medical physiology - Physiology of the digestive system. The pacemakers of the G.I.T

1. Group 9: Joseph Njihia.
Samson Ekisa
Assignment: Interstitial Cells of Cajal
INTERSTITIAL CELLS OF CAJAL
These are stellate mesenchymal cells with smooth muscle like projections found in the
walls of the gastro intestinal tract, and act as pacemakers to initiate contractions of the smooth
muscle layers of the G.I.T. their smooth muscle like features send long branched processes into
the intestinal smooth muscles, forming synaptic like contacts with numerous gap junctions.
Interspersed within the smooth muscle layers of the gastro intestinal tract, they also form a
network with each other.
The cells have distinct locations depending on the section of the G.I tract. In the stomach
and small intestines, they are located in the outer circular muscle layer near the Myenteric plexus
while in the colon, they are located at the outer sub mucosal border of the circular muscle layer.
These cells undergo cyclic changes in their membrane potentials due to the tonic periodic
opening of ionic channels of Na+, K+ and Ca2+, producing inward currents that generate slow
waves. This is the explanation of the spontaneous rhythmic fluctuations in membrane potentials
of between -65mv and -45mv in the GI tract smooth muscle cells with the exemption of the
stomach and the esophagus, a concept known as the Basic Electrical Rhythm (B.E.R). These
slow waves lead to occurrence of intermittent spike potentials which in turn lead to muscle
contraction. During the spike potentials, for depolarization to occur, the calcium and Na+
channels open and Ca2+ and Na+ flows into the cells, Ca2+ in numerous amounts, and Na+ in
smaller amounts. The sodium and calcium ion channels are thus referred to as calcium-sodium
channel. This channel however opens and closes slowly thus the long durations of contractions.
During repolarization, K+ flows out of the cell in great amounts. This, the spike potential occurs
at the peak of the slow wave to cause contractions. Why the contractions cannot occur during the
slow wave is because the slow waves do not allow for the entry of the calcium ions into the
muscle cells but spike potentials do.
Other factors are involved in stimulating the depolarization and repolarization of the
muscle cells such as stretching of the muscle cells, stimulation by acetyl choline, stimulation by
the parasympathetic nervous system and the stimulation by other gastrointestinal hormones, all
which stimulate depolarization. Repolarization is mainly stimulated by the effect of adrenaline
and norepinephrine and stimulation of the sympathetic nervous system which secrete
norepinephrine at the ends of its nerve fibers.

2. However, the frequency by which the interstitial cells of cajal excite the smooth muscle
cells vary in different regions of the gastrointestinal tract. In the stomach, they cause excitations
of a frequency of 4/min, 12/min in the duodenum, and 8/min in the ileum and 3-4/min in the
colon. At the cecum, the rate is about 2/min and rises to about 6/min in the sigmoid. To note,
these frequencies are recorded as the basic electrical rhythm rate and not the spike potential rate.
It is because the interstitial cells only initiate the B.E.R which will now later, at its peak, initiate
the spike potentials and eventually contractions.
Some writers and scholars have been able to classify the interstitial cells of cajal in two
classes; The Intermascular Interstitial cells and The Myenteric cells. The intermascular cells are
located in the colon-at the sub mucosal border of the smooth muscle layer, while the Myenteric
interstitial cells are the ones located stomach and small intestines- on the outer circular muscle
layer near the Myenteric plexus.
In summary, these cells are located in the walls of the gastrointestinal tract and acts as the
pacemakers in the same. They act through calcium-sodium channels to initiate the slow-wave
which initiates the spike potentials which in turn cause contractions of the muscles. Their activity
is enhanced by the action of the parasympathetic nervous activity and slowed by the sympathetic
nervous activity.