fibers originate in bipolar cells in the vestibular ganglion in the internal acoustic meatus. The cochlear branch is responsible for hearing and is distributed to the hair cells of the spiral organ. The cochlear fibers originate in bipolar cells in the spiral ganglion in the modiolus. Deafness

Deafness is significant or total hearing loss. Sensorineural deafness occurs by impairment of hair cells or damange of the cochlear branch of the (XIII) cranial nerve. Sensorineural deafness can occur due to atherosclerosis, which decreaes blood supply to the ears, or by loud noise which destroys the hair cells. The louder the sound, the quicker the hearing loss. Deafness begins with the loss of sensitivity of high pitched frequencies.

Conduction deafness is due to destruction of the external or middle ear mechanisms for relaying sounds to the cochlea. Weber’s test is a hearing test used to differentiate between sensorineural and conduction deafness. The stem of a vibrating fork is held to the forehead. People with normal hearing can hear the sound equally in both ears, however if someone only hears the sound better in the affected ear, it’s a conduction deafness. If the sound is heard in the normal ear, it’s sensorineural deafness.

Auditory Pathway

When the stereocilia of the hair cells in the spiral organ bend, they release a neurotransmitter that causes a nerve impulse in sensory neurons attached to the hair cells. The sensory neuron’s cell bodies are in the spiral ganglia. Nerve impulses pass along the axons which form the cochlear branch of the vestibulocochlear nerve. The axons synapse with the neurons in the cochlear nuclei in the medulla oblongata. Some axons cross over the medulla and ascend in the tract called the lateral meniscus, and synapse in the inferior colliculus on the midbrain. Some axons terminate in the superior olivary nucleus (SON) in the pons on each side. Slight differences in the timing of the nerve impulses arriving from the two ears at the superior olivary nucleus can determine the location of the sound. Axons from the SON also ascend in the lateral meniscus tracts on both sides and end in the inferior colliculus. From each inferior colliculus, nerve impulses are conveyed to the medial geniculate nucleus in the thalamus and the primary auditory area of the cerebral cortex in the temporal lobe.

Equilibrium Pathway

The hair bundles in the semicircular canals, utricle or saccule bend and cause nerve impulses in the sensory neurons. The cell bodies of the hair cells are in the vestibular ganglia. Nerve impulses pass along the axons and form the vestibular branch of the (XIII)th nerve. These axons synapse with sensory neurons in vestibular nuclei, in the medulla oblongata and the pons. Furthermore, the vestibular nuclei receive input from the eyes and somatic receptors. The remaining axons enter the cerebellum through the inferior cerebellar peduncles.

The vestibular nuclei combine information from vestibular, visual and somatic receptors and send them to: (1) the nuclei of nerves (III), (IV) and (VI), which all control eye movement; (2) nuclei of (XI) nerve to control head and neck movements, maintaining equilibrium; (3) the vestibulospinal tract which sends impulses down the spianl cord to maintain equilibrium in skeletal muscles; and (4) the ventral posterior nucleus in the thalamus and then to the vestibular area in the parietal lobe of the cerebral cortex for conscious awareness of movements of the head. The eye

The accessory structures of the eye

These include the eyelids, eyelashes, eyebrows, lacrimal (tearing) apparatus, and extrinsic eye muscles.

Eyelids

The upper and lower eyelids are also called palpebrae. The palpebrae protect the eyes from overexposure, foreign objects and it spreads lubricating fluid over the eyeballs. The upper eyelid is more mobile than the lower one and is moved by the levator palpebrae superioris muscle. The levator palpebrae superioris comes from the sphenoid bone into the optic canal and inserts on the skin of the upper eyelid and the upper border of the tarsal plate by the superior tarsal muscle. The levator is directed by the oculomotor nerve and paralysis of the nerve can cause drooping of the upper eyelid. Twitches in the eye are due to stress or fatigue and fade away in a matter of seconds. The palpebral fissure is the space in between the eyelids that exposes the eyeball. The angles of the palpebrae are the lateral commissure, which is closer to the temporal bone, and the medial commissure, which is nearer to the nasal bone. Some people, mostly of Asian decent, have a fold of skin called the epicanthus that covers the medial commissure. The lacrimal caruncle is in the medial commissure and is a small, red elevation that contains sebaceous and sudoriferous glands. White discharge exudes from the caruncles.

