Avoidance reaction occurs when the cell hits an obstruction, providing an anterior, mechanical stimulus: - The cell will then reverse. - It will then stop and rotate. - Now facing a new direction, the cell will move off in that direction.
This process will continue until the cell is able to negotiate it's way around the obstruction.
Movement of Paramecium cells is caused by control of calcium ions inside the cell and membrane potentials. The simplest explanation for the avoidance reaction is that membrane potential controls the influx of calcium ions, which regulates the beat frequency and angles of cilia on the surface of the cell.