Logan Steele
Muscle spindles are sensory receptors located within skeletal muscles that detect changes in muscle length and the speed of change in muscle length. When a muscle is stretched, the muscle spindle is also stretched, and its nerve activity increases. This results in a muscle contraction, which prevents the muscle from being overstretched. This process is known as the stretch reflex or myotatic reflex, and it is an involuntary response that occurs without input from the brain. The stretch reflex helps to maintain the muscle at a constant length and protect it from injury.
| Characteristics | Values |
|---|---|
| Definition | Muscle spindles are sensory receptors that detect changes in muscle length and the speed of change in muscle length. |
| Location | Muscle spindles are located within the body of a skeletal muscle, in the belly of a skeletal muscle. |
| Function | Muscle spindles convey length information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception. |
| Sensory Fibres | Primary type Ia sensory fibres and secondary type II sensory fibres. |
| Motor Neurons | Gamma motor neurons (also known as fusimotor neurons), beta motor neurons, and alpha motor neurons. |
| Reflex Type | Monosynaptic reflex, polysynaptic reflex (tonic stretch reflex), spinal cord reflex, spinal reflex, tendon reflex. |
| Disorders | Basal ganglia disorder, spasticity, ataxia, spastic hypertonia (spastic paralysis), sensory neuropathies, coordination disorders, neuromuscular diseases. |
| Uses | Understanding muscle spindles can help with personal training and developing power, as well as understanding more complex reflexes. |
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What You'll Learn
Muscle spindles are stretch receptors
Muscle spindles are small sensory organs with an elongated shape, involved in proprioception. Imagine a muscle spindle as a thread spiralled or wrapped around muscle fibres near the muscle belly. As the muscle lengthens or stretches, it pulls on the spindle, causing it to lose its spiral shape and stretch. This signals the muscle to contract, after which the spiral regains its shape, protecting the muscle from overstretching. This process is called the stretch reflex.
When a muscle spindle's associated muscle is stretched, the spindle can cause two things to happen. Functionally, muscle spindles are stretch detectors, sensing how much and how fast a muscle is lengthened or shortened. When a muscle is stretched, this change in length is transmitted to the spindles and their intrafusal fibres, which are subsequently stretched. The muscle spindle then signals the muscle to contract to prevent it from stretching too far, too quickly.
The stretch reflex (or myotatic reflex) is a muscle contraction in response to stretching a muscle. The function of the reflex is generally to maintain the muscle at a constant length, but the response is often coordinated across multiple muscles and even joints. The stretch reflex is a spinal cord reflex and is completely involuntary, requiring no input from the brain. The speed of the response is critical to its protective function, as a delay could result in a more serious injury.
The optimal length of a muscle to generate force is 1.2 times its resting length. This length generates the most tension and primes the elastic properties of the muscle without causing damage. Stretching a muscle beyond this length increases the risk of injury.
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Muscle spindles detect muscle length and speed of change
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in muscle length and the speed of change in muscle length. They convey length and speed information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception.
Imagine a muscle spindle as a thread spiralled or wrapped around muscle fibres near the muscle belly. As the muscle lengthens or stretches, it pulls on the spindle, causing it to lose its spiral shape and stretch. This signals the muscle to contract, after which the spiral regains its shape, protecting the muscle from being overstretched. This process is called the stretch reflex.
When a muscle spindle's associated muscle is rapidly stretched, the spindle can cause two things to happen. Functionally, muscle spindles are stretch detectors, sensing how much and how fast a muscle is lengthened or shortened. When a muscle is stretched, this change in length is transmitted to the spindles and their
The stretch reflex is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length, but the response is often coordinated across multiple muscles and even joints. The older term deep tendon reflex is now criticised as misleading, as tendons have little to do with the response, and some muscles with stretch reflexes have no tendons.
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The stretch reflex is a spinal cord reflex
The stretch reflex, also known as the myotatic reflex, is a spinal cord reflex. It is a muscle contraction in response to stretching a muscle. This response is crucial for muscle protection. When a muscle lengthens, the muscle spindle located inside the muscle is stretched, causing the spindle to lose its spiral shape. This stretch activates the muscle spindle, which then sends an impulse to the spinal cord. This impulse results in the activation of more motor neurons at the spinal level that send an impulse back to the muscle, causing it to contract with greater force to resist the stretch. This process is completely involuntary and enables a rapid response that protects the muscle from being overstretched or torn.
