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The stretch reflex, or myotatic reflex, is a muscle contraction in response to stretching a muscle. It is an involuntary and nearly instantaneous movement that occurs without conscious thought. The function of the reflex is to maintain muscle length at a constant level. When a muscle is stretched, the muscle spindles within it are also stretched, increasing nerve activity and resulting in muscle contraction. This reflex is important for muscle protection and posture maintenance. It can be activated by external forces, such as a load placed on the muscle, or internal forces, such as the stimulation of motor neurons. The stretch reflex can be further categorized into phasic and tonic reflexes, with the former involving synchronous motor neuron discharge and the latter resulting in sustained muscle contraction for posture maintenance or alteration.
| Characteristics | Values |
|---|---|
| Reflex type | Stretch reflex (myotatic reflex), patellar reflex, knee-jerk reflex, deep tendon reflex |
| Function | Muscle protection, maintaining muscle length, posture maintenance, involuntary control of movement |
| Structure | Muscle spindles, afferent sensory neurons, efferent motor neurons, alpha motor neurons, gamma motoneurons |
| Response | Contraction of stretched muscle, relaxation of opposing muscle |
| Latency | Short, medium, or long |
| Modulation | Supraspinal control, spinal control |
| Sensitivity | Controlled by gamma motoneurons |
| Phasic/tonic | Phasic (brief stimulation of muscle spindles), tonic (prolonged discharge of motor neurons) |
| Clinical significance | Localizing neurological damage, assessing muscle function, examining pain processing |
| Enhancement | Jendrassik maneuver |
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What You'll Learn
Muscle spindles
The muscle spindle has both sensory and motor components. The sensory component conveys information to the central nervous system (CNS) via afferent nerve fibres. This information includes changes in the length of individual muscles and the speed of stretching. The CNS then computes the position and movement of our extremities in space, which is a requirement for motor control, maintaining posture, and a stable gait. The sensory information conveyed by the muscle spindles is done through primary type Ia sensory fibres and secondary type II sensory fibres.
The motor component of the muscle spindle is provided by motor neurons, specifically up to a dozen gamma motor neurons (also known as fusimotor neurons) and, to a lesser extent, one or two beta motor neurons. The activation of these neurons causes a contraction and stiffening of the end parts of the muscle spindle fibres. Gamma motor neurons modify the sensitivity of the muscle spindle sensory afferents to stretch.
The muscle spindle is involved in the stretch reflex, also known as the muscle stretch reflex or myotatic reflex. When a muscle spindle's associated muscle is stretched, the spindle detects the change in length and signals its muscle to contract to prevent it from stretching too far, too quickly. This is accomplished through the activation of motor neurons via the stretch reflex, causing the muscle to resist the stretch.
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Spinal control
The stretch reflex involves the activation of muscle spindles, which are sensory organs found within muscles. When a muscle is stretched, the muscle spindles detect the change in length and velocity, sending a signal to the central nervous system (CNS). This signal is transmitted through afferent sensory neurons to the spinal cord, specifically to the dorsal root ganglion. The CNS then processes this information to compute the position and movement of the limbs, ensuring motor control and maintaining posture and gait stability.
In response to the incoming signal, the CNS sends an outgoing signal through efferent motor neurons back to the muscle. This signal causes the muscle to contract, resisting further stretching. Additionally, a secondary set of neurons triggers the relaxation of the opposing muscle, allowing for coordinated movement. The stretch reflex is a rapid response due to the short distance travelled by the signal between the muscle and the spinal cord.
The sensitivity of the stretch reflex is regulated by gamma motoneurons, which tighten or relax the fibres within the muscle spindles. This regulation ensures that the stretch reflex remains active even as muscle fibres shorten, maintaining its function. The stretch reflex can be further modulated by higher levels of the neural hierarchy, with supraspinal control involving signals travelling above the spinal cord before returning to the same spinal cord segment.
The stretch reflex plays a crucial role in various movements, such as the knee-jerk reflex, where tapping the patellar tendon causes a rapid contraction of the knee extensor muscle. It also contributes to maintaining posture, as seen in the jaw-stretch reflex, which helps restore the postural position of the mandible during rapid head movements. The autogenic inhibition reflex is another example where the stretch reflex is involved in motor control, preventing muscle damage by distributing work evenly across all muscle fibres.
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Supraspinal control
The stretch reflex, also known as the muscle stretch reflex or myotatic reflex, is a muscle contraction in response to a muscle stretch. The function of this reflex is to maintain the muscle at a constant length, and the response is often coordinated across multiple muscles and joints. This reflex is accomplished through several structures, including muscle spindles, which lie parallel to the muscle fibres and sense changes in length and velocity.
