Physiology of Nausea and Vomiting Flashcards
What do the following mean
- Nausea
- Retching
- Vomiting
- Nausea - A feeling of sickness with an inclination to vomit
- Retching - Making the sound and movement of vomiting (without expulsion)
- Vomiting - To eject matter from the stomach through the mouth
Describe the cortex, hypothalamus and brainstem.
- Cortex - composed of surface of brain and involved in thoughts, memories and emotions
- Hypothalamus - autonomic processes e.g BP, body temperature regulation
- Brainstem - divided into midbrain, pons and medulla
Describe the blood-brain barrier.
- Blood vessels wrapped by glial cells - form barrier, preventing substances in vasculature entering brain parenchyma
- EXCEPTIONS - Circumventricular organs - where this barrier doesn’t exist e.g area postrema and CTZ - neurons in these areas can respond to substances in circulation/CSF
Describe the vagus nerve.
- 10th cranial nerve
- Exits from brainstem and travels down thorax and viscera
- Has both efferent and afferent components
What does afferent and efferent information refer to, in the context of the GI tract?
- AFFERENT - information such as stomach and intestinal distension, gut hormones and emetic mediators
- EFFERENT - motor commands from CNS to GI tract - influences gastric motility, acid secretion and smooth muscle contraction
Where is nausea and vomiting controlled?
Brainstem
What are the 3 key brain regions responsible for triggering nausea and vomiting?
- Area postrema – this is an area which lacks a blood brain barrier and can therefore respond to circulating substances in the CSF
- Nucleus tractus solitarius – this region receives input from the AP as well as visceral inputs from the gastrointestinal system
- Dorsal motor nucleus of the vagus – this region contains motor neurons which travel to the stomach via the vagus nerve
The neurons in the central vomiting centre contain receptors for a number of neuroactive agents. Give examples of these receptors.
- 5HT3 (Serotonin)
- Neurokinin 1 Receptors (NK1)
- Dopamine 2 and 3 receptors (D2/3)
- Histamine 1 receptors (H1R)
When the receptors in the central vomiting centre are activated, what do the neurons do?
Send projections to the NTS which synapse onto muscarinic type 1 receptors
Describe stimulation of nausea. PART 1
- Enterochromaffin cells secrete serotonin - enters circulation as well as activating vagal afferents
- Enteric neurons regulate gastric motility - also send afferents via vagal and spinal neurons back to CNS
- Enteric neurons release neurotransmitters e.g ACh, dopamine and substance P
Describe stimulation of nausea. PART 2
- Emetic substances cause serotonin release from enterochromaffin cells or stimulate neurotransmitter release from enteric neurons
- When released in high enough quantities, neurotransmitters can stimulate nausea
How do certain viruses e.g rotovirus cause nausea?
- Stimulate insertion of calcium channels into enterochromaffin cells.
- Entry of calcium stimulates the exocytosis of serotonin-containing vesicles from their basolateral membrane
How does 5-HT , substance P and dopamine activate their receptors?
- Travel in circulation and pass into the area postrema/chemoreceptor trigger zone and activate their respective receptors
- 5HT has also been shown to activate 5HT3 receptors on vagal afferent neurons, which synapse directly in the NTS
Describe motion sickness.
- Mismatch of information being received from the visual and vestibular system
- Reading whilst in moving vehicle (vestibular system detects motion, visual system not)
What is the mechanism of vomiting?
- Cardiac sphincter relaxes
- Diaphragm and abdominal muscles contract
- Increase in intragastric pressure and the movement of gastric contents down their pressure gradient, into the oesophagus and out the mouth
