Your fight-or-flight system switches on moments before you even wake up
The sympathetic nervous system is famous for panic: racing heart, wide pupils, goosebumps. But it never fully rests. It quietly tunes blood sugar, body temperature and blood flow all day, and in the minutes before waking it spontaneously ramps up, priming the body to get moving.
It is one of the autonomic nervous system's divisions, which run the body's unconscious business, and it is usually cast as the counterweight to the parasympathetic system that governs resting and digesting. Its nerve cells start in the middle stretch of the spinal cord, from the first thoracic vertebra down to about the second or third lumbar, so anatomists speak of a thoracolumbar outflow. Short first-stage neurons leave the cord and meet long second-stage neurons in chains of ganglia beside the spine or clusters near the aorta, and those long fibres reach almost every organ.
The chemistry follows a two-step pattern. At the ganglia, the first neuron releases acetylcholine; the second then releases norepinephrine onto its target. There are notable exceptions. Most sweat glands respond to acetylcholine instead, except on the palms and soles. The adrenal medulla behaves like a modified ganglion whose cells pour mostly epinephrine, better known as adrenaline, straight into the blood. Nerves to the kidney use dopamine to control how much blood gets filtered.
In a stress response, the system reroutes circulation. Alpha-1 receptors tighten vessels in the skin, gut and kidneys, while beta-2 receptors, stimulated by adrenal epinephrine, open vessels in skeletal muscle, heart, lungs and brain. Blood therefore drains from organs that can wait and floods the ones needed for exertion. At the same time the heart speeds up, airways widen, the large intestine slows and blood pressure rises.
The sensory side works differently. Signals travelling back from internal organs are not split into sympathetic and parasympathetic fibres, and when they do register as pain, the brain often misreads them as coming from skin at the same spinal level, the phenomenon of referred pain. Research continues on the system's darker roles: a review in a Japanese hypertension journal notes that persistent sympathetic overactivity helps raise blood pressure, and a mouse study linked a stroke-induced sympathetic surge to rapid muscle wasting.
Source: Sympathetic nervous system
