Negative spikes in neurons
Intracellular recordings of action potentials always show the same format with which you are familiar. There is polarisation which the stimulus reduces, a threshold is reached, there is a positive spike, then repolarisation, hyperpolarisation, a refractory period an a return to the baseline polarisation.

The term "negative spike" applies to extracellular recordings. These measure the potential between a point close to but outside the cell and a reference point. The inflow of cations into the cell causes a drop in extracellular potential, which appears as a negative spike in the recording.