Sirs Hodgkin and Huxley
A spike is an "all-or-none" phenomenon, a digital one, a 0/1. Along with synaptic potential, spikes (action potentials) form the 2 types of potential in the nervous system: the action potential, located at the axon of the pre-synaptic cell and the synaptic, at the dendritic spine of the post-synaptic cell (a reminder: synaptic potential is either EPSP or IPSP). We say spikes "overshoot" when momentarily they go above zero voltage (remember, resting potenital is -70mV) and they "undershoot" when momentarily the dive even below their resting potential, upon attenuation phase. This is also called "after-hyperpolarization) (lesson 4, video 1, 09:30-).