Touch something hot and your hand pulls back before you have even thought about it. That speed is the work of your nervous system — a vast electrical network of cells that detects change, carries the signal, decides what to do, and acts, all in a fraction of a second. This chapter follows that signal from the single neuron to the whole brain.
A multicellular animal is a community of trillions of cells. For the body to behave as one organism, those cells must be linked so that a change sensed in one place produces the right response somewhere else. Coordination is the way an organism detects changes (stimuli) and adjusts its activities to give a suitable, organised response. Animals coordinate by two cooperating systems:
| Feature | Nervous coordination | Chemical (hormonal) coordination |
|---|---|---|
| Carried by | Electrical impulses along neurons | Chemical hormones in the blood |
| Speed | Very fast (milliseconds) | Slower (seconds to days) |
| Duration of effect | Brief — stops when impulses stop | Often long-lasting |
| Path / target | Precise — to a specific muscle or gland | Widespread — to all cells with receptors |
This chapter deals with the first of these — the nervous system. Its job is rapid, precise, short-lived control: pulling a hand from a flame, steadying your balance, reading these words.
The nervous system is built from neurons (nerve cells), the longest cells in the body. A neuron is specialised to carry electrical impulses over long distances at high speed. Each one has the same basic parts:
A nerve impulse is not electricity flowing like in a wire — it is a fast, self-renewing wave of changing electrical charge across the neuron's membrane, created by ions moving in and out. To understand it, start with a resting neuron.
A neuron that is not firing is polarised: the inside of the membrane is negative relative to the outside, at roughly −70 mV. This difference is maintained by the sodium–potassium (Na⁺/K⁺) pump, which uses ATP to push 3 Na⁺ out for every 2 K⁺ in, leaving the inside short of positive charge. This is the resting potential — the neuron poised and ready to fire.
When a stimulus is strong enough to reach the threshold, voltage-gated sodium channels fly open and Na⁺ rushes into the axon. The inside suddenly becomes positive — about +40 mV. This rapid reversal of charge is depolarisation, and the spike it produces is the action potential (the nerve impulse itself). It obeys the all-or-none law: below threshold nothing happens; at or above threshold a full-size impulse always fires.
Almost at once the sodium channels close and potassium channels open, so K⁺ flows out and the inside becomes negative again — this is repolarisation, restoring the −70 mV resting state. For a brief moment afterwards the membrane cannot fire again, no matter how strong the stimulus: the refractory period. This ensures the impulse travels in one direction only and limits how fast impulses can follow each other.
In a myelinated axon the impulse cannot form under the insulating sheath; it can only "happen" at the bare nodes of Ranvier. So the action potential jumps from node to node — this is saltatory conduction (Latin saltare, to leap). Because it skips the insulated stretches, the impulse travels far faster (up to ~120 m s⁻¹) and uses less energy than in an unmyelinated fibre.
Neurons do not actually touch. Where one neuron's axon meets the next cell there is a microscopic gap, the synapse (synaptic cleft). The impulse is electrical, but it cannot leap the gap — so it is passed across by a chemical:
A reflex action is a rapid, automatic, involuntary response to a stimulus — like pulling your hand off a hot plate or the knee-jerk when the tendon is tapped. It does not wait for the brain to "decide"; the spinal cord deals with it at once, which is why it is so fast and protective. The pathway the impulse follows is the reflex arc:
In the withdrawal reflex: heat (stimulus) is detected by skin receptors; a sensory neuron carries the impulse to the spinal cord; there an interneuron passes it straight to a motor neuron; the motor neuron fires the arm muscle (effector), which contracts and pulls the hand away. The brain is informed afterwards — which is why you feel the pain a moment after you have already moved.
The whole system is divided into the central nervous system (CNS) — the brain and spinal cord, which coordinate everything — and the peripheral nervous system (PNS) — the nerves connecting the CNS to the rest of the body.
The brain is the master coordinator, protected by the skull and the meninges and bathed in cerebrospinal fluid. Its main regions:
| Region | Main role |
|---|---|
| Cerebrum | The large, folded "thinking" part — controls voluntary actions, intelligence, memory, reasoning, the senses and speech. |
| Cerebellum | Coordinates muscles, balance and posture, making movements smooth and accurate. |
| Medulla oblongata | Controls vital involuntary actions — heartbeat, breathing rate, blood pressure, swallowing. |
(Between these sit the hypothalamus — temperature, hunger, thirst and links to hormones — and the thalamus, which relays incoming sensory signals.)
The spinal cord runs from the medulla down inside the backbone. It carries impulses between the body and the brain, and it is the centre for spinal reflexes (like the withdrawal reflex above). In cross-section it has central grey matter (cell bodies) shaped like a butterfly, surrounded by white matter (myelinated axons).
The PNS is all the nerves outside the CNS. Part of it, the somatic system, controls voluntary actions of skeletal muscle. The other part, the autonomic nervous system (ANS), runs the involuntary organs and has two opposing branches:
The two coordinating systems work together. The hypothalamus in the brain is the bridge: it is part of the nervous system, yet it controls the pituitary gland and so the hormones too. A fright is sensed by the nerves, but it is the adrenaline they trigger that keeps your body primed long after the impulse has passed — fast nervous action handing over to slower, longer chemical control.
Understanding the neuron and the impulse explains a great deal: why local anaesthetics work (they block Na⁺ channels so no impulse forms), why multiple sclerosis is so disabling (the myelin sheath is destroyed, slowing or stopping conduction), how nerve gases and many drugs act at the synapse, and why reflexes can protect us faster than thought. The same chain — detect, conduct, transmit, respond — runs from a single ion crossing a membrane up to the whole behaving animal.