Hearing and Balance Flashcards
(34 cards)
What 3 systems need to integrate signals to maintain good posture and balance?
Vestibular, proprioception and vision (and important connections with cerebellum and motor systems)
What are the 2 organs of equilibrium?
Semi circular canals and otiliths
Explain the bony labyrinth
3 semicircular canals, from top - anterior, lateral and posterior. Ampulla is the dilated region at the end of the canals. Fenestra vestbuli (oval window) Vestibule Fenestra cocheae (round window) Cochlea
Explain the membranous labyrinth
Utricle, saccule, cochlear duct.
Endolymph - low Na, high K (inside membranous labyrinth)
Perilymph - high Na low K (between bony labyrinth and membranous labyrinth).
Explain hair cells
Sensory receptors of the vestibular system
They have no axon but the release their neurotransmitter directly from their cell body onto the axon of an adjacent cell.
Apical surface - kinocilium is the tallest sterocillia (all rest = stereocilia and are 40-70 actin rich).
Stereocilia are connected by tip links - only join up and down and all stereocilia point in the direction of the kinocilium.
Tip links are linked to mechanotransducer channels.
What happens when stereocillia are pulled in the direction of the kinocilium?
Causes the channels to open, Ca++ and K+ flow in and depolarise the cell - cause VOCC to open and stimulate NT release to an afferent fibre.
What happens if they are pulled away from the kinocillium?
The channels that would have been slightly open at rest close and the cell hyperpolarises. Decrease frequency of impulse. No NT release.
What is the ampullary crest?
Inside the ampulla (dilated region at end of canals) and is where the hair cells sit. It is enlarged epithelium.
What is the cupula?
It is a gelatinous mass that the hair cells are embedded into.
Explain what happens in the semi-circular…
Semi circular canals respond to angular acceleration. All the hair cells point in the direction of the saccule. Therefore each amuplla responds to depolarisation in one direction and hyperpolarisation in the other. The semicircular canals work in pairs. Fluid moves in the opposite direction from movement.
So, looking from top (utricle is at front), if turning left - fluid moves to the right. Hyperpolarises right ampulla, decrease firing and depolarise left, increase firing.
What are otiliths and where are they found?
They are crystals of calcium carbonate and are found in the utricle. They are heavy, rest on macula - otiliths sags in direction of tilt and bends hair cells. MOVEMENT is nto required - static head position can be detected.
Explain the oreintation of the stereocilia in the utricle and the consequences of this organisation.
They are all point towards a curving landmark - the striola. Therefore tilit or linear acceleration in many directions can be responded to. Complex signals will be sent to the brain to accurately measure head position
How is the macula orientated in the saccule? Why?
Vertically when the person is upright. Therefore responds to vertically directed linear force - and directs the position of the head in space.
Organisation of the cilia in the saccule?
All point away from the striola.
Where does the vestibulocochlear nerve enter the brainstem?
cerebellopontine angle
Explain the projections from the 4 vestibular nuclei…
Lateral vestibular nuclei - to cerebellum and limb motor neurones to maintain posture and balance.
Medial vestibular nuclei - to the extraocular muscles of the eye and also the the neck muscles to control the vestibuloocular reflex - head moves - move eyes in opposite direction but same angle.
What is Meniere’s disease?
Too much endolymph and distension of the membranous labyrinth
Causes - severe vertigo, nausea, nystagmus, hearing loss and tinnitus
What is benign positional vertigo?
When otoconia dislodges from utricle to the semi circular ducts. When head moves, grsavity dependant movement of otoconia causes abnormal fluid displacement in the affected semicircular canals - resultant vertigo.
What is acoustic neuroma?
benign tumor of the myelin forming cells of VIII located in the cerebellopontine angle.
What is the function of the ear?
To transduce sound waves into neural signals that can be processed by the brain.
What are sound waves?
They are alternate compression and rarefraction (to make less dense).
Give 4 features of a sound wave and do they correlate to anything?
Waveform - amplitude plotted against time…sine waves.
Phase
Amplitude (dB) = LOUDNESS
Frequency (Hz) = PITCH (cycles/second)
What frequency range can human ears detect?
20Hz to 20kHz.
Explain the external ear
Pinna, concha and auditory meatus gather sound and focus it on the ear drum (tympanic membrane) - increase the pressure 30-100fold. The pinna and concha provide cues about the eleveation of the sound source.
