Lec:1 : Anatomy, Physiology and Immunology
of the Pharynx :
The pharynx is a tubular, fibromuscular space extending from
the skull base to the inlet of the esophagus (upper
esophageal sphincter). Anatomically and clinically, the
pharynx consists of a nasal part (nasopharynx), an oral part
(oropharynx), and a laryngeal part (hypopharynx). The entire
pharynx is bounded externally by several muscle systems,
which perform diversefunctions and are continuous distally
with the muscles of the esophageal wall.
The primary function of the pharynx and esophagus is to
coordinate the act of swallowing, which is regulated by a
complex interaction of various cranial nerves and peripheral
muscular and connective-tissue structures located in the oral
cavity, pharynx, and esophagus. The pharynx also contains the
tonsillar ring, a series of lymphoepithelial organs that are
important in the immune response to infection. Finally,
portions of the pharynx function as a variable resonance
chamber for modulating vocal sounds.
Nasopharynx, Oropharynx,
andHypopharynx
Anatomical Extent
Nasopharynx: This highest part of the pharynx extends from
the bony skull base to an imaginary horizontal line at the
level of the velum . It communicates with the nasal cavity via
the choanae and with the middle ear via the orifice of the
Eustachian tube. The nasopharynx is bounded superiorly by
the floor of the sphenoid sinus and pharyngeal roof. Also in
this region is the pharyngeal tonsil, which forms part of the
tonsillar ring .Medial to the Eustachian tube orifice, the tubal
cartilage forms a projecting lip called the torustubarius. The
concavity behind it is termed the pharyngeal recess
(Rosenmullerfossa) . The nasopharynx is bounded posteriorly
by the curve of the first cervical vertebra, with its overlying
prevertebral cervical fascia and prevertebralmusculature.
Oropharynx: The oral cavity communicates via the faucial
isthmus with the oropharynx, which extends inferiorly from
the lower boundary of the nasopharynx to the upper margin
of the epiglottis . It is bounded anteriorly by the tongue base
and lingual tonsil and posteriorly by the second and third
cervical vertebrae with their prevertebral fascia. It is
bounded laterally by the faucialpillars , which flank the
palatine tonsils.
Hypopharynx: The lowest pharyngeal segment is the
hypopharynx, which extends from the superior border of the
epiglottis to the inferior border of the cricoid cartilage plate
of the larynx , where it joins with the esophagus. Lying
posterior to the hypopharynx
are the third through sixth cervical vertebrae. Its anterior
wall is formed by the back of the larynx, which protrudes
into the hypopharynx and forms two lateral mucosal
pouches (piriform sinuses), which rejoin at the level of the
esophageal inlet.
Mucosal Lining
The mucosa that lines the nasopharynxconsists of several rows of
ciliated epithelium. At the oropharynxthis gives way to a stratified,
nonkeratinized squamous epithelium, which also lines the
hypopharynx. Pharyngeal MusculatureThe muscular boundaries
of the pharynx are formed by the constrictor pharyngis muscle group.
The highest of these muscles, the constrictor pharyngis superior, begins
at the level of the nasopharynx just below the tough, fibrous
pharyngobasilar fascia, which in turn is suspended from the bony skull
base. Just below the superior constrictor muscle are the overlapping
constrictorpharyngismediusand inferior muscles, the latter of which joins
distally with the esophageal musculature
While most of the constrictor pharyngis muscle fibers run obliquely,
the lowest portions of
anatomical weak spots in the pharyngeal wall . These weak spots are
sites of predilection for the development of pulsion (Zenker)
diverticula in the hypopharynx
Three additional pairs of external muscles are distributed to the
pharyngeal wall and assist in controlling vertical movements of the
pharynx: the stylopharyngeus, the salpingopharyngeus, and the
palatopharyngeus
Neurovascular Supply
The pharynx receives its blood supply from the territory
of the external carotid artery (branches of the facial
artery, maxillary artery, ascending pharyngeal artery,
lingual artery, and superior thyroid artery). The veins of
the pharynx drain into the internal jugular vein.
The lymphatic drainage of the upper portions of the
pharynx is through the retropharyngeal lymph nodes,
while the lower portions drain to the parapharyngealor
deep cervical nodes.
Nerve supply:The muscles and mucosa of the pharynx
receive their motor and sensory innervation from the
pharyngeal plexus, which in turn receives fibers from
the glossopharyngeal and vagus nerves. The plexus itself
is located on the outer aspect of the constrictor
pharyngismedius muscle.
Parapharyngeal Space
The parapharyngeal space encompasses an anatomicallywell- defined
region with the shape of an inverted pyramid whose base is formed by
the inferior surface of the petrous bone and whose apex is at the
lesser horn of the hyoid bone. The parapharyngeal space is divided
anatomically into two parts, the retropharyngeal space and the
lateralpharyngeal space. The latter in turn is subdivided by the
common connective-tissue sheath of the muscles
arising from the stylohyoid process (stylopharyngealaponeurosis) into
a prestyloid and a retrostyloid part.
The prestyloid part communicates with the parotid compartment. It
contains the lateral and medial pterygoidmuscles, lingual nerve, optic
ganglion, and maxillary artery. Its lower part is directly adjacent to the
tonsillar compartment. The retrostyloid part of the lateral pharyngeal
space is traversed by neurovascular bundles made up of the internal
carotid artery, internal jugular vein, and lower cranial nerves (IX–XII).
The retropharyngeal space contains smaller arterial and venous
vessels and, most notably, the retropharyngeal lymph nodes that
drain the nasopharynx.
