unit 22 review
The ear is a complex sensory organ responsible for hearing and balance. It consists of three main parts: the outer, middle, and inner ear. Each part plays a crucial role in converting sound waves into electrical signals that the brain can interpret.
The hearing process involves sound waves traveling through the ear structures, causing vibrations that stimulate hair cells in the cochlea. The vestibular system, located in the inner ear, maintains balance by detecting head movements and position relative to gravity.
Key Structures of the Ear
- Outer ear consists of the pinna (auricle), external auditory canal, and tympanic membrane (eardrum)
- Middle ear is an air-filled cavity that contains the three smallest bones in the body, the ossicles
- Malleus (hammer) attaches to the tympanic membrane
- Incus (anvil) connects the malleus to the stapes
- Stapes (stirrup) is the smallest bone and interfaces with the oval window of the inner ear
- Inner ear is composed of the cochlea, a snail-shaped structure responsible for hearing, and the vestibular system, which maintains balance and equilibrium
- Cochlea contains the organ of Corti, which houses hair cells that convert mechanical vibrations into electrical signals
- Semicircular canals, utricle, and saccule are part of the vestibular system and detect rotational and linear acceleration
Sound Wave Basics
- Sound waves are longitudinal pressure waves that travel through a medium (air, water, or solid materials)
- Frequency, measured in Hertz (Hz), determines the pitch of a sound
- Higher frequencies produce higher-pitched sounds, while lower frequencies produce lower-pitched sounds
- Amplitude, or intensity, determines the loudness of a sound and is measured in decibels (dB)
- Sound waves are characterized by compression (high-pressure) and rarefaction (low-pressure) regions
- The human ear can typically detect frequencies between 20 Hz and 20,000 Hz (20 kHz)
- Sounds below 20 Hz are called infrasound, while those above 20 kHz are ultrasound
Outer Ear Function
- Pinna (auricle) collects and funnels sound waves into the external auditory canal
- Its shape helps localize sound by filtering and amplifying specific frequencies
- External auditory canal (ear canal) directs sound waves toward the tympanic membrane (eardrum)
- Ceruminous glands in the canal secrete cerumen (earwax), which protects against foreign particles and microorganisms
- Tympanic membrane (eardrum) is a thin, cone-shaped membrane that vibrates in response to sound waves
- Outer ear acts as a resonance chamber, amplifying frequencies between 2,000 and 5,000 Hz, which are important for speech comprehension
Middle Ear Mechanics
- Tympanic membrane vibrations are transmitted to the ossicles (malleus, incus, and stapes) in the middle ear
- Ossicles act as a lever system, amplifying the force of the vibrations and overcoming the impedance mismatch between air and the fluid-filled inner ear
- This amplification is essential for efficient energy transfer and hearing sensitivity
- Tensor tympani and stapedius muscles can contract reflexively in response to loud sounds, reducing the transmission of vibrations and protecting the inner ear from damage
- Eustachian tube connects the middle ear to the nasopharynx, equalizing pressure between the middle ear and the atmosphere
- Yawning, swallowing, or chewing gum can help open the Eustachian tube and relieve pressure buildup (ear popping)
Inner Ear Processes
- Stapes transfers vibrations to the oval window, a membrane-covered opening in the cochlea
- Vibrations create pressure waves in the perilymph, a fluid within the cochlear ducts
- Pressure waves cause the basilar membrane, which supports the organ of Corti, to vibrate
- Different frequencies resonate at specific locations along the basilar membrane (tonotopic organization)
- High frequencies near the base and low frequencies near the apex of the cochlea
- Hair cells in the organ of Corti bend in response to basilar membrane vibrations
- Stereocilia (hair-like projections) on top of hair cells are deflected, opening ion channels and allowing potassium influx
- This influx depolarizes the hair cells, releasing neurotransmitters and generating electrical signals in the cochlear nerve fibers
Neural Pathways of Hearing
- Cochlear nerve (part of vestibulocochlear nerve or cranial nerve VIII) transmits electrical signals from hair cells to the cochlear nuclei in the brainstem
- Auditory information is then processed in the superior olivary complex, lateral lemniscus, and inferior colliculus
- These structures are involved in sound localization and auditory reflexes
- Medial geniculate nucleus of the thalamus receives auditory input and relays it to the primary auditory cortex in the temporal lobe
- Primary auditory cortex (Brodmann areas 41 and 42) processes basic characteristics of sound, such as pitch, intensity, and timbre
- Secondary auditory cortex (Brodmann area 22) and associated areas are involved in higher-level processing, such as speech perception, music appreciation, and auditory memory
Balance and Equilibrium
- Vestibular system in the inner ear is responsible for maintaining balance and equilibrium
- Semicircular canals detect rotational acceleration (head rotation)
- Three orthogonal canals (anterior, posterior, and lateral) filled with endolymph fluid
- Ampulla at the base of each canal contains a gelatinous structure called the cupula and hair cells
- Endolymph movement deflects the cupula and hair cells, generating electrical signals
- Otolith organs (utricle and saccule) detect linear acceleration and head position relative to gravity
- Contain hair cells embedded in a gelatinous matrix with calcium carbonate crystals (otoconia)
- Gravitational pull and linear acceleration cause the otoconia to shift, deflecting hair cells
- Vestibular nerve (part of cranial nerve VIII) transmits signals from the vestibular system to the brainstem and cerebellum
- Vestibular nuclei integrate information from the vestibular system, visual system, and proprioceptors to maintain balance and coordinate eye and head movements
Common Ear Disorders
- Conductive hearing loss results from problems in the outer or middle ear that prevent sound from reaching the inner ear effectively
- Causes include earwax blockage, ear infections (otitis media), perforated eardrum, and otosclerosis (stiffening of the ossicles)
- Sensorineural hearing loss occurs due to damage to the cochlea, hair cells, or auditory nerve
- Causes include age-related hearing loss (presbycusis), noise-induced hearing loss, ototoxic medications, and genetic factors
- Tinnitus is the perception of ringing, buzzing, or other sounds in the absence of an external stimulus
- Can be caused by exposure to loud noise, ear infections, Ménière's disease, or certain medications
- Ménière's disease is characterized by episodes of vertigo, fluctuating hearing loss, tinnitus, and a feeling of fullness in the affected ear
- Believed to be caused by an abnormal buildup of endolymph in the inner ear (endolymphatic hydrops)
- Benign paroxysmal positional vertigo (BPPV) is a common cause of vertigo triggered by specific head movements
- Occurs when otoconia dislodge from the otolith organs and migrate into the semicircular canals, causing abnormal endolymph movement and vertigo