Neuroscience 1906

The Integrative Action of the Nervous System

Charles Scott Sherrington

The nervous system is not a switchboard but an integrator — the reflex is its unit, the synapse its valve.

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In depth · the introduction

Your body is built from billions of separate cells, yet it moves as one. Sherrington asked how — and found the answer in something as humble as a reflex.

The big idea

A knee-jerk, a hand snatched off a hot stove — these reflexes feel too simple to matter. Charles Sherrington saw them as the key to the whole nervous system. He realised that the brain and spinal cord don't just relay signals like wires; they integrate them — adding some up, cancelling others out, and turning a flood of competing messages into one coordinated movement.

Studying reflexes, he worked out the rules of the junction where one nerve cell talks to the next — the synapse, a word he had helped invent. He showed that to bend a joint, the body excites one muscle while actively switching off its opposite (reciprocal innervation), and that many signals funnel down onto a single 'final common path' to the muscle, which can obey only one of them at a time.

How it came about

Around 1900, scientists had just learned that the nervous system is made of separate cells, neurons, rather than one continuous web. But nobody knew how those cells worked together to produce coordinated action. Sherrington, an English physiologist, attacked the problem from the bottom up, with painstaking experiments on the reflexes of the spinal cord.

He gathered the results into ten lectures given at Yale in 1904 and published them in 1906 as The Integrative Action of the Nervous System. The book reasoned its way to the properties of the synapse before anyone could see one, purely from how reflexes behaved in time. It reshaped physiology, and in 1932 Sherrington shared the Nobel Prize for this line of work.

Why it mattered

Sherrington turned the nervous system from a switchboard into something that thinks in miniature — a machine that weighs competing demands and decides. His idea that a nerve cell adds up excitation and inhibition and only then acts is the basic picture of how every neuron works, and even how the artificial 'neurons' in modern AI are imagined. Reciprocal innervation and the final common path are still taught to every doctor and every student of movement.

A way to picture it

Think of a single door into a crowded room, with many people all trying to give the doorkeeper orders at once — some pushing to open it, some pulling it shut. The doorkeeper can't obey everyone; he adds up the pushes, subtracts the pulls, and does the one thing the balance demands. The motor neuron is that doorkeeper, the 'final common path', and every muscle is moved by the running total of all the excitement and all the inhibition arriving at once.

Where it sits

Sherrington sits at the head of modern brain science. Just before him, Santiago Ramón y Cajal proved the nervous system is built from separate cells; Sherrington showed how those cells act together. After him, Alan Hodgkin and Andrew Huxley — also in this Library — cracked the electrical impulse that races along a single neuron, and his own student John Eccles revealed the chemistry of the synapse he had only inferred. His balance of excitation and inhibition is a cousin of every system in this Library that integrates many inputs into one output, all the way to the artificial neuron behind today's AI.

The original document

Original source text

Charles S. Sherrington · The Integrative Action of the Nervous System · New York: Charles Scribner's Sons, 1906 · the ten Silliman Lectures delivered at Yale University, 1904

The lectures set out from a single question: how does a body made of countless separate cells act as one individual? Sherrington's answer is that the nervous system integrates — it binds the parts into a coordinated whole — and that the elementary act of integration can be studied in its simplest form, the reflex. The book builds, lecture by lecture, from that unit up to the unified animal.

The simple reflex as the unit of integration

He treats the reflex — a stimulus at a receptor producing a coordinated response — as the indivisible unit of nervous action, while warning that a truly 'simple' reflex is a convenient abstraction: in the living animal no reaction stands alone, each is shaped by all the others competing for the same machinery.

The synapse: a one-way valve between neurons

From the timing of reflexes — their delay, their one-way conduction, their fatigue and summation — Sherrington infers that the junction between one neuron and the next has special properties, unlike conduction along a nerve fibre. For this junction he had earlier established the name synapse. He could not see it; he reasoned it into existence from how reflexes behave.

Reciprocal innervation of antagonistic muscles

A limb's flexor and extensor pull against each other, so to bend a joint the body must both excite one muscle and relax its antagonist. Sherrington shows that the same reflex that excites a muscle simultaneously inhibits its opponent — reciprocal innervation — making inhibition as active and central a process as excitation.

The final common path

Many separate reflex arcs converge on the same motor neuron running out to a muscle. That motoneuron is the final common path: it must serve all the arcs that claim it, and at any instant it can obey only one, so the convergence forces the nervous system to choose — to integrate competing demands into a single coordinated act.

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From these pieces the later lectures assemble the coordinated reflexes — the scratch-reflex, the flexion and crossed-extension reflexes of standing and stepping, postural tone — and close on the nervous system as the organ that compounds reflexes into the behaviour of the whole individual.

Liverpool · 1906