Unconscious activities in your body, like digestion, breathing, and heartbeat, are controlled by the Autonomic Nervous System. This has two divisions: Sympathetic and Parasympathetic. The Sympathetic division is the fight or flight, or survival portion. It is vital in a crisis or stressful scenario, and can be thought of like your body's "gas pedal." Conversely, the Parasympathetic division is the “brake pedal,” integral for rest, growth, digestion, and relaxation. This is the mode we want to live in the majority of our lives. When there is nerve interference, especially in the upper neck, it can drive our body into a Sympathetic dominant state. This means our body’s physiology will change like it is in a stressful, survival-like situation. For example, blood pressure will rise, stress hormones will increase, and the digestive and immune systems will be suppressed.
A 2014 randomized control study described the effects of chiropractic adjustments to the upper and lower neck on blood pressure and the autonomic nervous system. The findings revealed that adjustments served to shift the body back into a parasympathetic, “rest-relax-growth” mode, including a decrease in blood pressure.
Win NN, Jorgensen AMS, Chen YS, Haneline MT. Effects of Upper and Lower Cervical Spinal Manipulative Therapy on Blood Pressure and Heart Rate Variability in Volunteers and Patients With A Randomized Controlled, CrossOver, Preliminary Study. Journal of Chiropractic Medicine. 2015;14(1):1-9. doi:10.1016/j.jcm.2014.12.005.