Neuro: Hearing

• The external (outer) ear collects sound waves
• The middle ear (tympanic cavity) is a small, air-filled cavity in the temporal bone
• The middle ear contains auditory ossicles, including the malleus, incus, and stapes, the oval window, and the round window
• The internal (inner) ear is also called the labyrinth

External Ear

• The external ear collects sound waves and passes them inward.
• Structures include the auricle or pinna, external auditory canal
• The auricle is made of elastic cartilage
• The rim of the auricle is called the helix, and the inferior portion is the lobule
• The external auditory canal is a curved 1" tube of cartilage and bone leading into the temporal bone
• The external auditory canal leads to tympanic membrane (eardrum)

Tympanic Membrane

• The the tympanic membrane, also called the eardrum, is a thin semitransparent partition between the external ear and middle ear
• It is made of epidermis, collagen, and elastic fibers
• A perforated eardrum has a hole present
• Symptoms of a perforated eardrum include: pain, ringing, hearing loss, and dizziness
• Perforations may be caused by explosion, scuba diving, cotton swab, or ear infection
• Perforations generally heal within a month

Ceruminous Glands

• Ceruminous glands produce cerumen, also known as ear wax
• The combination of hairs and cerumen can help prevent dust and foreign objects from entering the ear
• These prevent damage to the delicate skin of the external ear canal by water and insects
• Cerumen usually dries up and falls out of the ear canal
• Some people produce large amounts of cerumen, which can lead to impacted cerumen, which muffles incoming sounds
• Impacted cerumen may need periodic ear irrigation or removal of wax with a blunt instrument

Middle Ear

• The middle ear is an air filled cavity in the temporal bone lined by epithelium
• It is separated from the external ear by the eardrum and from the internal ear by the oval and/or round windows
• The middle ear contains 3 auditory ossicles connected via synovial joints: malleus, incus & stapes
• The handle of the malleus attaches to the internal surface of tympanic membrane
• The head of the malleus attaches to the incus body; incus articulates with head of stapes
• Two tiny skeletal muscles attach to ossicles

Tensor Tympani

• The tensor tympani limits movement and increases tension on the eardrum to prevent damage to the inner ear from loud noises
• The tensor tympani is innervated by the mandibular branch of the trigeminal nerve

Stapedius

• Stapedius is the smallest skeletal muscle in the body, it dampens large vibrations of the stapes to protect the oval window and decrease the sensitivity to hearing
• The facial nerve supplies the stapedius
• Paralysis of the stapedius can lead to hyperacusis, which is an increased sensitivity to sound
• Muscle contraction takes a fraction of a second

Auditory Tube

• The auditory tube leads to the nasopharynx, the tube is also called the eustacian tube
• it's normally closed at its medial end, but it opens during swallowing and yawning, allowing air to enter or leave the middle ear, which equalizes atmospheric air pressure
• Lack of equalization can cause pain, hearing impairment, ringing in the ears and/or vertigo
• It may also serve as a route for pathogens to travel from the nose and throat to the middle ear which causes otitis media
• Connection of the auditory tube to the mastoid bone allows for mastoiditis following otitis media

Internal Ear

• The internal ear, also called the labyrinth, is a complicated series of canals
• The labyrinth consists of two main divisions: an outer bony labyrinth and an inner membranous labyrinth
• The bony labyrinth is a series of cavities in the petrous portion of the temporal bone.
• The bony labyrinth can be divided into 3 areas based on shape: semicircular canals, vestibule, and the cochlea which contains receptors for hearing.
• The bony labyrinth is lined with periosteum and contains a fluid called perilymph which is chemically similar to cerebrospinal fluid, surrounds the membranous labyrinth
• The bony labyrinth is a set of tubelike cavities in temporal bone
• It includes the semicircular canals, vestibule, and cochlea lined with periosteum & filled with perilymph
• Bony labyrinth surrounds & thus protects the membranous labyrinth

Membranous Labyrinth

• The membranous labyrinth is a set of membranous tubes which contains sensory receptors for hearing & balance
• It contains the utricle, saccule, ampulla, 3 semicircular ducts & cochlea
• It is series of sacs and tubes lying inside and having the same general form as the bony labyrinth
• Lines with epithelium
• Contains fluid called endolymph, chemically most similar to interstitial fluid
• The vestibule constitutes the oval central portion of the bony labyrinth including two sacs called the utricle and saccule
• Anterior to the vestibule is the cochlea, a bony spiral canal that makes almost three turns around a central bony core called the modiolus
• Projecting upward and posterior from the vestibule are the three bony semicircular canals
• These are arranged at approximately right angles (X-Y-Z axis)
• The anterior& posterior semicircular canals are oriented vertically, while the lateral semicircular canal is oriented horizontally.
• One end of each canal enlarges into a swelling called the ampulla
• The portions of the membranous labyrinth that lie inside the semicircular canals are called the semicircular ducts
• The semicircular ducts connect with utricle of vestibule

Vestibulocochlear nerve

• Vestibulocochlear nerve = CN VIII
• The vestibular branch of the vestibulocochlear nerve consists of ampullary, utricular, and saccular nerves
• The cochlear branch has spiral ganglion in bony modiolus

Hearing

• The following events are involved in hearing:
• 1. The auricle directs sound waves into the external auditory canal.

