Flashcards in Hearing and Balance Deck (92):
What is the frequency of the human hearing range?
Where are air pressure waves converted into mechanical energy?
In the middle ear
Where is the middle ear located and how does it communicate with the nasopharynx?
It is located medial to the tympanic membrane and communicates with the nasopharynx via the eustachian tube
What are the main two nerves of the auditory system that are branches of the facial nerve?
What is the tensor tympani innervated by?
CN V -> V3 -> Medial Pterygoid Nerve
What cranial nerves can be seen normally?
CN II - through the eye into retina
CN VII - chorda tympani in the ear
What is the inner ear comprised of?
Bony and membranous labyrinths
What is the bony labyrinth filled with?
What is the membranous labyrinth filled with?
What are the three methods for the conduction of sound?
Air - poo conduction
Osseus - through bones in the skull
Ossicular - most efficient with the direct coupling of the inner and outer ear systems
What is the hearing structure and what is it contained within?
Cochlea and it is contained within the modiolus
What is the function of stria vascularis?
What is connected to the oval window to conduct sound waves through it?
The cochlear duct lies between two bony channels. What separates it above and below?
Above it is separated by Reisner's Membrane and by the Basilar Membrane below
How are different frequencies of sound responded to? Different intensities?
Frequencies are related to the location on the basilar membrane with high pitches where it is the stiffest. Loudness is coded based on the number of action potentials fired.
What is the function of the Organ of Corti?
It converts pressure waves into action potentials.
How many rows of outer hair cells are there? Inner hair cells?
Describe the outer hair cells.
The outer hair cells are long and flexible, and are used to modulate the tectorial membrane. These are the ONLY receptors which can be directly modified by the CNS, and can change their length and stiffness.
Describe the inner hair cells.
The inner hair cells are short and stiff, and are used primarily to detect sound. They are NOT directly modified by the CNS. Due to the unique innervation ratio of the inner hair cells, each nerve fiber has a characteristic frequency of excitation based on it’s position on the basilar membrane.
Where does most of hearing come from?
99% of hearing comes from the inner row of hair cells
What is the role of the outer row hair cells?
Its main role is to protect the inner row cells
Where does the first action potential occur?
In the bipolar cells, not the hair cells
What does injury to the cochlear nerve of nuclei cause?
Deafness in that area
What does injury above the cochlear nuclei cause?
Less loss of hearing as it becomes bilateral above the nucleus.
The extensive bilaterality of the auditory system above the cochlear nuclei contributes to the prevention of total deafness in general brain dysfunctions. Unless there is a lesion in CN VIII or the cochlear nuclei, an individual will most likely NOT be deaf due to CNS problems.
What are the methods for the localization of sounds?
Interaural Difference - difference in sound heard between the two ears
Changes in Pitch and Intensity - as sound moves towards/away from us
What is the charge of endolymph?
What range of sound does the ear hear best in?
What does the bending hair cells towards the kinocilium result in?
The bending of the hair cells TOWARDS the kinocilium mechanically opens K+ channels in the hairs, and allows the K+ ions to depolarize the hair cell.
What does the bending of hair cells away from the kinocilium result in?
Movement of the basilar membrane in the opposite direction causes the stereocilia to bend AWAY from the kinocilium, resulting in the closure of the K+ channels and hyperpolariztion of the hair cells and a decrease in the release of neurotransmitter.
What do lesions in Wernicke's area cause?
Lesions in Wernicke's area will result in failure to comprehend the auditory signals, and if the lesion extends posteriorly into the PTO (association areas), then visual input for language may also be affected. The patient will be unable to understand either visual or spoken language.
What do lesions of Broca's area cause?
Lesions in Broca's area will not affect comprehension or formation of language, but will cause a major disruption of speech output and the verbal production of language, due to the loss of input to the motor cortex.
What do lesions of the arcuate fasciculus cause?
Lesions of the arcuate fasciculus will also disrupt verbal output.
What is found in the outer ear?
What is found in the middle ear?
Ossicles -> Oval Window
What is found in the inner ear?
Perilymph and Endolymph
Describe the route of the auditory system.
Vibrations pass from the tympanic membrane through the middle ear ossicles, which in turn cause the oval window and the fluid in the inner ear to vibrate
Describe the connections of the ossicles.
The malleus is attached to the tympanic membrane on one end and fuses at the other end with the incus to form an L‐shaped lever arm. Incus articulates on the stapes, which is attached to the oval window and pivots on the anterior rim of this membrane.
What is the perilymph secreted by?
Secreted by the periosteal lining of scalae tympani and vestibuli
How are the hair cell cilia arranged?
