S2: The Oesophagus and Its Disorders I

Question 1

Anatomy of the oesophagus

Answer 1

• Fibromuscular tube about 25 cm in length
• Lined by striated squamous epithelium
• Lies posterior to the trachea
• Begins at the end of the laryngopharynx and joins stomach a few cm from diaphragm
• It extends from the lower border of the circoid cartilage (C6) to the cardiac orifice of the stomach (T12)

Question 2

Function of the oesophagus

Answer 2

• Transport food to the stomach
• Secretes mucus which lubricates food and neutralises any acid that may come up in the gut (not enough mucus produced can damage epithelium in oesophagus leading to barretts)

Question 3

Describe the muscle structure of the oesophagus

Answer 3

• Skeletal muscle surround the upper 1/3 of the oesophagus, surrounding the oesophagus below the pharynx
• Smooth muscle is present in the last portion from the lower oesophageal sphincters surrounding the lower 2/3
• The oesophagus has longitudinal muscle running along the outside and circular in the inside. A lot of it is attached to the cricoid cartilage.

Question 4

What does relaxation of the sphincters (UOS + LOS) do?

Answer 4

The relaxation of the sphincters (UOS and LOS) promotes the transport of ingested food into the stomach. This is a highly coordinated muscular process which involves contraction and relaxation of the oesophagus which transports food through the GIT.

Question 5

Describe structure of the two oesophageal sphincters

Answer 5

• The Upper Oesophageal Sphincter (UOS) is composed of striated muscle and constricts to avoid air from entering oesophagus
• The Lower Oesophageal Sphincter (LOS) lies close to the diaphragm and is composed of smooth muscle and acts as an flap valve. The LOS is a high pressure zone and has intrinsic and extrinsic components.

Question 6

What can malfunction of intrinsic and extrinsic component of LOS lead to?

Answer 6

Malfunction of the intrinsic and extrinsic components of the LOS lead to GORD, where acidic chyme can move up the oesophagus.

Question 7

List structures associated with UOS

Answer 7

• Hyoid bone
• Cricopharyngeus
• Thyropharyngeus
• Cranial cervical oesophagus muscles

Question 8

Why is thyropharyngeus and cricopharyngeus weak areas?

Answer 8

The thyropharyngeus and cricopharyngeus muscles have areas of weak spot due to the area’s sparse muscle layer.

The sparse muscle near the middle, can sometimes form an outpouch called Zenker’s diverticulum.

Question 9

Function of longitudinal and circular muscle in oesophagus

Answer 9

A bolus of food that is travelling through the oesophagus will be massaged through by the longitudinal and circular muscle.

If you have the bolus food in area (of oesophagus), then the circular muscle will relax and longitudinal will contract. This allows food to pass into stomach.

Question 10

What is the neuronal control of the upper oesophageal sphincter (UOS)?

Answer 10

• Afferent inputs to motor neurones and also capable of mediating contraction/relaxation
• Vagal nerves will elicit contractile and relaxation responses so that bolus of food does indeed pass down the oesophagus to the stomach.
• Contractile properties are mediated by Ach and Ach right at the end. The NANC pathways do have Ach at the ganglion but downstream NO to cause gastric relaxation.

Question 11

Describe the intrinsic component of LOS (structure, neuronal innervation, clinical relevance)

Answer 11

• Composed of thick oesophageal smooth muscle (circular on right side)
• Oblique and sling fibres of stomach of left lateral side to help prevent regurgitation. These fibres are very responsive to cholinergic innervation
• Has myogenic activity -resting tone
• Under neurohormonal influence (NO, Ach - less responsive to Ach)

The oblique angle/Angle of His is underdeveloped in infants, oesophagus makes a vertical junction with the stomach so reflux is common in infants.

Question 12

Describe the extrinsic component of the LOS (structure, neuronal innervation, clinical relevance)

Answer 12

• Formed by the crural diaphragm
• Forms a channel which oesophagus enters abdomen
• Fibres of crural portion of the diaphragm possess a pinchcock like action. This blocks any reflux of acidic chyme from stomach into the oesophagus which if not treated leads to GORD and this can result in Barretts and then cancer.

