Anatomy of the Ear and Hearing

• The ear is responsible for hearing and equilibrium.

Anatomy of the Ear

• The external ear consists of the pinna and ear canal.
• The tympanic membrane, or eardrum, seals the end of the ear canal.
• The middle ear is an air-filled space connected to the pharynx by the Eustachian tube.
• The inner ear contains the cochlea for hearing and the vestibular apparatus for equilibrium.

Hearing: Sound

• Sound is made of pressure waves.
• Molecules are crowded together at the peaks of sound waves, resulting in high pressure.
• Molecules are far apart at the troughs of sound waves, resulting in low pressure.

Frequency and Pitch

• Frequency is the number of wave peaks per second, measured in hertz (Hz).
• The human hearing range is approximately 16-20,000 Hz, covering about 10 octaves
• Humans perceive frequency as pitch, where low frequencies correspond to lower pitched sounds and high frequencies to higher pitched sounds.
• Acuity is highest 1000-3000 Hz.
• Middle C is ~261.6 Hz, adult male voices ~100 Hz, female ~150 Hz.
• Some bats and dolphins can hear up to 200 kHz
• Elephants and crocodiles can hear infrasonics.

Amplitude and Loudness

• Amplitude is the pressure difference between the peak and trough of a sound wave.
• Loudness is determined by amplitude and frequency.
• Larger amplitude results in louder sounds, at any given frequency, where a Larger amplitude results in a louder sound.

How Eardrums Vibrate

• Sound waves cause the eardrum to vibrate.
• The eardrum separates the outer ear from the middle ear.

Ossicles and Vibration Transmission

• A chain of small bones transmits vibrations through the middle ear.
• The eardrum vibrates the malleus (hammer) bone, which moves the incus (anvil), then the stapes (stirrup).
• The stapes pushes against the oval window, a membrane between the middle and inner ear.
• The ossicles, the smallest bones in the body, act as a lever system, carrying vibrations from the eardrum to the much smaller oval window.

Cochlea

• The oval window leads into the cochlea, where receptor cells are located.

Cochlear Ducts and Fluids

• Uncoiling the cochlea reveals its anatomy more clearly.
• The vestibular duct (scala vestibuli) and tympanic duct (scala tympani) contain perilymph, a fluid similar to plasma.
• The two ducts communicate at the helicotrema.
• The cochlear duct (scala media) contains endolymph, which is similar to intracellular fluid.

Wave Transmission

• The oval window vibrates, setting up waves in the perilymph.
• Wave energy enters the cochlea at the oval window and exits into the middle ear through the round window.

Organ of Corti and Hair Cells

• The cochlear duct contains the organ of Corti.
• The organ of Corti sits on the basilar membrane and under the tectorial membrane.
• The organ of Corti contains hair cells, the auditory receptor cells, which are mechano-receptors ~20,000 per cochlea and are epithelial cells, not neurons.
• Each hair cell has 50-100 stiff "hairs" called stereocilia, extending into the tectorial membrane.
• The stereocilia will bend as waves will deform the basilar and tectorial membranes.

Hair Cell Activation

• When its cilia bend toward the longest cilium, the hair cell depolarizes and releases neurotransmitter, activating a primary sensory neuron.
• The axons of these neurons form the auditory nerve (cochlear nerve), a branch of cranial nerve VIII.
• When its cilia bend away, the hair cell hyperpolarizes and releases less neurotransmitter, therefore less excitation of the neuron.

Basilar Membrane and Frequency Response

• The basilar membrane is narrow and stiff near the oval window, and wider and more flexible near the helicotrema.
• The basilar membrane responds to different frequencies at different points: High-frequency waves maximally displace the membrane at the oval-window end, and low-frequency waves maximally displace the other end, thus the brain deduces frequency by which hair cells are most active.
• The brain interprets pitch from the pattern of basilar membrane motion

Auditory Processing

• Signals pass from each ear to both sides of the brain.
• Primary auditory cortex (A1) is in the temporal lobe.

Sound Localization

• The brain localizes sounds based on loudness and timing.
• A sound louder in the right ear compared to the left, indicates it's coming from the right side.
• Louder sounds create a higher firing frequency from auditory sensory neurons.
• If sound reaches the right ear before the left, it's coming from the right side.

Hearing Loss Types

• There are 3 types of hearing loss.
• In conductive hearing loss, sound cannot be transmitted through the external or middle ear.
• In sensorineural hearing loss, the hair cells or other parts of the inner ear are damaged, of which mammals cannot replace these cells.
• 90% of hearing loss in the elderly (presbycusis) is sensorineural.
• In central hearing loss, there is damage to the cortex, where typically the patient has trouble recognizing sounds.

Clinical Hearing Tests

• Rinne Test:
• Use: Differentiates conductive from sensorineural hearing loss.
• Procedure: Hold tuning fork against mastoid bone, then beside ear.
• Normal Result: Sound is louder through the ear canal.
• Conductive Loss: Sound is louder through the bone.
• Weber Test:
  • Use: Differentiates conductive from sensorineural hearing loss.
  • Procedure: Hold tuning fork to the patient's forehead (midline), and ask in which ear the sound is louder.
  • Sensorineural Loss: Sound is louder in the good ear.
  • Conductive Loss: Sound is louder in the bad ear with sounds heard through that ear canal.

Equilibrium

• Different parts of the vestibular apparatus sense head position and motion.
• The utricle and saccule have hair cells activated when the head tilts relative to gravity.
• The semicircular canals are fluid-filled hoops that detect head rotation.
• Head rotations cause fluid in the tubes to slosh leftward, thus activating hair cells.

Equilibrium Pathways

• Vestibular hair cells activate primary sensory neurons of the vestibular nerve (a branch of cranial nerve VIII).
• These neurons either pass directly to the cerebellum or synapse in the medulla, then proceed to the cerebellum or up through thalamus to cortex.
• The brain uses vestibular information to infer position and motion, and to maintain upright posture.