the nerve supply to the hair cells decreases.
Blood Vessels
The main supply comes from the labyrinthine artery which arises from the basilar
or anterior inferior cerebellar artery. The veins unite to form the labyrinthine
vein which opens into the inferior petrosal sinus or the sigmoid sinus. Small
veins pass via the aqueducts of the vestibule and cochlea to the superior and
inferior petrosal sinuses respectively.
Nerve Supply
The vestibulocochlear (acoustic) nerve is formed by cochlear and vestibular
parts in the internal acoustic meatus from which it emerges on the lateral side
of the sensory root of the facial nerve and enters the brain stem between the
pons and the medulla. The cochlear part is composed of fibres which are the
central processes of bipolar cells in the spiral ganglion in the modiolus of the
cochlea. The peripheral processes of the ganglion cells pierce the bony spiral
lamina to reach the inner and outer hair cells of the organ of Corti. Other
fibres follow a spiral course on the internal part of the basilar membrane. The
vestibular part consists of the processes of the bipolar cells of the vestibular
ganglion in the internal acoustic meatus. From the superior part of the ganglion
fibres pass via the superior vestibular nerve, the utricle, the ampullae of the
frontal and horizontal semicircular ducts and the anterior part of the macula of
the saccule. The inferior part of the ganglion sends fibres via the inferior
vestibular nerve to the macula of the saccule and the ampulla of the sagital
semicircular duct.
PHYSIOLOGY OF HEARING
External and Middle Ears
Airborne sound consists of vibrations of the atmosphere, that is, of alternate
phases of condensation and rarefaction. The purpose of the auditory apparatus is
to convert these vibrations in air to vibrations in the inner-ear fluids, and
then to nerve impulses to be transmitted along the auditory nerve to the higher
centres of hearing.
The auricle collects the sound waves to some extent, and they pass along the
external acoustic meatus to the tympanic membrane which is set in motion. The
vibrations of the tympanic membrane are transmitted to the malleus, incus and
stapes. The malleus and incus rotate around a common fulcrum and transmit
vibrations to the stapes in the oval window, causing vibrations to be set up in
the endolymphatic and perilymphatic compartments of the inner ear. The
conversion of sound from air into fluid is accomplished by the middle-ear
structures. To some extent the lever system of the malleus and incus helps, but
the main effect comes from the tympanic membrane. This system increases the
sound pressure at the footplate to a degree which causes the fluids of the inner
ear to vibrate. The stapes moves in a rocking rather than a piston motion and,
as fluids cannot be compressed, these vibrations are transmitted to the round
window membrane. This reciprocal action of the oval and round windows is
essential. In the normal ear the presence of the tympanic membrane and an
air-containing middle ear prevents the sound-pressure waves from reaching the
round window and opposing the outward movement of the round window membrane.
This protection of the round window is lost where there is a large perforation
of the tympanic membrane, and this is one of the factors which may produce
deafness.
The tympanic membrane is at its most efficient when the air pressure in the
external acoustic canal and the middle ear is equal. This is achieved by the
eustachian tube which normally opens during each act of swallowing. In this way
the air pressure on both sides of the tympanic membrane can be kept equal. The
stapedius and tensor tympani muscle seem to have a protective function; loud
sound causes a reflex contraction of the muscles and this serves to stiffen up
the conducting mechanism and possibly to protect the inner ear from damage.
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
inferior turbinate is a separate bone attached to the maxilla. Each turbinate
overhangs a channel or meatus corresponding in length to the turbinate beneath
which it is situated. All three reach forwards from the posterior aperture of
the nose, called the posterior naris or choana. The superior meatus is confined
to the posterior third of the lateral wall of the nasal cavity; the middle
meatus runs forward about two-thirds of its length; and the inferior meatus
extends the whole length of the lateral wall of the cavity. The space above the
superior turbinate is called the spheno-ethmoidal recess. Between the three
turbinates and the nasal septum, which separates the two nasal cavities, is a
space called the general nasal meatus.
The meatuses are of clinical importance in respect of their contents. The
nasolacrimal canal opens into the anterior end of the inferior meatus.
