Sense Organs Part 2: the Ear and the Eye

Lecture 13

The Ear  Anatomy of the Ear o Outer Ear . Essentially a funnel for conducting the airborne vibrations to the eardrum . Auricle (pinna) – portion of the ear outside the skull  Shaped and supported by elastic cartilage  Directs sound into the auditory canal . Auditory canal – passage through the temporal bone  Runs from the external auditory meatus slightly upward through an S-shaped course to the tympanic membrane (eardrum)  Has ceruminous glands that produce cerumen (earwax) o Middle Ear – Tympanic Cavity . Runs from the tympanic membrane to the oval window . Posteriorly it is continuous with the mastoidal air cells in the mastoid process  This cavity is normally flattened and closed, but swallowing allows air to enter or leave the tympanic cavity o Swallowing equalizes pressure on both sides of the eardrum . The eardrum is innervated by vagus and trigeminal nerves and is highly sensitive to pain . Equalizing pressure prevents pain and keeps the sense of hearing from being dampened  The cavity contains three auditory ossicles o The malleus, incus, and stapes o The “footplate” end of the stapes fits into the oval window, where the inner ear begins . The ossicles are held in place by ligaments, but there are also muscles that attach to these bones . The stapedius attaches to the stapes . The tensor tympani attaches to the malleus o In response to loud noises, these muscle contract and dampen the vibration of the ossicles (the tympanic reflex) . Despite the reflex, sensory cells can be permanently damaged by exposure to loud noises o Inner Ear . Extends inward from the oval window . Housed in the labyrinth  Bony labyrinth is the outer portion of passages through bone  The membranous labyrinth is a maze of fleshy tubes inside the bony labyrinth  The labyrinth is filled with two types of fluid o Perilymph is fluid, similar to cerebrospinal fluid, that is found between the membranous labyrinth and the bony labyrinth o Endolymph is fluid found within the membranous labyrinth . The vestibule is a large chamber in the bony labyrinth.  It contains the organs of equilibrium  The membranous labyrinth begins at this chamber  Arising from the anterior side of the vestibule is the cochlea . Cochlea – the organ of hearing  A coiled tube that forms a snaillike spiral, allowing a long tube to fit in a small space  It winds 2 ½ times around a bony structure called the modiolus  The cochlea has three fluid-filled chambers separated by membranes o The upper chamber is called scala vestibuli . Filled with perilymph . Runs from oval window to the apex o The lower chamber is called scala tympani . Filled with perilymph . Runs from the apex to the round window (just below the oval window) o The upper and lower chambers are actually connected at the apex, in a channel called the helicotrema o The middle chamber a triangular space called the cochlear duct . Filled with endolymph . The ceiling of the cochlear duct is the vestibular membrane . The floor of the cochlear duct is the basilar membrane . Within the cochlear duct, supported by the basilar membrane is the organ of Corti (called the spiral organ in some books)  This the is structure that converts vibrations to nerve impulses) . Organ of Corti  Epithelium composed of hair cells and supporting cells  Hair cells have stereocili (long, stiff, microvilli) that extend upward  Resting on the stereocilia is a gelatinous tectorial membrane  Hair cells are not neurons, but they synapse with nerve fibers at their base  Auditory Function o When sound waves vibrate the eardrum, the three auditory ossicles transfer the vibrations to the inner ear o The footplate of the stapes moves the fluid in the inner ear, and the fluid movement causes differences in the movements of the basilar membrane compared to the movements of the tectorial membrane . This causes bending of the stereocilia on the hair cells to move back and forth, and the hair cells release neurotransmitters o Differentiating sounds: . Loudness:  Measured in decibels  Loud sounds produce more vigorous vibrations of the organ of Corti over a broader area of the basilar membrane . Frequency:  pitch  High-pitched sounds cause the free end of the basilar membrane, near the tip of the cochlea to vibrate more than the attached, basal end  Low-pitched sounds cause the basal end to vibrate more