The stretch reflex is also important for maintaining posture and balance. For example, when a person standing upright begins to lean to one side, the postural muscles on the opposite side will stretch, and the stretch reflex will quickly counter this by activating the muscle spindle, keeping the person upright. Similarly, if someone encounters a sudden change in the ground while walking or standing, the stretch reflex will mediate a rapid correction to prevent them from falling.
The stretch reflex is also involved in achieving optimal contraction length. The optimal length of a muscle to generate force is 1.2 times its resting length, as this generates the most tension without causing damage to the muscle. By stretching a muscle immediately before contracting it, the muscle is primed to create more force. This can be observed in actions such as jumping, where a person will first briefly flex their knees and hips to lower themselves before jumping up.
The stretch reflex is a spinal process, with the signal travelling between the muscle and the spinal cord and returning to the muscle from the same spinal cord segment. This is the shortest distance for a reflex signal to travel, resulting in a fast response. The stretch reflex can be tested as part of a neurological examination, such as the patellar reflex (knee-jerk) test, to assess the integrity of the peripheral nerves and reflex arc components.
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Muscle spindles regulate muscle stiffness
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 fibres. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.
The stretch reflex (myotatic reflex), or more accurately muscle stretch reflex, is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length but the response is often coordinated across multiple muscles and even joints. The older term deep tendon reflex is now criticized as misleading. Tendons have little to do with the response, and some muscles with stretch reflexes have no tendons.
When a muscle is stretched, the stretch reflex regulates the length of the muscle automatically by increasing its contractility as long as the stretch is within the physiological limits. When a muscle lengthens, the muscle spindle located inside the muscle is stretched, and the rate of neural firing of muscle spindle afferents increases. This augments alpha motor neuron activity in the anterior horn cell pool, causing the muscle fibres to contract and therefore resist the stretching. Another subset of neurons then directs the antagonistic muscles to relax by the mechanism of reciprocal inhibition and in this way, the entire reflex process functions to maintain the muscle at a constant length.
Gamma motor neurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibres within the spindle. In the early 1950s, John Eccles and his colleagues used the stretch reflex as a model to study the physiology of synapses. Merton, a Cambridge physiologist, proposed a servo control model for the generation of movement based on the stretch reflex. He felt that the muscle spindles and the γ-motor neuron system were an integral part of the servo mechanism controlling the length of the muscle and suggested that the reflex helped control posture by increasing the activation of the muscle in direct proportion to the degree of its stretch.
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Muscle spindles and the golgi tendon organ (GTO) work together
Muscle spindles are sensory receptors located within muscles that detect changes in muscle length and the speed of change in muscle length. When a muscle lengthens, the muscle spindle is stretched, and this activates the muscle spindle, causing it to send an impulse to the spinal cord. This results in the activation of more motor neurons, which send an impulse back to the muscle, telling it to contract with greater force to decrease the speed at which the muscle is being stretched. This is known as the stretch reflex.
The golgi tendon organ (GTO), on the other hand, is a tree-like sensory ending enclosed in a spindle-like connective tissue capsule that lies near the junction of a tendon and a muscle. GTOs are considered muscle tension receptors. When GTOs are stimulated, they cause the associated muscle to relax by interrupting its contraction. This process is called autogenic inhibition.
Muscle spindles and GTOs are both proprioceptors that play a role in flexibility. They work together reflexively to regulate muscle stiffness and help you stretch safely and effectively. GTOs sense muscular tension within muscles when they contract or are stretched. If there is too much muscle tension, GTOs will inhibit the muscle from creating any force, thus protecting the muscle from injury. GTOs and muscle spindles work together through their reflexive actions to prevent injury. For example, when a muscle is stretched, the stretch reflex regulates the length of the muscle automatically by increasing its contractility as long as the stretch is within physiological limits.
The muscle spindle and GTO can also work together to produce reciprocal inhibition, where the contraction of one muscle inhibits the opposing muscle from contracting, preventing further stretching. This is important in alerting the brain that nearby joints and soft tissues are in danger of being stretched too far.
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Frequently asked questions
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 fibres.
The muscle spindle reflex, also known as the stretch reflex, is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length but the response is often coordinated across multiple muscles and even joints.
The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. This process is involuntary and automatic, protecting the muscle from being overstretched and preventing injuries.