Communication between the paranasal sinuses and the nasal cavity takes place
through openings, or ostia. The frontal, anterior ethmoidal and maxillary
sinuses open into the middle meatus; the posterior ethmoidal sinuses drain into
the superior meatus; the sphenoidal sinus communicates with the superior meatus.
The middle meatus contains several structures of importance (Fig. 19, 23). An
enlargement is found at the anterior end of the middle meatus, which is part of
the ethmoid bone, known as the unciate process. A little farther back can be
seen another eminence which is called the bulla ethmoidalis, which represents a
protrusion into the meatus of one air cells of the ethmoidal labyrinth.
In the normal nose these parts can rarely be seen from the front. Between these
two enlargements is a groove which is known as the hiatus semilunaris, into
which the ostium of the maxillary sinus opens. The hiatus semilunaris, when
followed upwards, leads to a narrowing called the infundibulum. In many cases
the infundibulum continues upwards becoming the fronto-nasal duct. Owing,
however, to the irregularity of the development of the frontal sinus and the
anterior ethmoid cells, it is possible that the fronto-nasal duct may open from
an anterior ethmoid cells.
The nasal septum separates the two nasal cavities and is partly osseous and
partly cartilaginous. The perpendicular plate of the ethmoid and the vomer bone
constitute the upper and posterior part, while the septal cartilage completes
the septum anteriorly, stretching from the dorsum of the nose above to the nasal
crests of the maxillary and palatine bones below. The main arterial supply of
the nasal septum arises from the septal branch of the sphenopalatine artery
(maxillary a.- E.C.A.), and this anastomoses with the greater palatine artery
(maxillary a.- E.C.A.), septal branches of the superior labial (facial a.-
E.C.A.), anterior ethmoidal (ophthalmic a.- I.C.A.) and posterior ethmoidal
(ophthalmic a.- I.C.A.) arteries at the antero-inferior part of the septum, or
Little’s (Kiesselbach’s) area (Fig. 20), which is of importance in epistaxis.
The lateral nasal wall is supplied by lateral branches from these vessels.
Venous drainage from the nasal cavity is through the sphenopalatine foramen to
the pterygoid plexus, but some veins join the superior ophthalmic vein in the
orbit, while others enter the anterior facial vein. Lymphatic vessels from the
anterior part of the cavity join cutaneous lymphatics to the submandibular
glands, and so to the superior deep cervical glands. Posteriorly the lymphatic
drainage is to the medial deep cervical glands.
The nasal mucous membrane consists of a layer of fairly dense connective tissue
containing large blood vessels and some unstriped muscle fibres. There is
erectile or cavernons tissue comprising irregular thin-walled blood spaces in
the anterior and posterior ends of the inferior turbinate. A layer of elastic
tissue fibres is present beneath the basement membrane, and this layer allows
the mucosa to return to normal size when the vascular engorgement of the
erectile tissue has worn off. The surface epithelium is columnar ciliated lying
upon several layers of cuboidal cells resting upon the basement membrane. There
are many mucous glands beneath the basement membrane, their ducts penetrating
the membrane to open on the surface.
There are two nerve supplies to the nasal cavity - sensory and secretory. The
main sensory nerve supply is derived from the maxillary division of the
trigeminal nerve through branches arising in the pterygopalatine ganglion. The
lateral and medial internal nasal branches of the ophthalmic division of the
trigeminal nerve supply the anterior part of the nasal cavity, while the floor
and anterior end of the inferior turbinate are served by the anterior dental
branch of the infra-orbital nerve (maxillary division of the trigeminal nerve).
Secretory nerve fibres supplying the glands and unstriped muscle belong to the
sympathetic and parasympathetic systems. Sympathetic fibres, which produce
vasoconstriction and diminished secretion, arise from the superior cervical
ganglion via the nerve of the pterygoid canal to the pterygopalatine ganglion.
Parasympathetic fibres, which produce vasodilatation and increased secretion,
are carried in the greater superficial petrosal nerve and the nerve of the
pterygoid canal to the pterygopalatine ganglion from which postganglionic fibres
are distributed.
