Heart muscle differs from its counterpart, skeletal muscle, in that it exhibits rhythmic contractions.
Rhythmic contractions of the entire ovary occur at ovulation in many vertebrates and have been described in rabbits.
That these rhythmic contractions originate in the cardiac muscle can be substantiated by observing cardiac development in the embryo (see above); cardiac pulsations begin before adequate development of nerve fibres.
Though the overall picture suggests that there is a pacemaker (a set of specialized cells capable of rhythmic contractions) near the pelviureteric junction, this has never been satisfactorily demonstrated in the tissue.
Because atrial and ventricular cells are normally quiescent, exhibiting action potentials only after the muscle is depolarized to the critical membrane potential (Ecrit), the source of the rhythmic contractions of the heart must be sought elsewhere.
The leg muscles of all insects, and the wing muscles of many, require action potentials to initiate every contraction; however, the wing muscles of other insects consist of fibrillar muscle, which requires only occasional action potentials to maintain its rapid rhythmic contractions.
