Pyramidal Tract Pathway
Stretch Reflex (Myotatic Reflex)
The myotatic stretch reflex (MSR) is an involuntary muscle contraction that occurs in response to overly stretching a muscle. Special receptor cells called muscle spindles have the central role in this reflex, which is why the reflex is also called the muscle spindle reflex.
Muscle spindles are stretch receptors located in the cells of all skeletal muscles. When a muscle fiber stretches, the muscle spindle stretches too and transmits information about the degree of the stretch to the spinal cord. In turn, the spinal cord prevents further stretching by initiating the contraction and shortening of the muscle. At the same time, the contraction of the antagonist muscle is inhibited. This results with shortening of the affected muscle and prevention of its further stretching and tearing. So, if a stretch reflex is activated, the muscle will involuntarily contract. This reflex is beneficial as it helps keep the muscle length and tension in its physiological range.
In a clinical setting, the stretch reflex is tested by tapping a tendon or a muscle with a neurological hammer (for example, jaw jerk reflex, knee jerk reflex, etc). Because of the way it’s tested, the stretch reflex is often wrongfully termed as “deep tendon reflex”. Upon being tapped, tendons stretch, but they also stretch the connected muscle fibers, which then stretches the muscle spindles. So, the role of the tendon is only to mediate the stretching, rather than to initiate the reflex. This explanation is further elaborated by McGee in his textbook “Evidence-Based Physical Diagnosis” (4th edition). The author backs this explanation using the jaw jerk reflex as an example. The masseter muscle is the object of examination in this reflex. However, it doesn’t have a tendon, but rather its stretch reflex is caused exclusively by tapping on the muscle fibers and consequently stimulating their muscle spindles.
Similar to muscle spindles, there is another type of proprioceptors called Golgi tendon organ. These receptors are located in the muscle tendons and detect their degree of tension. If the tendon is too stretched, these receptors transmit that information to the spinal cord which then inhibits the contraction of the muscle to prevent further tendon stretching. Since it results in muscle relaxation, this reflex is known as the inverse stretch reflex or tendon reflex.
Stretch reflex pathway
Stretch reflex is the fastest reflex humans have. It is a monosynaptic reflex with the following reflex arc:
- Muscle stretch stimulates muscle spindles
- Afferent sensory neuron that innervates the spindle sends the information through the dorsal horn of the spinal cord
- The afferent neuron synapses with alpha motor neuron that innervates the affected muscle
- Alpha motor neuron stimulates the affected muscle and causes it to contract
- Interneuron that synapses with the afferent neuron causes the antagonist muscle to relax
Here an animated example how the reflex arc translated to the knee jerk reflex in which the quadriceps muscle is stimulated:
Stretch receptor stimulation
The stretch receptor (muscle spindle) is stimulated by tapping the patellar tendon of the quadriceps muscle.
1A afferent (sensory) neuron which innervates the muscle spindle carries the information about the stretching to the spinal cord.
The alpha motor neuron that innervates the quadriceps muscle stimulates it, which results with fast contraction.
Via an interneuron, the antagonistic muscles (in this case knee flexors) get inhibited.
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The outcome of the reflex is the contraction of the effector muscle, in this case quadriceps femoris.
- Palastanga, N., & Soames, R. (2012). Anatomy of Human Movement (6th ed.). Edinburgh, Scotland: Churchill Livingstone.
- Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., & Kruse, D. H. (2022). Anatomy and Physiology (2nd ed.). OpenStax. OpenStax | Free Textbooks Online with No Catch
- Hall, J. E., & Guyton, A. C. (2016). Guyton and Hall Textbook of Medical Physiology (13th ed.). Elsevier, Philadelphia PA
- Mescher, A. L. (2013). Junqueira’s Basic Histology (13th ed.). McGraw Hill.
- Pawlina, W. (2016). Histology Text and Atlas (7th ed.). Wolters Kluwer.
- McGee, S. R. (2018). Evidence-based physical diagnosis (4th ed.). Elsevier.