Output — the motor system turns the code into behaviour

The chain becomes behaviour at the output. The cerebellum is error-based supervised learning, giving adaptation plus an opposite-sign after-effect. The reflex arc is the minimal sensorimotor loop — stretch-reflex negative feedback (≈9.4×) with a top-down gamma-motor gain — and the motor unit is the final common path: size-principle recruitment and force-frequency (twitch → tetanus ≈3.9×).

The chain becomes behaviour at the output. The cerebellum is error-based supervised learning — a separate learning system from the hippocampus — giving adaptation plus an opposite-sign after-effect (the climbing-fibre error is the sister of the value error of §6). The reflex arc is the minimal sensorimotor loop: stretch-reflex negative feedback (≈9.4×, homologous to the §7 setpoint), active reciprocal inhibition, and a gamma-motor gain that is top-down control. The motor unit is the final common path: size-principle recruitment (Henneman) and force-frequency (twitch → tetanus ≈3.9×).

Cerebellum

The cerebellum learns by being told its error. It is supervised learning — a teaching signal (the climbing fibre) drives a delta-rule reduction of motor error toward zero — and it is a separate learning system from the hippocampal, unsupervised auto-associator of §5. Its signature is adaptation followed by an opposite-sign after-effect when the perturbation is removed. The teaching error here is the sister of the reward-prediction error that tags value in §6: different learners, related error signals.

Reflex arc

The reflex arc is the minimal sensorimotor loop, and it reuses the control motifs already seen. The stretch reflex is negative feedback that resists a length change — on the order of 9.4× correction — homologous to the hypothalamic setpoint of §7. Antagonist muscles are held in active reciprocal inhibition, and the gamma motor neurons set the loop's gain from above, which is the same top-down threshold control that §7 called will. The reflex is not a separate trick; it is the setpoint loop at the spinal scale.

Motor unit

The motor unit is the final common path: every command, however it arose, becomes force here. Recruitment follows the size principle (Henneman): small units first, large units as demand grows, so force rises smoothly. Within a unit, force is set by firing rate through the force-frequency relation, twitch summing toward tetanus on the order of 3.9×. Low force is set by recruitment, high force by rate — a dual code.

Closing the loop

This closes the chain. A scalar drive — assembled from value (§6) and will (§7) — passes through cerebellar adaptation, the reflex loop, and motor-unit recruitment to become graded force = behaviour. The ions of §2 reach the world. What remains unmeasured is absolute: exact gains, latencies, unit counts, and force-per-Hz are open (§9); the structure is direction-matched, the magnitudes are not.