Canonical Question
GIT Pharm – Gastric fluid
Master answer
Physiology of Vomiting
Vomiting
- Involuntary, forceful and rapid expulsion of gastric contents through the mouth
Triggers
- Excessive GI tract distension
- Stimulation of CTZ
- Directly by certain drugs eg apomorphine, morphin
- Rhythmic motion of the body stimulating the vestibular labyrinth of the inner ear
- Cerebral excitation secondary to odours, pain, stress
Central control of vomiting
- Vomiting center (5HT3, NK1, Muscarinic and Histamine), near NTS
- Inputs
- Chemoreceptor trigger zone (NK1, 5HT3, Dopamine), area postrema outside BBB
- Vestibular system (Histamine and Muscarinic) via CNVIII
- GI Tract (5HT3, stretch and chemoreceptors) via vagal
- Higher centres
- Efferent arc
- via CN V,VII,IX,X,XII and spinal nerves to abdominal wall musles and diaphragm
- Inputs
| Afferent signal | Sensor | Efferent signal | Effect |
|---|---|---|---|
| Chemo/ baroreceptor input Drugs Stim from NTS Stim from GIT (5HT3) | CTZ (area postrema) | 5HT3, D2 and Opioid neurons | Direct stimulation of vomit centre in lateral reticular formation |
| Surgery/ Rhythmic motions | Labyrinths | ACh and H1 neurons | |
| Memory/ Emotions Sensory (sight, smell, taste) | Cortex | ||
| Severe painful stimuli | Pain | H1 neurones | |
| Irritation Manipulation during surgery Distension | GIT | NK1, NAdr, ACh neurons | |
| Via CNX, to NTS | Stimulation of CTZ then Vomit centre | ||
| Via 5HT3 to CTZ | CTZ to vomit centre | ||
| Manipulation of pharynx via CN9 to.. | NTS | Direct to CTZ | To vomit centre |
Vomiting act
- Antiperistalsis as the prelude to vomiting
- At the onset of vomiting, strong intrinsic contractions occur in both the duodenum and the stomach
- Partial relaxation of the lower oesophageal sphincter (LOS)
- Deep inspiration
- Raising of the hyoid bone and larynx to open the upper oesophageal sphincter
- Glottic closure
- Lifting of the soft palate to close the posterior nares
- Strong down ward contraction of diaphragm and simultaneous contraction of all the abdominal wall muscles
- Complete relaxation of the LOS
Gladwin / Sakurai 2016
Pharmacology of Ondansetron
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2014A Q03 | Describe the neural integration of vomiting. (60% of marks) Describe the pharmacology of ondansetron. (40% of marks) | historical_member | — |