The olfactory nerves - some twenty filaments - derive from the olfactory bulb,
enter the nasal cavity through the cribriform plate of the ethmoid, and are
distributed in a network in the mucous membrane in the upper third of the nasal
septum and the lateral wall of the nasal cavity. The perineural sheaths of these
filaments communicate directly with the pia-arachnoid and thus may transmit
infection to the meninges.
The paranasal sinuses
The paranasal sinuses, arranged in pairs and in relation to each nasal cavity,
comprise two groups, anterior and posterior (Fig. 22, 23). The former includes
the maxillary sinus, the frontal sinus and the anterior ethmoidal cells, all of
which communicate with the middle meatus. The posterior group consists of the
posterior ethmoidal cells and the sphenoidal sinus communicating with the
superior meatus.
The maxillary sinus is also known as the maxillary antrum. It exists at birth as
a small but definite cavity adjacent to the middle meatus, and it enlarges
gradually to reach its maximum dimensions about the twenty-first year with the
eruption of the upper wisdom tooth. The sinus expands in the maxilla during the
eruption of the primary dentition until it reaches the level of the floor of the
nasal cavity about the seventh year. In adult life it is somewhat pyramidal in
shape, its roof being formed by the floor of the orbit, its floor being in close
proximity to the roots of the second dentition; its posterior wall lying in
relation to the pterygopalatine fossa; its medial wall adjoining the lateral
wall of the nasal cavity; and its anterolateral walls being superficial. The
opening into the middle meatus, the maxillary ostium, is near the upper part of
the cavity of the sinus, and is thus unfavourably placed for drainage. There may
be one or more accessory ostia posterior to the main one.
The frontal sinus is rudimentary at birth, being represented by a small upward
prolongation from the anterior end of the middle meatus, the nasofrontal duct.
During childhood this duct enlarges upwards to reach the level of the orbital
roof about the ninth year. Thereafter the sinus extends for a variable distance
as a result of absorption of cancellous bone between the outer and inner tables
of the frontal bone. The anterior wall is formed by the outer table; the
posterior wall is related to the inner table which separates it from the frontal
lobe of the brain; its floor forms part of the orbital roof; and the medial wall
is a septum separating the two frontal sinuses. The opening of the frontal sinus
is in its floor, and communicates with the middle meatus through the nasofrontal
duct.
The ethmoid sinuses constitute a cell labyrinth and are present at birth as
prolongations of the nasal mucosa into the lateral mass of the ethmoid bone. In
adult life they vary in number, size and shape, and for clinical purposes they
are classified as anterior and posterior, depending upon whether they
communicate with the middle or the superior meatus. Medially the cell labyrinth
lying in relation to the upper half of the nasal cavity, laterally - to the
orbit, from which the cells are separated laterally by the lamina papyracea and
to the maxillary sinus. The cells abut anteriorly on the frontal process of the
maxilla, posteriorly they are related to the sphenoid sinus. Superiorly the
ethmoid cells related to the frontal sinus and anterior cranial fossa.
The sphenoid sinus occupies the body of the sphenoid bone, and may be present at
birth as a small indentation of nasal mucosa. The sinus varies greatly in size
in the adult. The lateral wall is contiguous with the internal carotid artery,
the cavernous blood sinus, first branch of the V cranial nerve, III, IV and VI
cranial nerves (Fig. 24); the roof is related to the frontal lobe, the olfactory
tract, the optic chiasma and the pituitary gland lying in the hypophyseal fossa;
the floor adjoins the pterygoid canal, roof of the nasal cavity, nasopharynx;
while the medial wall is a septum separating it from its neighbour. Anterior
wall is contiguous with ethmoidal cells. Behind the posterior wall lies the
posterior cranial fossa. The ostium is placed high up in the cavity of the
sinus.
BASIC PHYSIOLOGY
The nose is both a sense organ and a respiratory organ. In addition, the nose
performs an important function for the entire body by providing both physical
and immunologic protection from the environment. It is also important in the
formation of speech sounds.