From superficial to deep, each eyelid consists of the epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, tarsal plate, tarsal glands and conjunctiva. The tarsal plate is connective tissue that gives form and support to the eyelids. Each tarsal plate has sebaceous glands known as tarsal or Meibomian glands, that secrete fluid to prevent the eyelids from sticking to each other. The ends of the plates are anchored to the orbital margin by lateral and medial palpebral ligaments. The medial palpebral ligament draws the lids laterally and is anterior to the lacrimal sac. The superior tarsal muscle connects the levator with the tarsal plate and consists of smooth muscle directed by sympathetic fibers. Infection of the tarsal glands produces a cyst called a chalazion.

Conjunctiva

The conjunctiva is a protective mucous membrane consisting of nonkeratinized stratified columnar epithelium cells. It acts as the connection between the eyelids, sclera and cornea. The palpebral conjunctiva lines the inner part of the eyelids while the bulbar conjunctiva passes from the inner eyelids to the surface of the eyeball. The palpebral conjunctiva contains the openings of the lacrimal canaliculi, allowing tears within the conjunctival sac to drain into the nasal cavity. The bulbar conjunctiva is translucent and colorless, except when its vessels are dilated as a result of inflammation (conjunctivitis). Mainly, it’s continuous at the limbus with the anterior epithelium of the cornea. The plica semilunaris, a conjunctival fold at the medial angle of the eye, helps protect the eye against foreign bodies. The space between the lids and eye are lined by conjunctiva, and is called the conjunctival sac. The mouth of the sac is the palpebral fissure, which varies in size when the" eye is open". The reflections of the conjunctiva from the lids to the eye are known as fornices. The lacrimal glands open into the superior fornix.Over the sclera the conjunctiva is vascular while at the cornea it is avascular. Dilation of the blood vessels in the bulbar conjunctiva is known as having bloodshot eyes.

Eyelashes and eyebrows

Eyelashes extend from the border of each eyelid and eyebrows arch above the upper eyelids and protect the eyes from perspiration and ultraviolet light. Sebaceous ciliary glands release lubricating fluid into the hair follicles of the eyebrows and lashes. Infection of these glands caused by bacteria causes a sty.

Lacrimal Apparatus

The lacrimal apparatus is a group of structures that produces and drains lacrimal fluid, also known as tears. The lacrimal apparatus comprises (1) the lacrimal gland and its ducts and (2) passages for draining: the lacrimal canaliculi and sac and the nasolacrimal duct

The lacrimal gland, lodged in a fossa at the roof of the orbit, rests on the lateral rectus and the levator muscles. The main portion is in the orbital, but a process called the palpebral part projects into the upper lid. A dozen lacrimal ducts leave the palpebral part to enter the superior conjunctival fornix where there are small accessory lacrimal glands. The half of the lacrimal secretions that does not evaporate drains into the lacrimal sac. The lacrimal glands are supplied by parasympathetic fibers of the (VII) nerve and secrete lacrimal fluid, which drains into lacrimal ducts, which empties tears. Tears pass over the anterior surface of the eyeball to two small openings called lacrimal puncta. Then the tears pass through two ducts called the lacrimal canals, which lead to the lacrimal sac and then to the nasolacrimal duct. This carries the fluid into the nasal cavity under the inferior nasal concha. An infection of the lacrimal sacs is known as dacryocystitis.

Lacrimal fluid contains salts, water and lysozyme. The fluid protects and nourishes the eyeball. If an irritating substance makes contact with the conjunctiva, then the lacrimal glands oversecrete and tears accumulate. It dilutes the irritating substance. When humans cry, the lacrimal glands produce excessive lacrimal fluid that spills over the eyelids and fills the nasal cavity to produce a runny nose.

