**_🧠Neurology🧠 - Auditory and Vestibular Systems Flashcards
What is the function of the vestibular organ?
Capture low frequency motion (movement)
What is the function of the hearing organ?
Capture high frequency motion (sound)
What is amplitude and frequency?
Amplitude - loudness, sound pressure, subjective attribute correlated with physical strength
Frequency - pitch, oscillations per second, perceived tone
What is the range of human hearing?
20-20000Hz
0dB to 120dB
What are the 3 parts of the ear?
Outer ear
Middle ear
Inner ear
What is the function of the outer ear?
Capture sound, and focus it onto the tympanic membrane
Modest amplification (10dB) of upper range speech frequencies via resonance
Protect the ear from external threats
What is the function of the middle ear?
Mechanical amplification (20-30dB)
What is the inner ear, and give its function
The cochlea
Transduces vibration into nervous impulses
Does so in a way that captures both frequency and intensity of the sound
Outline the structure of the cochlea
3 compartments:
Scala vestibuli
Scala media
Scala tympani
Vibrations travel into the scala vestibuli via the oval window, through the scala media, where it is translated to nervous impulses, to the scala tympani and out the round window
What are the compartments of the cochlea?
Scala vestibuli, scala media, scala tympani
Scala vestibuli and tympani both bone structure containing perilymph (high in Na)
Scala media is a membranous structure containing endolymph (high in K), where the Organ of Corti (hearing organ) is located
Where does the organ of Corti lie?
Lies in the basilar membrane
How does the basilar membrane allow for different frequencies to be interpretated?
Arranged tonotopically, meaning different parts of the membrane will oscillate according to the frequency of the incoming vibrations
What does the organ of Corti consist of?
Inner hair cells
Outer hair cells
Tectorial membrane
(Hair cells attach to the auditory nerve)
What is the function of the inner hair cells?
Carry afferent information on the auditory nerve
Transduction of sound into nerve impulses
What is the function of the outer hair cells?
Carry efferent information on the auditory nerve
Modulation of the sensitivity of the response
Which hair cells are in constant contact with the tectorial membrane?
Outer hair cells
How do the hair cells achieve transduction?
Stereocilia of the hair cells are deflected (moved) when the tectorial membrane comes into contact with them (during a sound vibration)
This deflection opens the K+ channels, causing the cell to depolarise, releasing neurotransmitters to the afferent nerve, which itself then depolarises
Higher amplitudes cause greater deflection, and so more K+ channel opening
How does the direction of deflection effect the signal produced?
Deflection in one direction causes depolarisation
The other way causes hyperpolarisation
Deflection towards the kinocilium will cause depolarisation
What is the longest stereocilia called?
Kinocilium
What is the first part of the auditory pathway?
Spiral ganglions via the vestibulo-cochlear nerve (VIII) travel to the ipsilateral cochlear nuclei in the brainstem (pons)
What happens after auditory information reaches the cochlear nucleus?
Auditory information crosses at the superior olive level
All connections after this point are bilateral
Where does auditory information go after it leaves the superior olive?
To the inferior colliculus, then the medial geniculate body
Terminates at the auditory cortex in the parietal lobe
What are the different types of hearing loss?
Anatomical:
Conductive
Sensorineural
Central
Timing:
Sudden hearing loss
Progressive hearing loss
What is conductive hearing loss?
Problem in the outer or middle ear - areas of sound conduction
What is sensorineural hearing loss?
The sensory organ (cochlea) or nerve (auditory nerve) is affected
90% of all hearing loss
What is central hearing loss?
Problem originates in the brain or brainstem
Very rare
What is sudden vs progressive hearing loss?
Sudden hearing loss occurs over minutes to days
Progressive hearing loss occurs over months to many years
How can hearing thresholds be accurately measured?
Audiogram
A normal hearing threshold is 0-20dB
How does conductive hearing loss compare to sensorineural loss in terms of patients hearing?
Conductive hearing loss is along all frequencies while sensorineural hearing loss affects some frequencies more than others
What structures make up the vestibular organ?
Anterior, lateral and posterior canals connected to the utricle, have an ampulla on one side
Utricle
Saccule
What are the otolith organs?
Utricle and saccule are the otolith organs
Their cells are located on the maculae, placed horizontally in the utricle and vertically in the saccule
What is the maculae?
The maculae contain the hair cells, a gelatinous matrix and the otoliths on top
Otoliths are carbonate crystals that help the deflection of the hairs
What is the function of the utricule vs saccule?
Utricule detects horizontal movement
Saccule detects vertical movement
What are the semi-circular canal planes?
The orientation of the canals in the head defines three planes.
Anterior and posterior canals form a 90° angle. Lateral canals are horizontal to the other canals.
Therefore they work in pairs to evaluate head movement
How do vestibular hair cells compare to cochlear hair cells?
Same concept of hair deflection resulting in depolarisation/hyperpolarisation
How do hair cells generate a signal?
Hair cells have a resting potential which has a basal discharge to the nerve
Movements towards kinocilium causes depolarization and an increase in nerve discharge
Hairs moving away from the kinocilium generates hyperpolarization and a reduction in nerve discharge
Where do nerve impulses from the vestibular organs go?
Primary afferents end in vestibular nuclei in the brainstem (pons)
Impulses travel to nuclei for eye movements, and down the vestibulospinal tract (for posture related reflexes)
Which major blood vessel supplies the vestibular system?
What is the vestibulo-ocular reflex?
Eye remains fixed on a point even with rapid head movements
How does the vestibulo-ocular reflex come about?
Keeps images fixed in the retina
Connection between vestibular nuclei and the abducens and oculomotor nuclei
Eye movement in opposite direction to head movement, but same velocity and amplitude
