The Oesophagus and its Disorders I Flashcards
Anatomy of the Oesophagus
What is the oesophagus lined by? what does it lie posterior to? when does it begin? when does it join the stomach? key segment relations?
What is it’s function? (2)
what can happen if it fails one of its functions?
What promotes the transport of ingested food into the stomach?
The oesophagus is a fibromuscular tube (25cm in length), that is lined by striated squamous epithelium.
The oesophagus lies posterior to the trachea and begins at the end of the laryngopharynx and joins the stomach a few cm from the diaphragm (at the cardiac orifice). Extends from the level of the lower border of cricoid cartilage (C6) to the cardiac orifice of the stomach (T12)
Its function is to transport food to the stomach (eating gullet), importantly it also secretes mucus which lubricates food and to neutralise any acid that may come up from the gut.
If you do not produce enough mucus it can lead to damage to the epithelial structures of the oesophagus (can lead to Barretts)
What promotes the transport of ingested food into the stomach?
- Relaxation of the sphincters (UOS and LOS)
- This is a highly coordinated muscular process; involves contraction and relaxation of the oesophagus which transports the food through the GIT
Muscle Structure of the Oesophagus
Where is the skeletal muscle? where does it run up to?
Where is the smooth muscle? where does it run from?
What are the 2 sphincters? WHat are they made up of? what are their function?
What is the instrinic component and what is it under the influence of? what is the extrinsic component and its function?
What is gord and why do you get it?
The muscle running on the outside and inside?
what is UoS and what is it between? what kind of structure is it? what three muscles are here?
other structures associated with UoS?
How might killian triangle/zenker’s diverticulum emerge? What is it and what callows it to appear? (also called pharyngeal diverticulum)
what is a raphe?
Pharyngeal raphe?
what is intubation?
Skeletal muscle surrounds the upper third of the oesophagus, surrounding the oesophagus below the pharynx.
Smooth muscle is present in the last portion from the lower oesophageal sphincter, surrounding the lower two thirds.
The oesophagus has two sphincters:
- The upper oesophageal sphincter (UOS) is composed of striated muscle and constricts to avoid air entering into the oesophagus. (Musculo-cartilaginous structure) This is very important in terms of its function. The UOS area is a high-pressure zone.
- The lower oesophageal sphincter (LOS) lays close to the diaphragm, is composed of smooth muscle and acts as a flap valve. The LOS is a high-pressure zone and has intrinsic and extrinsic components.
The intrinsic component is oesophageal muscles under neurohormonal influence (NO, Ach can control the level of constriction)
The extrinsic component is the diaphragm muscle which acts as a pinch-cock in terms of restriction of food from the stomach moving up the oesophagus and damaging the epithelial cells.
Malfunction of the intrinsic and extrinsic components of the LOS lead to GORD, where acidic chyme can move up the oesophagus.
The oesophagus has longitudinal muscle running along the outside and circular in the inside. A lot of it is attached to the cricoid cartilage.
UoS is an area of high pressure zone between pharynx and cervical oesophagus; it’s a musculocartilaginous structure; composed of posterior surface of thyroid cartilage, cricoid cartilage, hyoid bone and three muscles (cricopharyngeus, thyropharyngeus, and cranial cervical oesophagus).
The 3 muscles spread upwards, posteriorly and they insert into oesophageal submucosa after crossing the muscle bundles of the opposite side.
Thyrophrangeus is obliquely oriented and the cricophrayngeus is transversely oriented. Between the 2 muscles, there is a zone of sparse musculature – Killian Triangle can form here, Zenker’s diverticulum might emerge.
Other structures associated with the UOS is
- Hyoid bone
- Cricopharyngeus
- Thyropharyngeus
- Cranial cervical oesophagus muscles
The space muscle near the middle, can sometimes form an out pouch called Zenker’s diverticulum.
A bolus of food that is travelling through the oesophagus will be massaged through by the longitudinal and circular muscle.