Extrinsic eye muscles

Eyes sit in the depressions of the skull known as orbits. The orbits stablize and anchor the muscles to produce eye movements. The extrinsic eye muscles extend from the walls of the orbit to the sclera of the eye and are surrounded by periorbital fat. The superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique muscle and inferior oblique muscle are the six extrinsic muscles that move the eye. Except for the inferior oblique muscle, these skeletal muscles originate from the posterior aspect of the orbit. The four recti form a common tendinous ring that surrounds the optic canal and part of the superior orbital fissure. All the structures that enter the orbit through the optic canal and the adjacent part of the fissure lie at first within the cone of recti. The extrinsic eye muscles receive impulses from cranial nerves III, IV, and VI. The small motor units have smooth movement for the eyes.

The superior oblique muscle comes from the sphenoid bone to the optic canal. It’s superior to the medial rectus, passes the trochlear and attaches to the frontal bone. The inferior oblique muscle rises from the maxilla, at the floor of the orbit, and inserts on the posterior sclera. The superior oblique is directed by the trochlear, the lateral rectus is directed by the abducens, and the rest of the muscles are directed by the oculomotor nerve.

The recti extend from the posterior aspect of the orbit to the anterior aspect of the sclera. The lateral and medial recti are an abductor and adductor, respectively. The superior and inferior recti elevate and depress respectively, and because they are in the lateral position, they can only move when the eye is abducted. The superior and inferior oblique muscles depress and elevate, respectively, and because of their lateral position, they can move only when the eye is adducted.

Paralysis of an extrinsic eye muscle is described by (1) restricted movement in the field of action of the paralyzed muscle and (2) the presence of two separate images (diplopia) when eye moves in the direction of the paralyzed muscle.

Actions of the extraocular muscles

Eye equilibrium is maintained by all of the eye muscles and move in unison as the two eyes move together. Movements are around the vertical axis (abduction and adduction), lateromedial axis (elevation and depression) and the anteroposterior axis (extorsion and intorsion).

Anatomy of an Eyeball

The adult eyeball measures 2.5 cm in diameter, and only 1/6th of the eyeball is exposed, the rest if rested in the orbit. If the eye is too short in relation to the lens, near objects are focused behind the retina (hypermetropia: farsightedness or longsightedness). If the eye is too long in relation to the lens, distant objects are focused in front of the retina (myopia: nearsightedness or shortsightedness)

The retina is regarded as an extension of the wall of the brain, and develops from neural ectoderm, where the lens and the anterior epithelium of the cornea come from the somatic ectoderm. Neural crest and mesoderm also participate in ocular development. The eyeball has three layers from superficial to deep: the fibrous tunic, vascular tunic and retina.

Fibrous tunic

The fibrous tunic is a strong, dense collagenous connective tissue layer.

The sclera is the outer white covering of the eye. It provides protection for the internal eye structures, maintains eye shape, and is an important site for muscle attachment. Posteriorly, the optic nerve fibers pierce the sclera through a weak plate called the lamina cribrosa. External to the sclera, the eyeball is covered by a thin fascial sheath (Tenon's capsule) that extends from the optic nerve to the sclerocorneal junction. The sheath separates the eyeball from the orbital fat and acts as a socket where the eye moves as in a ball-and-socket joint. It blends with the sheaths of the muscles of the eye. Hormonal disturbances (especially hyperthyroidism) can cause swelling of the orbital fat and extra-ocular muscles, causing protrusion of the eyes (exophthalmos).

The cornea is superficial to the colored iris and allows light to enter the eye. Its curved surface refracts the light into the eye. The cornea is continuous with the conjunctiva and the junction is called the limbus. The cornea is directed by the opthalmic nerve of the trigeminal nerve. The outer surface of the cornea is coated by the bulbar conjunctiva. The sclera is opaque and the cornea is translucent because corneal tissue is avascular and consists of five layers: the substantia propria enclosed by anterior and posterior epithelia and limiting laminae.. The canal of Schlemm connects the aqueous fluid in the anterior area of the eye and veins in the sclera. It drains the aqueous fluid. When the cornea does not conform to a sphere but is more curved in one axis than in another, the condition is termed astigmatism.

