chapter 9: hearing and equilibrium

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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