 The Vestibular Apparatus o equilibrium – coordination and balance . static equilibrium – perception of the orientation of the head when it is stationary . dynamic equilibrium – perception of motion or acceleration  linear acceleration – change in velocity in a straight line o detected by the saccule and utricle  angular acceleration – change in the rate of rotation o detected by the semicircular ducts o Utricle . Has a 2-by-3 mm patch of hair cells and supporting cells called a macula utriculi that lies nearly horizontally on the floor of the utricle . Has 40 to 70 stereocili and one motile true cilium called a kinocilium . The tips of the stereocili and kinociium are embedded in a gelatinous otolithic membrane  The membrane is weighted with calcium carbonate granules called othliths  Because the membrane is weighted, it has higher inertia, resistance to change in movement . As the body is suddenly moved forward, the otolithic membrane of the saccule utriculi briefly lags behind the rest of the tissues  This bends the stereocilia backward, and stimulated the hair cells, sending signals to the brain . As the body is suddenly moved backward, the othlithic membrane lags behind in the opposite direction  This bends the stereocilia forward, stimulating the hair cells, which send signals in a different pattern to the brain o Saccule . Has a macula called the macula sacculi that lies nearly vertically on the wall of the saccule . The structure is essentially identical to the utricle, except that the hair cells project vertically . The stereocilia can be bent upward or downward as the otolithic membrane lags behind the body during vertical acceleration. o Semicircular ducts . Three membranous ducts housed in osseous semicircular canals . All three are in different planes . The ducts are filled with endolymph . Each duct opens into the vestibule and has a dilated sac at one end called an ampulla  Within the ampulla is a mound of hair cells and supporting cells called the crista ampullaris  The hair cells have stereocilia and a kinocilium embedded in the cupula o The cupula is a gelatinous membrane that extends to the roof of the ampulla . When the head rutns, the duct rotates, but the endolymph lags behind and pushes the cupula  This bends the sterocilia and stimulates the hair cells

The Eye  Accessory Structures of the Orbit o Eyebrows – keep perspiration from the eyes o Eyelids – Close to moisten the eye with tears, sweep debris from the surface, block foreign objects from the eye, protect from visual stimuli o Conjunctiva - thin mucous secreting epithelial membrane over anterior surface of eye and posterior surface of eyelid – prevents eyeball from drying out . forms conjuntival sac- prevents foreign objects from passing beyond the front of the eye o Lacrimal gland – Located in the superolateral corner of the orbit, it secretes tears to cleanse and lubricate the eye surface o Extrinsic eye muscles- muscles responsible for movement of the eye  Anatomy of the Eyeball o Tunics . Fibrous tunic  Sclera – white portion of the eye that consists of collagenous fibers and covers most of the eye  Cornea – anterior transparent region of modified sclera that admits light . Vascular tunic  Choroid – highly vascular, deeply pigment area behind the retina (and within the sclera)  Ciliary body – thickened extension of the choroids that forms a muscular ring around the lens  Iris- adjustable diaphragm that adjusts the diameter of the pupil (opening through which light enters the eye) . Tunica interna –  retina, which internally lines the posterior two thirds of the eyeball o Optical components – transparent parts that admit light rays . Cornea – previously described . Aqueous humor – serous fluid secreted by the ciliary body  Found between the cornea and the iris . Lens – composed of tightly compressed cells called lens fibers  Suspended behind the pupil by suspensory ligaments o The ligaments connect to the ciliary body which can adjust tension on the ligaments to change the shape of the lens . Vitreous humor – transparent jelly that fills the space behind the lens o Neural components . Retina – the inner layer of the eyeball that contains the photoreceptors  Photoreceptors are cells that absorb light and generate a chemical or electrical signal o Each photoreceptor has an outer segment and an inner segment . The outer segment points towards the wall of the eye and is specialized to absorb light . The inner segment faces the interior and contains mitochondria and other organelles. . Light has to go through the inner layer to reach the parts that are actually light sensitive  Photoreceptors are of two types o Rod cells are responsible for night vision and cannot distinguish colors from each other o Cone cells are responsible for color vision . Some respond best to deep blue light . Some respond best to green . Some respond best to orange-yellow o The fovea centralis is the area of keenest vision, because it has a high concentration of cone cells  The retina also has a posterior layer called the pigment epithelium o Composed of darkly pigmented cuboidal cells o It absorbs light that is not absorbed first by the receptor cells so that it does not reflect back into the eye, degrading the visual image  Formation of an Image o Light rays enter the eye, become focused on the retina, and produce a tiny inverted image o The cornea refracts incoming light rays toward the back of the eye o The lens makes adjustments to fine focus the image - accommodation . When you focus on something far away, the lens flattens to a thickness of about 3.6 mm . When you focus on something closer than 6 m, the lens thickens to about 4.5 mm  The visual projection pathway o The optic nerves leave each orbit through the optic foramen o The nerves then converge and form an X, called the optic chaism, below the hypothalamus and in front of the pituitary gland o Within the chaism, half the fibers of each optic nerve cross to the opposite side of the brain (fibers from the medial half of the eye cross over, those from the lateral half stay to the same side) o From there, nerve fibers run to the thalamus, and from there many fibers run to the occipital lobe