Vascular tunic

The vascular tunic or uvea is the middle eyeball layer. It has three parts: 1) the choroid is the posterior portion that lines most of the internal surface of the sclera. Its blood vessels provide nutrients for the retina. The part near the choroid is a smooth ciliary ring (pars plana), whereas near the iris is a ridged crown (pars plicata). The choroid also contains melanin, which is why it is such a dark color. The melanin absorbs excess light rays so the image is clear on the retina. Albinos have no melanin, also in the eye so they need to wear sunglasses indoors.

In the anterior area, the choroid becomes the ciliary body. It extends from the ora serrata which is the jagged anterior of the retina. The ciliary body also contains melanin so it is very dark. The ciliary body has ciliary processes which connect to suspensory ligaments, which connect to the lens. The ciliary processes circle the iris and produce aqueous humor. The ciliary body also has ciliary muscles that are also connected to the suspensory ligaments, which then convex or concave the lens or near or far vision. The ciliary muscle comprises two main sets of smooth-muscle fibers: (1) longitudinal fibers connect the sclera (anterior) to the choroid (posterior), and (2) oblique fibers enter the base of the ciliary processes.

The iris is the colored portion of the eyeball and is shaped like a flat doughnut and the opening is called the pupil. It is suspended between the cornea and lens. It has melanocytes and radial smooth muscle fibers. The amount of melanin determines the eye color of the iris. When there’s large amounts of melanin, the stroma of the iris is black and brown, when few amounts of melanin it is blue. The anterior surface of the iris has a fringe known as the collarette. The pattern of radial striations in the iris is unique from one person to another and can be used for identification. A congenital, radial defect of the iris is termed a coloboma. The two distinct fiber arrangements of smooth muscle from the sphincter pupillae muscle is a flat thin band of muscle fibers at the pupillary bounder of the iris. The sphincter pupillae is directed by the parasympathetic fibers by the short ciliary nerves. The dilator pupillae muscle attaches to the outer circumference of the sphincter pupillae and projects like the spokes of a wheel towards the iris. The iris regulates the amount of light enter the pupil. The pupil is the opening to the retina and choroid, therefore it’s black. Autonomic reflexes regulate pupil diameter in response to light levels. When there’s a bright light, the parasympathetic fibers of the (III) nerve cause the sphincter muscle of the iris to contract, decreasing the pupil size. In dim light the sympathetic neurons cause the dilator pupillae muscle of the iris to contract, increasing the pupil size.

Retina

The inner coat of the eyeball is the retina. The retina is the beginning of the visual pathway. The retina is nourished by the choroid and the opthalmic artery of the trigeminal nerve. The central artery travels in the optic nerve and divides at the optic disc. The optic disc is the where the optic nerve exits the eyeball. The central retinal artery is bundled together with the optic nerve and the central retinal vein. The retina has a pigmented layer and a neural layer. The pigmented layer has melanin epithelial cells between the choroid and the neural part of the retina, which also helps absorb rays. The neural layer is a multilayered outgrowth that processes visual data before sending nerve impulses into axons that form the optic nerve. There are three layers of retinal neurons: the photoreceptor layer, the bipolar cell layer and the ganglion cell layer, which are separated by the outer and inner synaptic layers. Light passes through the ganglion and bipolar cell layers before it reaches the photoreceptors. In the bipolar cell layer of the retina are also horizontal cells and amacrine cells. The cells form direced neural circuits that modify the signals being transmitted along the pathway from photoreceptors to bipolar cells to ganglion cells.

The retina has two types of photoreceptors: rods and cones. Rods allow us to see in dim light because they are of a low threshold. Cones are high threshold photoreceptors which produce color vision. A person who loses rod vision can’t see at night. There is an outer pigment layer where the rods and cones lie, and it consists of three parts: (1) an outer segment that detects the light stimulus, (2) an inner segment which contains metabolic mechanisms, and (3) a synaptic terminal which is facing the bipolar neurons. The outer segment has many photopigment molecules.

