Your body sends every message one of two ways: down a wire, or over the air

Two systems, working in opposite ways

Your body moves information around using two systems, and almost everything about them is opposite [1].

The nervous system sends electrical signals down dedicated fibres to specific targets [1]. Chemical messengers called neurotransmitters carry the signal across the tiny gap between one cell and the next [1].

The other system is glands. They release chemical messengers called hormones into the bloodstream, which carries them everywhere [1]. Hormones run the slow, whole-body business: metabolism, digestion, blood pressure, growth [1].

One is a private line. The other is a broadcast.

The wire is fast, narrow, and expensive

Speed is the wire’s whole advantage, and it is enormous.

When an electrical signal reaches a nerve ending, the packets of neurotransmitter fuse with the membrane within about 4 milliseconds [2]. That fast, tightly time-locked form of release finishes in less than a few milliseconds [3]. Nerves also have slower release modes, running over seconds to minutes, but the classic one is near-instant [3].

Travel speed depends on the fibre. The slowest uninsulated fibres carry a signal at roughly 1 metre per second [4]. A faster pain fibre runs at about 15 metres per second [4]. Insulating a fibre makes it conduct roughly ten times faster than an uninsulated fibre of the same width [4]. That gap is why a stubbed toe gives you a sharp first pain, then a duller burning second pain a moment behind it [4].

Speed has a bill. A fibre’s width and its insulation set its conduction speed, but the same two things set how much energy it burns and how much room it takes [5]. Very wide, heavily insulated fibres are therefore rare [5]. You cannot wire a whole body for maximum speed.

The broadcast is slow, wide, and lingering

A hormone runs on a completely different clock.

Adrenaline is about as fast as hormones get, and it is still slow next to a nerve. Given as a drug, it reaches peak blood levels at about 2 minutes, with a half-life of 2.6 minutes [6]. Half-life is simply the time it takes for half of it to disappear from the blood.

Most hormones last far longer. DHEA, a hormone made by the adrenal glands, clears with a half-life of 1 to 3 hours [7]. Its stored form, DHEAS, lasts about 13.7 hours [7]. The difference is a delivery detail. DHEAS clings more tightly to albumin, the main carrier protein in blood [7]. Albumin binds hormones including thyroid hormone, keeps them circulating, and lasts about 16 hours in the blood itself [8].

Cortisol, the main stress hormone, is made at 8 to 30 milligrams a day on a daily rhythm that peaks near waking [9].

Nothing on that clock could pull your hand off a hot pan.

The receiver decides who is listening

Here is the part that makes an undirected broadcast usable. Hormones reach every cell in the body, but only cells carrying the matching receiver respond.

Steroid and thyroid hormones work through nuclear receptors. These are proteins sitting inside the cell that wait for their particular hormone, then bind DNA directly and switch genes on or off [10]. A cell without that receiver simply ignores the message.

This is not an abstraction. About 70% of breast cancers carry the oestrogen receiver, which is exactly why hormone-blocking treatment works in those tumours and not in the rest [11].

The broadcast is loud and has no address on it. The precision lives at the far end.

Where the two systems turn out to be one

The clean split breaks down in a few places, and those places are the most revealing thing in the whole design.

Sympathetic nerves normally run in a two-cell chain. A nerve leaves the spinal cord, meets a second nerve, and that second one acts on the organ [12]. The adrenal gland is the exception. The spinal-cord nerve runs straight through to it and lands on chromaffin cells, which act as the missing second nerve [12]. Instead of touching an organ, those cells squirt adrenaline straight into a vein [12]. They are described as the neuroendocrine arm of the sympathetic nervous system: nerve and gland in one [13].

The same chemistry runs both roles. Noradrenaline is the messenger sympathetic nerve endings use on organs; adrenaline is the main hormone the same tissue pours into the blood [14].

The pituitary does the identical trick with different hormones. Nerve cells in the hypothalamus send fibres down into the back of the pituitary. When those cells fire, their endings release oxytocin and vasopressin straight into the bloodstream [15]. A nerve, working as a gland.

This is why a fright has two halves. The nerve half is instant. The hormone half arrives seconds later and outstays it, and the body can remain switched on for minutes to hours after the threat has gone [16].

The honest limit

A message that lingers is a feature when the situation is real and a problem when it is not.

The stress chain is built with brakes on. The hypothalamus releases one hormone, the pituitary releases a second, the adrenal cortex makes cortisol, and rising cortisol shuts the chain back down [17]. It is designed to end.

When it does not end, the same messengers do damage. Sustained overactivation of the sympathetic system, and the flood of adrenaline-type chemicals that comes with it, can injure the heart [18]. Whether that is happening in any one person is a question for a doctor, not a reader.