The Nose as an Olfactory Organ
The human sense of smell is poorly developed compared to most mammals and
insects. Despite that, it is still very sensitive in the human and is almost
indispensable for the individual. For example, taste is only partially a
function of the taste buds since these can only recognize the qualities of
sweet, sour, salty, and bitter. All other sensory impressions caused by food
such as aroma and bouquet are mediated by olfaction. This gustatory olfaction is
due to the fact that the olfactory substances of food or drink pass through the
olfactory cleft during expiration while eating or drinking. The sense of smell
can stimulate appetite but can also depress it. It also provides warning of
rotten or poisonous foods and also of toxic substances, e.g., gas. The sense of
smell is particularly important in the field of psychology: Marked affects may
be induced or inhibited by smells. It should also be remembered that a good
sense of smell is essential for those in certain occupations, e.g., cooks,
confectioners, wine, coffee, and tea merchants, perfumers, tobacco blenders, and
chemists. Finally, the physician needs a "clinical nose" for making his
diagnosis.
The olfactory area of the nose is relatively small. It contains the olfactory
cells, i.e., the bipolar nerve cells, which are to be regarded as the sensory
cells and first-order neurons. They are collected into about 20 fibers in the
olfactory nerves which run to the primary olfactory center of the olfactory
bulb.
From here the neurons of the bulb run via the olfactory tract to the secondary
olfactory center. The tertiary cortical olfactory field lies in the dentate and
semilunate gyri.
The mode of action of the scent molecules on the olfactory cells is not known
with certainty. There are numerous current theories of the mechanism of action,
including: emission of scent corpuscles, selective absorption, specific
receptors on the sensory cells, enzymatic control, molecular vibrations,
electrobiologic processes such as changes in cell membrane potential, etc.
It is certain that only volatile substances can be smelled by humans. These
substances must be soluble in water and lipids. Only a few molecules suffice to
stimulate the sense of smell. 10-15 molecules per ml of air are sufficient
stimulation on average to exceed the threshold.
It is said that there are about 30000 different olfactory substances in the
atmosphere; of these, humans can perceive about 10000 and are able to
distinguish among 200.
The sense of smell, like other senses, demonstrates the phenomenon of
adaptation. The sensitivity of the olfactory organ depends also on hunger:
several olfactory factors can be smelled better if the subject is very hungry
than shortly after eating, a very useful physiologic regulation.
Anosmia and hyposmia may be caused by obliteration of the olfactory cleft
(polyps, etc.), causing respiratory anosmia. Inability of the olfactory
substances in food and drink to pass from the mouth and throat to the olfactory
epithelium of the nose because of obstruction of the nasal cavity or the choana
is described as gustatory anosmia. Central anosmia is caused by a disorder of
the central nervous parts of the olfactory system in the presence of a patent
airway. Causes include: traumatic rupture of the olfactory nerve, cerebral
contusion, and cerebral diseases. Essential anosmia is due to local damage to
the olfactory epithelium, e.g., due to influenza, with an open olfactory cleft.
The Nose as a Respiratory Organ
In the human the only physiologic respiratory pathway is via the nose. Mouth
breathing is unphysiologic and is only brought into play in an emergency to
supplement nasal respiration. The physiology of the airstream through the normal
nose in inspiration and expiration may be summarized as follows:
The average ventilation through a normal nose in physiologic breathing is 6
l/min, and 50 to 70 l/min in maximal ventilation.
The internal nasal valve or limen nasi is the most narrow point in the normal
nose. It thus acts like a nozzle, and the speed of the airstream is very high at
this point.
The nasal cavity between the valve and the head of the turbinates acts as a
diffusor, i.e., it slows the air current and increases turbulence. The central
part of the nasal cavity with its turbinates and meatus is the most important
part for nasal respiration. The column of air consists of a laminar and a
turbulent stream. The proportion between laminar and turbulent flow considerably
influences the function and condition of the nasal mucosa.
The airstream passes in the reverse direction through the nasal cavities on
expiration. The expiratory airstream demonstrates considerably less turbulence
in the central part of the nose, and thus offers less opportunity for heat and
metabolic exchange between the airstream and the nasal wall than on inspiration.
The nasal mucosa can thus recover during the expiratory phase. Inspiration
through the nose followed by expiration through the mouth leads rapidly to
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