Overview Introduction The ear can be divided into the outer ear involved in directing sound into the auditory canal middle ear an air-filled space that contains tympanic membrane auditory ossicles (which amplifies sound) which is comprised of the malleus incus stapes this inserts into the oval window and therefore when the ossicles vibrate it displaces the fluid within the inner ear note that the tensor tympani muscle decreases movement of the malleus this muscle is innervated by the tensor tympani nerve off of CN V3 (mandibular division of the trigeminal nerve) stapedius decreases movement of the stapes this muscle is innervated by the stapedius nerve off of CN VII (facial nerve) this is in response to loud noise Eustachian tube inner ear a fluid-filled space that consists of a bony labyrinth that is comprised of the cochlea structures scala vestiubli, tympani, and media perilymph is located outside the ducts the scala vestibuli and tympani contain this fluid which has an ↑ sodium (Na+) concentration vestibule semicircular canals membranous labyrinth that is comprised of the cochlear duct utricle saccule semicircular canals note that endolymph is located inside the ducts the scala media contains this fluid which has an ↑ potassium (K+) concentration organ of Corti located on the basilar membrane in the cochlea and contains hair cells with protruding cilia that are embedded in the tectorial membrane these hair cells synapse to primary sensory neurons in which their cell bodies are found in the spiral ganglion tonotopy high frequency vibrations are best received by the cochlear base (by the oval window) due to its thin and rigid nature low frequency vibrations are best received by the cochlear apex (by the helicotrema) due to its wide and flexible nature Auditory pathway external ear → tympanic membrane → middle ear ossicles → oval window → labyrinth scala media (cochlear duct) also receives vibrations, which result in movement of the basilar and tectorial membrane leading to activation of mechanoreceptor cilia on hair cells which in turn excite primary sensory neurons (which have their cell bodies in the spiral ganglion) and ultimately will have axons sent into the cochlear nerve (of CN VIII) from the cochlear nerve, electrical impulses subsequently arrive at the dorsal and ventral cochlear nuclei and then bilaterally ascends superior olivary nuclear complex → inferior colliculus → medial geniculate nucleus → primary auditory cortex