36) Symptoms of GI disease: nausea, vomiting and pain

Question 1

Why is the role of nausea, vomiting and pain?

Answer 1

• It is the GI tract’s alarm system that recognises something has gone wrong and needs to be dealt with

Question 2

How are nausea, vomiting and pain related?

Answer 2

• Nausea and pain are served by separate systems and neural pathways
• Nausea and vomiting are part of a single system which have the same triggers while being served by the same neural pathway.
• They set up a continuum of mild queasiness (feeling nauseous) on one end and forceful expulsion of the contents of the upper GI at the other.

Question 3

What is nausea?

Answer 3

• Nausea is a sensation which is generated within the brain of the person feeling ill
• It is triggered by emetic stimuli
• It is personal and so cannot be detected or measured
• It is normally associated with physiological change (which can be measured)
• It is very unpleasant and bad episodes of sickness can trigger aversions

Question 4

What are aversions?

Answer 4

• A strong dislike to do something (e.g. eat a particular type of food)
• It can arise from being taught about things or from past experiences

Question 5

What is vomiting?

Answer 5

• Vomiting (emesis) is the physical act of expelling contents of the upper GI tract via the mouth
• It is stimulated by emetic stimuli
• It is forceful that is caused by a complex set of co-ordinated reflexive events that are coordinated by the brainstem
• It is associate with the sensation of relief.

Question 6

What is the relationship between nausea and vomiting?

Answer 6

• Vomiting is closely linked to nausea as they have the same triggers and achieve the same purpose
• Nausea happens first (it is a prodrome) and warns the person that vomiting is about to occur
• Nausea may clear up if the stimulus clears up before it turns into vomiting
• As the stimulus of nausea gets more forceful/ lasts longer it can cause vomiting to occur
• Vomiting can occur without prior nausea (e.g. sticking a finger down your throat)

Question 7

What are different types of emetic stimuli?

Answer 7

• Poisoning (e.g. contaminated food, poisonous plants, chemical agents)
• Excessive alcohol
• GI infections (e.g. norovirus)
• GI disease (e.g. gastritis)
• Obstruction
• Excessive eating
• Travel sickness
• When other people are sick
• Emotional upset
• Metabolic disturbance
• Pregnancy
• IV drugs (e.g. morphine)
• Raised intracranial pressure

Question 8

How do our mouth and nose protect the body against ingested toxins?

Answer 8

• Through taste and smell which can prevent potential ingestion
• We have a built in dislike of bitter flavours
• The tongue have bitter receptors which sends warning signals to the brain
• We also learn from elders what bitter substances are safe for ingestion

Question 9

How do abdominal afferents protect the body against ingested toxins?

Answer 9

• There are sensory afferents within the stomach and upper part of the small intestines which carry information to the brainstem (via visceral sensory nerves) and can evoke a vomiting reflex
• These afferents are driven by chemoreceptive cells in the walls of the gut which respond to naturally occurring toxins, damaging chemicals and inflammatory mediators
• However since the chemoreceptive cells are found in the cell wall (surrounded by rich blood supply and extracellular fluid), substances that are circulating in the blood can trigger responses in the chemoreceptive cells to produce vomiting reflexes
• This is why some non-ingested toxins (e.g. chemotherapy agents) can cause vomiting
  There are visceral sensory nerves that run into the brainstem.

Question 10

How does the area postrema protect against ingested toxins?

Answer 10

• The chemoreceptor trigger zone is designed to detect toxins in the blood stream
• They are found in the area postrema in the brainstem below the NTS
• It is the only part of the brain where there is no blood brain barrier as the capillary walls are leaky
• This means toxins and other substances can freely get into the extracellular fluid of the area postrema
• Blood can also enter the foramen magnum (hole at the base of the brain where the spinal chord exists) during increased cranial tension
• In doing so they encounter chemoreceptors which can respond to the toxins and cause nausea and vomiting
• However since it is detecting toxins in the blood non ingested toxins (e.g. chemotherapy) will also have the same effect

Question 11

How does the vestibular system protect the body from ingested toxins?

Answer 11

• It is the organ of balance which is a potent trigger for emesis
• It is located in the inner ear and sends its axons out through the cranial nerve and into the NTS in the brainstem
• Poisoning produces aberrant activity in the vestibular neural pathways
• It is the recognition of this aberrant activity that causes nausea and vomiting
• In doing so it expels any poison that was not detected by any of the chemoreceptor cells (and so is thought to be the last line of defence)
• The vestibular system is also used to explain motion sickness as usual means of transport (such as cars, boats, etc) may produce aberrant activity in the vestibular neural pathways

Question 12

How is ingestion of toxins prevented?

Answer 12

• Higher centres within the brain can log information and send them to the NTS when needed which causes us to feel nauseous.
• Through learning and aversion we avoid harmful substances that can contain toxins
• Nausea creates unpleasantness which teaches us to avoid things that causes nausea and so leads to aversions
• However some aversions may lead to complete avoidance (i.e. eating a bad piece of chicken that makes a person really ill may hinder their ability to face chicken in the future)
• The brain can also create incorrect associations

Question 13

What is the nucleus tractus solitarius (NTS)?

Answer 13

• It is the vomiting centre that is located in the medulla of the brainstem
• It receives four different types of warning inputs from different sources
• In response to the inputs the NTS sends out signals to different places to produce nausea and vomiting

Question 14

What are the different inputs of the NTS?

Answer 14

• Vestibular system
• Higher centres
• Abdominal afferents
• Area postrema

Question 15

Why do some actions/food make us sick?

Answer 15

• Limbic areas of the brain have logged our responses to past actions/foods and those that have made us sick.
• These limbic areas (higher centres) send impulses down to the NTS which makes us nauseous

Question 16

Why do we feel sick when we see others feel sick?

