Special Senses: Equilibrium & Hearing Honors Anatomy & Physiology

Ear Anatomy External Ear Pinna (Auricle) elastic cartilage covered w/thin skin Directs sound waves into auditory canal External Acoustic Meatus (auditory canal) Short curved tube carved into temporal bone Lined w/Ceruminous glands – secrete cerumen (ear wax) to repel insects and trap pathogens Tympanic Membrane (Ear Drum) Connective tissue membrnae Sound waves cause it to vibrate

Ear Structures Middle Ear Air-filled mucosa-lined cavity Phargnotympanic (Eustachian) tube links middle ear to nasopharynx Flattened until needed to equalize w/external air pressure Important for free vibrations of ear drum Auditory Ossicles (smallest bones in body) Malleus (Hammer) Incus (Anvil) Stapes (Stirrup) Transmit vibrations to oval window (small opening) of inner ear

Internal Ear (Labyrinth) Osseous Labyrinth Filled w/perilymph (similar to CSF) Vestibule Houses receptors that respond to gravity & changes of head position Cochlea Snail coil the size of a split pea Spiral organ of Corti – receptor organ for hearing Semi-Circular Canals Oriented in X,Y,Z planes Receptors respond to rotational movements of the head Membranous Labyrinth Sacs & ducts contained w/in the bony labryinth Filled w/endolymph – K+ rich intracellular fluid

Properties of Sound Sound – alternating areas of high and low pressure causing molecules to vibrate…travels slower than light Illustrated as a sine wave defined by: Frequency – number of waves that pass a given point in a certain time Human hearing 20-20,000 waves per second (Hz) Wavelength – distance between 2 consecutive crests Pitch – perceived sound frequencies (higher freq = higher pitch) Amplitude – height of sine wave – sound intensity…interpreted as loudness measured in decibels (dB)

Physiology of Hearing Sound waves vibrate tympanic membrane transferred to auditory ossicles and then the oval window Bulging membrane of round window creating pressure waves within perilymph of vestibular duct Pressure waves distort basilar membrane, vibrating hair cells against tectoral membrane Information relayed via cochlear nerve Hair cells – receptors of inner ear Stereocilia:80-100 microvilli on free surface moved by external forces causing displacement in one direction increasing neurotransmitter release, displacement in opposite direction decreases neurotransmitter release Pitch determined by which part of cochlear duct is stimulated Volume determined by how many cells are stimulated

Vestibular Apparatus: Equilibrium & Orientation Dynamic Equilibrium Maintains balance during sudden movements Semicircular Ducts (anterior, posterior & lateral) Rotational Motion Ampulla – swollen region w/sensory “hair” cells Movement of endolymph stimulates hair cells Static Equilibrium Maintains posture and stability when motionless Vestibule Gravity & Linear Acceleration Osteolith (CaCO3) sits on hair cells and can shift Vestibular branch of vestibular cochlear nerve integrate sensory info from both sides, and relay info to cerebellum, cerebral cortex, & brain stem

Dynamic Equilibrium Static Equilibrium

Hearing Deficits Conduction Deafness Sensorinueral Deafness Blocks transfer of vibration from tympanic membrane to oval window Ear wax buildup, trapped water, scarring or perforation of tympanic membrane, overgrowth of ossicles Sensorinueral Deafness Within the cochlea or auditory pathway Broken sterocilia (loud sounds), drugs destroy hair cells, bacterial infection kill hair cells Cochlear implants convert sound into electrical signals can be inserted into temporal bone

American Sign Language Can you spell your name?