Auditory/vestibular System special sensory

• Sound and balance information is relayed by cranial nerve VIII (vestibulocochlear nerve).
• Hearing and balance share anatomical and physiological features.
• Peripheral pathologies can affect both systems.

Auditory System

• The auditory system senses and analyzes sound.
• Sound is a wave of mechanical energy, where air molecules are set in motion producing rarefaction and compression waves, radiating from a source.
• Frequency is quantified in Hertz (Hz) as cycles per second; Frequency is measured by amplitude.
• The audible range for humans is 20-20,000 Hz.
• Sound intensity is measured by pressure amplitude.
• Loudness of the sound is correlated with the intensity of the sound
• The decibel scale (dB) measures amplitude of pressure waves. It is a logarithmic scale
• The perceptible range is about 120 dB for humans.
• Pitch perception correlates to frequencies. Low pitch sounds have low frequencies, and high pitch sounds have high frequencies.

General Ear Anatomy

• The outer ear consists of the pinna, external auditory meatus, and tympanic membrane
• The middle ear contains 3 middle ear bones/ossicles (malleus, incus, stapes) and muscles, round window, and oval window
• The inner ear contains the cochlea (hearing) and semicircular canals, utricle, and saccule (balance)
• The pinna collects sound waves, which travel down the external auditory meatus and cause the tympanic membrane to vibrate.
• Vibration of the tympanic membrane causes the 3 middle ear bones to oscillate, causing fluid movement in the cochlea and the activation of hair cells and the cochlear nerve.
• In the outer ear sound waves enter, causing the tympanic membrane to fluctuate
• The tensor tympani, when contracted, causes the tympanic membrane to become stiff. Trigeminal nerve V3 innervates this muscle.
• The middle ear bones (malleus, incus, stapes) move in response to movement in the tympanic membrane in an air environment.
• Middle ear bones transmit movements to the inner ear structures which are in a fluid environment; the oval and round windows are in the middle ear
• The ossicles act as levers to reduce the magnitude of the movements of the tympanic membrane while increasing their force at the oval window.
• The stiffness of the ossicles helps to compensate for the difference in impedance between air and fluid environments (impedance matching; from low-impedance air to high-impedance fluid).
• This feature allows for optimal transfer of energy between air and fluid media.
• Recall that the stapedius muscle is attached to the stapes, contraction of the muscle will allow the ossicles to become stiff
• Cranial nerve VII (Facial) innervates the stapedius muscle.

Inner Ear: Cochlea

• The cochlea converts sound waves into fluid waves, then into ionic signals, and finally into action potentials.
• It is shaped like a snail, has 2.5 turns, and a base and apex.
• The cochlea is made of bony and membraneous parts.
• The membranous labyrinth has 3 parts:
• Scala vestibuli: Contains perilymph; opens to the oval window from the stapes.
• Scala media (cochlear duct): Contains endolymph. The Vestibular (Reissner's) membrane separates it from scala vestibuli.
• Scala tympani: Contains perilymph (high [Na+], low [K+]).
• The basilar membrane separates scala media from tympani and runs the length of the cochlea
• The Organ of Corti sits on the basilar membrane.
• Scala vestibuli and tympani are connected at the apex (helicotrema),

Receptor Organ: Organ of Corti and Sensory Receptors: Hair Cells

• The inner hair cells form 95% of the synapses with the primary sensory afferent, so they are important for sound transduction.
• The outer hair cells outnumber the inner hair cells 4:1 but make up only 5% of the connections with the primary sensory afferent neurons.
• The outer hair cells can increase the movement of the basilar membrane so it acts as a cochlear amplifier.

Sound Transmission

• Sound waves in the air strike the tympanic membrane, which then vibrates.
• The vibrations of the tympanic membrane are transferred to the middle ear bones, which then also vibrate.
• The vibrations of the stapes are transmitted via the oval window which then create pressure waves in the perilymph within the scala vestibuli and tympani.
• The pressure waves also push on the endolymph and the basilar membrane of the scala media.
• Pressure waves are transmitted to the scala tympani and dissipate back into air via the movement of the round window.
• Deformation of the basilar membrane of the scala media causes the tectorial membrane to move, bending the stereocilia of the hair cells and activating the cochlear nerve afferents.
• Deformation of the basilar membrane of the scala media causes the tectorial membrane to move, bending the stereocilia of the hair cells.
• The pivot points for the tectorial and basilar membrane are offset.
• Movement of the basilar membrane up and down introduces a shearing force on the tectorial membrane.

