Action potentials in neurones are followed by a hyperpolarization, whi
ch can last up to several seconds. This hyperpolarization has several
phases that are mediated by the activation of different types of Ca2+-
activated K+ currents. Patch-clamp studies have revealed two families
of Ca2+-activated K+ channels of small (SKCa) and high (BKCa) conducta
nce. Activation of BKCa channels contributes to action-potential repol
arization, while SKCa channels are thought to underlie the afterhyperp
olarization (AHP). In addition, AHPs in neurones can be divided into t
wo distinct types that are easily separated by kinetic and pharmacolog
ical criteria. It is now clear that only one type of AHP can be explai
ned by activation of SKCa channels while a new type of Ca2+-activated
K+ channel underlies the other. Modulation of this channel by a range
of transmitters is a key determinant of the excitability of many neuro
nes.