Peripheral somatic activation and spontaneous firing patterns of neurons in the periaqueductal gray of the cat.

A total of 50 neurons in the periaqueductal gray (PAG) of the cat were studied. Sixty percent of the units were excited by nociceptive pinch stimulation. These units were also excited by non-nociceptive stimulation such as tapping, hair movement and/or joint rotation. Non-nociceptive stimulation excited 20% of the units, whereas nociceptive stimulation did not drive these units. Their receptive fields were large, and often encompassed the entire body. Spontaneous discharges of PAG neurons revealed great irregularity.