This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Principles of Gastrointestinal Function, Subject name: Physiology
Pacemaker is formed by w hich of the follow ing cells in GIT - 
Select and explain. 
A. P-ceils, B. Oxyntic cells, C. Cajal cells, D. Parietal cells
C: Cajal cells
Answer: C: Cajal cells 
Explanation: Ans. is 'c' i.e., Cajal cells o The smooth muscle cells of gastrointestinal tract has spontaneous rhythmic fluctuations in membrane potential between about -65 and -45 mV.o This is called basic electrical rhythm (BER).o This BER is initiated by pacemaker cells called intersititial cells of CajaLo During these cyclic fluctuations, if the resting membrane potential crosses the threshold for excitation, an action potential or a burst of action potential may develop.o Thus, BER itself does not cause muscle contraction, but when spike potentials (action potentials) superimposed on the most depolarizing portions of BER. muscle contraction occurs,o Pacemaker cells (interstitial cells of Cajal) are stellate mesenchymal cells with smooth muscle like features,o Pacemaker cells are absent in the esophagus and proximal stomach.o In the distal stomach and small intestine they are located in outer circular muscle layer near myentric plexus,o In the colon they are at the submucosal border of circular muscle layer.o In stomach and small intestine, pacemaker frequency is in descending gradient and the pacemaker with highest frequency usually dominates (like heart ).