The Ear

External Ear Structures & Functions Pinna—Collects sound waves and channels them into the external auditory canal. External Auditory Canal—Directs the sound waves toward the tympanic membrane. Tympanic membrane—Receives the sound waves and transmits the vibration to the ossicles of the middle ear.

Middle Ear

Middle Ear Structures & Functions Ossicles—Transmit vibrations from the tympanic membrane to the oval window of the inner ear. Auditory tube—To equalize the pressure of the middle ear with the atmospheric pressure, thus preventing its rupture.

Middle Ear Structures & Functions Oval window—Transmits vibrations from the stapes to the perilymph of the cochlea’s upper chamber, the scala vestibuli.

Inner Ear Structures Cochlea Semicircular Canals Vestibule Vestibule

Inner Ear Oval Window

Inner Ear Cochlea

Vibrations by stapes and oval window produces pressure waves that displace perilymph fluid within scala vestibuli. Vibrations pass to the scala tympani. Movements of perilymph travel to the base(end) of Cochlea where they displace the round window. As sound frequency increases, pressure waves of the perilymph are transmitted through the vestibular membrane to the basilar membrane.

Cochlea Unrolled Sprial Organ of Corti

Spiral Organ of Corti Receptor organ of hearing Different frequencies of vibrations (compression waves) in cochlea stimulate different areas of Organ of Corti Interpreted as differences in pitch

Organ of Corti

Sensory hair cells (stereocilia) located on the basilar membrane. –Arranged to form 1 row of inner cells. Extends the length of basilar membrane. Multiple rows of outer stereocilia are embedded in tectorial membrane. When the cochlear duct is displaced, a shearing force is created between basilar membrane and tectorial membrane, moving and bending the stereocilia.

Organ of Corti Ion channels open, depolarizing the hair cells, releasing glutamate that stimulates a sensory neuron. Greater displacement of basilar membrane, bending of stereocilia; the greater the amount of NT released. Increases frequency of APs produced.

Effects of Different Frequencies 1.Displacement of basilar membrane results in pitch discrimination. 2.Waves in basilar membrane reach a peak at different regions depending upon pitch of sound. 3.Sounds of higher frequency cause maximum vibrations of basilar membrane.

Inner Ear Round Window

Cochlea Oval Window Round Window

Steps of Hearing 1.The auricle directs sound waves into the external auditory canal. 2.When sound waves strike the eardrum, the alternating high and low pressure of the air cause the eardrum to vibrate back and forth.

Steps of Hearing 3.The central area of the eardrum connects to the malleus which also starts to vibrate. 4.The vibration is transmitted from the malleus to the incus and then to the stapes. 5.As the stapes moves back and forth, it pushes the membrane of the oval window in and out.

Steps of Hearing 6.The movement of the oval window sets up fluid pressure waves in the perilymph of the cochlea. 7.As the oval window bulges inward, it pushes on the perilymph of the scala vestibuli.

Steps of Hearing 8.Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the round window. 9.As the pressure waves deform the walls of the scala vestibuli and scala tympani, they also push the vestibular membrane back and forth.

Steps of Hearing 10. This creates pressure waves in the endolymph inside the cochlear duct. 11. The pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane.

Steps of Hearing 12.Bending of the sterocilia produce receptor potentials that lead to the generation of nerve impulses in the cochlear division of cranial nerve VIII.

Inner Ear Vestibule

Saccule Utricle

Utricle and Saccule Utricle: –More sensitive to horizontal acceleration. Saccule: –More sensitive to vertical acceleration.

Vestibule Monitors position of head in space Responds to linear acceleration & direction Maculae is the receptors for static equilibrium

Structure of the Macula

Macula Receptor for Static Equilibrium

Macula of the Utricle Utricle—The hair cells of the macula are oriented in a horizontal plane, therefore, when the head is vertical they are vertical. Movement in a horizontal direction, which changes their position, stimulates neurons which conduct this information to the brain. This results in the head being repositioned.

Macula of the Saccule Saccule—The hair cells of the macula are oriented in a vertical plane. Movement in a vertical direction, which changes their position, stimulates neurons which conduct this information to the brain. This results in the head being repositioned.

Utricle and Saccule Utricle: During forward acceleration, otolithic membrane lags behind hair cells, so hairs pushed backward. Saccule: Hairs pushed upward when person descends.

Utricle and Saccule

Sensory Hair Cells of the Vestibular Apparatus Hair cell receptors: –Stereocilia and kinocilium: When stereocilia bend toward kinocilium; membrane depolarizes, and releases NT that stimulates dendrites of VIII. When bend away from kinocilium, hyperpolarization occurs. –Frequency of APs carries information about movement.

Inner Ear Semicircular Canals

The canals project in 3 different planes

Semicircular Canals The christa ampularis is the receptor for dynamic equilibrium Responds to rotational (angular) movements Changes in rotatory velocity movements

Semicircular Canals The endolymph of the semicircular canal provides inertia so that the sensory hair cells will be bent in a direction opposite to that of the angular acceleration. As the head rotates to the right, for example, the endolymph causes the cupula to be bent toward the left stimulating the hair cells.

Semicircular Canals Provide information about rotational acceleration. –Project in 3 different planes. Each canal contains a semicircular duct. At the base is the crista ampullaris, where sensory hair cells are located. –Hair cell processes are embedded in the cupula. Endolymph provides inertia so that the sensory processes will bend in direction opposite to the angular acceleration.

Semicircular Canals Dynamic Equilibrium