Page 104 - Edited - Webster HEAD AND NECK - part 2-Merge PDF

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SOME FURTHER NOTES ON THE EXTERNAL AND MIDDLE EARS

EXTERNAL EAR

Sensory innervation.

The skin of the tragus, anterior part of the concha and anterior wall of the external meatus (HN 292) is
supplied by V3. Otherwise, the skin of the pinna (auricle) is innervated by spinal nerve C2 (HN 49).
(Remember that Cl has no cutaneous sensory branches.) In addition, general sensory afferents carried in the
facial nerve innervate the skin of the anterior wall of the external meatus, and the external surface of the ear
drum (HN 251). This accounts for the appearance of herpetic vesicles in the external ear during herpes zoster
(shingles) of the geniculate ganglion - see HN 252. It is said that the cutaneous sensory innervation of the
posterior wall of the external auditory meatus is provided by the vagus (cranial nerve X) and that this accounts
for the fainting or vomiting occasionally met with when syringing wax from ears, or removing foreign bodies
(especially in children), since this nerve's parasympathetic components are crucially concerned with the
control of respiration and blood pressure

MIDDLE EAR

(In the following, think of electrical impedance as a measure of resistance to flow of alternating current.)

The middle ear is an acoustic impedance transformer. The elasticity of a medium determines its impedance
to transmitting sound waves: when such waves meet an interface between media with different impedances,
transmission across the interface is imperfect. Sound travelling in air (high elasticity, low impedance), and
meeting water (lower elasticity, higher impedance) is largely reflected from the surface of the water. (The
shrieking and yelling echoing around in a public indoor swimming pool is silenced when you put your head
under the water. Not a fair example, but you get the drift - I hope.) Electrical circuits with different
impedances can effectively be linked by using an impedance transformer. Similarly, the problem of
transferring sound across an "impedance barrier" can be overcome by interpolating an impedance-matching
transformer. For sound, this is a mechanical device, and such is the middle ear mechanism, placed between
the air of the external auditory meatus and the liquid - effectively water in the present context - of the inner
ear. How it works will be examined a little more closely in Neuroscience. Note, however, that this transformer
is subtle - it can be "tuned" (i.e. made more or less effective) by contraction/relaxation of the tensor tympani
and stapedius muscles. These muscles contract markedly, for example, when you are exposed to very loud
noises, in an attempt to prevent damage to the inner ear. Paralysis of stapedius in facial palsy leads to
hyperacusis - even moderately low sounds, such as bells ringing, become extremely unpleasant and even
painful.

The functioning of the transformer is also affected by factors other than the muscles. These include:
(a) Pressure Differences Between Middle and Outer Ears

Increased tension of the tympanic membrane because of a pressure differential between the atmosphere and
tympanic cavity is common and the result of falling pressure in the middle ear. The commonest cause is a
cold, when the auditory tube is blocked. Even without a cold, there can be few people whose ears have not,
sometime or another, "popped" on blowing the nose or swallowing (the auditory tube is normally closed until
opened by the pull of salpingo-pharyngeus and tensor tympani) - when hearing noticeably "brightens". The
auditory tube may be congenitally blocked. The patency of an auditory tube is easily tested: the examiner
observes the tympanic membrane through an auroscope, asks the subject to perform the Valsalva manoeuvre,
with the glottis open and nose pinched, and watches the ear drum for movement. (If a patient cannot
understand what is required in the Valsalva manoeuvre, he/she is offered a balloon fitted with a nozzle which
is fitted into one nostril. With the other nostril closed, the patient is asked to blow up the balloon.....)

\NewCMedPhysSc\25 HN 301 Deafness.