The seconds-fast response
The HPA axis is powerful but slow — it takes minutes to raise cortisol. When a car swerves toward you, minutes is far too late. So the body keeps a second, faster wing of the stress response: the fight-or-flight response, driven by the sympathoadrenal system. This one works in seconds because it skips the slow chemistry of an axis and uses nerves directly.
Here is the elegant part. A sympathetic nerve runs straight from the spinal cord to the adrenal medulla — the inner core of the adrenal gland. The medulla is, in effect, a modified cluster of nerve cells. When the nerve fires, instead of signalling a muscle it tells the medulla to dump epinephrine (adrenaline) into the blood. This is neuroendocrine signaling at its most dramatic: a nerve impulse becomes a hormone surge almost instantly.
What adrenaline actually does
Epinephrine belongs to a family of messengers called catecholamines. Within a heartbeat it reprioritises the whole body for sudden physical effort. The pattern is easy to remember if you picture the body bracing for a sprint.
- Heart rate and force rise, so oxygen-rich blood is pumped harder.
- Airways widen and breathing quickens to take in more oxygen.
- The liver breaks down its glycogen stores by glycogenolysis, pushing glucose into the blood as instant fuel.
- Blood is shunted toward muscles and away from digestion and skin; pupils widen and the mind sharpens.
Two timelines, one threat
It helps to see the two systems as a relay team handling the same emergency on different clocks. The sympathoadrenal wing acts in seconds and fades in minutes — the jolt. The HPA axis acts over minutes to hours and sustains the body through a longer challenge — the staying power. Epinephrine gets you out of the road; cortisol keeps your blood sugar up and your reserves mobilised while you recover and stay alert afterwards.