Chapter 9 Hearing and Equilibrium Equilibrium and Hearing

Chapter 9: Hearing and Equilibrium

Equilibrium and Hearing • Both of these senses are provided by the internal ear which is located in the temporal bone. • Equilibrium informs us of our body’s position in space • Hearing enables us to detect and interpret sound waves • Both use hair cells which are mechano receptors

Anatomy of the Ear • Divided into three anatomical regions – External ear • Collects and direct sound waves toward middle ear – Middle ear • Amplify sound waves and transmit them to inner ear – Internal ear • Contains the sensory organs for hearing and equilibrium

External Ear • Includes the auricle or pinna which surround the entrance to the external acoustic meatus • Ends at the tympanic membrane.

Middle Ear • Connected to the nasopharynx by the auditory tube (eustachian tube). • Encloses and protects the auditory ossicles which connect the tympanic membrane to the internal ear. – Malleus: attached to tympanic membrane – Incus: middle bone – Stapes: attached to the oval window of the inner ear.

Internal Ear • Senses of equilibrium and hearing are provided by the receptors within the internal ear. • These receptors are protected by the bony labyrinth which is fused with the temporal bone • The bony labyrinth surrounds the membranous labyrinth which is a collection of tubes and chambers.

• The membranous labyrinth is filled with endolymph and between the bony and membranous labyrinths is another fluid called perilymph. • Bony labyrinth has three parts – Vestibule: receptors for gravity and acceleration – Semicircular canals: rotation of the head. – Cochlea: hearing.

Equilibriium • Dynamic equilibrium : aids us maintaining our balance when the head and body move suddenly • Static equilibrium: maintains our posture and stability when the body is motionless. • Semicircular canals monitor rotational movement of the head which is part of dynamic equilibrium • Structures in the maculae respond to gravity and linear acceleration.

Hearing • The receptors for hearing are hair cells similar to those of equilibrium. • Their placement in the cochlea shields them from stimuli other than sound • The auditory ossicles convert the pressure waves of air to pressure pulses in the perilymph at the oval window. • The pressure pulses stimulate hair cells along the cochlear spiral.

• The frequency (pitch) of the perceived sound is determined by which part of the cochlear duct is stimulated. (units hertz) • The intensity (volume) of the perceived sound is determined by how many hair cells at that location are stimulated. (units decibels)

6 steps • 1. Sound waves arrive at the tympanic membrane. • 2. Movement of the tympanic membrane causes displacement of the auditory ossicles. • 3. The movement of the stapes at the oval window establishes pressure waves in the perilymphs of the inner ear.

• 4. The pressure waves distort the basilar membrane on their way to the round window of the tympanic duct. • 5. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane. • 6. Information about the region and intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII.

Aging and the Senses • Smell: olfactory receptor cells are regularly replaced by cell division but this decreases with age. The receptors also become less sensitive. • Taste: reduction in number and sensitivity of taste buds. Begin life with around 10, 000 taste buds but number declines quickly after age 50.

Vision and age • With age the lens loses its elasticity and stiffens. Seeing objects close up becomes a problem—called presbyopia. • Cataracts: loss of transparency in the lens. • Gradual loss of rods with age: need more light to read. • Macular degeneration: growth and proliferation of blood vessels in the retina.

Hearing • The tympanic membrane loses some elasticity. • It becomes difficult to hear high pitched sounds. • Progressive hearing loss that occurs with aging is presbycusis.

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