How does the nervous system work?

Your nervous system is your body's wiring. It carries messages as tiny electrical pulses, and it has two parts. We are facing the person, so their right side is on your left.

The central nervous system (CNS) is the brain and the spinal cord. The brain decides; BrainClear shows its regions in depth. The spinal cord is a cable about 45 cm long, as thick as your finger, inside the bones of your back. It does not reach the bottom of your spine: it ends at about your waist (the L1–L2 bones), and the lower nerve roots hang down like a horse's tail, the cauda equina.

The peripheral nervous system (PNS) is everything else: the nerves. 31 pairs of spinal nerves leave the cord between the bones, one pair per level: 8 in the neck, 12 in the chest, 5 in the lower back, 5 in the sacrum and 1 at the tailbone. They join into big nerves: the ulnar nerve (the one you hit on your "funny bone"), the femoral nerve in the front of the thigh, and the sciatic nerve, the thickest in your body, about as wide as your thumb.

12 pairs of cranial nerves leave the brain directly, for your eyes, face, ears and tongue (see EyeClear, EarClear and TongueClear). One of them, the tenth, is special: the vagus nerve wanders all the way down to your heart, lungs and gut.

A nerve is a bundle of thousands of axons, the long wires of nerve cells. Some carry messages in (sensory: touch, pain, where your limbs are; see SkinClear) and some carry orders out (motor: move this muscle).

A nerve is made of the long wires of neurons, nerve cells. Each has short branches called dendrites that collect messages, a cell body (soma), and one long axon that sends the message on. A motor neuron's axon can run from your lower spine to your big toe: about a metre long, but thinner than a hair.

Every neuron is a tiny battery. Pumps keep sodium (Na⁺) mostly outside and potassium (K⁺) mostly inside, so the inside sits at about −70 mV: the resting potential.

When the voltage rises past about −55 mV, the threshold, sodium gates snap open and Na⁺ rushes in. The inside swings to about +35 mV in half a millisecond. Then potassium gates open, K⁺ flows out, and the voltage falls back, dipping a little below rest. That flip, over in about 2 ms, is an action potential, or spike. It is all or nothing: a stronger push gives more spikes, not bigger ones.

Each spike triggers the next patch of axon, so it travels. Bare axons are slow, about 0.5–2 m/s, a brisk walk. Many axons are wrapped in fatty myelin, with little bare gaps called nodes of Ranvier. The spike jumps from node to node, up to about 120 m/s, faster than a racing car. Thicker axons are faster too. BrainClear shows the same spike starting inside the brain.

That is why a stubbed toe hurts twice: a sharp first pain on fast Aδ fibres, then a dull ache on slow C fibres a second later.

Neurons don't quite touch. Between the end of one axon and the next cell is a gap about 20 nanometres wide, a thousandth of a hair: the synaptic cleft. The whole junction is a synapse.

When a spike reaches the axon terminal, calcium flows in and tiny bubbles called vesicles fuse with the membrane, spilling thousands of molecules of neurotransmitter into the gap. They cross in less than a millionth of a second and click into receptors on the far side, opening gates there. The whole hand-over takes about half a millisecond. Then the transmitter is cleared away, ready for the next spike.

Different transmitters do different jobs. Glutamate is the main excitatory ("go") signal: it nudges the next cell towards threshold. GABA is the main inhibitory ("stop") signal: it pulls it away. One input moves the voltage by less than 1 mV, so a neuron only fires when many go signals beat the stop signals. Dopamine works more slowly, turning other signals up or down; it matters for movement, motivation and learning.

Where a motor nerve meets a muscle is a special synapse, the neuromuscular junction. It uses acetylcholine (ACh), and it is built never to fail: each spike releases about 125 vesicles, far more than needed, so every spike makes the muscle fibre fire and twitch. MuscleClear shows what happens inside the muscle next. BrainClear shows synapses inside the brain.

Some jobs are too urgent to wait for the brain. A reflex is an automatic reaction wired through the spinal cord: the message comes in, turns around in the cord, and goes straight back out.

The knee jerk is the simplest. A tap below the kneecap stretches the thigh muscle. Stretch sensors called muscle spindles fire; the message runs up a fast sensory nerve, into the back of the cord, and connects through one synapse to the motor neuron for the same muscle. The muscle contracts and your leg kicks. At the same time an inhibitory interneuron tells the opposite muscle, the hamstring, to relax. The whole loop takes about 30–40 ms. Doctors tap your knee to check this circuit is intact.

Touch something hot and a withdrawal reflex pulls your hand back. It uses a few more synapses, and pain nerves are slower, so it takes about 70 ms. The pain message also travels up to your brain, but you only feel "hot!" after your hand has started moving.

Compare that with choosing to react. Seeing a light, deciding and pressing a button takes most people about 200–250 ms: your eyes, brain and muscles all have to take part. So a knee jerk is about seven times faster than a deliberate reaction, and even the slower hot-hand reflex is about three times faster.

Right now your heart is beating, your gut is digesting and your pupils are adjusting to the light, and you are not deciding any of it. That is the autonomic nervous system, the automatic part. It has two branches that pull in opposite directions, like an accelerator and a brake.

The sympathetic branch (orange) is fight or flight. Its nerves leave the spinal cord in the chest and lower back and pass through a chain of little knots, ganglia, beside the spine. It speeds up the heart, opens the airways, widens the pupils, makes you sweat, dries your mouth, slows digestion, and tells the adrenal glands on top of your kidneys to pour adrenaline into the blood.

The parasympathetic branch (green) is rest and digest. Most of it travels in the vagus nerve. It slows the heart, narrows the pupils, and gets saliva and digestion going. In fact your heart's own pacemaker would beat about 100 times a minute; at rest the vagus holds it back to about 60–80.

Most organs get both, and the balance between them sets how they run. A sudden fright flips it in under a second; adrenaline keeps you jumpy for minutes afterwards. Slow, deep breathing is one of the few ways you can nudge the balance back yourself. HeartClear and RespirationClear show the heart and breathing in detail.

Nerve cells in the brain and spinal cord mostly cannot regrow, so the best medicine is protection and quick help. This chapter explains common problems; it cannot tell you what is going on in your own body. For that, see a doctor.

A stroke is a blocked or burst blood vessel in the brain. Think FAST: Face drooping, Arm weakness, Speech slurred, Time to call an ambulance: 112 or 108 in India. In a big stroke the brain can lose about 2 million neurons a minute, and clot-busting treatment works best in the first few hours.

Epilepsy means repeated seizures, storms of electrical activity in the brain. It affects about 50 million people, about 10 million of them in India, and most can be seizure-free with treatment. If someone has a seizure: stay with them, time it, move hard things away, cushion the head, and when the jerking stops, roll them onto their side. Never put anything in their mouth or hold them down. Call an ambulance if it lasts over 5 minutes.

A spinal cord injury cuts the cable: messages can't pass the damaged level, so the higher it is, the more of the body is affected. Parkinson's disease is the slow loss of dopamine-making cells, causing tremor and slowness. Alzheimer's disease slowly damages memory and is the most common cause of dementia.

Everyday things matter. Sleep lets the brain clear waste and store what you learned: teenagers need about 8–10 hours. A helmet on a two-wheeler cuts the risk of a head injury by about two thirds, and a seat belt cuts front-seat deaths by nearly half. And pins and needles after sitting cross-legged is a squeezed nerve waking up; if numbness or tingling comes without pressure, or won't go away, see a doctor.