The activity of 44 single brain stem gigantocellular neurons was recorded in the freely moving rat during the following four states: quiet waking (W), waking with movement (W-M), slow-wave sleep (SWS), and rapid eye movement (REM) sleep. 2. Cells were classified into three groups on the basis of the states in which they maintained their highest rate of discharge. The three cell categories were: movement-REM (MOV-REM), movement (MOV), and quiet waking (QW) neurons. The MOV-REM neurons, comprising 68% of the cell population, discharged significantly more during waking-movement and REM sleep than during either W or SWS. The MOV neurons, 16% of the cells, showed significant increases in activity only when the rat moved. The QW neurons, also 16% of the cells, typically maintained high rates of discharge in the absence of movement. 3. The MOV-REM neurons were further divided into two subclasses of cells--phasically and tonically discharging neurons. The phasic MOV-REM cells appeared to participate in phasic motor events of REM sleep and corresponding movements during waking. The pattern of activity of the tonic MOV-REM neurons suggested that they may be involved in the generation and maintenance of the theta rhythm of the hippocampus during waking-movement and REM sleep. 4. No cells were found to discharge significantly more in REM sleep or SWS sleep than in the other states, (i.e., no REM or SWS selective cells were seen).
