Fig. 2. Different phases of long-term potentiation (LTP). (A) The early phase of LTP (E-LTP; solid circles) is usually induced by one train of high frequency stimulation (HFS; arrowhead) and lasts 1–2 h. A more enduring, late phase of LTP (L-LTP) is induced by three or more trains of HFS (open circles) and is associated with larger amplitudes and steeper initial slopes of fEPSPs for a longer period of time. Note the flat, stable plateau of elevated fEPSP slopes, which is LTP. (B) Different phases of LTP have different mechanisms. E-LTP involves covalent modifications of pre-existing molecules, whereas L-LTP requires transcription and de novo protein synthesis. Adapted from (Woo, 2003).