Your autonomic nervous system quietly stops you fainting every time you stand up
Rising from a chair should make your blood pressure crash. It doesn't, because an unconscious control network instantly tightens your arteries to compensate. The same system adjusts your heart rate with every breath, dilates your pupils and runs digestion, all without asking permission. It is the autonomic nervous system, and it rarely rests.
Its two best-known branches are usually described as an accelerator and a brake. The sympathetic branch mobilises the body quickly, triggering the fight-or-flight response. The parasympathetic branch works more slowly, promoting rest and digestion. The older idea that one simply excites and the other inhibits collapsed under too many exceptions; sexual arousal, for instance, needs both. A better picture is two systems constantly nudging vital functions in opposite directions to hold the body steady.
They leave the central nervous system from different places. Sympathetic fibres emerge from the spinal cord in the chest and lower back, while parasympathetic fibres start in four cranial nerves, most importantly the vagus, and in the lowest part of the spinal cord. Both use a two-neuron relay, with the first nerve cell handing the signal to a second in a cluster called a ganglion before it reaches the organ. The adrenal medulla is the exception, receiving its signal directly. Sympathetic ganglia sit close to the spine; parasympathetic ones sit near the organs they serve.
A third branch lives in the gut. The enteric nervous system, woven into the walls of the digestive tract, has about 200 million neurons and can regulate the gut independently while still talking to the brain. One of its two main networks runs the full length of the intestine, controlling movement; the other manages secretions and blood vessels.
Information flows inward too. Sensors in the carotid artery measure blood oxygen and carbon dioxide, while a brainstem region called the area postrema detects toxins in the blood and triggers vomiting. It also underlies the lasting aversion that keeps an animal from ever eating a food that once poisoned it. Because internal organs report pain through particular spinal segments, organ pain is often felt as coming from a patch of skin instead.
Source: Autonomic nervous system