The macula lutea is the center of the posterior retina. The fovea centralis is the small depression of the macula lutea which contains a dense population of only cones, which causes it to be the area of highest visual acuity. Moving further from the fovea centralis, less and less cones populate the retina and more and more rods are found. That’s why in our peripheral vision we can only see faint obects of no color. The optic disc is called the blind spot because rods and cones aren’t found here. Lens

The lens is biconvex and 1 cm in diameter, and is covered by a capsule and consists of cellular lens fibers. It is posterior to the pupil and iris. In the lens there are proteins called crystallins which are layered like an onion, create the refractory abilities of the lens, and lacks blood vessels. The lens capsule is anchored to the ciliary body by its suspensory ligaments (ciliary zonule). When seeing a distant object, the ciliary muscle is relaxed and elastic fibers in the choroid pull on the ciliary body, which keeps the zonular fibers and the lens capsule under tension. This pull results in flattening of the lens and focus images on the retina for a clear picture.

Interior of the Eyeball

The anterior cavity is between the cornea and the lens and holds aqueous humor. The cavity consists of two chambers. The anterior chamber is between the cornea and the iris. The posterior chamber is behind the iris and in front of the lens. Aqueous humor filters out of the blood capillaries and enters the posterior chamber and then flows forward between the iris and lens, through the pupil and into the anterior chamber, iridocorneal angle, trabecular meshwork, and scleral venous sinus, reaching the ciliary veins. Then it drains through the canal of Schlemm which is an endothelial channel at the sclerocorneal junction.

Intraocular pressure is the pressure in the eye produced by aqueous and vitreous humor, and depends on the ease of draining the aqueous humor. The pressure maintains the eye and prevents it from collapsing.

The larger posterior cavity is called the vitreous chamber, which is between the lens and retina. In it is the vitreous body which is a jelly like substance that contributes to intraocular pressure. The body also helps maintain the shape of the eye. The pressure of the vitreous body holds the retina flush against the choroid so the retina creates a even surface for images. The body also contains phagocytic cells to remove debri. The hyaloid canal is a narrow channel that runs through the vitreous body from the optic disc to the lens.

The visual pathway

Neurotransmitters released by rods and cones create changes in bipolar and horizontal cells that lead to the generation of nerve impulses. Amacrine cells synapse with ganglion cells and also transmit information. When bipolar, horizontal or amacrine cells transmit signals to ganglion cells, the cells initiate nerve impulses.

Pathway in the brain

The axons of the optic nerve pass through the optic chiasm. Some fibers cross to the opposite side and others remain uncrossed. After passing the optic chiasm, the fibers are now part of the optic tract and enter the brain and terminate at the lateral geniculate nucleus of the thalamus. Here they synapse with neurons whose axons form the optic radiations, which project to the primary visual areas in the occipital lobes.

Major causes of blindness

Cataracts are a common cause of blindness due to loss of transparency of the elns. The lens is clouded due to changes in the structure of the lens proteins. This usually occurs due to age, but also injury and exposure to UV rays. People who smoke also have an increased chance of developing cataracts.

Glaucoma is the most common cause of blindness. It is due to abnormally high intraocular pressure due to buildup of aqueous humor within the anterior cavity. The fluid compresses the lens into the vitreous body and puts pressure on the retina. Continuous pressure creates irreversible destruction of the neurons in the retina and damage to the optic nerve. Glaucoma is painless and the other eye compensates so much damage occurs before the condition is diagnosed.

Age-related macular disease (AMD) is also known as macular degernation, is a degenerative disorder of the retina and pigmented layer in people 50 years and older. In AMD, abnormalities occur in the macula lutea. Victims retain their peripheral vision but lose their vision straight ahead. It affects 13 million Americans and is 2.5 times more likely in smokers.