Bathmotropic
While bathmotropic, as used herein, has been defined as pertaining to modification of the "excitability" of the heart, it can also refer to modification of the "irritability" of heart muscle, and the two terms are frequently used interchangeably. Etymology. The term "bathmotropic" is derived from the Ancient Greek word "βαθμός" ("bathmós"), meaning "step" or "threshold". History. In 1897 Engelmann introduced four Greek terms to describe key physiological properties of the heart: inotropy, the ability to contract; chronotropy, the ability to initiate an electrical impulse; dromotropy, the ability to conduct an electrical impulse; and bathmotropy, the ability to respond to  direct mechanical stimulation. A fifth term, lusitropy, was introduced in 1982 when relaxation was recognized to be an active process, and not simply dissipation of the contractile event. In an article in the "American Journal of the Medical Sciences", these five terms were described as the five fundamental properties of the heart. Physiological explanation. The bathmotropic effect modifies the heart muscle membrane excitability, and thus the ease of generating an action potential. The ease of generating an action potential is related both to the magnitude of the resting potential and to the activation state of membrane sodium channels. During stage  4 of an action potential, the inside of a cardiac muscle cell rests at −90 mV. As the inner muscle cell potential rises towards −60 mV, electrochemical changes begin to take place in the voltage-gated rapid sodium channels, which permit the rapid influx of sodium ions. When enough sodium channels are opened, so that the rapid influx of sodium ions is greater than the tonic efflux of potassium ions, then the resting potential becomes progressively less negative, more and more sodium channels are opened, and an action potential is generated. The electrical potential at which this occurs is called  the threshold potential. As various drugs and other factors act on the resting potential and bring it closer to the threshold potential, an action potential is more easily and rapidly obtained. Likewise, when the sodium channels are in a state of greater activation, then the influx of sodium ions that allows the membrane to reach threshold potential occurs more readily. In both instances, the excitability of the myocardium is increased.