1. Intro and physiology Flashcards
Vomiting function
The main function of vomiting is to protect the animal from ingested toxins.
Common vomiting vs regurgitation
Vomiting is a very common clinical sign in small animal practice.
Dogs in particular have a propensity to eating disgusting things
and have a very well-developed vomiting reflex.
In contrast, regurgitation is a much less common clinical
sign.
Consequences
The causes and consequences
of vomiting can range from clinically inconsequential to life
threatening. In contrast, regurgitation is a much less common clinical
sign. Almost invariably, the patient who is truly regurgitating will
have serious disease.
Physiology of vomiting (4) vs regurgitation (4)
Vomiting or emesis is the forceful expulsion of gastric contents
through the oesophagus, mouth and, sometimes, nostrils. It is a neurologically
complex process resulting from the synchronised activity
of a number of abdominal, pharyngeal and thoracic structures. The
act of vomiting is coordinated in the medulla oblongata and cannot
occur without a functional vomiting centre..
In contrast, regurgitation
is a passive process, which involves the retrograde movement of food
and fluid from the oesophagus, pharynx and oral cavity without the
initiation of reflex neural pathways other than the gag reflex.
Essential neurological components of emetic reflex (4)
• Visceral receptors
• Vagal and sympathetic afferent neurones
• Chemoreceptor trigger zone (CRTZ)
• Vomiting centre within the reticular formation of the medulla
oblongata.
Stages in vominiting:
- Nausea (3 things happen + 4 signs)
- Retching (define)
- Vomiting (process, signs)
The first stage of vomiting is nausea. In this stage, gastric tone is reduced, duodenal and proximal jejunal tone is increased and duodenal contents reflux into the stomach. The patient often appears
depressed, hypersalivates and as a result may exhibit repeated swallowing and/or lip licking behaviour.
The nauseous stage is followed by retching (an unproductive effort to vomit, also known as ‘dry heaves’) and then vomiting. When the
animal vomits, the epiglottis is closed and the soft palate presses up against the nasopharynx. The abdominal muscles and the diaphragm contract. The contraction of the abdominal muscles is usually visible to an observant owner. The cardia then opens, the pyloric stomach contracts and vomiting occurs. Reverse peristalsis, cardiac rhythm
disturbances and increased colonic motility also occur during the vomiting process.
The closure of the epiglottis and pressing of the soft palate up against the nasopharynx protect against aspiration of gastric contents.
In contrast, during regurgitation, which is a passive process without neurological coordination, these actions do not occur. As a result, aspiration pneumonia is a common sequela to disorders which cause regurgitation and less likely in vomiting unless the animal is obtund.
Initiation of and the process of vomiting
- Humoral pathway (2)
- Neural pathway (1)
Vomiting is essentially initiated by either humoral or neural pathways.
The humoral pathway involves stimulation of the CRTZ within the medulla oblongata by blood-borne substances; the neural pathway
is through activation of the vomiting centre.
Vomiting centre input (5)
All animal species that vomit have a brainstem ‘vomiting centre’,
which is a group of several nuclei that act in concert to coordinate the somatomotor events involved in expelling gastric contents.
The vomiting centre receives input from vagal and sympathetic neurones, the CRTZ, the vestibular apparatus and the cerebral cortex.
It may also be stimulated directly by blood-borne toxins that can cross the blood brain barrier (BBB).
Central stimulation: via, 9 causes
Stimuli include nervousness,
unpleasant odours, pain and psychogenic factors. Opioids and benzodiazepine receptors have been implicated in centrally initiated vomiting,
but their exact mode of action has not been well characterised
pharmacologically. Centrally induced vomiting may also occur due to direct stimulation of the emetic centre by elevated cerebrospinal fluid
pressure, encephalitis or CNS neoplasia.
Vestibular apparatus (3 causes)
Labyrinthine dysfunction associated with motion sickness and middle/ inner ear infection also has input into the vomiting centre via neural pathways arising from the vestibular system. The CRTZ is involved in this pathway in the dog but not the cat.
Chemorreceptor trigger zone (explication,3 causes + ex)
The CRTZ is located in the area postrema in the floor of the 4th ventricle.
It has no BBB, therefore allowing toxins and chemicals that would normally be excluded from the CNS access to the brain. The CRTZ is stimulated by endogenous toxic substances produced in acute infectious diseases or metabolic disorders such as uraemia or diabetic ketoacidosis, as well as by drugs and exogenous toxins.
Peripheral receptors (4 locations, 4 stimulus)
Peripheral receptors are located mainly in the gastrointestinal (GI) tract, particularly the duodenum, but also in the biliary tract, peritoneum and urinary organs. The receptors may be stimulated by distension, irritation, inflammation or changes in osmolarity. There are a few receptors in the lower bowel, which explains why patients with inflammatory lower bowel disease may occasionally vomit.