The hair cell cilia, called stereocilia, are arranged in a V‐shape, which is pointed toward the spiral ligament.
What are the changes in K+ permeability when the hair cells move toward the kinocilium?
When stereocilia are moved toward the kinocilia, K+ permeability increases in the cilia and the hair cell depolarizes.
What are the changes in K+ permeability when the hair cells move away from the kinocilium?
When stereocilia are moved away from the kinocilia, K+ permeability decreases in the cilia and the hair cell hyperpolarizes.
What is the helicotrema?
Apex of the cochlea
Which hair cells respond better to higher frequencies?
Hair cells near the oval window preferentially respond to high frequencies
Which hair cells respond better to lower frequencies?
Lower frequencies cause optimal movements nearer the helicotrema, where the basilar membrane is wider and more distensible
Describe conduction deafness.
Normal responses to bone conduction, but impaired air (ossicular) conduction responses.
Generally, responses to broad range of frequencies are lost but lower‐ frequency loss is often more pronounced than that of higher frequencies.
Describe sensorineural deafness.
Characterized by loss of both air and bone conduction, affecting higher frequencies most often, and making it difficult for a person to understand constant sounds and usually associated with hair cell damage
Describe neural deafness.
Causes unilateral hearing loss commonly due to lesion of the auditory nerve
What system are involved in balance?
Proper balance requires functional vestibular, proprioceptive and visual systems to be present, but only 2 of the 3 systems are essential for balance.
What does the vestibular system measure and how is it interpreted?
The system measures the position of the head in space, and does not measure the body. It takes an acceleration measurement that is interpreted as velocity.
What are the 3 divisions of the bony and membranous labyrinths in the ear?
Semicircular Canal - 3 parts (anterior/posterior/horizontal)
Utricle - connects 3 canals (gravity and acceleration monitor)
Saccule - continuous with the cochlea (otolithic membrane)
Describe the effects of the rotation of the head on the cupola.
Movement of the head results in an inertial displacement of the fluid which pushes the cupola in the opposite direction due to the weight of the fluid (endolymph) in the canal.
How are the 3 semicircular canals oriented?
The horizontal canals are oriented towards the utricle.
The anterior and posterior canals are oriented away from the utricle.
How are the semicircular canals paired on the right and on the left?
The anterior canal on the right is paired with the posterior canal on the left.
The posterior canal on the right is paired with the anterior canal on the left.
The horizontal canals are paired.
What do otolithic granules respond to?
Otolithic granules are calcium carbonate crystals found in the saccule and the utricle, and have weight, so they respond to gravity.
What is the primary gravity sensor?
Besides gravity, what else does the utricle sense?
Horizontal linear acceleration
What does the saccule sense?
It detects general orientation and vertical linear acceleration
What results in the release of neurotransmitters and increased action potentials in the receptor?
Bending of the stereocilia TOWARDS the kinocilium causes depolarization of the receptor, an increased release of the NT, and an increase in the number of AP’s in the vestibular nerve.
What results in decreased action potentials in the receptor?
Bending of the stereocilia AWAY from the kinocilium results in hyperpolarization and a decrease in AP’s from the vestibular nerve.
Are there action potentials in the hair cells?
Describe the route of sensory input to the CNS.
Hair cell -> bipolar cell (AP’s) -> vestibular ganglion -> CN VIII -> Vestibular nuclei
What are the two divisions of the vestibular ganglion?
Superior: utricle, anterior portion of saccule, horizontal & anterior SSC
Inferior: posterior portion of saccule and posterior SSC
What are the functional and anatomical components of the lateral vestibular nuclei?
Utricle and Saccule - concerned with posture (extensor muscle groups) and vestibulo-ocular reflexes (via MLF).
What are the functional and anatomical components of the medial and superior nuclei?
SSC and some fibers from Utricle (very few fibers from Saccule) - concerned with vestibulo-ocular reflexes and coordination of eye and neck muscles.
What are the functional and anatomical components of the vestibular nuclei?
All Areas - projects to cerebellum, and coordinates balance with voluntary movements.
What do the ascending and descending fibers of the MLF control?
Ascending: Control eye movements
Descending: Controls head and neck movements
What is the flocculonodular node?
It is a region of the cerebellum that has important connections to the vestibular nuclei and uses information about head movement to influence eye movement.
What is the lateral vestibular tract?
• From the Lateral Vestibular Nucleus
• Uncrossed projection
• Spans the entire spinal cord in the ventral funiculus
• Proximal limb muscles
• Maintains balance by acting on the limbs
What is the medial vestibular tract?
