Mednotes

L4 GI Anatomy (Pharynx and oesophagus)

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tagsgi, lecture, anatomy
aliasesGI Anatomy - Pharynx and oesophagus

11 min read

Overview

Lecture 2 of a six-part GI anatomy series, covering the two conducting segments of the tract: the pharynx, whose function is to bypass the airway, and the oesophagus, whose function is to convey the bolus to the stomach. It works from the layout of the oral cavity and pharynx and their communications, through the pharyngeal wall (fascial layers, skull-base attachment, constrictor and levator muscles), the mucosal folds and subdivisions that line it, then the oesophagus with its layers, sphincters and constrictions, and finishes with the three stages of swallowing that use all of these structures.

Place in the GI tract

The lecture series runs: 1. oral cavity, 2. pharynx and oesophagus (this lecture), 3. abdominal cavity, oesophagus and stomach, 4. small intestine and large intestine, 5. liver, gallbladder and pancreas, 6. blood supply, lymphatics and nerve supply of the GI tract.

Region and its function:

  • Oral cavity: initial breakup
  • Pharynx: bypass the airway
  • Oesophagus: convey to stomach
  • Stomach: digestion, mechanical and acidic
  • Small intestine: absorption of nutrients
  • Large intestine: absorption of water
  • Rectum: storage
  • Anal canal: maintenance of continence

Relationships of the oral cavity and pharynx

The oral cavity is bounded by a roof (hard palate), lateral walls (cheeks) and a floor (tongue and other soft tissues), and opens anteriorly at the oral fissure. Within it, the oral vestibule lies outside the teeth and the oral cavity proper lies inside the dental arches; the soft palate lies at the back.

Two communications lead into the pharynx:

  • Oropharyngeal isthmus: from the oral cavity, behind the soft palate, into the oropharynx.
  • Pharyngeal isthmus: from the nasal cavities, above the soft palate, into the pharynx.

Inferiorly the pharynx continues as the oesophagus posteriorly, with the larynx and trachea lying anterior to it.

Because the pharynx can be reached from both directions, it is sampled clinically by two swab routes: the oropharyngeal swab through the wide-open mouth and the nasopharyngeal swab through the nostril (the CDC route recommended for COVID-19 testing on the slide).

Structure of the pharyngeal wall

The pharynx is an open-sided cylinder:

  • Open anteriorly.
  • Mobile: it moves in a cephalic direction and it constricts.
  • Wall layers, from inside outwards: mucous membrane; fibrous layer (pharyngobasilar fascia); muscle layers (inner levators, outer constrictors); fascial layer (buccopharyngeal fascia).

Basilar attachment. Seen from below, the pharynx attaches to the skull base along a line that runs via the pharyngeal tubercle, past the region of the pharyngotympanic (auditory) tube, and forward to the medial plate of the pterygoid process of the sphenoid. Related landmarks on this view: the scaphoid fossa of the sphenoid (attachment of tensor veli palatini), a roughening on the petrous part of the temporal bone (attachment of levator veli palatini), the choanae (posterior nasal openings), pterygoid hamulus, carotid canal, jugular foramen and external acoustic meatus.

Pharyngeal muscles

Outer layer: constrictors. Superior, middle and inferior constrictors overlap one another from above downwards, with the oesophagus continuing below the inferior constrictor. Posteriorly they meet in the midline at the pharyngeal raphe, which runs up to the pharyngeal tubercle on the skull base. The pharyngeal fascia occupies the gap above, between the skull base and the upper border of the superior constrictor. The palatopharyngeal sphincter lies on the deep surface of the superior constrictor. Also visible in this region: styloid process, stylohyoid ligament and stylopharyngeus.

Inner layer: levators (longitudinal muscles). Three, descending inside the constrictor layer:

  • Salpingopharyngeus, uppermost, from the auditory tube region
  • Palatopharyngeus, from the palate
  • Stylopharyngeus, from the styloid process

The deeper dissection of this layer also shows levator veli palatini, tensor veli palatini, the cartilaginous part of the pharyngotympanic tube, the palatopharyngeal sphincter and the palatine tonsil lying against the constrictors.

On the constrictor slide, one label at the top right of the lateral figure is hidden behind an added callout box and could not be read.

Subdivisions and mucosal folds

The pharynx is divided from above downwards into nasopharynx, oropharynx and laryngopharynx.

