Auditory System Flashcards
Describe the characteristics of a sound wave.
Has frequency (cycles per second in Hz) and intensity (magnitude of sound in dB). dB is a log scale.
What are the components of the outer ear? Describe path of sound waves through outer ear.
Ear pinna directs sound vibration to external auditory canal, then to tympanic membrane.
Describe path of sound waves through middle ear. Components?
Sound waves transferred to ossicles (malleus, incus, and stapes). Malleus has tensor tympani muscle (CN V) attached to it. Stapes has stapedius muscle attached to it (CN VII). Stapes attaches to oval window. Middle ear converts sound waves in air to waves in fluid in inner ear. Middle ear connected with nasopharynx; therefore, susceptible to middle ear infections (otitis media).
Describe components of inner ear.
Bony labyrinth, membranous labyrinth, and cochlea containing Organ of Corti.
Describe bony labyrinth
Has central region called vestibule, 3 semicircular canals, and cochlea. Filled with perilymph. Cochlear canal divided into scala vestibuli (above) and scala tympani (below). The two communicate with each other at helicotrema. Scala tympani ends at round window.
Describe membranous labyrinth.
Located within the cavities of bony labyrinth and filled with endolymph (low sodium, high potassium). Includes utricle, saccule, semicircular canal ducts, and cochlear duct (scala media). Scala media bound by vestibular membrane above and basilar membrane below, separating endolymph from perilymph.
Describe Organ of Corti.
Sensory area of cochlear duct and contains hair cells. Lies on the basilar membrane in scala media.
Describe the path of sound waves to hair cell starting from ear pinna.
Ear pinna -> EAC -> tympanic membrane -> amplified by ossicles (malleus, incus, stapes; modified by stapedius and tensor tympani, if needed) -> vibration of oval window -> pressure waves in perilymph -> oscillations of perilymph cause vibration of basilar membrane -> movement of hair cell cilia.
Describe the movement of ciliar/kinocilia and functional importance of direction of movement.
When stereocilia move toward kinocilia, potassium permeability increases and hair cell depolarizes.
When stereocilia move away from kinocilia, potassium permeability decreases and hair cell hyperpolarizes.
Where is the first AP?
In bipolar cells, NOT hair cells.
What results from the depolarization of hair cells?
Calcium influx -> neurotransmitter release -> action potential in bipolar cells.
Describe relationship between location of hair cells and frequency of sound perceived.
Hair cells near oval window = higher frequency.
Hair cells near helicotrema = lower frequency (basilar membrane is wider here).
Describe characteristics of outer and inner hair cells.
Outer hair cells are displacement sensitive. Only receptors that can be modified by CNS. 10 outer hair cells per bipolar cell.
Inner hair cells are velocity sensitive. NOT directly modified by CNS. 10 bipolar cells per inner hair cell.
Describe path of cochlear nerve starting from bipolar neurons.
Bipolar neurons -> cochlear nerve -> spiral ganglion -> dorsal and ventral cochlear nucleus (rostral medulla) -> lateral lemniscus (pons) -> inferior colliculus (midbrain) -> medial geniculate (thalamus) -> Herschl’s gyrus (area A-1).
Other notes:
Cochlear nerve + vestibular nerve = CN VIII.
Area 41 = Herschl’s gyrus. ARea 42 = auditory association area. Together = A-1 region.
