Abstract
The esophagus of Ascaris is a syncytial muscle organ of tubular shape in which the myofibrils are arranged radially between the lumen and the external surface. A resting potential of almost 40 mv (cytoplasm negative) is maintained by the extracellular organic anions (volatile fatty acids) found in the perienteric fluid. Replacement of these anions by Cl- ions results in a large depolarization. The resting potential is also decreased when the external pH is lowered. The leading phase of the action potential with a positive overshoot of about 18 mv elicits contraction of the myofibrils, development of negative pressure within the lumen, and suction of liquid and food particles. The mechanical energy stored in the elastic components of the cell is released when the myofibrils relax, thus injecting the contents of the lumen into the intestine. A fast and synchronous relaxation is elicited by a regenerative membrane polarization, a negative spike with a peak value of up to 108 mv produced by an increase in the permeability of the membrane to K+ ions. Cells completely depolarized in "chloride" saline are still able to generate such large potassium spikes.