CorpusHuman Anatomy
MuscularReceptor

Muscle spindle

Fusus neuromuscularis

Also known as: neuromuscular spindle, intrafusal fibres, stretch receptor, nuclear bag fibre, nuclear chain fibre

An encapsulated stretch receptor that lies parallel to the ordinary muscle fibres. It signals muscle length and its rate of change, and it drives the stretch reflex.

Overview

Intrafusal fibres per spindle≈3–12
Ia conduction velocity≈70–120 m/s

Each spindle (≈4–10 mm long) contains 3–12 specialised intrafusal fibres inside a fluid-filled connective tissue capsule. The spindle lies in the perimysium, in parallel with the force-producing extrafusal fibres. There are three kinds of intrafusal fibre. Dynamic nuclear bag (bag₁) fibres respond to the rate of stretch. Static nuclear bag (bag₂) and nuclear chain fibres respond to sustained length. Spindles are especially dense in muscles that need fine control, such as the small hand muscles, neck muscles and extraocular muscles.

Sensory endings: group Ia (primary, annulospiral) afferents wrap around the central part of every intrafusal fibre and signal both length and velocity. Group II (secondary, flower-spray) afferents end mainly on chain and bag₂ fibres and signal static length. Motor supply: γ-motor neurons (dynamic and static) make the contractile ends of the intrafusal fibres contract. This keeps the central sensory region taut when the whole muscle shortens, and α–γ co-activation therefore keeps the spindle sensitive throughout movement. Ia afferents make monosynaptic excitatory contacts with α-motor neurons of the same muscle, giving the stretch (myotatic) reflex. Through an inhibitory interneuron they also inhibit antagonist motor neurons (reciprocal inhibition).

Parts & subdivisions

  1. Nuclear bag fibres
    Nuclei bunched at the centre. Dynamic bag₁ senses velocity; static bag₂ senses length.
  2. Nuclear chain fibres
    Thinner fibres with nuclei in a single row. They sense static length.
  3. Ia afferents
    Large, fast (≈70–120 m/s) primary endings on all intrafusal fibres.
  4. Group II afferents
    Secondary endings that signal static length.
  5. γ-motor neurons
    Adjust the sensitivity of the spindle (fusimotor drive).

Anatomy

Innervation

Sensory: group Ia and group II afferents (cell bodies in dorsal root ganglia). Motor: γ-motor neurons from the ventral horn.

Function

  • Detects muscle length and the velocity of stretch
  • Drives the stretch reflex (e.g. the knee jerk) and muscle tone
  • Supplies proprioceptive input for movement and position sense

Clinical significance

Tendon reflexes

Tapping a tendon stretches the spindles, and Ia → α-motor neuron activation produces a twitch. Reflexes are reduced in lower motor neuron, root or sensory lesions and increased in upper motor neuron lesions (loss of descending inhibition, raised γ drive). Clonus and spasticity reflect hyperactive stretch reflexes.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.