Table 1.

Localization and proposed functional contributions of cellular components of the SIP syncytium in GI muscles

Type of Cell and Anatomical Localization	Common Name(s)	Proposed Function(s)	Representative References
Myenteric ICC (in region between circular and longitudinal muscle layers)	ICC-MY; ICC-MP	1) Pacemaker cells that generate electrical slow waves; 2) form networks that allow active (regenerative) propagation of slow waves; 3) voltage-dependent excitability due to expression of voltage-dependent Ca2+ channels; 4) role in setting resting membrane potential and basal excitability of SIP syncytium	7, 19, 44, 83, 99, 107, 113, 123
Intramuscular ICC (within bundles of smooth muscle cells and lying in close proximity to nerve varicosities)	ICC-IM; ICC-DMP in region of the deep muscular plexus of the small intestine	1) Transduction of neurotransmitter signals from enteric motor neurons; 2) mechano-sensitivity; 3) lack of voltage-dependent excitability due to generally low expression of voltage-dependent Ca2+ currents; 4) contribution to active propagation in gastric muscles; 5) mediation of inflammatory input via protease-activated receptors	8, 10, 14, 18, 33, 76, 104, 106, 112, 114
ICC within septa between bundles of muscle	ICC-SEP	1) Active propagation of slow waves into the depth of thicker muscles (as in human GI tract); 2) interactions with enteric neurons	42, 74, 115
ICC along the submucosal surface of the circular muscle layer in colon and to small extent in stomach	ICC-SM	1) Pacemaker activity in the colon	11, 100
ICC at the serosal surface of the longitudinal muscle layer	ICC-SS	Unknown	2, 13
Fibroblast-like cells found in all regions of the GI tract; generally common localization with ICC; also in close apposition with varicosities of enteric neurons	*PDGFRα+ cells	1) Mediation of purinergic inhibition (purinergic inhibitory junction potential); 2) mediation of inflammatory input via protease-activated receptors	5, 47, 69–71, 104
Smooth muscle cells	Circular SMCs, longitudinal SMCs, sphincteric SMCs, sling SMCs in stomach	1) Generation of forces in motility; 2) maintenance of sphincter tone; 3) contributions to setting resting excitability of SIP syncytium; 4) transduction of neural and hormonal responses; 5) mediation of Ca2+ sensitization mechanisms	32, 54, 75, 84, 91, 101, 103, 117

^*PDGFRα⁺ cells also have distinct localization and cellular morphologies when found within muscle bundles or in the myenteric region. They are often intertwined with ICC in these localizations. At present, no differences in cells within muscle bundles and cells in the myenteric region have been distinguished; thus, for the purposes of this overview description, these cells are not broken down into

^*PDGFRα⁺-MY and

^*PDGFRα⁺-IM classes.