The hippocampus functions in two different modes, each of which have their own specific pattern of neural and electrical activity that can be measured using an electroencephalogram (EEG). These two modes are called the theta mode and the large irregular activity (LIA) mode. The characteristics of each in the case of a rat's brain, are given below.
This mode of activity is the one seen during REM sleep and during active behaviour when the rat is alert. The EEG waves are large and regular, ranging in frequency from 6 to 9 Hz. The pyramidal cells and the granule cells which form the main bulk of cells in the hippocampus show little activity in this mode, meaning most cells are silent and the remainder of cells that are active fire at relatively high rates, up to as much as 50 spikes per second. Active cells only stay active for up to a few seconds.
Large irregular activity (LIA) mode
This mode occurs during non-REM, slow-wave sleep as well as during wakeful but immobile moments such as while watching the television or resting. EEG waves are sharp and random deflections last 200 to 300 ms. Between the waves, the pyramidal and granule cells are quiet, although not silent, and during a wave around 5 to 10% of the neurons emit at least one action potential over a 50 ms period.
The hippocampus contains a high proportion of receptors for the stress hormones glucocorticoids, which means the organ is more prone to stress compared with other regions of the brain. Steroids can reduce the excitability of some neurons in the hippocampus; they can inhibit the formation of new neurons; and they can cause shrinking (atrophy) of the dendrites in the pyramidal cells present in the CA3 region.
Evidence suggests that individuals who experience stress on a long-term basis have atrophy of the hippocampus that is increased compared with other brain regions. Atrophy of the hippocampus is also seen in Cushing's disease, a condition characterized by long-term high blood levels of cortisol, the primary stress hormone.