EPSPs and IPSPs are transient changes in the membrane potential, and EPSPs resulting from transmitter release at a single synapse are generally far too small to trigger a spike in the postsynaptic neuron. However, a neuron may receive synaptic inputs from hundreds, if not thousands, of other neurons, with varying amounts of simultaneous input, so the combined activity of afferent neurons can cause large fluctuations in membrane potential or subthreshold membrane potential oscillations. If the postsynaptic cell is sufficiently depolarized, an action potential will occur. For example, in low-threshold spikes depolarizations by the T-type calcium channel occur at low, negative, membrane depolarizations resulting in the neuron reaching the threshold. Action potentials are not graded; they are all-or-none responses.