Chapter 16 The Special Senses Hearing Outer ear

Chapter 16 The Special Senses Hearing

Outer ear La. Pointe Fall ’ 11 Slide # 2 • Anatomy of the ear – External Ear • Auricle or pinnae surrounds the ear • External acoustic meatus (ear canal or auditory canal) • ends on Tympanic membrane (ear drum) • Ceruminous glands - produce cerumen (ear wax)

Middle ear La. Pointe Fall ’ 11 Slide # 3 • Starts with the tympanic membrane • Communicates with pharynx via pharyngotympanic tube (auditory tube, Eustachian tube) • Middle ear encloses and protects the auditory ossicles • malleus, incus, stapes • oval window • tensor tympani muscle - stiffens the malleus and tympanic membrane • stapedius muscle - reduces movement of the stapes on the oval window

Anatomy of the Ear La. Pointe Fall ’ 11 Slide # 4

Middle and Inner Ear La. Pointe Fall ’ 11 Slide # 5 Balance Hearing Figure 15. 25 b

The Middle Ear - Frontal View Lateral La. Pointe Fall ’ 11 Slide # 6 Medial Figure 17. 21

The Middle Ear - Medial View La. Pointe Fall ’ 11 Slide # 7 Figure 15. 26

Inner ear La. Pointe Fall ’ 11 Slide # 8 • Membranous labyrinth contains endolymph (similar to intracellular fluid) • Bony labyrinth surrounds and protects membranous labyrinth • Perilymph lies between the two labyrinths (similar to cerebrospinal fluid) • Vestibular complex • Vestibule - contains the utricle and saccule , sense of gravity, linear acceleration • Semicircular canals - contains semicircular ducts, rotation, equilibrium • Cochlea contains the cochlear duct, sense of hearing

The Inner Ear green = perilymph La. Pointe Fall ’ 11 Slide # 9 blue = endolymph

Chambers of the Cochlea • • The cochlea is divided into three chambers: Scala vestibuli Scala media (chochlear duct) Scala tympani La. Pointe Fall ’ 11 Slide # 10

Perilymph and Endolymph La. Pointe Fall ’ 11 Slide # 11 Scala vestibuli Scala media Scala tympani Figure 17. 28 a

Vibrations of the inner ear La. Pointe Fall ’ 11 Slide # 12 Scala vestibuli (perilymph) Scala media (endolymph) Basilar membrane Scala tympani (perilymph) Tectoral membrane Marieb Figure 15. 34

Sound La. Pointe Fall ’ 11 Slide # 13 • Cochlear duct lies between the vestibular duct and the tympanic duct • Hair cells of the cochlear duct lie within the Organ of Corti • Frequency (cycles per second (Hz)) is determined by which region is stimulated • Intensity is the energy content of a sound, how many hair cells are activated and their degree of movement • Measured in decibels

Windows La. Pointe Fall ’ 11 Slide # 14 • Round window separates the perilymph from the air spaces of the middle ear • Oval window connected to the base of the stapes • Basic receptors of inner ear are hair cells • Contain stereocilia • Provide information about the direction and strength of stimuli

Organ of corti La. Pointe Fall ’ 11 Slide # 15 • Hair cells sit on basilar membrane • Steriocilia are in contact with the tectorial membrane • thin membrane attached to the cochlear duct

The Organ Of Corti (scala media) La. Pointe Fall ’ 11 Slide # 16

Pathway of sound La. Pointe Fall ’ 11 Slide # 17 • Sound waves travel toward tympanic membrane, which vibrates • Auditory ossicles conduct the vibration into the inner ear • Tensor tympani and stapedius muscles contract to reduce the amount of movement when loud sounds arrive • Movement at the oval window applies pressure to the perilymph (in the vestibular duct), and then to the endolymph of the cochlear duct. • Pressure waves distort basilar membrane • Hair cells of the Organ of Corti are pushed against the tectoral membrane

Sound and Hearing La. Pointe Fall ’ 11 Slide # 18 Figure 17. 28 a

Signal amplification La. Pointe Fall ’ 11 Slide # 19

Receptor cell depolarization High K+ solution La. Pointe Fall ’ 11 Slide # 20 High K+ solution K+

Pitch La. Pointe Fall ’ 11 Slide # 21 Figure 15. 32

Neural pathway La. Pointe Fall ’ 11 Slide # 22 • Sensory neurons of hearing are located in the spiral ganglion of the cochlea (first order neuron) • Afferent fibers form the cochlear branch of cranial nerve VIII • Synapse at the cochlear nucleus (second order neuron) • Goes to the inferior colliculus of the mesencephalon (third order neuron) Collaterals go to the olivary nucleus (that fine tunes the ear and adjusts for volume • then to the thalamus (fourth order neuron) • temporal lobe of the cerebral cortex

Neural Pathway (also see fig 16. 18) 4 3 La. Pointe Fall ’ 11 Slide # 23 Note that signals from each ear go to both sides of the brain! 2 1 Figure 15. 34