Answer 16

• When seeing others feel sick higher brain centres log that if they have ingested poison then there is a chance we may have ingested the same poison.
• These signals are sent to the NTS causing us to also feel nauseous

Question 17

Why do we feel sick when we see something we are told not to eat?

Answer 17

• We learn from elders what is acceptable to eat and what isn’t.
• Higher brain centres log this information and send impulses to the NTS which makes use nauseous

Question 18

How does the NTS initiate nausea?

Answer 18

• The NTS first sends inputs to different regions. This includes the higher centres of the brain, the autonomic efferents and the hypothalamus

Question 19

What happens when the NTS sends impulses to higher centres of the brain?

Answer 19

• The NTS first sends inputs to higher centres of the brain (e.g. the cerebral cortex and limbic areas) which produces the sensation of nausea
• The limbic areas also learn and generate avoidance responses

Question 20

What happens when the NTS sends impulses to the hypothalamus?

Answer 20

• The NTS sends signals to the hypothalamus causing increased secretion of ADH.
• By doing this the body saves water (by preventing urine production) to prepare for potential vomit and fluid loss

Question 21

What happens when the NTS stimulates autonomic efferents?

Answer 21

• The NTS activates many autonomic afferents which increase salivation
• Salivation creates a protective lining for the mucosa against the acidic stomach contents being thrown up
• The activation of autonomic efferents also causes vasoconstriction in the GI tract
• This reduces the absorption of poison (from the vomit) inside the GI tract
• Finally the activation of autonomic efferents sends impulses down vagus nerves which changes gut motility

Question 22

What changes in gut motility do we experience during nausea?

Answer 22

• Reduced mixing and peristalsis to prevent the toxins being released and absorbed
• Proximal stomach relaxes and becomes stretchy as it prepares the stomach to receive additional contents from the upper part of the small intestines
• There are giant retrograde contractions (which are peristaltic like contractions) which sweep up from the middle of the small intestines to return contents of the upper intestine to the stomach

Question 23

What is retching?

Answer 23

• Retching (also called dry heaving) is the co-ordinated contractions of abdominal muscles and the diaphragm repeatedly
• This creates waves of high pressure that slam against the stomach
• The stomach compresses however the anti-reflux barrier holds the contents in the stomach and prevents expulsion

Question 24

What happens that causes us to vomit after retching?

Answer 24

• The oesophageal sphincters and crural diaphragm relax and further waves of contraction expel stomach contents

Question 25

How are pain signals carried?

Answer 25

• There are pathways throughout the GI tract that are associated with afferents that can detect damaging stimuli
• They converge into two types of nerves (the greater splenic and the lesser splenic nerves) which join the spinal chord at the thoracic regions

Question 26

How is pain detected?

Answer 26

• There are primary afferents which act as receptors. The ending of these receptors in peripheral structures have sensitivity
• They respond to “noxious” stimuli which are stimuli that inflict damage to the tissue
• The afferent nerves respond by firing action potentials (being depolarised), to the brain, which increases as stimuli increases
• Since they respond to “noxious” stimuli they are called “nociceptors”
• The noxious stimuli they respond to are abnormal distension, inflammation (and their mediators) and muscle spam

Question 27

What is the positive feedback system of afferent nerves to inflammation?

Answer 27

• Along with detecting inflammation (leading to depolarisation and action potentials being fired) the receptors can also release inflammatory mediators
• These mediators exacerbate the inflammation leading to more depolarisation and more action potentials being fired by the afferent nerves
• Furthermore greater inflammation also leads to more proinflammatory mediators being released
• This helps to sustain inflammation as healing takes place
• However it can go wrong as it can sustain the inflammation even after the bacterial infection is destroyed

Question 28

What is somatic pain?

Answer 28

• Somatic pain is very precisely localised

Question 29

What is the mechanism for experiencing somatic pain?

Answer 29

• When damage is inflicted (e.g. a person is poked in the chest with a needle) it activates somatic nociceptors which project to the spinal chord and enter at the thoracic region
• They will then carry the signal up the spinal cord to the somatosensory cortex in the brain
• In the somatosensory cortex there is a map of the body (called the mass sensory homunculus)
• The signal is carried to a precise spot that corresponds to that exact spot of the body region on the map (mass sensory homunculus)

Question 30

How is the mass sensory homunculus created?

Answer 30

• The afferents from different parts of the body terminate in a very organised manner

Question 31

What is viscero-somatic convergence?

Answer 31

• Some nociceptors that are deep do not have a private pathway up to the mass sensory homunculus map
• Its nociceptors travel to the spinal column at the thoracic region (as usual) however this time they synapse with the same pathway of the region located above it (on the surface of the body)
• This results in a pain sensation located directly above the region of damage
• However often instead of synapsing with one pathway they synapse with multiple pathways causing pain to occur on multiple areas of the body
• It is the convergence of the visceral pathway with the somatic pathway that we call the viscero-somatic convergence

Question 32

How does the body receive sensory innervation?

Answer 32

• Each somatic and visceral part of the body receives its innervation from a particular part of the spinal chord
• The location of visceral innervation depends on their embryonic development

Question 33

What problems can arise with referred pain?

Answer 33

• Inflammation of one region can spread to other regions and so can cause the pattern of pain to change and evolve

Question 34

What is visceral pain?

Answer 34

• Poorly localised pain that is diffused due to the small number of afferents and imprecise wiring
• Each organ has a characteristic pattern on referral which matches their embryonic origin however the pain can evolve as other tissues are affected
• It is generally referred to regions of the body wall due to viscero-somatic convergence