Hair Cells

• Hair cells in the cochlea are made up of short to tall stereocilia with 1 long kinocilia.
• As the basilar membrane moves up and down, tip links stretch and compress, opening and closing mechanically-gated K+ channels.
• When motion deflects the hair cells in the direction towards the kinocilia, depolarization occurs.
• When the hair cells deflect away from the kinocilia, hyperpolarization occurs.
• There are mechanically gated ion channels near the cilia tips of the hair cells.
• Deflection towards the kinocilia opens the channel and allows the K+ to enter and depolarize the cell which in turn allows for neurotransmitter release to the vestibular neuron.
• At the apical surface, the hair cell is in contact with the endolymph, which has really high [K+], so that opening of K+ channels will lead to depolarization.
• Deflection away from the kinocilia closes the ion channels so that no K+ can enter, thus hyperpolarizing the hair cell.
• Hair cells transduce motion into a flow of ions which will allow the auditory afferent fibers in CN VIII to fire action potentials and transmit auditory information.
• The separation of endolymph and perilymph by a membrane is important.
• Damage that separates endolymph and perilymph causes mix, thus hair cell toxicity occurs and hair cells die, which is what occurs in Meniere's disease (more later).

Properties of the basilar membrane

• Mechanical properties of the basilar membrane vary along the length of the cochlea.
• At the base of the cochlea, the basilar membrane is narrow and stiff so it can receive high frequencies.
• At the apex of the cochlea, the basilar membrane is wider and relaxed so it can receive lower frequencies.
• The basilar membrane and the cochlea are tonotopically organized.
• These features of the basilar membrane contribute to the frequency tuning of the cochlea.

The auditory cortex

• The primary auditory cortex plays an essential role in our conscious perception of sound, including recognition of speech.
• The diagram shows the brain in lateral view, including the depths of the lateral sulcus, where part of the auditory cortex is normally hidden.
• Primary auditory cortex (in blue) demonstrates the tonotopic map for sound frequencies
• A neuron located in the area labeled “1000 Hz” will have a maximal response (highest frequency of action potentials) to a 1000 Hz sound.

Auditory: Contralateral pathway

• Afferent Neuron: Auditory nerves with cell bodies in spiral ganglion
• 1st CNS synapse: Auditory nerve synapses on dorsal and ventral cochlear nuclei (in rostral medulla)
• Dorsal cochlear nuclei projects its axons contralaterally via the lateral lemniscus to synapse on the inferior colliculus which projects its axons via the brachium of the inferior colliculus to synapse on the medial geniculate nuclei of the thalamus which projects its axons to the primary auditory cortex (a.k.a. transverse gyri of Heschl).
• Ventral cochlear nuclei project their axons contralaterally to the trapezoid body where the fibers synapse on the Superior Olivary complex which projects its axons via the lateral lemniscus to synapse on the inferior colliculus which projects its axons via the brachium of the inferior colliculus to synapse on the medial geniculate nucleus of the thalamus which projects its axons to the primary auditory cortex.
• Some axons from the ventral cochlear nuclei project up the pathway ipsilaterally.
• There are reciprocal connections between the two inferior colliculi.
• Thalamic Relay: Cell bodies from the inferior colliculus project their axons via the brachium of the inferior colliculus to the medial geniculate nucleus of the thalamus
• Cortical Area: Cell bodies of the medial geniculate nucleus of the thalamus project their axons to the primary auditory cortex (a.k.a. transverse gyri of Heschl and Brodman's area 41).
• The contralateral pathway for audition is dominant.
• Dorsal cochlear nucleus function is to detect differences in intensity of sound across frequencies
• Ventral cochlear nucleus function is to detect timing and frequency of sounds
• Superior Olivary nucleus function is the first place to compare input from both ears, analyze time of arrival and intensity of sound from both ears used for locating a sound source
• Inferior colliculus function is integrates function from all brainstem nuclei

Auditory pathway: Lesions

• Lesions can be a result of aging, loud noise exposure, pathology, ototoxins
• Unilateral lesion of cochlea, auditory nerve, cochlear nuclei results in impaired hearing in the ipsilateral ear
• Unilateral lesion above the Superior Olivary nucleus (for example, in the lateral lemniscus) results in inability to locate and analyze sounds from the contralateral field
• Lesions in the Auditory cortical areas results in speech comprehension problems (language comprehension problems)
• Signs / Symptoms of auditory lesions include hearing loss, tinnitus (ringing in the ears in the absence of sound waves), and speech comprehension impairment.

