What is a muscle spindle?

The muscle spindle is a micro-sensor located in almost all muscles. It provides the nervous system with information about the length and speed of muscle stretch, enabling posture and the myotatic reflex.

Anatomy

3–10 intrafusal fibers under a capsule (5–10 mm). Nuclear bags = sensitive to speed; nuclear chains = static length.

Double innervation

• Sensorial: Ia (speed), II (length).
• Gamma motoneurons: re-tension the spindle during contraction, preserving sensitivity.

Functions

• Myotatic reflex: stretching → Ia → α-motoneuron → contraction (40 ms). Inhibition of the antagonist.
• Tone: γ activity maintains baseline tension.

Fine motor control

Increasing γ command = more conscious hand (fine grasping). Fatigue = spindle gain ↓ → risk of falling.

Plasticity

• Plyometrics = increased Ia sensitivity, faster reflex.
• Neuropathies/spasticity = disturbed gamma loop (hyper or hypo-reactivity).

Summary

A muscle length/speed counter, the spindle communicates with α and γ motoneurons for posture, precision, and protection.

References

1. Enoka (2008). Neuromechanics.
2. Matthews (1981). J Physiol.
3. Purves (2019). Neuroscience.