The stretch reflex has both phasic and tonic components. A tonic stretch reflex involves a prolonged asynchronous discharge of motor neurons, causing sustained muscle contraction for posture maintenance or alteration. In contrast, a phasic stretch reflex consists of a synchronous motor neuron discharge caused by brief stimulation of muscle spindles or their afferent nerve pathways. An example of a phasic stretch reflex is the tendon jerk, while the active contraction provoked in a muscle by continuous stretch is a tonic reflex.
The patellar reflex, or knee-jerk, is a classic example of the stretch reflex. When a physician taps the patellar tendon, the knee extensor muscle stretches abruptly, activating the myotatic reflex and causing an extension of the lower leg. This reflex can be enhanced by the Jendrassik maneuver.
Heavy-load resistance training can lead to marked gains in eccentric muscle strength due to increased excitability of spinal motor neurons and decreased inhibition of these neurons. This results in improved neuromuscular performance in athletes and non-athletes alike, including older adults and clinical patients.
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Tonic stretch reflex
The stretch reflex, also known as the muscle stretch reflex or myotatic reflex, is a muscle contraction in response to stretching a muscle. The function of the reflex is generally believed to be maintaining the muscle at a constant length, but the response is often coordinated across multiple muscles and even joints. The stretch reflex can be a monosynaptic reflex, which provides automatic regulation of skeletal muscle length. The signal entering the spinal cord arises from a change in muscle length or velocity.
The stretch reflex has both phasic and tonic components. A tonic stretch reflex is one in which a stimulus produces a prolonged asynchronous discharge of motor neurons, causing sustained muscle contraction for the maintenance or alteration of posture. In contrast, a phasic stretch reflex consists of a synchronous motor neuron discharge caused by brief stimulation of muscle spindles or their afferent nerve pathways. The tendon jerk is a phasic stretch reflex. The active contraction provoked in a muscle by continuous stretch is a tonic reflex and is the basis of muscle 'tone'.
The Tonic Stretch Reflex Threshold (TSRT) is a biological biomarker of deficits in motor control. The TSRT indicates the joint angle at and beyond which spasticity will be manifest in the muscle. It is determined by measuring the Dynamic Stretch Reflex Thresholds (DSRTs) by stretching the resting muscle at different velocities.
The stretch reflex is accomplished through several different structures. In the muscle, there are muscle spindles, whose intrafusal muscle fibres lie parallel to the muscle and sense changes in length and velocity. The afferent sensory neuron 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 carries the signal from the spinal cord back to the muscle.
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Autogenic inhibition reflex
The autogenic inhibition reflex is a protective mechanism that prevents muscles from tearing. It is a sudden relaxation of a muscle in response to excess tension. This automatic lengthening reflex is controlled by the central nervous system and regulated by proprioceptors in the muscles and tendons, primarily the Golgi tendon organs (GTOs). GTOs are stretch receptors that signal the amount of force developed by a muscle.
The GTO reflex, also known as the autogenic inhibition reflex, is triggered when too much contraction force is applied to a muscle, causing it to simultaneously lengthen and relax. This reflex is an autonomic inhibitory response that protects the muscle from potential tears or ruptures. It operates as a feedback mechanism, controlling muscle tension and contraction to ensure stability and prevent injury.
The autogenic inhibition reflex plays a crucial role in motor control, particularly in distributing the workload evenly across the entire muscle. When certain muscle fibres bear a heavier load, their GTOs become more active, inhibiting their contraction. This, in turn, prompts less active muscle fibres to contract more to compensate, resulting in a more efficient distribution of work across all motor units.
The reflex also has implications for treating dysfunctional muscles. Positive stimulation of the GTO typically leads to a reduction in the AI reflex and a significant increase in muscle resistance. This can result in improved muscle function, often experienced by patients as a rapid and noticeable change.
While the GTO was initially believed to only have a protective function, preventing muscle damage under high tension, recent evidence suggests that it is sensitive to lower force levels as well. This indicates that the autogenic inhibition reflex may be more extensively involved in normal motor control than previously thought.
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Frequently asked questions
A stretch reflex is a muscle contraction in response to a muscle being stretched. It is also known as the myotatic reflex, deep tendon reflex, or muscle stretch reflex.
The stretch reflex is accomplished through several structures, including muscle spindles, sensory neurons, and motor neurons. When a muscle is stretched, the muscle spindles detect the change in length and convey this information to the central nervous system. This results in increased alpha motor neuron activity, causing the muscle to contract and resist further stretching.
The patellar reflex (knee jerk) is a commonly known example of the stretch reflex. The jaw-stretch reflex is another example, which helps maintain and restore the postural position of the mandible during rapid head movements.
