Sensory transduction - auditory and vestibular systems Flashcards
Name the otolith organs
Utricle and saccule
Name the semicircular canals
Anterior vertical, posterior vertical and horizontal
Give a brief overview of hearing i.e. how is sound processed?
Sound -> vibration of air -> vibrate the eardrum, malleus, incus and stapes -> vibration spreads to cochlea -> vibration of air is converted to movement/vibration of fluids in the cochlea -> vibration in the cochlea is captured by hair cells -> transduction -> perceived in the auditory cortex
Name the 3 fluid-filled cavities of the inner ear
Scala vestibuli, Scala media, Scala tympani
What separates the scala vestibuli from the scala media?
Reissner’s membrane
What separates the scala media from the scala tympani?
The basilar membrane
Where does transduction in the inner ear occur?
Organ of Corti hair cells, basilar membrane and tectorial membrane
Which fluid filled cavity is closed off at the apex?
Scala media
Where do the scala tympani and scala vestibuli connect?
Helicotrema
Where is the perilymph continuous?
In the scala tympani and scala vestibuli
The scala tympani meets the round window (T/F/)
False - meets the oval window!
The scala vestibuli meets the round window (T/F)?
True
The basilar membrane is flexible and vibrates in sync with fluid motion. What factors determine the distance a particular frequency of sound travels?
The width and flexibility of the BM determines the distance a particular frequency travels i.e. a non-spatial parameter -> spatial representation
Where are hair cells located in the inner ear?
Between the basilar membrane and reticular lamina, tips in the tectorial membrane
Where do hair cells synapse?
On bipolar neurons with cell body in the spiral ganglion
Describe hair cell transduction
Tectorial membrane vibrates -> hair cells cilia bend -> depending on how they bend, neurotransmitter is released -> neurotransmitters are captured in nerve fibres
When hair cells bend most, they fire most. Why is this?
Hair cells are tuned to different frequencies
What is different about the hair cell potassium current?
The K current is inwards! Higher K concentration than normal in endolymph (conc gradient is inwards)
There are two types of hair cells, inner and outer hair cells. What is the supply and function of each?
Inner HC - main source of afferent signal in auditory nerve (CN VIII). Multiple outer HC innervated by a single afferent
Outer HC - primarily get efferent inputs. Control stiffness, amplify membrane vibration
What is sometimes referred to as the ‘cochlear amplifier’?
The outer hair cells
How do outer hair cells responds to sound?
With both a receptor potential and a change in length. Change in length -> increased movement of basilar membrane -> increased bending of inner HC increased -> transduction
Name the 3 branches of CN VIII
Dorsal cochlear nucleus, posteroventral cochlear nucleus and ante rove trial cochlear nucleus
What is tonotopy?
Spatial arrangement of where sounds of different frequency are processed in the brain
What does the medial superior olive do?
Computes sound arrival at the 2 ears, ipsilateral ear earlier than contralateral. This generates an interaural time difference
The lateral superior olive neuron gets an excitatory input from the ipsilateral antero-ventral cochlear nucleus. The contralateral AVCN input excites a neuron in the MTNB (medial nucleus of the trapezoid body) which sends an inhibitory signal to the LSO neuron. What happens if the aural signal is closer to the ipsilateral ear?
LSO will have a net depolarisation but will have net inhibition if the sound is closer to the other ear. Thus each LSO neuron gets a depolarising (from ipsilateral ear) and hyperpolarising (from contralateral ear) signal and each aural signal stimulates an LSO neuron from each hemisphere. The balance between the two determines the net excitation that is forwarded to the inferior colliculus and the lateral lemniscus
The vestibular organ answers two basic questions; where am I going? Which way is up? How does it do this?
Head angular acceleration (semicircular canals) - head rotation
Head linear acceleration (saccule and utricle) - translational motion and gravity
How many degrees of freedom do all 3D moving bodies have?
Six degrees of freedom - 3 translational (linear movements in the x, y and z axes) and 3 rotational (body’s rotation relative to the x, y and z axes)
Which part of the internal ear is responsible for translational degrees of movement?
Otolith organs - utricle and saccule
Which part of the internal ear is responsible for rotational degrees of movement?
The semicircular canals
What is the function of the semicircular canals?
Detect head movements via rotatory acceleration
What is the crista?
Sheet of cells where hair cells are clustered
What is the ampulla of the inner ear?
Bulge along canal, contains crista
How are kinocili oriented?
In the same direction so that they’re all excited or inhibited together
Where do cilia project into?
Gelatinous cupula
How do the semicircular canals work?
They are filled with endolymph. Movement causes fluid to circulate and displace cupula
Which semicircular canals share the same plane?
The horizontal canals
Which semicircular canals share contralateral planes?
The posterior canal shares a plane with the contralateral anterior canal
What do the otolith organs do?
Sense linear acceleration and gravity
What do the otolith organs consist of and what movements do they sense?
Consist of the utricle and the saccule. Utricle - movement in the horizontal plane. Saccule - movement in the vertical/sagittal plane
Where do hair cells lie within the otolith organs?
The macula
What is the name of the structure that divides the hair cells into two populations with opposing polarities?
Striola
What are otoconia?
Crystals of calcium carbonate that lie on the otolithic membrane
Other than dividing the hair cells into 2 populations of opposite polarity what else does the striola do?
Arranges otoconia into narrow trenches dividing each otolith. Allows otolith organs to have multidirectional sensitivity
What is the function of the vestibulo-ocular reflex?
Keeps the eyes still in space when the head moves
What is the function of the vestibulo-colic reflex?
Keeps the head still in space - or on a level plane when you walk
What is the function of the vestibular-spinal reflex?
Adjusts posture for rapid changes in position
Name some examples of substances that can cause ototoxicity
Aminoglycoside antibiotics e.g. gentamicin. Chemotherapy agents. Alcohol (transient)
Why can alcohol cause dizziness?
Alcohol is less dense than water. When you drink alcohol, it enters the blood and then the cupula. The cupula becomes less dense and floats in the endolymph more. The cupula bends more than usual away from the ground which bends hair cells, as if you are rotating, even when you are still. This gives you the sensation of spinning
