Welcome, everyone. Today, we are excited to present our project, which investigates the relationship between the parasympathetic nervous system and pain modulation in healthy adults. Parasympathetic activity is assessed using heart rate variability and is known to influence pain modulation in healthy adults and patient populations. The human nervous system can facilitate and inhibit pain, which are measured by temporal summation and conditioned pain modulation. The autonomic nervous system responds to acute stressors, such as nociceptor activation, and may play a crucial role in influencing these pain processes. Our study aims to fill the gaps in understanding the relationship between the state of the autonomic nervous system and pain modulation. We also assessed if sex plays a role in this relationship, since pain processing is different in males and females. Testing was done in two parts, separated by a 60-minute washout period. In each session, we recorded resting electrocardiogram for 10 minutes to assess baseline parasympathetic activity. We identified the R waves and ectopic beats. The RR time intervals between normal sinus rhythm beats, which is the time between successive R waves, were saved as a time series, which you can see in figure 1. The RR time series was analyzed in the time and frequency domains. Time domain variables included PNN50, the percentage of successive RR intervals differing by greater than 50 milliseconds, and RMSSD, root mean square of squared differences between beats. For frequency domain analysis, the RR time series underwent a fast Fourier transformation to determine the proportion of high frequency sine waves between 0.15 and 0.40 hertz that make up the signal. The high frequency percent power reflects parasympathetic activity. In part 1, we measured condition pain modulation, which is when pain is inhibited by the presence of another painful stimulus applied at a different body location. As you can see in figure 3, the painful stimulus was cold water immersion of the hand in water that was 6 to 8 degrees Celsius. Condition pain modulation was measured using pressure pain thresholds measured at the knee before and during cold water immersion. Pressure pain thresholds were also measured after the hand came out of the water to assess lasting pain inhibition. Heat temporal summation was also measured by applying 10 mildly painful pulses every 1.5 seconds to the knee. The participant rated the pain during each heat pulse on a visual analog scale and the difference between the highest pain rating and the first comprised heat temporal summation. In part 2, we measured pressure pain thresholds and heat temporal summation before and after 20 minutes of noxious electrical stimulation. The stimulation parameters were 400 microseconds pulse duration, 1 second ramp up, 10 seconds on, and 10 seconds off. Amplitude was set as high as the participant could tolerate and was adjusted throughout testing to maintain the highest tolerated pain rating. In the cold water immersion procedure, participants immersed their non-dominant hand in cold water at a temperature of 6 to 8 degrees Celsius for 2 minutes with pain assessed on a scale of 0 to 100. Participants also completed two patient reported outcome measures including the DAS-21 that measures the emotional states of depression, anxiety, and stress and pain sensitivity questionnaire that assesses the pain perception to imagine physical stimuli that may be experienced in daily life. Ten females and nine males between 19 and 34 years of age have participated thus far. Tables 1 and 2 show the results of conditioned pain modulation and heat temporal summation, testing that show increased pressure pain thresholds during cold water immersion and reduced heat temporal summation after cold water immersion. These are the expected responses that suggest that the participants in this study have intact pain modulation systems. Figure 6 on the left illustrates that the application of cold did not produce lasting inhibition of pressure pain thresholds, but did show a reduction in heat temporal summation. Figure 6 on the right shows the results from the application of noxious electoral stimulation and the 20-minute bout of noxious electoral stimulation produced lasting inhibition of pressure pain thresholds and heat temporal summation. No correlations were observed between the baseline state of the parasympathetic nervous system and the amount of inhibition of pressure pain thresholds or reduction of heat temporal summation. When assessing sex-related differences, no differences were observed by sex in the changes in pressure pain threshold and heat temporal summation in response to cold or noxious electrical stimulation. This may be due to a small number of participants in this report. Our findings provide evidence that the baseline state of the parasympathetic nervous system has no effect on pain modulation in young healthy individuals. The results also demonstrate that cold water immersion and noxious electrical stimulation produced different effects and that cold water immersion did not produce lasting pressure pain threshold inhibition while noxious electrical stimulation did. The results also show that the baseline state of the parasympathetic did. The lack of inhibition after the cold water immersion may be due to several things. Differences in the nociceptors that are activated by cold versus noxious electrical stimulation or differences in the duration of the painful stimuli. Future studies are needed to determine if the same results are observed in people with chronic pain. Thank you for your attention and we are happy to answer any questions that you have.

parasympathetic nervous system

pain modulation

heart rate variability

cold water immersion

noxious electrical stimulation