Physiology of Hearing and Balance Flashcards
What is sound?
A pressure wave caused by oscillating molecules (set in motion by vibrations)
What does frequency dictate?
Pitch
What does amplitude dictate?
Intensity
What range can human detect sounds in?
20-20000 Hz
How does the external ear act as a receiver for sound?
Shaped to receive airborne sounds (huge variation in shape)
What does the ossicular chain of the middle ear acts?
Piston/lever arm = varying efficiency depending on frequency of sound transmitted
What is the tympanic to OW ratio in the middle ear?
18:1 (impedance matching air to liquid) = leads to pressure gain of 22x
What is the ratio of the malleus handle to the incus long process in the middle ear?
1.3:1 = gain of 20-25dB (theoretical gain in 34dB)
What is the Eustachian tube?
Ventilation pathway for middle ear mucosa = has bony and cartilaginous portions
What is the resting state of the cartilaginous tube of the Eustachian tube?
Closed = opened by tensor veli palatini and levator palatine muscles
What does dysfunction of the cartilaginous tube in the Eustachian tube cause?
Middle ear negative pressure
What are the oval and round windows of the ear?
Two openings of the cochlea to the middle ear = have in-phase movement
What do the oval and the round windows of the ear allow?
Transmission of pressure wave in enclosed canal and vibration of the basilar membrane
Where is the pressure wave transmitted to from the oval and round windows of the ear?
Perilymph
How is the inner ear structured?
Curved spiral lamina = two and a half turns around central modiolus (need to consider ultrastructure of cochlea)
Where is the scala media (endolymph) of the cochlea located?
Suspended between scala tympani and scala vestibuli (perilymph)
What is within the cochlea of the ear?
Coiled tube of liquid = actively maintained in specific concentration to power cells
How does sound stimulate the 8th cranial nerve and then the central pathways?
Wave passes through cochlea, maximally stimulating a particular area of the basilar membrane, then the organ of Corti here depolarises and fires to stimulate nerve
Where does sound activity culminate?
In the superior temporal gyrus
What is the input and output displayed by the modified Romburg’s test for balance?
Input = vestibular Output = vestibulospinal
What is the structure of the inner ear like?
Complex structure termed the labyrinth = bony and membranous structures
What are the five key vestibular end organs present in the inner ear?
SVN = lateral/anterior superior semi-circular canal, utricle IVN = posterior semi-circular canal, saccule
What are the hair cells of the inner ear?
Kinocilium and stereocilia = resting potential is 90 spikes/s, deflection towards increases
What are the otolith organs of the ear?
Utricle and saccule
What is present in the maculae of the otolith organs?
Stereocilia = project upwards into a gelatinous matrix with otoconia
How are the stereocilia of the otolith organs orientated?
In all directions = all movements perceived by changes in resting firing rate
How does the brain interact with the otolith organs?
Perceives position and movement when tilting head/linear motion (lend weight and due to gravity)
How are the semi-circular canals of the inner ear orientated?
At 90 degrees to each other so paired, equal and opposite
What sits within the ampulla of the semi-circular canals of the inner ear?
Cupula
What does bending of the semi-circular canals of the inner ear cause?
Causes stereocilia to deflect
What causes the semi-circular canals of the inner ear to deflect?
Movement of perilymph
What does VOR stand for?
Vestibulo-ocular reflex
What feeds into the cupula of the inner ear?
The christa ampullaris
What occurs in the vestibulo-ocular reflex?
Movement of head to left causes inhibition
Movement of head to right causes excitation
When may nystagmus occur during the vestibulo-ocular reflex?
When turning towards a particular side (ie on left side when turning left), on left side when there is neuritis
What is the input and output to the vestibulo-ocular reflex?
Input = vestibular Output = vestiblo=ocular
What is oscillopsia?
Objects appear to move or tremble when they are still = occurs when there is no vestibular input
What is nystagmus?
Rhythmic abnormal eye movements
What is the vestibulo-oculo reflex?
Activation of vestibular system of inner ear
What does the modified Romburg’s test assess?
Tests for presence of two out of three = neurological function for balance, assesses proprioception, vison and vestibular function
What is the central pathway for balance?
Complex series of communications in vestibul nuclei
Where do vestibul nuclei have output to?
Vestibulospinal, medial longitudinal fasciculus and ocular muscles, medial lemniscus and thalamus
Where does the vestibulospinal tract have outputs to?
Motor output to neck, back and leg muscles
Where do the medial longitudinal fasciculus and ocular muscles have output to?
Motor output to eyes
Where do the medial lemniscus and thalamus send outputs to?
The cerebrum
What is the function of the hair cells in the ear in terms of sound?
Transduction = conversion of mechanical bending force into electrical impulse
How are stereocilia of the ear arranged?
In height order with tip links connecting them together
When does hyperpolarisation and depolarisation of the ear hair cells occur?
Depolarisation occurs when deflected towards longest
Hyperpolarisation occurs when deflected away
Where is the primary auditory complex of the brain?
Left posterior temporal gyrus = Brodman areas 41/42
When does firing of the cilia in the otolith organs stops?
Stops with linear motion so you know movement has stopped (but continues with head tilt)
How are the otolith organs orientated?
360 degree orientation so all planes of motion detected
What type of hair cells are present in otolith organs?
Type 1 and 2
What increase and decreases the resting potential of the otolith organs?
Direction of bend towards or away from the kinocilium
