VPB 121: Veterinary Physiology - II (2+1)

EMETIC REFLEX

Emesis is the regurgitation of materials from stomach and upper intestine through oesophagus to oral cavity.

• This reflex is initiated by irritating stimuli from vicera.(pharynx, stomach, duodenum, heart, uterus) or from external auditory canal.
• Protective mechanism to remove the irritating stimuli.
• Motion sickness results due to irritation of the vestibular receptor organs. This particular event involves the  participation of flocculonodular lobe of cerebellum.
• This reflex is preceded by salivation & swallowing.
• The nerves of afferent limbs are glossopharyngeal, vagus or vestibular and visceral afferent of the spinal cord (for abdominal/pelvic visceral).
• Emetic center is located in the caudal medulla.
• Cells present in chemo receptor triggerzone of medulla can also initiate emesis on stimulation by specific chemical (apomorphine, ergot alkolids morphine, glycosides or metobolites of systemic acidosis, alkolosis, uremia)
• The central neural mechanism of efferent limb includes nerves supplying to
  • Smooth muscles of G.I tract.
  • Skeletal muscle of larynx, pharynx, thorax, abdominal wall.
• Effernt nerve of emeticreflex causes closure of glottis with inhibition of inspiration; These together produce negative pressure in the intrathoracic and oesophagus.
• Relaxation of cardia, fundus, body of the stomach and oesophagogastric cardiac sphincter prior to act of vomiting.
• Followed by forceful contraction of the abdominal muscle.
• This result in movement of  content of stomach to oesophagus.
• Closure of nasopharynx prevents the entry of contents to respiratory organs.
• Powerful antiperistalsis with negative I/oesophageal pressure expels the content to oral cavity.
• Creation of negative pressure by forceful inspiration against closed glottis and strong abdominal muscle contraction is important than antiperistalsis.
• Relaxation of pyloris allow contents from duodenum to oesophagus and then to oral cavity.
• Efferent fibers also reaches salivatory nucleus to initiate salivation before emesis to prevent oral mucosa from acid contents.
• Fibers project to respiratory and cardiac centre  to modify appropriately the B.P and the rate of respiration.