The Pharyngeal raphe is a raphe that serves as the origin and insertion for several of the pharyngeal constrictors (thyropharyngeal part of the inferior pharyngeal constrictor muscle, middle pharyngeal constrictor muscle, superior pharyngeal constrictor muscle).
Raphe: a groove, ridge, or seam in an organ or tissue, typically marking the line where two halves fused in the embryo (in particular: the connecting ridge between the two halves of the medulla oblongata or the tegmentum of the midbrain)
Anaesthetics press on the cricoid cartilage when intubating to prevent gastric reflux. This is called Sellick manoeuvre
Intubate: To put a tube in, commonly used to refer to the insertion of a breathing tube into the trachea for mechanical ventilation. For example, as a life-saving measure, an emergency room physician might intubate a patient who is not breathing adequately so that the lungs can be ventilated
Control of function of UOS
2 things
- Has 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.
Components of the lower oesophageal sphincter
what is the intrinsic component?
how does the stomach help prevent regurgitation? what is it responsive to?
what is the extrinsic component? what is it’s main function anf how does it prevent GORD?
What are the antireflux barriers? (3 things)
another function of the extrinsic component in terms to respiration?
WHat relaxes and what contracts to allow bolus of food to go through oesophagus?
As stated earlier, the LOS has intrinsic and extrinsic components
The intrinsic component of LOS is composed of thick oesophageal smooth muscle and have myogenic activity (i.e. have a resting tone) but they are less responsive to Ach.
Oblique and sling fibres of the stomach help to prevent regurgitation. These fibres are very responsive to cholinergic innervation.
The extrinsic component of LOS is formed by the crural diaphragm encircling the LOS. It also forms a channel through which the oesophagus enters the abdomen.
The fibres of the crural portion of the diaphragm possess pinchcock-like action. This blocks any reflux of acidic chyme into the oesophagus which if not treated leads to GORD and this can result in Barrett’s and then cancer.
The antireflux barriers include two sphincters: LOS and the diaphragmatic sphincter, and the unique anatomic configuration at the gastroesophageal junction (e.g., the mucosal folds)
action (extrinsic sphincter; diaphragmatic sphincter)- myogenic tone
The extrinsic component LOS consists of the diaphragm muscle; functions as an adjunctive external sphincter that raises the pressure in the terminal oesophagus related to the movements of respiration.
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.
Innervation of the Oesophagus
what supplies the upper part? via what? interruption?
what supplies the lower part? via what? interruptions? what is released for contraction and relaxation?
what must work together to push food into the stmoach?
what part of vagus nerve innervates the upper and lower part?
what is the group of nerves around oesophagus called?
sympathetic nerve that supplies oesophagus?
phrenic nerve that innervates the diaphragm?
We have cholinergic (via Ach) innervation of the oesophagus and there are also times when Ach plays a part further up but then downstream (in neuronal pathway) is non-cholinergic, these are NANC nerves.
There is innervation to control the tone of the lower oesophageal sphincter
Ach is released at the oesophageal sphincters to contract the intrinsic sphincters, NO and VIP will relax the intrinsic sphincters.
Extrinsic and intrinsic sphincters will work together to push food into the stomach.
Upper oesophageal end has vagal innervation, info gets send to the brain via afferents.
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.
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. (also, Splanchnic nerves (thoracic sympathetic trunks))
The lower part contains smooth muscle that is innervated by visceral motor neurones of vagus nerve with 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)
- These nerves release VIP + NO (relax) and Ach (contraction)
How NO causes relaxation of smooth muscle cells
NO interacts with AC which converts GTP to cGMP; cGMP activates PKG which acts on myosin phosphatase on smooth muscle cells and causes smooth muscle relaxation
Myosin phosphatase is an enzyme which dephosphorylates the regulatory chain of myosin II. The dephosphorylation reaction occurs in smooth muscles which initiates the relaxation of smooth muscles. Thus, myosin phosphatase undoes the muscle contraction process initiated by initiated by myosin light-chain kinase
Swallow reflex
what receptors are stimulated? where do the afferent firbres go? what happens next? where do the efferent fibres go? what do the vagal efferent do? postganglionic transmitters?