Weak points in the wall of the hypopharynx
Three muscular weak points exist in the lower
posterior wall of the hypopharynx. The first is the
Killian triangle, located between the constrictor
pharyngis inferior and the uppermost
fibers of the cricopharyngeus muscle. The second
area of weakness is the KillianJamieson region
between the oblique and transverse fibers of the
constrictor pharyngis.
The third is the Laimer triangle, which is bounded
above by the cricopharyngeus and below by the
uppermost fibers
of the esophageal musculature .The Killian triangle
is a particularly common site for the formation of
hypopharyngeal diverticula.
Physiology of Swallowing
Normal swallowing requires a coordinated
interaction of various anatomic structures in
the oral cavity, pharynx, larynx, and esophagus.
From a functional standpoint, the voluntarily
initiated oral phase of swallowing is
distinguished from an “involuntary” pharyngeal
phase and esophageal phase, which are
controlled through reflex mechanisms .
During the oral phase of swallowing, food is
broken down and moistened to form a bolus
that is moved toward
the oropharynx. This is accomplished mainly by
pressing the food against the hard palate with
the tongue (
➀).
The pharyngeal phase begins when the bolus comes into contact
with receptors in the throat (especially on the tongue base), eliciting
an involuntary swallowing
reflex (
➁). The afferent impulses for this reflex travel through the
glossopharyngeal and vagus nerves, while the efferent neurons that
supply the pharyngeal muscles arisefromcranial nerves V3, VII, IX, X,
and XII.
The extensive nerve supply highlights the complexity of swallowing as
well as the potential vulnerability of this process. While the
involuntary swallowing reflex is triggered during the pharyngeal
phase, the velum is elevated to close off the nasopharynx
➂. The
larynx is also sealed off by elevation of the epiglottis
➃. This is
accompanied by a reflex adduction of the vocal cords
➄, allowing the
food to pass through the piriform sinuses toward the esophagus
while bypassing the larynx
➅.
The esophageal phase of swallowing begins with a primary peristaltic
wave, which isreflexly initiated in response to movement of the bolus
through the pharynx (cranial nerves IX, X)
➆. Secondary peristalsis is
additionally triggered in the esophagus by the pressure of
the bolus against the esophageal wall
➇. Through the coordinated
action of these mechanisms, the bolus is transported into the
stomach within 7–10 seconds.
Structure and Function of the Tonsillar Ring Anatomy
The tonsillar ring (Waldeyer’s ring) is composed of a series of
lymphoepithelial “organs” called the tonsils. This tissue is
structurally similar to lymph nodes but lacks afferent lymphatic
vessels.
The tonsils are named for their location, consisting of a
pharyngeal tonsil, the paired palatine tonsils, and the unpaired
lingual tonsil at the base of the tongue. Additionally,
smaller condensations of lymphoepithelialtissue are found in
the pharyngeal recess.
and in the “lateral bands” (tubopharyngeal folds) on the
posteriorwall of the oropharynx and
nasopharynx.Theepitheliumof the tonsils also varies by
location. While the pharyngeal tonsil is covered mainly by
multiple rows of ciliated epithelium, the palatine and lingual
tonsilsare covered by stratified, nonkeratinizedsquamous
epithelium.
Structure of the Palatine Tonsil
The palatine tonsil has special immunologic importance among
the tissues of the tonsillar ring owing to its distinctive
morphology. Its surface is invaginated by crypts—fold-like tissue
indentations that are lined by porous epithelium and substantially
increase the surface area of the tonsil. This arrangement facilitates
contact between inspired or ingested antigens and the
subepithelial lymphatic tissue. 5 Pharynx and Esophagus primary
follicles are formed during embryonic development and
differentiate into secondary follicles after birth .the secondary
follicles mainly contain B lymphocytes at various stages of
differentiation, along with scattered T lymphocytes. Besides the
lymph follicles, there are also extrafollicular areas with B and T
lymphocytes that enter the lymphatic tissue through the post
capillary venules.
Functional Importance of the Tonsils
in the Immune System
The palatine tonsil in particular is considered to be an “immune organ” that
plays a significant role in the defense against upper respiratory infections. By
analogy with comparable lymphoepithelial tissue masses in the bronchi and
intestinal tract, the lymphatic tissue in the tonsillar ring is also termed the
mucosa-associated lymphatic tissue (MALT) of the upper respiratory tract.
Accordingly, this tissue has the ability to mount specific immune reactions in
response to various antigens. The activity of this lymphatic organ is especially
pronounced during childhood, when immunologic challenges from the
environment induce hyperplasia of the palatine tonsils . Following this “active
phase” of immune initiation, which lasts until about 8–10 years of age, the
lymphatic tonsillar tissue becomes less important as an immune organ, and
there is a corresponding decline in the density of lymphocytes in all regions of
the tonsils. While the tonsils become less important immunologically with
ageing, the tonsillar tissue continues to perform immune functions even at an
advanced age, although this should not alter the decision to remove the tonsils
if a valid indication for tonsillectomy exists .While the tonsils are “learning”
their immune function during childhood, extreme tonsillar hyperplasia (“kissing
tonsils”) may develop, leading to functionally significant narrowing of the faucial
isthmus, with eating difficulties and obstructed breathing. Especially when
recumbent, these children may experience significant respiratory dysfunction,
with periods of apnea. They also have an increased long-term risk of developing
corpulmonale. Consequently, there should be little hesitation in recommending
tonsillectomy, even in small children.