Conductive Hearing Loss

• Lesions interfere with transmission of sound waves
• Damage to the outer or middle ear occurs
• Damage to the tympanic membrane, otitis externa, otosclerosis, or otitis media can be the cause
• Treatment usually involves the use of hearing aids that amplify the sound.
• Problems include lack of directionality, and all frequencies are amplified, hence not effective in noisy environments

Sensorineural Hearing Loss

• Damage to the cochlea or CN VIII occurs
• Acoustic neuroma (vestibular Schwannoma)
• Damage to hair cells happens (hair cells do not regenerate!)
• Meniere's Disease: tinnitus, fluctuating hearing loss, episodic vertigo
• Treatment: Cochlear implants if the auditory nerve is intact.
• Problems: Invasive, and not good frequency resolution.
• Sensorineural lesions can be caused by: Acoustic neuroma (vestibular Schwannoma) (most common type of CNS hearing loss), damage to CN VIII due to a Schwannoma, damage to hair cells through genetics, high intensity sounds, ototoxic drugs, and Meniere's Disease
• Meniere's Disease Triad of symptoms: tinnitus (ringing in ear), fluctuating hearing loss, and episodic vertigo
• Meniere's Disease Etiology: Idiopathic, Traumatic, Post-Syphilis, Viral
• Meniere's Disease Pathophysiology: Trauma/damage to endolymphatic sac cant absorb endolymph fluid which results in fluid overload
• Vestibular membrane ruptures from buildup of inner fluid and pressure
• Hair cell toxicity occurs from the mixing of endolymph and perilymph fluids, which results in hair cell death

Testing for Hearing Loss

• Tests are focused on determining whether the loss is sensorineural (vibrations through bone) or conductive (vibrations through air).
• Rinne's Test: Tuning fork placed on mastoid process and normal patient can hear sound on the same side
• When patient cant longer hear the sound, the tuning fork is placed 2.5 cm in front of external auditory meatus
• If patient cannot hear the sound, then there is conductive hearing loss in the external or middle ear
• Weber's Test: One must know which is the affected ear. Used to determine conductive vs. sensorineural deficit in the affected ear
• Tuning fork is placed in the middle of the forehead
• If sound (vibration) is perceived equally well in both ears, then hearing is normal If sound is perceived or is louder in the normal (unaffected ear), then patient has a sensorineural deficit
• If sound is perceived or is louder in the affected ear, then patient has a conductive deficit

The Vestibular System

• Processes sensory information underlying motor responses to self-motion, head position, and spatial orientation relative to gravity.
• Senses and analyzes linear and angular acceleration, and orientation with respect to gravity.
• Helps stabilize gaze, head, and posture.

Receptor Organ and Cell

• The inner ear has a bony compartment and within the bony compartment is a membraneous compartment that contains 3 semicircular canals, a utricle, and a saccule.
• Perilymph: The space between the bone and membrane (high [Na+], low [K+]).
• Endolymph: Is within the membrane (high [K+]).
• The vestibular apparatus consists of 2 chambers (utricle and saccule) and 3 semicircular canals.
• Mechanoreceptors (hair cells) reside within the receptor organ, located in the macular area of the utricle and saccule, and in the ampulla, the swelling at the base of each semicircular canal.
• Utricle and Saccule respond to linear accelerations of the head and static head position relative to the gravitational axis
• Semicircular canals are specialized for rotational acceleration of the head
• Hair cells are the sensory receptor cells that transduce movement due to head motion or gravity into a flow of ions which will allow the vestibular neuron (a bipolar neuron) to fire an action potential
• Deflection of the hair cells' stereocilia towards the kinocilium leads to depolarization of the hair cell
• Deflection away from the kinocilium leads to hyperpolarization

Utricle and Saccule

• Utricle and saccule are in an area called the macula
• Hair cells are arranged such that kinocilium and stereocilia are oriented in the same direction. The macula therefore has a map of directional space.
• An otolithic membrane lies atop the hair cells' stereocilia, composed of a gelatinous substance with calcium carbonate crystals (otolithic stones).
• Forces on the head will act on the otolithic membrane, displacing it and thus bending the kinocilia and stereocilia of the hair cells.
• The utricle and saccule detect static balance and linear acceleration.
• Utricle hair cells are arranged in a horizontal plane, and the saccule hair cells are arranged in a vertical plane, detecting head tilt and some linear acceleration.