Excitation of receptors in pharynx (oesophageal peristalsis and relaxation).
The afferent stimulus travels to the sensory nucleus (nucleus solitarius, smaller upper inset).
A programmed set of events from the dorsal vagal nucleus and the nucleus ambiguus mediates oesophageal peristalsis and sphincter relaxation.
Efferent impulses pass to the pharyngeal musculature and the tongue.
Also the vagal efferent fibres communicate with myenteric neurons that mediate relaxation of LOS (larger lower inset).
Postganglionic transmitters = NO, VIP
Contraction of the crural diaphragm is controlled by
location and molecule
inspiratory centre in the brainstem and the nucleus of the phrenic nerve.
The crural diaphragm is innervated by right and left phrenic nerves through ACh
Functions of the Oesophagus
what is the main function?
How is swallowing tiggered? 3 nerves?
what 3 efferent nerves are sent? to where?
where does integration of impulses happen? (3 locations)
The oesophagus transports food from the pharynx to the stomach, so it helps with swallowing (deglutition).
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.
How is Swallowing Initiated?
what is the voluntary action?
what is the involuntary action?
journey the food goes through (locations)
what does swallowing inhibit? how? and why?
what will move food towards the stomach?
what moves any food remnants?
how do muscles move food down the gi tract? the movement pattern?
What causes swallowing difficulty?
Swallowing is a combination of voluntary and involuntary mechanisms.
Voluntary action is the collection of material on the tongue and pushing it backwards into the pharynx (skeletal muscle, mucus membrane).
There are then waves of involuntary contractions once the food passes the pharynx.
Mouth -> Oropharynx -> Laryngopharynx -> Oesophagus
This is an example of reflex responses (massaging of food down oesophagus once it passes pharynx).
Importantly there is inhibition of respiration (breathing) as the nasopharynx 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.
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.
During swallowing, there is a coordinated opening and closing of the upper and lower oesophageal sphincters
Progressive muscular contractions and relaxations move the food towards the stomach and along the GIT
Swallowing difficulty (oropharyngeal dysphagia) is caused by the inability of the UOS to open or discoordination of the timing between the opening of UOS and the pharyngeal push of ingested bolus
Secondary Peristalsis
what happens to UOS once food has passed? what happens to glottis?
What about LOS?
How does secondary peristalsis come about and why does it come about?
This secondary wave of peristalsis is very important.
Relaxation of the UOS allows food to pass through, the UOS then closes as soon as food passes. The glottis then opens and 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).
What Prevents Reflux of Gastric Contents?
3 ways
There are times when food (acid-laden) may get up into the oesophagus. So, there are mechanisms in place to prevent the gastric reflux of food up the oesophagus.
1) The LOS normally closes after the food has passed.
2) The pinchcock effect of the diaphragm helps block material entering into the oesophagus.(side-to-side compression between “2 pillars” of the crus)
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 VIA:
- > 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.
Overall, we can see there is an anti-reflux barrier in the region of the gastro-oesophageal junction.
summary
Oesophagus; features and its functions
Anti-reflux barriers stop the reflux of gastric contents into the oesophagus
Anti-reflux barriers: LOS, diaphragmatic sphincter, and gastro-oesophageal junction
Both sphincters maintain tonic closure of the sphincter mechanism
Both sphincters relax upon swallowing but can also relax without a swallow, as part of a reflex called transient spontaneous relaxation (TSR) of LOS
LOS: smooth muscles; myogenic (tonic contraction) and neurogenic properties; vagal innervation (NO, VIP)
Diaphragmatic sphincter: striated muscles that exhibit tone and contract due to the excitatory nerves. Swallowing and TSR of LOS relax it
The loss of inhibitory mechanisms of sphincters leads to achalasia (next lecture)
Dysfunctional anti-reflux barriers → GORD or dysphagia.
Increased frequency of TSR of LOS → GORD