Question 13

Describe innervation of the oesophagus

Answer 13

We have cholinergic (via Ach) innervation of the oesophagus and there are also times when Ach plays a part further up but then down stream (in neuronal pathway) is non-cholinergic, these are NANC nerves.

The vagus nerve and splanchnic nerves (thoracic sympathetic trunk) innervate the oesophagus.

• The upper part of the oesophagus contains striated muscle that is supplied by somatic motor neurones of the vagus nerve without interruptions. These nerves seem to come from the nucleus ambiguus.
• The lower part contains smooth muscle that is innervated by visceral motor neurones of vagus nerve without interruptions (synapse with postganglionic neurones cell bodies in oesophagus and splanchnic plexus). These have fibres coming from DVN and NTS.

The oesophagus is also encircled by nerves of the oesophageal plexus.
Phrenic nerve innervates the diaphragm.
Parasympathetic and sympathetic innervation of oesophagus (via vagal nerves and spinal nerves)

Question 14

What is released to contract and relax intrinsic oesophageal sphincter?

Answer 14

Ach is released at the oesophageal sphincters to contract the intrinsic sphincters, NO and VIP will relax the intrinsic sphincters.

Question 15

What 3 parts of the brain integrate impulses from oesophagus?

Answer 15

Upper: Nucleus Ambiguus (NA)

Lower: DVN (Dorso vagal nucleus) and NTS (nucleus tractus solitarius)

Question 16

What does deglutition mean?

Answer 16

Swallowing

Question 17

Neural control of swallowing

Answer 17

Swallowing is triggered by afferent impulses in the trigeminal, glossopharyngeal and vagus nerves.
The efferent fibres then go back and pass to the pharyngeal musculature and the tongue (these efferent neurones going back in trigeminal, facial and hypoglossal nerves).

There is integration of impulses in the nucleus of tractus solitaries (NTS) and the nuclear ambiguus.
Swallowing is coordinated with opening and closing of the upper and lower oesophageal sphincters.

Question 18

Mechanism of swallowing (deglutition)

Answer 18

Swallowing is a combination of voluntary and involuntary mechanisms

Voluntary:
- Collection of material on the tongue and pushing it backwards into the pharynx (skeletal muscle, mucus membrane)

Involuntary
- Contractions pushing food down once it has passed the pharynx
- This is an example of reflex response
- The ring of peristaltic waves will move food towards the stomach (4cm/sec).
There is a second wave of peristalsis that moves any food remnants in the oesophagus along.

Importantly there is inhibition of respiration (breathing) as the nasopharnyx gets closed off and there is closure of the glottis (around vocal cords) by the epiglottis folding over it, this prevents food entering the trachea.

Question 19

Describe secondary peristalsis during swallowing

Answer 19

Relaxation of UOS allows food to pass through and then UOS closes as soon as food passes. The glottis opens and then breathing resumes

The LOS opens at the same time the UOS does and stays open throughout swallowing and then closes after the material has passed.

A large amount of food material doesn’t reach the stomach after the first peristaltic wave. Hence stimulation of receptors upon distension of the oesophageal lumen by food will cause repeated waves of peristalsis (secondary peristalsis).

Question 20

What prevents reflux of gastric contents (anti-reflux barriers)?

Answer 20

1. The LOS normally closes after the food has passed. Abdominal pressure acting on the intra-abdominal parts of the oesophagus also help prevent reflux, the valve-like effect of the oblique angle (sling fibres) between oesophagus and smooth muscle also help.
   
   2. The pinchcock effect of the diaphgram helps block material entering into the oesophagus.
   3. There is also a plug-like action of the mucosal folds (these are present around the fundal area), these can help block food passing up.
      Overall we can see there is an anti-reflux barrier in the region of the gastro-oesophageal junction.

Question 21

What is transient spontaneous depolarisation (TSD)?

Answer 21

A reflex in which both sphincters can relax without swallowing

An increase in TSD is a risk for GORD