Self and Memory 225“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitylong- term potentiation (LTP), first reported by Lomo and Bliss in the
1970s, in which a burst of high frequency stimulation leads to long-lasting
enhancement of excitatory post-synaptic potential (EPSP).^18 A record-
ing of LTP is shown in Figure 10.7. The synaptic mechanism of LTP,
explained in Figure 10.8, depends on the NMDA receptor and involves
gene activation and protein synthesis in the post-synaptic neuron. This
results in the growth (both size and number) of the dendritic spines and
strengthening of synaptic connections.^19 LTP is believed to be universal
for all animals.
LTP occurs in many parts of the brain other than the hippocampus.
Further, the phenomenon of long-term depression (LTD), the opposite
of LTP, has also been described. Synapses that exhibit LTP can also
exhibit LTD, which is typically induced by sustained stimulation at low
frequency (2 stimuli per second for 5 seconds).^20 Some neurons use LTD
as a mechanism of learning, such as the cerebellar Purkinji cells.^21
Another remarkable fact relevant to hippocampal function is the
discovery of neurons that are capable of charting real-time location and
physical orientation of the animal (rat), made in the 1970s by O’Keefe
Fig. 10.7. Demonstration of long-term potentiation (LTP). Recording of excitatory
post-synaptic potential (EPSP) from a hippocampal neuron of the rodent brain as a
result of repeated stimulation of its pre-synaptic fiber. Four trains of 100 Hz stimulation
(1 second duration, 5 minutes apart) given at zero time (arrow) leads to long-term sus-
tained responsiveness that lasts for days or weeks, corresponding to long-term memory.