Transcript Hearing
Hearing The Ear • The ear is designed to capture disturbances in the air in the form of moving waves of energy known as sound waves • The ear consists of three portions, the outer ear, the middle ear, and the inner ear • Each of these portions contain structures that help direct sound waves from the exterior to the receptors in the inner ear The Outer Ear • The outer ear is made up of the auricle and the external auditory canal • The external auditory canal (EAC) penetrates through the skull at the external auditory meatus and leads to the middle ear The Outer Ear • The EAC contains glands in its lining of skin that secrete a waxy substance called cerumen which, along with tiny hairs, help to keep dirt from entering the ear • The auricle collects sound waves and directs them down the EAC to the eardrum, which is the entrance to the middle ear The Middle Ear • The middle ear contains an air-filled space inside the temporal bone called the tympanic cavity • It also includes the eardrum, or tympanic membrane, and three small bones called the auditory ossicles • The tympanic cavity connects to the throat by way of the eustachian tubes The Middle Ear The Middle Ear • The eustachian tubes allow air pressure to be equalized on both sides of the tympanic membrane so that it can vibrate freely • Unfortunately, infections can travel from the mouth to the throat to the middle ear by way of mucous membranes • The tympanic membrane, or eardrum, vibrates in response to sound waves it receives from the EAC The Middle Ear • It is connected to the auditory ossicles (malleus, incus, and stapes or “hammer, anvil, and stirrup”) which amplify and transmit those vibrations to the oval window • Within the inner ear, vibrations from the stapes cause fluid to move, stimulating the receptors for hearing The Inner Ear • The inner ear is also known as the labyrinth because it is made up of a complicated series of passageways; there is a bony portion and a membranous portion • The bony labyrinth is a series of canals within the temporal bone • It has three regions; the semicircular canals, the vestibule, and the cochlea Bony Labyrinth The Inner Ear • Within each of these bony regions is a membranous region which parallels its outline • Between the outer walls of the bony labyrinth and the walls of the membranous labyrinth is a fluid called perilymph • Within the membranous labyrinth is a fluid called endolymph The Inner Ear • The cochlea resembles a snail shell in that it coils and spirals; generation of nerve impulses occurs inside of this structure at the organ of Corti • Stimulating the organ of Corti is a round-about process and requires the transmission of waves through many different structures The Inner Ear • The cochlea is separated into two compartments by a thin shelf of bone lined with membrane • The upper compartment is the scala vestibuli, which extends from the oval window to the end of the cochlea • The lower compartment is the scala tympani, extending in the opposite direction from the end of the cochlea to the round window located inferior to the oval window From Vibration to Nerve Impulse Transmission The Inner Ear • Within the membranous labyrinth is a portion called the cochlear duct • The cochlear duct passes between the two tubular compartments (scala) to end as a closed sac • The two sides of the cochlear duct have different names: the vestibular membrane separates it from the scala vestibuli, the basilar membrane separates it from the scala tympani Cross-sections of Cochlea The Inner Ear • On top of the basilar membrane is the organ of Corti with is made up of hair cells with cilia that extend into the endolymph of the cochlear duct • They contact a delicate, gelatinous membrane that lies over them called the tectorial membrane • The basal ends of the hair cells are in contact with the nerve fibers of the cochlear nerve that connects to the brain Organ of Corti Cochlear Sensitivities The Inner Ear • Movement of the basilar membrane causes the hair cells of the organ of Corti to bend against the tectorial membrane • The movement of the hairs causes the hair cell membranes to release neurotransmitters • If the movement is great enough, an action potential is generated From Vibration to Nerve Impulse Transmission Sense of Equilibrium • Detected by receptor cells in the inner ear • Has two aspects: static equilibrium and dynamic equilibrium • Static equilibrium refers to the sensation of body position • Dynamic equilibrium refers to the sensation of rapid movements Static Equilibrium • Comes from receptors in the vestibule of the inner ear • Gives information regarding the exact position of the head • The membranous labyrinth contains two sacs called the utricle and the sacule which are connected to each other by a small duct • Within their walls is a small, flat region of special cells known as the macula Macula Static Equilibrium • The macula contains two types of cells: supporting epithelial cells and hair cells • The hair cells are receptors and each contain a long strand that extends into a thick, jelly-like mass • Lying in this mass is a layer of calcium carbonate crystals, called otoliths • The position of the head is monitored by the movement of otoliths in the macula Static Equilibrium • When you tilt your head to one side, the otoliths shift their position because of gravity • Their movement causes the jelly-like mass to move and pull on the hair cells • The bending causes release of neurotransmitters Dynamic Equilibrium • Detects rapid movements, mostly of the head • The receptors that sense this aid in balancing and are located within the fluid-filled semicircular canals of the inner ear • Within the membranous labyrinth of each of these canals is an expanded region, called the ampulla • The ampulla contains the sensory organs, called the cristae Cupula Dynamic Equilibrium • Each crista contains groups of supporting cells and a group of hair cells • Hair cells are the receptors and embedded in a jelly-like mass known as the cupula • When the head shifts position rapidly, the cupula moves, bending the hair cells which release neurotransmitters in response Last Slide • I feel that it’s rude for you to pack up your things before I’m finished talking so I put in a “Dummy Note” to make you think there was more ; 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