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The stretch reflex

The knee jerk explained. A stretched muscle's spindle makes the same muscle contract directly through the cord and relaxes its antagonist, keeping the muscle at a constant length.

Updated 2026-10-023 sources

Depth:

Why it matters

Reflexes combine the spinal sensory and motor components so that a sensory input directly generates a motor response [1]. Tapping a tendon is one of the quickest ways to check that a lower motor neuron is working [1].

What it is

A deep tendon reflex is commonly known as a stretch reflex; it is elicited by a strong tap to a tendon, as in the knee-jerk reflex [1]. Its receptor is the muscle spindle, the stretch receptor of skeletal muscle [2].

How it works

  1. When a skeletal muscle is stretched, a muscle spindle receptor is activated [3].
  2. The axon from the spindle causes direct contraction of the same muscle [3].
  3. A collateral of the spindle fibre also inhibits the motor neuron of the antagonist muscles [3].
Spinal cordRootsMusclesMuscle spindlestretch receptorSensory axondorsal rootMotor neuronsame muscleMotor neuronantagonist muscleSame musclecontractsAntagonistless drive
Stretch reflexStretching a muscle activates its spindle receptor; the receptor’s axon makes the same muscle contract directly, and a collateral inhibits the motor neuron of the antagonist muscle.
  • Excitatory synapse (filled arrowhead)
  • Inhibitory synapse (bar)
  • Modulatory (open circle)
  • Signal or data flow, not a synapse (dashed)
  • Midline crossing (decussation)
Figure 1. Stretch reflex. Stretching a muscle activates its spindle receptor; the receptor’s axon makes the same muscle contract directly, and a collateral inhibits the motor neuron of the antagonist muscle.[3] Schematic
  • The reflex helps to keep muscles at a constant length; the knee jerk, tested with a hammer on the patellar ligament, is the common example.[3]

Simplified: The source says a collateral of the spindle fibre inhibits the antagonist’s motor neuron without naming the cells in between; the diagram draws it as one inhibitory link.

Text description of this diagram
  1. Stretching a muscle activates its spindle receptor; the receptor’s axon makes the same muscle contract directly, and a collateral inhibits the motor neuron of the antagonist muscle.
  2. Muscle spindle (left side, Muscles) to Sensory axon (left side, Roots); signal or data flow (not a synapse).
  3. Sensory axon (left side, Roots) to Motor neuron (left side, Spinal cord); excitatory synapse; direct contraction of the muscle.
  4. Sensory axon (left side, Roots) to Motor neuron (left side, Spinal cord); inhibitory synapse; collateral.
  5. Motor neuron (left side, Spinal cord) to Same muscle (left side, Roots); excitatory synapse; neuromuscular junction.
  6. Motor neuron (left side, Spinal cord) to Antagonist (left side, Muscles); signal or data flow (not a synapse).

Simple somatic reflexes do not include the higher centres used for conscious or voluntary movement [3]. Golgi tendon organs, a separate receptor, transduce the stretch of tendons [2].

Why it is built this way

The reflex helps to maintain muscles at a constant length: if a muscle is stretched, it contracts to compensate for the change in length, which is useful for stability [3, 1].

How we see and measure it

In the arm, the common reflexes tested are those of the biceps, brachioradialis, triceps and finger flexors; in the leg, the knee jerk of the quadriceps and the ankle reflex of the gastrocnemius and soleus [1]. The tendon at the muscle's insertion is struck with a rubber mallet [1].

In theatre and clinic

Reflexes show that the lower motor neuron is functioning [1]. Strong deep tendon reflexes, with weakness and spasticity, suggest an upper motor neuron lesion; lost reflexes with flaccid paralysis suggest a lower motor neuron lesion [1]. This site describes these signs for learning only.

Common misconceptions

Misconception: The knee jerk is a voluntary response the brain decides on.

It runs through the cord: the spindle's axon makes the muscle contract directly, without the higher centres used for voluntary movement [3].

Check yourself

What does the spindle collateral do?

It inhibits the motor neuron of the antagonist muscle [3].

What does a brisk knee jerk with spasticity suggest?

An upper motor neuron lesion [1].

Read next

References

  1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 16.4 The Sensory and Motor Exams. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/16-4-the-sensory-and-motor-exams
  2. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 14.1 Sensory Perception. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/14-1-sensory-perception
  3. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 14.3 Motor Responses. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses

For learning only. This page does not diagnose, predict outcomes or recommend treatment. Corrections are welcome: how to suggest one.