Figure 8.3i Amplitude Decrease. A Ca²⁺ cell without spontaneous activity was stimulated with depolarizing current. The amplitude of APs after the first one decreases due to inactivation of voltage-gated Na⁺ and/or Ca²⁺ channels. Afterhyperpolarization amplitude decreases due to inactivation of the K⁺ current responsible for repolarization. Spike failure (note the depolarization after the last AP) occurs when the membrane reaches AP threshold but there are too few active Na⁺ channels to trigger a full AP.