A new study from Neuroscience Research Australia (NeuRA) has uncovered the underlying brain mechanisms that cause some people to experience an increase in blood pressure after being exposed to long-lasting pain, while others experience a decrease in blood pressure.
The team at NeuRA have developed a world-first technique that allows them to measure muscle sympathetic nerve activity (MSNA), which is a determinant of blood pressure, while using functional magnetic resonance imaging (fMRI) to see which areas of the brain are involved in causing the change in blood pressure.
An increase in MSNA results in an increase in blood pressure, while a decrease in MSNA results in a decrease in blood pressure.
For this study, fine microelectrodes were inserted into the side of the knee where spontaneous bursts of MSNA were measured. At the same time, fMRI images identified which areas of the brain lit up in a similar fashion.
The study revealed several core areas of the brain that were strongly coupled with MSNA bursts.
- prefrontal and cingulate cortices,
- nucleus accumbens,
- caudate nucleus and
- dorsomedial hypothalamus
areas increased in activity in those who experienced an increase MSNA, or remained at baseline or decreased in people whose MSNA decreased.
These areas are involved in higher-order emotional and cognitive functions, researchers said.
However, pain catastrophising, pain vigilance and anxiety levels were not predictors of change in blood pressure. In addition, there was no difference in pain parameters such as the intensity, speed or descriptors.
These results suggest that the differing responses in blood pressure due to muscle pain result from the activation of a neural pathway thought to be responsible for cardiovascular responses to psychological rather than physiological stressors. However, researchers warn this may not fully explain why some people with chronic pain go on to develop hypertension.
“Although there is a well-documented relationship between psychological variables such as pain catastrophising and the development of chronic pain, our findings suggest that these variables do not predict the MSNA response to pain and likely do not play a role in hypertension involved in chronic pain,” said Prof Macefield.
Central circuitry responsible for the divergent sympathetic responses to tonic muscle pain in humans,
Sophie Kobuch, Azharuddin Fazalbhoy, Rachael Brown, Luke A Henderson, Vaughan G Macefield,
Human Brain Mapping, doi: 10.1002/hbm.23424, published 3 October 2016.