• From the Medial Vestibular Nucleus
• Bilateral projection
• Cervical spinal cord only via the medial ventral funiculus
• Neck muscles
• Maintains head erect
What is the function of the saccadic system?
Rapid movements to "catch up" to a target which has moved, using highly stereotyped responses.
What is smooth pursuit?
Voluntary movement requiring a moving stimulus/target.
What are vergence eye movements?
Eyes converge on near objects, diverge on far objects and also perceives distance.
What is the function of the vestibule-ocular reflex?
Stabilizes the image on the retina during a rotation of the head and faster than visual tracking.
Describe the vestibule-ocular reflex.
-> Head is rotating to the right
• The right horizontal canal is activated
• Right vestibular nucleus is “activated’
• The left 6th nucleus (via PPRF) is activated and the left lateral rectus muscle contracts
• The left PPRF “activates neurons in the right 3rd nucleus and the right medial rectus contracts
<- Both eyes begin to move to the left
What are the reasons for dizziness?
Vestibular input without vision
• While spinning in a chair with your eyes closed (the constant motion eventually results in the cupula membrane returning to its baseline) and you suddenly open your eyes
Sense of motion via the visual system, but without vestibular “confirmation” (“a disconnect”)
• Looking out a car window when an adjacent car moves away (false sense of motion)
Sense of motion via the vestibular system, but without visual “confirmation” (“a disconnect”)
• In the cabin of a boat during a storm (motion sickness)
What is the optokinetic reflex?
The optokinetic reflex is a combination of a saccade and smooth pursuit eye movements. It is seen when an individual follows a moving object with their eyes, which then moves out of the field of vision at which point their eye moves back to the position it was in when it first saw the object.
What is a unique feature of the optokinetic reflex?
It does not attenuate.
What is the caloric test?
In medicine, the caloric reflex test is a test of the vestibulo-ocular reflex that involves irrigating cold or warm water or air into the external auditory canal.
Cold -> Eyes turn opposite
Warm -> Eyes turn same
What are the vestibular hair cells and where are they located?
Especially sensitive mechanoreceptors located within the cristae ampullares of the semicircular canals and the maculae of the utricle and the saccule.
Bundles of hairs or stereocilia (specialized microvilli) extend from the surface
of the hair cell.
What are the crista ampullaris?
A ridge of epithelium, the ampullary crest, within the ampulla of each semicircular canal containing sensory hair cells and supporting cells. Stereocilia protrude apically into a gelatinous cupula that covers the entire crista ampullaris.
What is the macula utriculus?
Sensory epithelium within the utricle containing sensory hair cells oriented in the horizontal plane. This structure is similar to the cupula of the crista ampullaris except that it contains small crystals of calcium carbonate known as otoliths, which add to the mass.
What is the macula sacculus?
Sensory epithelium within the saccule. It is identical in structure to the macula utriculus but is vertically oriented and thus sensitive to vertical movement.
What are the secondary vestibular connections?
Spinal Cord Projections
Brain Stem Connections
What is the paramedian pontine reticular formation?
Located near the abducens nucleus and therefore has been called the parabducens nucleus.
Stimulation results in horizontal eye movements.
What is the ascending medial longitudinal fasciclus?
Contains crossed and uncrossed projections originating from several nuclei, including the abducens, trochlear, and oculomotor nuclei; the PPRF; and the vestibular nuclei.
Interconnects nuclei of the extraocular muscles to coordinate conjugate eye movements.
What is the function of vestibule-ocular reflexes?
Movement of the eyes in compensation for head movements.
What does a lesion of the unilateral abducens nerve cause?
At rest, the affected eye may deviate medially because of unopposed action of the medial rectus.
What does a lesion of the unilateral abducens nucleus cause?
Lateral gaze paralysis‐inability to move both eyes to side of lesion because lesion disrupts connections between the abducens and
What does a lesion of the MLF rostral to the abducens nucleus cause?
Unilateral lesion (anterior internuclear ophthalmoplegia)‐patient is unable to move affected eye medially; horizontal nystagmus present in opposite abducting eye.
Bilateral lesions‐cause dissociated horizontal eye movements with attempted gaze to either side.
What does a lesion of the pontine paramedian reticular formation cause?
Disruption of saccadic eye movements in horizontal plane; partial lesions cause abnormally slow saccades.
Smooth pursuit and vergence remains intact, indicating that relevant pathways bypass PPRF
What does a lesion of the frontal eye fields cause?
Patient is unable to move eyes voluntarily away from side of lesion.
Reflex eye movements away from lesion may still occur.