Sensory Processing mechanisms in periaqueductal gray matter.

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Characteristics of somatosensory inputs to the periaqueductal gray matter (PAG) neurons were investigated using barbiturate-anesthetized Wistar albino rats. Results were as follows :(1) there were at least 2 types of PAG neurons according to their receptive fields. One was large type (PAG-L), and the other was small type (PAG-S).(2) Both types of neurons were activated by nonoxious (touch/ heat) and noxious (mechanical, heat and chemical) stimulation.(3) In a majority of PAG-L type neurons, wind-up phenomenon was observed.(4) Microiontophoretically applied morphine onto the PAG-S type neurons increased their spontaneous activities.(5) The threshold value of noxious heat stimuli applied to the PAG-S type neurons was higher than that applied to the PAG-L type neurons.