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.

Click to add title• Click to add text

Click to add title• Click to add text