Specialized cardiac myocyte that is subendocardially interspersed with the regular cardiac muscle cell. They are uninucleate cylindrical cells, associated end-to-end in long rows, continue from the node to the atrioventricular bundle; relatively short compared to ordinary myocytes but are nearly twice their diameter. [ GOC:tfm http://www.ncbi.nlm.nih.gov/pubmed/19939742 FMA:0412046911 ]