The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a

muscle contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such a ...

in response to stretching within the muscle. The

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

functions to maintain the muscle at a constant length. The term deep tendon reflex is often used by many health workers and students to refer to this reflex. "Tendons have little to do with the response, other than being responsible for mechanically transmitting the sudden stretch from the reflex hammer to the muscle spindle. In addition, some muscles with stretch reflexes have no tendons (e.g., "jaw jerk" of the masseter muscle)". As an example of a spinal reflex, it results in a fast response that involves an afferent signal into the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...

and an efferent signal out to the muscle. The stretch reflex can be a monosynaptic reflex which provides automatic regulation of

skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of m ...

length, whereby the signal entering the spinal cord arises from a change in muscle length or velocity. It can also include a polysynaptic component, as in the tonic stretch reflex. When a muscle lengthens, the

muscle spindle Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...

is stretched and its nerve activity increases. This increases

alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their co ...

activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle. There are several theories as to what may trigger gamma motoneurons to increase the reflex's sensitivity. For example, ''alpha-gamma co-activation'' might keep the spindles taut when a muscle is contracted, preserving stretch reflex sensitivity even as the muscle fibers become shorter. Otherwise the spindles would become slack and the reflex would cease to function. This reflex has the shortest latency of all spinal reflexes including the Golgi tendon reflex and reflexes mediated by pain and cutaneous receptors.

Structures

The stretch reflex is accomplished through several different structures. In the muscle, there are muscle spindles, whose extrafusal muscle fibers lie parallel to the muscle and sense changes in length and velocity. The afferent sensory neuron is the structure that carries the signal from the muscle to the spinal cord. It carries this action potential to the dorsal root ganglion of the spinal cord. The efferent motor neuron is the structure that carries the signal from the spinal cord back to the muscle. It carries the

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

from the ventral root of the spinal cord to the muscle down the alpha motor neuron. This synapses on the first structure discussed, the extrafusal fibers of the muscle spindle.

Examples

A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the opposite side will stretch. The muscle spindles in those muscles will detect this stretching, and the stretched muscles will contract to correct posture. Other examples (followed by involved

spinal nerve A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the ...

s) are responses to stretch created by a blow upon a muscle tendon: *

Jaw jerk reflex The jaw jerk reflex or the masseter reflex is a stretch reflex used to test the status of a patient's trigeminal nerve (cranial nerve V) and to help distinguish an upper cervical cord compression from lesions that are above the foramen magnum. The m ...

(CN V) * Biceps reflex C5/C6 * Brachioradialis reflex C6 * Extensor digitorum reflex C6/C7 * Triceps reflex C7/C8 * Patellar reflex L2-L4 (knee-jerk) * Ankle jerk reflex S1/S2 Another example is the group of sensory fibers in the calf muscle, which synapse with motor neurons innervating muscle fibers in the same muscle. A sudden stretch, such as tapping the Achilles' tendon, causes a reflex contraction in the muscle as the spindles sense the stretch and send an action potential to the motor neurons which then cause the muscle to contract; this particular reflex causes a contraction in the soleus-gastrocnemius group of muscles. Like the patellar reflex, this reflex can be enhanced by the

Jendrassik maneuver The Jendrassik maneuver is a medical maneuver wherein the patient clenches the teeth, flexes both sets of fingers into a hook-like form, and interlocks those sets of fingers together. The tendon below the patient's knee is then hit with a refl ...

Spinal control

Spinal control of the stretch reflex means the signal travels between the muscle and spinal cord. The signal returns to the muscle from the same spinal cord segment as where it entered the spinal cord. This is the shortest distance for a reflex signal to travel, thus creating a fast response. These responses are often referred to short latency stretch reflexes.

Supraspinal control

Supraspinal control of the stretch reflex means the signal travels above the spinal cord before traveling back down to the same segment it entered the spinal cord from. The responses from these pathways are often termed medium or long latency stretch reflexes, because the time course is longer due to distance it needs to travel. The central nervous system can influence the stretch reflex via the gamma motoneurons, which as described above control the sensitivity of the reflex.

Clinical significance

The patellar reflex (knee jerk) is an example of the stretch reflex and it is used to determine the sensitivity of the stretch reflex. Reflexes can be tested as part of a neurological examination, often if there is an injury to the central nervous system. To test the reflex, the muscle should be in a neutral position. The muscle being tested needs to be flexed for the clinician to locate the tendon. After the muscle is relaxed, the clinician strikes the tendon. The response should be contraction of the muscle. If this is the knee jerk reflex, the clinician should observe a kick. The clinician rates the response. The

clasp-knife response Clasp-knife response refers to a Golgi tendon reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination. It is one of the characteristic responses of an upper motor neuron lesion. It get ...

is a stretch reflex with a rapid decrease in resistance when attempting to flex a joint. It is one of the characteristic responses of an

upper motor neuron lesion An upper motor neuron lesion (also known as pyramidal insufficiency) Is an injury or abnormality that occurs in the neural pathway above the anterior horn cell of the spinal cord or motor nuclei of the cranial nerves. Conversely, a lower motor ne ...

References

External links

* * {{Reflex Somatic motor system Reflexes