Pectinate muscles are specialized Intracardiac muscle
- Pectinate muscles are located mainly in the right atrium , more in right atrial appendage , sparse in left atrium
- Has muscle fibers arranged in a comb like fashion.
- Has less mechanical activity, no significant contribution to atrial contractility.
- Can stretch and improve the voluminous nature of right atrium
- The pectinate muscle folds act as RA volume reserve during adverse loading conditions . It helps RA dilate with out much wall stress.
- Rarely it can be a cell of origin for focal atrial tachycardia
Image Source : Greys anatomy
The concertina like effect of pectinate muscle .
The atrial infolding increases the surface area of atrial chamber at times of dilatation , like the music instrument .So, these macro folds ( like intestinal villi ) help overcome the constantly changing volume status of right atrium.Since the variation left atrial blood flow is not that much , the pectinate muscles are not well developed in the left atrium.
The same rules would apply for why there is excess trabeculations in right ventricle than left ventricle .
Dr.S.Venkatesan MD 
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