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

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Abstract
Responses of 66 primate [Macaca fascicularis] spinothalamic neurons to natural stimulation of the urinary bladder and testicle were studied with extracellular recording techniques to elucidate the neural basis for referral of visceral pain. Of 53 cells located at the thoraco-lumbar junction or in sacral segments 38 responded to noxious cutaneous stimuli, and 84% of these exhibited phasic and/or tonic excitatory responses to distension of the urinary bladder. Of 20 of these units, 17 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 observed. Twelve sacral units having inputs from deep receptors of the tail exhibited mixed excitatory and inhibitory responses to bladder distension. Two cells located at the thoraco-lumbar junction responded only to cutaneous tactile stimuli and 13 cells located at the lumbosacral enlargement were tonically inhibited by bladder distension. Spinothalamic neurons that convey nociceptive input from the skin may also respond to noxious visceral stimuli. Such viscero-somatic convergence provides a neural substrate for the phenomenon of cutaneous referral of visceral pain.