The frequency and pattern of action potentials constitute the code used by neurons to transfer information from one location to another. The ups and downs of an action potential: see fig. 4. 1: the first part, called the rising phase, is characterized by a rapid depolarization of the membrane. Na+ ions enter the fiber through these channels: the entry of na+ depolarizes the membrane (i. e. the cytosolic surface of the membrane becomes negative) If this depolarization (generator potential) achieves a critical level, the membrane will generate an action potential. The critical level of depolarization that must be crossed in order to trigger an action potential is called threshold: the depolarization that causes action potentials arises in different ways in different neurons. The generation of multiple action potentials: the rate of action potential generation depends on the magnitude of the continuous depolarizing current, the firing frequency of action potentials reflects the magnitude of the depolarizing current.