Convergence of cutaneous and pelvic visceral nociceptive inputs onto primate spinothalamic neurons

The responses of 66 primate spinothalamic neurons to natural stimulation of the urinary bladder and testicle were studied with extracellular recording techniques in order to elucidate the neural basis for referral of visceral pain. Thirty-eight out of 53 cells located at the thoraco-lumbar junction or in sacral segments responded to noxious cutaneous stimuli, and 84% of these also exhibited phasic and/or tonic excitatory responses to distension of the urinary bladder. Seventeen out of 20 of these units, all located at the thoraco-lumbar junction, were excited by compression of the ipsilateral testicle. The response was graded with the compressive force. Excitatory responses to noxious heat and an irritant chemical (KCl) applied to the exposed testicular surface were also observed. Twelve sacral units having inputs from deep receptors of the tail exhibited mixed excitatory and inhibitory responses to bladder distension. A further 2 cells located at the thoracolumbar junction responded only to cutaneous tactile stimuli, and 13 cells located at the lumbosacral enlargement were tonically inhibited by bladder distension. It is concluded that spinothalamic neurons that convey nociceptive input from the skin may also respond to noxious visceral stimuli. Such viscerosomatic convergence provides a neural substrate for the phenomenon of cutaneous referral of visceral pain.