Named folds and recesses:

  • Nasopharynx: pharyngeal opening of the pharyngotympanic (auditory) tube, with the torus tubarius around it and the torus levatorius (fold overlying levator veli palatini); the pharyngeal recess behind the torus; the salpingopharyngeal fold running down from it; a fold overlying the palatopharyngeal sphincter; the pharyngeal tonsil (adenoid) in the roof.
  • Oropharynx: the palatoglossal arch anteriorly and the palatopharyngeal arch posteriorly, with the palatine tonsil between them; the oropharyngeal isthmus; the lingual tonsils on the tongue base; the valleculae, anterior to the epiglottis.
  • Laryngopharynx: the laryngeal inlet, with a piriform fossa on each side of it, leading into the oesophagus.

A swallowed bolus passes down through the pharynx into the oesophagus by running lateral to the laryngeal inlet, that is, through the piriform fossae.

Three slides carry unlabelled images that the deck does not annotate, so no structures are named from them: two posterior views of the opened pharynx (showing the choanae and conchae above, the opened pharyngeal wall exposing the posterior larynx and piriform recesses, and the great vessels and nerves on each side), and a lateral barium-swallow / videofluoroscopy still of the head and neck showing contrast filling the pharynx and upper oesophagus. A further slide titled "Piriform fossa" shows only a photograph of a blue whale's paired blowholes; the slide does not state how the image relates to the piriform fossa.

Oesophagus

  • Length 25 cm.
  • Wall layers: mucosa, submucosa, muscle, adventitia.
  • Has oesophageal sphincters (upper and lower; see swallowing below).
  • Constrictions, with the distances given on the slide [the slide does not state the reference point for the measurements]:
    • Cricopharyngeus sphincter, 15 cm
    • Aortic arch, 22 cm
    • Left main bronchus, 27 cm
    • Diaphragm, 38 cm

These match the accompanying illustration, in which the oesophagus descends through the thorax, is crossed by the aortic arch and the great vessels, passes the bronchial tree, and reaches the diaphragm.

One slide in this section contains only a truncated bullet reading "Perf" and nothing else; the intended word and its content are not on the slide.

Swallowing

Three stages.

Stage 1 (voluntary)

  1. Food gathered on the dorsum of the tongue.
  2. Tongue pressed to the hard palate.

Stage 2 (rapid and involuntary)

  1. Soft palate: elevation and tension.
  2. Larynx: elevation.
  3. Levator and constrictor muscles act.
  4. Upper oesophageal sphincter relaxes.
  5. Subsequent contraction of that sphincter.

Stage 3 (involuntary)

  1. Peristalsis in the oesophagus.
  2. Lower oesophageal sphincter relaxes.
  3. Subsequent contraction of that sphincter.

The deck also lists three YouTube clips on swallowing as further viewing.

Self-test

  1. List the eight regions of the GI tract given in the lecture and the single function assigned to each.
  2. Describe the boundaries and anterior opening of the oral cavity.
  3. Distinguish the oral vestibule from the oral cavity proper.
  4. Name the two isthmuses through which the oral and nasal cavities communicate with the pharynx, and state which structure separates them.
  5. Explain why the pharynx can be swabbed by either the nasal or the oral route.
  6. Describe the pharynx as a tube: its shape, its two forms of mobility, and the four layers of its wall from inside outwards.
  7. Describe the line of attachment of the pharynx to the skull base, and name the bony landmark that gives attachment to each of tensor veli palatini and levator veli palatini.
  8. List the constrictor muscles of the pharynx and explain how they are arranged relative to each other and in the midline posteriorly.
  9. List the three levator muscles of the pharynx with the origin of each, and state whether they lie inside or outside the constrictors.
  10. Name the three parts of the pharynx from above downwards, and give two named features found in each.
  11. Distinguish the valleculae from the piriform fossae by position.
  12. Describe the route a swallowed bolus takes through the pharynx relative to the laryngeal inlet.
  13. State the length and the four wall layers of the oesophagus, and list its four constrictions in order with the distance given for each.
  14. Describe the events of stage 2 of swallowing in order, and state whether it is voluntary.
  15. Distinguish stage 3 of swallowing from stage 2 in terms of the structures involved and the control of each.
  16. Explain why both oesophageal sphincters are described as relaxing and then contracting rather than simply relaxing.
  17. A fish bone lodges in the recess lateral to the laryngeal inlet. Name the recess and the part of the pharynx it lies in.
  18. An endoscope meets resistance at about 22 cm and again at about 38 cm. Which constriction is responsible at each depth?
  19. A patient has weakness of the muscles that elevate the larynx and tense the soft palate. Which stage of swallowing is impaired, and which listed events of that stage would fail?
  20. Trace a bolus from the dorsum of the tongue to the stomach, naming in order the openings, pharyngeal regions and sphincters it passes, and the muscle groups that move it.

Answers

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