Specific localization of gap junction protein, connexin45, in the deep muscular plexus of dog and rat small intestine
The innermost layer of the circular musculature of the ileum of the guinea-pig, mouse, rat, rabbit, cat and dog is made of special small and dark muscle cells (sd cells), which can be identified by light and electron microscopy. These cells have a higher surface to volume ratio than ordinary muscle cells. They show all the organelles of smooth muscle cells and are in close relationship with a great number of extrinsic and intrinsic nerve fibres. They send numerous thin cytoplasmic processes towards the bulk of the circular layer, but nexuses between the sd cells and ordinary muscle cells are not found. It is suggested that some of the nerve fibres in this part of the circular layer are motor, setting the sd cells to a given length, and some are afferent in luminal pressure.