Spindles sense length and drive the monosynaptic stretch reflex
Expansion
Muscle spindle: intrafusal fibres lying in parallel with the main muscle, sensing length and rate of change of length.
- Ia afferents (fast) sense both; II afferents sense static length
- Gamma motor neurones contract the intrafusal fibres, keeping the spindle sensitive as the muscle shortens: alpha-gamma coactivation
Stretch (myotatic) reflex: tapping the tendon stretches the spindle, the Ia afferent synapses directly on the alpha motor neurone, and the muscle contracts. It is monosynaptic, with a latency of about 30 ms, and includes reciprocal inhibition of the antagonist.
Golgi tendon organ is the counterpart: it lies in series with the muscle, senses tension rather than length, and via Ib afferents produces inhibition, protecting against excessive force.
Clinical interpretation: absent reflexes suggest a lower motor neurone, nerve or sensory afferent lesion; brisk reflexes with clonus suggest an upper motor neurone lesion, since descending inhibition of the gamma system is lost.