Semicircular Canals

• At the base of each semicircular canal is where hair cells reside with the ampulla, covered in gelatinous substance (cupula)
• Arranged uniformly, ampullar hair cells creates a directional space map
• Head rotation causes endolymph to flow
• Endolymph flow pushes the cupula to one or the other side
• Semicircular canals detects rotation among three different planes

Vestibular: Contralateral Thalamocortical Pathway

• Afferent Neuron: Vestibular nerve with cell body in vestibular ganglia (Scarpa's) is neuron #1 and will synapse on 1 of the 4 vestibular nuclei in medulla/pons
• 1st CNS Synapse: Vestibular nerve synapses on 4 vestibular nuclei (inferior, medial, lateral, superior)
• Inputs to vestibular nuclei: Vestibular ganglia, contralateral vestibular nuclei, fastigial nucleus of the cerebellum (vestibulocerebellum), proprioceptive information from the neck and postural muscles Outputs of the vestibular nuclei: Spinal cord (lateral and medial vestibulospinal tracts), oculomotor/abducens nuclei, and reticular formation
• Note: Direct projection from the vestibular ganglion to synapse on the fastigial nucleus of the cerebellum (thus bypassing the vestibular nuclei in the brainstem). Other efferents of the Vestibular nuclei:
• Inferior vestibular nuclei project their axons via the medial longitudinal fasciculus (MLF) to cerebellum
• Medial/Superior vestibular nuclei project axons via MLF to cranial nerves 3,4,& 6 for head and eye movements
• Thalamic Relay: All 4 vestibular nuclei send axons bilaterally to the thalamus, but most fibers terminate in the contralateral nuclei, mainly in the VP nuclear group of the thalamus. Cortical Area: Parietal cortex near somatosensory of face (Broadman's area 3a)

Descending Projections

• The medial vestibulospinal tract mediates the vestibulo-colic reflex
• The lateral vestibulospinal tract mediates the vestibulospinal reflex
• Vestibulo-cerebellum is a key structure in both reflex pathways
• With body rotation, neck muscles are activated to turn the head in the opposite direction of the body rotation (medial)
• Helps maintain a consistent position of the head in space during body movements
• Semicircular canal information synapses on the medial vestibular nuclei whose cell bodies project their axons via the medial longitudinal fasciculus (MLF) or as the medial vestibulospinal tracts to synapse on neck muscles.
• Head and body tilt upon linear acceleration activates antigravity muscles (axial and extensor muscles) via the vestibulospinal reflex, which acts to counteract the tilt or acceleration.
• Utricle, saccule, and vestibulocerebellum information synapses upon the inferior and lateral vestibular nuclei and these cell bodies project their axons as the lateral vestibulospinal tracts to activate antigravity muscles in response to head tilt and linear acceleration.

Vestibulo-Ocular Reflex (VOR)

• The VOR is a reflex that provides gaze stabilization during movement of the head and body to maintain a stable visual field, resulting in eye movements in the opposite direction
• Head turns to the right causes the endolymph in right horizontal semicircular canal to move, causing hair cells to depolarize, vice-versa in left horizontal (hyperpolarization)
• VOR depends on the push-pull effect of the semicircular canal on each side of the head
• Vestibular nerve sends action potentials to synapse on the medial and superior vestibular nuclei
• Cell bodies of the medial and superior vestibular nuclei project their axons to the contralateral abducens nucleus to innervate the ipsilateral lateral rectus muscle, and the left oculomotor nucleus to innervate the left medial rectus muscle
• This allows the eyes to move in the opposite direction of the head movement

Rotatory Test

• Rotatory tests test the integrity of the semicircular canals. The vestibular sense depends on the balance of input coming from each canal.
• Nystagmus: Head rotation to the right activates the R canal and inhibits the L canal.
• Results in slow eye movement in the opposite direction of the head movement (VOR)
• Slow eye movement followed by fast eye movement in the same direction as the head movement to bring the eyes back to center of the orbit Such combination leads to nystagmus. This eye movement also helps you fixate at a specific object through a moving train window
• Direction of the fast movement of nystagmus labels direction of the head movement
• Spontaneous or innappropriate nystagmus indicates a lesion in the vestibular system, cererbellum, or can be caused by some antiepileptic medications

Evaluation

• Caloric Testing tests 1 side of the semicircular canals by irrigation using either cool or warm water that moves through the external auditory meatus Remember that the nystagmus is named after the direction of the fast phase of the eye movement
• Mnemonic: COWS (Cold, Opposite, Warm, Same)
• Applying cold water (30 degrees C) directs eyes towards irrigated ear where nystagmus directs towards opposite side
• Applying warm water (44 degrees C) is the opposite side where nystagmus directs towards the same ear
• Note: cold water thickens the endolymph which inhibits canal from moving, mimicking head movement by that opposite side. This is what elicits the said nystagmus.
• Warm water rises the endolymph, and stimulates canal while mimicking head movement and irrigation.

Vestibular

• Spontaneous nystagmus
• Decreased antigravity muscle reflexes
• Motion sickness
• Vertigo: a perception of body motion or when spinning takes place, even when there is no real motion