Physiology of hearing and balance Flashcards
Provide a quick summary of the main functions of the outer, middle and inner ear
- external ear - sound receiver
- middle ear - amplifier
- inner ear - sound transducer / analyser
What frequency range of sound can humans hear ?
20-20,000 Hz
What is the main functions of the eustachain tube ?
- To ventilate the middle ear space, ensuring that its pressure remains at near normal
- Secondary function to drain any secretions due to infection or debris from the middle ear space
What is the function of the oval and round windows in the cholea ?
Permits transmission of pressure wave in the enclosed canal to the cholea which results in it being transmitted into the cholea by allowing the pressure waves to come into contact and cause vibrations in the basilar membrane
Give an overview of sound stimulation
- Sound is collected by the external ear
- ‘Amplified’ by the middle ear & (tympanic membrane & ossicles) and transmitted to the OW
- Wave passes through the cochlea maximally stimulating a particular area of the basilar mambrane. The organ of corti here depolarises and fires – stimulating the (chochlear division) of the VIIIth nerve and then the central pathways.
- Culminating in activity in superior temporal gyrus (in the brain)
What change in mode of sound transmission occurs when the sound waves pass through the oval window and come into contact with the basilar membrane ?
- Change from mechanical (vibrations) into electrical impulse
- The hair cells on the basiliar membrane convert the sound waves into electrical impulses
What are the 5 vestibular organs involved in balance sensation in the inner ear ?
Saccule, utricle and ampullae of the lateral posterior and superior semicircular canals
What are the main systems involved in balance ?
- Ears
- Eyes
- Propioception
- Cardiac
What is otoconia ?
Small crystals of calcium carbonate in the saccule and utricle of the ear that under the influence of acceleration in a straight line cause stimulation of the hair cells by their movement relative to the gelatinous supporting substrate containing the embedded cilia of the hair cells
How do the otolith organs (saccule and utricle) perceive movement?
- Have stereocilia projecting upwards into a gelatinous matrix with otoconia
- These lend weight & due to gravity brain perceives movement when tilting head or travelling in elevator - linear motion
(think the crystals stimulate the sterocilia when moving as they then move)
Which organs in the ear sense linear motion ?
Utricle and saccule (otolith organs)
How does the arrangement of the stereocilia allow for all linear movements to be perceived in all directions ?
The stereocilia are arranged in all directions
Which organs in the inner ear detect rotational movements?
Semicircular canal system
How does the semicircular canals detect rotational movements ?
- Each canal is filled with endolymph and contains motion sensors within the fluids.
- At the base of each canal, the bony region of the canal is enlarged which opens into the utricle and has a dilated sac at one end called the osseous ampullae.
- Within the ampulla is a mound of hair cells and supporting cells called crista ampullaris. These hair cells have many cytoplasmic projections on the apical surface called stereocilia which are embedded in a gelatinous structure called the cupula.
- As the head rotates the duct moves but the endolymph lags behind. This deflects the cupula and bends the stereocilia within. The bending of these stereocilia alters an electric signal that is transmitted to the brain.
- Hence detecting the movement
What is the vestibulo-ocular reflex ?
A rotation of the head is detected, which triggers an inhibitory signal to the extraocular muscles on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes in keeping with the movement of the head