Sound Waves

• When sound waves strike the tympanic membrane, the alternating high- and low-pressure of the air causes the tympanic membrane to vibrate back and forth.
• The ear drum vibrates slowly in response to low-frequency (low-pitched) sounds and rapidly in response to high-frequency (high-pitched) sounds.
• The central area of the eardrum connects to the malleus, which also starts to vibrate
• The vibration is transmitted from the malleus to the incus and then to the stapes.
  • As the stapes moves back and forth, it pushes the membrane of the oval window in and out.
• The fluid in the perilymph of the cochlea vibrates.
• As the oval window bulges inward, it pushes on the perilymph in the scala vestibuli.
• Lastly pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane.
• Vibration of the sound waves leads to bending of the hair cell stereocilia, which lead to the generation of nerve impulses.

Hair Cells

• Have free ends containing stereocilia overlayed by the tectorial membrane
• Base synapse with cochlear nerve ending transform sound wave vibrations into action potentials that travel along the cochlear nerve.
• Movement of the hair cells opens and closes K+ channels, opening of leads to depolarization of the hair cell, influx of Ca+2, and release of neurotransmitter, closing of K+ channels does the opposite
• Higher frequency is higher pitch -Auricle collects sound waves
• The Eardrum vibrates, since malleus is attached to the eardrum from malleus to incus to stapes

Scala Vestibuli and Tympani

• Stapes pushes on oval window producing fluid pressure waves in scala vestibuli & tympani, this means the oval window vibration is 20X more vigorous than eardrum
• The Pressure fluctuations inside cochlear duct move the hair cells against the tectorial membrane
• Microvilli are bent producing receptor potentials

Sound Vibration

• Results from the alternating compression and decompression of molecules (ie. the air)
• The unit of measurement for frequency is called a hertz (cycles per second) _The human ear can detect sounds between 20 and 20,000 Hz however, we can hear best between 1000 and 4000 Hz
• Frequency of a sound vibration is perceived as pitch (2k-5k)
• Volume of sound is its intensity/amplitude the greater the amplitude of a vibration, the louder the sound
• Different frequencies cause maximal vibration of the basilar membrane at different places in the canal

Hair Cells:

• They lie beneath the vibrating region of the basilar membrane
• They transduce the mechanical force into an electrical signal (receptor potential)
• Sounds of the same pitch vibrate the same region of the membrane
• louder sounds cause a greater vibration amplitude, which increases the amount of electrical impulse sent

Signals

• Bending of the stereocilia of hair cells in the spiral organ causes release of glutamate, this generates nerve impulses in the sensory neurons that innervate the hair cells – cell bodies in the spiral ganglia
• The cochlear branch of CN VIII then sends these axons to the cochlear (ipsilateral) and superior olivary nuclei (bilateral) within the medulla and pons, respectively
• This also allows us to locate the source of a sound

Conditions

• Damage to CN VIII or the hair cells themselves leads to Nerve deafness
• The louder the sound, the quicker the loss of hearing
• Conduction deafness- can be caused by perforated eardrum otosclerosis
• vibrations that are not "conducted" to hair cells

Tinnitus

• Ringing in the ears
• Two leading theories on cause are damaged hair cells and abnormal processing of auditory signals
• Endolymph levels fluctuate

Otitis Media

• Middle ear infection, that is cause as a sore throat
• Middle ear infection- more susceptible in children

Equilibrium

• 2 types of equilibrium: static and dynamic
• Static means measurement of head position relative to gravity and does not involve movements
• Vestibular apparatus (saccule and utricle)

Dynamic Equilibrium

• Measurement of head position during movement (of head or body) involves semicircular ducts
• Dynamic balance can involve utricle/saccule only with linear movement

Macula

• The walls of the utricle and saccule or in a small thickened region are called the macula
• Receptors for static equilibrium are within the macula
• The Walls also provide sensory information on the head positions

Saccule

• Sacs with a thickened region called the macula Contains: hair cells surrounded by supporting cell with many stereocilia, has gelatinous otolithic membrane Also has - Dense calcium cabonate crystals
• The Gravity tilts macula down

Rotation

• 3 ducts at right angles to each other
• Two vertical ducts are the anterior and posterior ducts;
• Lateral duct Permits detection of rotational acceleration
• Hair cells and supporting cells move with head

Hair Cells

-Move with Head leads to movement in endolymph

• Produce receptor potentials–>> nerve impulses transmit to Vestibular brane
• Posterior- detects sagittal change The vestibular branch of vestibulocochlear nerve goes to, then synapse with Sensory Vestibular nucli The brain has a major integrating centre for equilibrum in the Medulla and Pons Eyes can also have the receptor proprioceptors also

Cranial Nerves

• Vestibular nucei controlls cranial nerves3, 4, 6 which contribute coupled balance
• Vestibular nucli has input from eyes and muscles too
• Enter cerebellum through inferior pedicules Age and changing eyesight-Presbyopia,Cataracts, weakened eye musles leads to macular degenretion Decrease to tear production Sharpness of vision as well as depth are reduced in colour perception
• After 50-individuals experience loss of smell Glaucoma is a disease with high eye pressure Otitus Media is bacterial in children