Muscle spindle · Lobeworks/17

The muscle spindle is a sensory receptor that measures a muscle's length and its changes, a small capsule of thin muscle fibres lying alongside the ordinary ones, whose middle is wrapped by sensory endings that fire when the muscle is stretched, and it is the main source of the sense of where the limbs are (propriocept

Muscle spindle. The muscle spindle is a sensory receptor that measures a muscle's length and its changes, a small capsule of thin muscle fibres lying alongside the ordinary ones, whose middle is wrapped by sensory endings that fire when the muscle is stretched, and it is the main source of the sense of where the limbs are (proprioception).

Two kinds of ending share the spindle and split a job that the skin gives to separate receptors. The primary ending (group Ia, among the thickest and fastest sensory fibres of the body, up to about 120 m/s) answers with a burst while the stretch is happening and keeps a lower rate while it is held: it carries both the dynamic and the static message. The secondary ending (group II) answers mostly to the length reached, the static message alone. The spindle also has motor nerves of its own: gamma motor neurons tighten the ends of its fibres while the muscle shortens, so it stays taut and keeps reporting instead of going slack.

The knee jerk is the spindle's reflex. A tap on the tendon stretches the thigh muscle, its Ia endings fire, and in the spinal cord they excite the motor neurons of the same muscle across a single synapse, so the leg kicks.

It measures length; force is another receptor's job. When the muscle contracts by itself the spindle is unloaded and goes quiet unless the gamma system keeps it taut; tension is the Golgi tendon organ's message.

One receptor, both answers. It is the clearest case of a phasic and a tonic response side by side in the same organ.

Ia says the muscle is being stretched, II says how stretched it is.

Speed and position travel on two fibres from the same small capsule.

Questions: Muscle spindle or Golgi tendon organ: which reports length and which force? The spindle reports length and the tendon organ force, and the reason is where each one sits. The spindle lies in parallel with the muscle fibres, so it is stretched when the muscle is stretched and unloaded when the muscle contracts; the tendon organ lies in series, between muscle and tendon, so it is squeezed whenever the muscle pulls. Stretch a relaxed muscle and the spindle fires; make it contract and the tendon organ does. Why does the body nearly fall silent in a flotation tank? Because nothing changes and nothing has to be held. The water is at skin temperature and still, so the phasic receptors have no movement to report and the tonic ones adapt with no edges or stretch to measure; floating, no muscle carries the body, so the spindles and tendon organs are unloaded. Whether that quiet is why people come out calmer is not settled: one hour lowered anxiety sharply in an open-label study of 50 patients, and the authors name reduced sensory and motor input as one plausible contributor among several. Why does the brain fire gamma motor neurons while a muscle contracts? To keep the length sensor working. The spindle lies in parallel with the muscle, so when the muscle shortens its middle goes slack and its endings fall silent. Gamma motor neurons contract the ends of the spindle's own fibres, which pulls its middle taut again, so it goes on reporting length and can catch an unexpected stretch at any point of the movement.