神经系统的整合作用

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.

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.

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.

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.

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.

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.