Differential effects of urinary bladder distension on high cervical projection neurons in primates

Projection neurons located in high cervical segments of primates are generally excited instead of inhibited by cardiopulmonary spinal inputs, which enter thoracic dorsal roots. Thus, high cervical neurons with axons that either ascend to the thalamus or descend to thoracolumbar spinal segments can process and transmit excitatory cardiac information. The purpose of this study was to determine whether the excitatory effects observed to cardiopulmonary afferent stimulation are a universal response in high cervical projection neurons to spinal visceral inputs. Urinary bladder distension (UBD) was used to stimulate visceral afferent inputs that enter lumbosacral dorsal roots. Effects were determined on extracellular activity of either spinothalamic tract (STT) neurons or descending propriospinal neurons that were recorded in high cervical segments of anesthetized monkeys. Results showed that 17/34 STT neurons were inhibited by UBD and 3/34 STT neurons were excited. Widespread visceral inputs, therefore, can excite high cervical STT neurons but the majority of responsive STT neurons were inhibited by UBD. Effects of UBD on high cervical descending propriospinal neurons were significantly different from responses in STT neurons. Extracellular activity of fewer propriospinal neurons was affected by UBD and responses were more variable; 3/26 neurons were inhibited, 5/26 neurons were excited and one neuron was excited/inhibited by UBD. These results showed that the generally excitatory responses of high cervical projection neurons to cardiopulmonary inputs were not duplicated by stimulation of sensory input from the urinary bladder. Furthermore, results of this study indicated that effects of sensory inputs on spinal neurons might vary depending on axonal projections of the neurons examined.