Stretch reflex
Encyclopedia
The stretch reflex is a muscle contraction
in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length.
When a muscle lengthens, the muscle spindle
is stretched and its nerve activity increases. This increases alpha motor neuron
activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length.
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, gamma co-activation might keep the spindles taut when a muscle is contracted, preserving their stretch-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.
Other examples (followed by involved spinal nerve
s) are responses to stretch created by a blow upon a muscle tendon:
Another example is the group of fibers in the calf muscle, which synapse with motor neurons supplying 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
.
Inhibitory signals arrive at gamma neurons through the lateral reticulospinal tract from Brodmann area 6
, the paleocerebellum and the red nucleus
. Facilitatory signals arrive through the ventral reticulospinal tract from Brodmann area 4
, the neocerebellum and the vestibular nucleus.
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.
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length.
When a muscle lengthens, the muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
is stretched and its nerve activity increases. This increases alpha motor neuron
Alpha motor neuron
Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax. The reflex functions to maintain the muscle at a constant length.
Gamma motoneurons
Gamma motoneurons
Gamma motoneurons , also called gamma motor neurons, are the efferent component of the fusimotor system, the system by which the central nervous system controls and modifies muscle spindle sensitivity. The fusimotor system refers to the combination of muscle spindles and γ-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, gamma co-activation might keep the spindles taut when a muscle is contracted, preserving their stretch-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
Golgi tendon reflex
The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. In a Golgi tendon reflex, skeletal muscle contraction causes the muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the...
and reflexes mediated by pain and cutaneous receptors.
Examples
A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the side will stretch. Because of this, stretch receptors in those muscles contract to correct posture.Other examples (followed by involved spinal nerve
Spinal nerve
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...
s) are responses to stretch created by a blow upon a muscle tendon:
- Biceps reflex C5/C6
- Brachioradialis reflex C6Brachioradialis reflexThe brachioradialis reflex is observed during a neurological exam by striking the brachioradialis tendon directly with a hammer when the patient's arm is relaxing...
- Extensor digitorum reflex C6/C7Extensor digitorum reflexThe Extensor digitorum reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C6 and C7 spinal nerves. It is also known as Braunecker-Effenberg reflex, or BER.-Testing:...
- Triceps reflex C7/C8Triceps reflexThe triceps reflex, a deep tendon reflex, is a reflex as it elicits involuntary contraction of the triceps brachii muscle. It is initiated by the Cervical spinal nerve 7 nerve root...
- Patellar reflex L2-L4Patellar reflexThe patellar reflex or knee-jerk is a deep tendon reflex and is a myotatic reflex.-Mechanism:Striking the patellar tendon with a tendon hammer just below the patella stretches the sensory nerve fiber of the femoral nerve which synapses at the level of L4 in the spinal cord, completely independent...
(knee-jerk) - Ankle jerk reflex S1/S2Ankle jerk reflexThe ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsi-flexed. A positive result would be the jerking of the foot towards its plantar surface....
- Plantar reflex L5-S2Plantar reflexThe plantar reflex is a reflex elicited when the sole of the foot is stimulated with a blunt instrument. The reflex can take one of two forms. In normal adults the plantar reflex causes a downward response of the hallux...
- Jaw jerk reflexJaw jerk reflexThe jaw jerk reflex or the masseter reflex is a stretch reflex used to test the status of a patient's trigeminal nerve . The mandible—or lower jaw—is tapped at a downward angle just below the lips at the chin while the mouth is held slightly open. In response, the masseter muscles will jerk the...
Another example is the group of fibers in the calf muscle, which synapse with motor neurons supplying 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
Jendrassik maneuver
The Jendrassik maneuver is a medical maneuver wherein the patient 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 reflex hammer to elicit the patellar reflex. The elicited response is compared...
.
Supraspinal control
The central nervous system can influence the stretch reflex via the gamma motoneurons, which as described above control the sensitivity of the reflex.Inhibitory signals arrive at gamma neurons through the lateral reticulospinal tract from Brodmann area 6
Brodmann area 6
- Human :Brodmann area 6 is part of the frontal cortex in the human brain. Situated just anterior to the primary motor cortex , it is composed of the premotor cortex and, medially, the supplementary motor area, or SMA...
, the paleocerebellum and the red nucleus
Red nucleus
The red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...
. Facilitatory signals arrive through the ventral reticulospinal tract from Brodmann area 4
Brodmann area 4
Brodmann area 4 comprises the primary motor cortex of the human brain. It is located in the posterior portion of the frontal lobe.Brodmann area 4 is about the same as the precentral gyrus...
, the neocerebellum and the vestibular nucleus.
Pathology
The clasp-knife responseClasp-knife response
Clasp-knife response refers to a stretch 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 a upper motor neuron lesion...
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.