Internal Ear

The vibrations, transmitted by the stapes, produce displacement of the basilar

membrane and shearing movements between the hair cells and the tectorial

membrane of the organ of Corti which initiates nerve impulses in the fibres of

the auditory nerve. The greater the degree of displacement the more hair cells

and hence the more nerve fibres are stimulated. It has been found that the basal

portion of the cochlear duct responds to high-frequency sounds while the apex

responds maximally to low-frequency stimuli. There is a gradation in response

from the basal turn to the apex. The nerve fibres supplying each area of the

cochlea, therefore, are stimulated by different frequencies. These fibres then

transmit impulses to the auditory nuclei in the brain stem, and from there the

fibres pass through the mid-brain to the auditory cortex where the impulses are

perceived as sound.

The nerve impulses in response to sound are generated by the hair cells of the

organ of Corti. The exact mechanism is uncertain. The electrical potentials

which are set up, however, follow accurately the wave form and changes in

Intensity of the stimulating source. These are known as cochlear microphonics

and may readily be picked up by an electrode placed on the promontory near the

round window. The VIIIth nerve action potential may also be picked up and is now

utilized in an objective test of hearing known as electrocochleography.

PHYSIOLOGY OF THE VESTIBULAR APPARATUS

The balance of the body is maintained by coordination of information from three

systems: (1) proprioception, i.e. sensation from muscles, joints, tendons and

ligaments; (2) the eyes; (3) the vestibular system.

The vestibular system consists of the semicircular canals, the utricle and the

saccule. The utricle and saccule respond to linear acceleration. The greatest

linear acceleration to which the body is normally subjected is gravity (10 m per

sec2), and it is alterations in the position of the head in relation to the

direction of gravity which stimulate selective parts of the utricle and saccule.

Impulses from the utricle and saccule not only give information about the

position of the head in space but initiate reflexes which tend to keep the head

in the upright position and are contributory to the maintenance of muscle tonus.

The semicircular canals respond to angular (rotatory) acceleration, and

stimulation of the semicircular canals gives rise to the sensation of rotation

and to reflex movements of the eyes and body to counter the movement. Angular

acceleration around any axis will stimulate at least one pair of semicircular

canals. The mechanism can most easily be explained by considering rotation in a

rotating chair about a vertical axis. Acceleration to the right will cause

movement to the left of endolymph within the membranous horizontal canal and

deflection of the cupula on the crista in the expanded ampulla of the canal.

There is a constant impulse rate of 10-20 impulses per second in the fibres of

the nerves leaving the crista. Movement of the endolymph and cupula towards the

ampulla causes an increase in this impulse rate. Movement away from the ampulla

causes a reduction in the impulse rate. Acceleration to the right will cause an

increase in the impulse rate from the crista of the right horizontal canal and a

reduction of the rate on the left side. This difference in impulse rate is

interpreted by the central nervous system and gives rise to the sensation of

rotation to the right. The eyes will move to the left at a rate proportional to

the degree of stimulation, but as the eyes can only move a limited amount

laterally, a central reflex will return them to mid-position and the vestibular

stimulation will again move them to the left. This constitutes nystagmus with a

relatively slow vestibular component and a very much faster central component.

When the acceleration stops, rotation will continue at constant velocity, the

cupula on the crista will return to rest and there will be no sensation of

rotation and no nystagmus. If the rotation is suddenly stopped, i.e. the

equivalent of an acceleration to the left, the endolymph and cupula will

continue to move to the right, the impulse rate from the left horizontal canal

will reduce and the rate from the right canal will increase. There will be a

sensation of rotation to the left, although the head and body are at rest; there

will be nystagmus with the slow phase to the right and the quick phase to the

left. It will be seen that destruction of one labyrinth with abolition of the

resting impulse rate will produce imbalance of impulses arriving in the central

nervous system from the two sides, resulting in the same effect as vestibular

stimulation, i.e. sensation of rotation, nystagmus, and, if the stimulation is

large enough, nausea, vomiting, pallor of the skin and sweating. Rotation

carried out in this way, using controlled accelerations, may be used for testing

semicircular canal function but it has the disadvantage of stimulation of the

canals of both sides at the same time.

Part 2

THE NOSE, PARANASAL SINUSES AND PHARYNX

ANATOMY OF THE NOSE AND PARANASAL SINUSES

The external nose

The external nose is shaped as a triangular pyramid with its root above and its

base directed downwards, and perforated by two nostrils, separated by a median

septum. The free angle of the external nose is the apex, connected to the root

by the dorsum, the upper part of which is termed the bridge. Each side of the

external nose ends in a rounded eminence, the ala nasi, which forms the outer

boundary of the nostril. The skin over the apex of the nose is thick and

adherent, and contains many sebaceous glands. The external framework is osseous

and cartilaginous. The nasal bones form the bridge, and each is united above

with the frontal bone and laterally to the frontal process of the maxilla. Three

paired cartilages, the upper lateral (triangular), lower lateral (greater alar),

lesser alar cartilages, and one unpaired cartilage, the septal (quadrangular),

complete the external framework (Fig. 15, 16). The chief muscles acting upon the

external nose are the compressors and dilators of the ala nasi, and are supplied

by the facial nerve. In confirmed mouth breathers the dilators tend to atrophy

from disuse so that the anterior nares become narrow and slit-like.

Blood supply to the external nose derives from the maxillary (E.C.A.), facial

(E.C.A.) and ophthalmic arteries (I.C.A.), while venous drainage is through the

anterior facial and ophthalmic veins, the latter being a tributary of the

cavernous sinus. Lymphatic drainage follows the anterior facial vein and opens

into the submandibular glands, but other lymphatics drain into the pre-auricular

glands.

The nasal vestibule

This is the name given to the entrance to the nasal cavity, within the nostrils.

It is lined by skin which contains hair follicles, and it ends at the

mucocutaneous junction. The part between the two nasal vestibules, containing

the anterior end of the nasal septum, is called the columella. A very important

structure is the internal nasal valve (Fig. 17). It lies at the junction of the

vestibule and the nasal cavity.

The nasal cavity

Boundaries of the nasal cavity. Inferiorly the floor of the nasal cavity is

formed by the maxilla and by the palatine bones. The roof of the nasal cavity is

formed, in front, by the lateral nasal bones. Superiorly is the cribriform

plate. This is a bony lamina of the ethmoid bone, which is perforated to permit

the passage of the filaments of the olfactory nerves. Posteriorly the sphenoid

bone forms part of the roof.

The lateral wall of each nasal cavity is convoluted in appearance due to the

three conchae or turbinates (Fig. 18). The superior and middle turbinates

constitute the medial surface of the lateral mass of the ethmoid bone. The