Hearing

The Reverend Dr. David CM Taylor dcmt@liverpool.ac.uk

This page is © David Taylor and The University of Liverpool, 1999

 Carpenter, RHS, Neurophysiology 3rd Edition 1996, Arnold chapter 6

 Bray, Cragg, MacKnight, Mills & Taylor, Lecture Notes on Human Physiology 3rd Edition, 1994 Blackwell Scientific Publications chapter 7 (part 4)

 Kandel, Schwartz & Jessell, Principles of Neural Science 3rd Edition, 1991, Elsevier chap 32

 

If you are studying on 2LS50, our emphasis in the lectures has been on the peripheral mechanisms, rather than the central processing and pathways, which is covered rather well in the text books. You will be assessed on your understanding of the peripheral mechanisms. Try to pull in the material you gathered whilst performing your practical work.

 

Sound

 Is a travelling pressure wave

 has two variables

 frequency (pitch)

 amplitude (loudness)

 variations in pressure are small !

 less than 0.6% of atmospheric pressure for most intense sound imaginable

Decibels

We use the decibel scale, which is important because equal increments in the scale represent equal increments in sensation

Not surprisingly we need a reference value, it is the pressure which corresponds to the quietest sound that Alexander Graham Bell could hear (2x10-5Nm-2). Clearly some people can hear quieter sounds than that, which are given -ve decibel numbers. The expression below gives the intensity of a sound (SPL - sound pressure level) in decibels.

 

 

Impedance

 molecules in water have a greater resistance to movement than molecules in air.

 greater presure is needed to give the same velocity or displacement

 impedance of water is 1350 x that of air (which is 400 Nsm-3)! This is why you have to shout to be heard under water.

 

So there are two problems

We need

 an impedance transformer

 to amplify the sound

 a sound transducer

 to convert the vibrations into electrical signals

 

Sound is altered before it even reaches the eardrum.

 

Outer Ear

 Certain frequencies pass through ear better than others

 resonance

 The ear is also direction sensitive

 best location 45O from front

 

Middle Ear

Has the function of impedance matching, in order of importance...

 greater area of tympanic membrane than oval window MOST IMPORTANT !

 lever action of the ossicles

 buckling of the tympanic membrane

 

Inner Ear

Is where the sound is converted into an electrical signal. The sound causes the oval window of the cochlea to vibrate, and this in turn causes the basilar membrane to vibrate. It is wider at the apex of the cochlea and narrower at the base. If you remember making a ruler "twang" at school, you will know that a large displacement gives a low frequency sound.

The movement of the basilar membrane causes the movement of the hair cells within the Organ of Corti. Since they possess stereocillia which are embedded in the tectoral membrane (which is rigid), the stereocillia bend,

 

The cells of the stria vascularis generate a potential difference. Older forms of loop diuretic drugs blocked the ion pumps in the stria vascularis as well as in the loop of Henle. This resulted in iatrogenic (look it up....) deafness, due to the inability to maintain an ionic gradient between the scala media and the scala tympani (sm, st respectively, sv stands for stria vestibularis).

The movement of the hairs on the hair cells changes the resistance to current flow, and the current flow causes the release of transmitter, which excites the primary afferent neurone terminals.

 

 

Because different parts of the basilar membrane are caused to vibrate by different frequencies, the auditory nerves exhibit tuning curves.

The fine tuning of the mechanism owes a considerable amount to the outer hair cells, which are disturbed by some types of antibiotic.

 

Surround Inhibition

 Is important and similar in principle to surround inhibition within the cutaneous sensory system.

 It allows us to distinguish between two tones of similar frequency.

 

Sensorineural deafness

 auditory nerve

 tumor

 Cochlear

 noise trauma

 drugs

 infections

 congenital

 presbycusis

 

Conductive deafness

before the cochlea

 immobilization of ossicles

 frequency dependent attenuation

 can compensate with hearing aids

 sometimes responds to antibiotics

 sometimes responds to surgery

 

Tests for deafness

WHICH YOU NEED TO KNOW AND UNDERSTAND !

Weber's test - tuning fork middle of skull

 inner ear damage - heard on healthy side

 middle ear damage - heard on affected side

 

Rinne's test - tuning fork on mastoid process, when no longer heard then use sound conduction

 if hear it, Rinne +ve, then O.K.

 if not, Rinne -ve, then conductive deafness

back to top | back to index |