Joule discovered in 1840 that the “calorific effects of equal amounts of electricity emitted are proportional to the resistances opposed to its passage.” Joule heating is a function of friction of the charge current within a conductor, and resistive heat generation is independent of energy storage within capacitive or inductive circuit elements.
As a result, all stimulation of the nervous system generates heat in the electrical components and neural conductors through which electrical current flows. The effects of neural stimulation are usually related to induced electrical currents and potential gradients rather than thermal effects. However, the thermal effects of stimulation are always present.
Last summer, devastating wildfires raged in Siberia, Alaska and the frozen regions of Canada. They were caused in part by rising global temperatures, which accelerated the ability of bacteria to metabolize plant and animal matter in the soil.
These environmental conditions illustrated a fundamental principle of physics: temperature is a key factor in chemical reactions. Even small variations can have disastrous consequences, even in freezing temperatures. A wildfire can start if you heat a Siberian peat faster than it releases carbon into the environment.
Researchers have discovered that the physics of this environmental phenomenon are also related to brain activity. They also found that even small increases in temperature while stimulating the brain can dramatically alter brain activity with negative consequences.
Steven Schiff, MD, vice president of global health in neurosurgery at Yale School of Medicine, specializes in the intersection of engineering and neurosurgery, allowing him to apply physics principles to biological processes in the brain.
All forms of electrical and magnetic stimulation of the brain deposit thermal energy there because activity in wires generates heat. According to Schiff and his co-authors, temperature changes in the brain must be caused by electrical brain stimulation devices, such as Deep Brain Stimulation, used in patients with epilepsy and Parkinson’s disease.
The firing of neurons is also affected by temperature changes in the brain. Molecular pumps coat the membranes of nerve cells and electrically charge them with the energy released during brain activity. The researchers showed that if cells are heated faster than the charges can adjust, they can produce more or less neuronal activity than normal. Even small temperature changes caused by electrical stimulation of the brain (less than 1oC) can cause significant changes in neuronal activity. Neurons can become silent as they warm up. They can get very excited if you let them cool down to their average temperature.
Schiff, the study’s lead author, said: “Seeing these dramatic effects on brain activity from small temperature changes means we now have to account for such small temperature changes.”
“Long ago, physicist James Joule taught us that there is no way around this problem. When electrical current is passed through small conductive wires to generate electric or magnetic fields to stimulate the brain, heat is generated in both the wires and the conductive brain.
William Stacey, MD, Ph.D., associate professor of neurology and biomedical engineering at the University of Michigan, said, “This paper is a real tour de force of combining several physical behavior models to reexamine some ‘old norms.’
“Combining modeling with smart experimentation yielded the very intriguing and unexpected result that heat could suppress neural firing. This model could also provide some new methods for manipulating neural activity.”
Goldenholz said, “He would be very interested to see how Dr. Schiff’s findings are applied to seizure treatment and neuromodulation in the future.”
The findings were also important to Daniel M. Goldenholz, MD, Ph.D., assistant professor of epilepsy at Harvard and author of a recent paper on why focal cooling is essential for the future of focal epilepsy treatment. Dr. Schiff and his team emphasize the importance of temperature changes in brain tissue. It is likely to be relevant in epilepsy treatments, including focal cooling. These fluctuations need to be better understood and accounted for to improve the accuracy of their therapies.
This paradigm-shifting paper was presented in December 2022 at the meeting of the American Epilepsy Society in Washington, DC, and was well received.
It remains to be seen how these temperature changes affect the patient and how they can be used to improve outcomes. Surgeons have previously observed in clinical settings that a common side effect of implanting nervous system stimulators is that the activity of the stimulated brain is often reduced by electrical or magnetic stimulation.
The article provides a convincing explanation for this phenomenon. According to dr. Schiff could help clinicians calibrate the use of these devices more accurately.
The results show that while electrical interaction with brain tissue is inseparable from thermal effects when electrodes are used, magnetic induction allows anyone to separate Joule heating from induction effects by contrasting AC and DC actuation of magnetic coils with the same energy deposition in the conductors.
- TaeKen Kim, Andrew J Whalen et al. Thermal effects on neurons during brain stimulation. Neural technique. DOI: 10.1088/1741-2552/ac9339