Alpha-gamma coactivation · Lobeworks/17

Alpha-gamma coactivation is the motor command by which the nervous system fires gamma motor neurons together with alpha motor neurons, so that while the ordinary muscle fibres contract the muscle spindles inside the muscle are tightened too and keep reporting its length instead of going slack.

Alpha-gamma coactivation. Alpha-gamma coactivation is the motor command by which the nervous system fires gamma motor neurons together with alpha motor neurons, so that while the ordinary muscle fibres contract the muscle spindles inside the muscle are tightened too and keep reporting its length instead of going slack.

The problem it solves is mechanical. The spindle lies in parallel with the muscle fibres; when the muscle shortens, the spindle's two ends come closer, its sensory middle is unloaded and its Ia and II endings fall silent, exactly when the brain most needs to know where the limb is going. Gamma motor neurons contract the striated ends of the spindle's own thin fibres, which stretches its middle again and keeps it within its working range. Alpha neurons move the muscle; gamma neurons retune the sensor that watches it.

The brain adjusts its own sensors while moving. The setting is not fixed: gamma drive can rise for a novel, difficult or precise task (the fusimotor set view), and recordings show it differs between shortening and lengthening contractions, so simple coactivation is one pattern among several.

It keeps the stretch reflex ready at any length. A spindle kept taut answers to an unexpected stretch whatever the muscle's length, so the spinal cord can correct a load that pulls the limb off course.

Seen in the laboratory as silence. Stimulate only the alpha neurons and the spindle pauses during the twitch; add gamma activity and it keeps firing.

While the muscle shortens, the brain tightens its length sensor so it never goes slack.

The body's measuring instruments are adjusted by the same commands that move it.

Questions: 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.