Which cranial nerves are associated with hearing?

The organ of hearing and equilibrium The Ear The organ of hearing and equilibrium

What is Sound? Pressure disturbance originating from a vibrating object Compressions and rarefactions of particles in a medium

What sound looks like http://www.youtube.com/watch?v=4shodbQMcmM

Pitch Frequency Number of waves that pass a given point in a given time Hearing range is 20 Hz to 20,000 Hz Higher frequency = higher pitch Lower frequency – lower pitch

Which disturbance is a higher pitch?

Loudness Amplitude Height of the wave (crest) Intensity of the sound = energy Humans: 0.1 decibels to 120 dB (pain threshold = 130dB)

Which disturbance is louder?

Hearing Auditory area of temporal lobe cortex Sound waves through the air must be perceived What type of receptors are involved?

Intro video http://www.youtube.com/watch?v=pCCcFDoyBxM

Three Parts External ear Middle ear Inner Ear

External ear: Auricle/Pinna

External ear: auditory canal Lined with skin, bearing hairs, sebaceous glands, and modified sweat glands Secretes sticky cerumen (earwax): purpose? 1 inch long

Impacted eardrum

Boundary between external and middle ear: tympanic membrane Connective tissue, thin Rich supply of nerves and blood vessels Cone shaped, apex points into ear Vibrates in response to sound waves entering the ear

Middle ear Small, air-filled chamber Eardrum on one side, bone on other

Otitis media

Otitis media: middle ear inflammation

Or this…

Middle ear: eustachian tube Linked to nasopharynx Closed by a membrane most of the time Opens briefly when yawn or swallow Equalizes middle ear pressure and external air pressure

Middle ear: ossicles Smallest bones in body Suspended by tiny ligaments, linked together by joints Malleus secured to ear drum Stapes – base fits into oval window (entrance to cochlea)

Middle ear: ossicles Transmit vibratory motion of eardrum to oval window Sets fluids of inner ear in motion Eventually excites the hearing receptors

Problems… Chronic ear infections – ossicles can fuse “glue ear”: stapes becomes fixed to the oval window Protection: Muscles help protect the eardrum and reduce sound transduction when accosted by loud noises

Inner ear Complicated structure Behind eye socket Secure site Delicate receptor machinery

Inner ear: two divisions Bony labyrinth: the cavity – filled with perilymph (fluid) Membranous labyrinth – inside the cavity, floating in perilymph, filled with endolymph

Inner ear: vestibule Equilibrium receptor region Otoliths – increase sensitivity Respond to pull of gravity Report changes in head position

Inner ear: semicircular canals Oriented in 3 planes Anterior Posterior Lateral Equilibrium receptors Conducts to vestibular nerve of 8th cranial nerve

Inner ear: cochlea Converts the physical vibrations of sound waves and converts them into electrical impulses

Inside the cochlea http://www.youtube.com/watch?v=dyenMluFaUw

Inner ear: cochlea Lined with receptors for hearing Stimulated by bending of hairs: mechanoreceptors Organ of corti: receptor organ for hearing

Vestibulocochlear nerve (cranial nerve 8) Cochlear branch Conducts to primary auditory cortex of temporal lobe Can damage this nerve when exposed to prolonged periods of loud noise Drugs = ototoxic agents

Hearing test http://pedsohns.ucsf.edu/upload/medialibrary/2fa/2fa3732df5888aefd57ec0442570a239.jpg