Temperature-sensitive ascending neurons in the spinal cord of pigeons

Ascending neuronal activity in the lateral funiculus of the spinal cord of pigeons (spinalized at about C4, recordings at about C6) has been studied with regard to effects of temperature changes with a thermode in the vertebral canal between Th4 and C8. 2. Both warm-sensitive (35) and cold-sensitive (14) neurons were found. According to the change in impulse frequency during steplike thermal stimuli, different reaction types could be distinguished. Twenty-four warm-sensitive and seven cold-sensitive units showed a proportional frequency change without any dynamic reaction. Three other warm-sensitive neurons had an additional dynamic reaction (excitatory overshoot during warming, inhibition during cooling). Five warm-sensitive and three cold-sensitive units showed no static sensitivity but responded with outstanding dynamic frequency changes during rising or falling temperature. The activity of some neurons stopped suddenly above (4) or below (3) a critical temperature, which was always near the normal spinal temperature (about 41 degrees C). Altogether the reaction to rapid temperature changes was consistently greatest near the normal body temperature. 3. The mean static sensitivity of 17 warm-sensitive units was + 4.2 plus or minus 1.3 imp./sec. degrees C (mean value and s. d.) and that of three cold-sensitive ones minus 2.3 plus or minus 0.3 imp./sec. degrees C in the range 35 degrees minus 45 degrees C (vertebral canal temperature). The temperature coefficient (Q10) which was calculated for the same neurons showed great variations with mean values of about 5 for both warm- and cold-sensitive units.