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Muscle spindles are stretch receptors within skeletal muscles that detect changes in muscle length and the speed of change in muscle length. They are fusiform structures 0.5–3.0 mm in length, found longitudinally oriented at the edge of muscle fasciculi. The muscle spindle has both sensory and motor components. Almost every muscle contains muscle spindles, although they are mostly absent in facial muscles. They are distributed throughout flexor and extensor muscles, with a greater density in small muscles that perform precise movements. Muscle spindles play an important role in regulating muscle contraction by activating motor neurons via the stretch reflex to resist muscle stretch.
| Characteristics | Values |
|---|---|
| Definition | Stretch receptors within the body of a skeletal muscle |
| Function | Detect changes in the length of the muscle and the speed of change in muscle length |
| Mechanism | Convey length information to the central nervous system via afferent nerve fibres |
| Muscle Contraction | Activating motor neurons via the stretch reflex to resist muscle stretch |
| Muscle Relaxation | Stimulated Golgi Tendon Organ (GTO) causes its associated muscle to relax by interrupting its contraction |
| Muscle Density | Muscle spindle density is not uniform across the musculoskeletal system |
| Muscle Length | Less than a centimetre long |
| Muscle Composition | 2-12 intrafusal fibres (usually about 10 in humans) in an envelope of connective tissue |
| Muscle Distribution | Distributed throughout flexor and extensor muscles, more densely in small muscles serving fine movement |
| Muscle Response | The strength or degree of the muscle response is determined by the speed at which the stretch occurs |
| Muscle Protection | Protective reflex to increase force and decrease stretch |
| Muscle Tuning | Tuned by spinal gamma motor neurons that receive top-down and peripheral input |
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What You'll Learn
Muscle spindles are stretch receptors
Spindles are distributed throughout flexor and extensor muscles, more densely in small muscles serving fine movement. In most muscles, the spindles are distributed throughout the belly of the muscle with their greatest concentration near the main intramuscular nerves. The muscle spindle has both sensory and motor components. The muscle spindle consists of a group of fine muscle fibres, called intrafusal muscle fibres, 4–10 mm long, whose central portions are not contractile. Typically, the connective tissue capsule encloses two different types of intrafusal fibres. The nuclear chain intrafusal fibres have a set of aligned nuclei in the centre. The nuclear bag fibres have a clump of nuclei randomly arrayed in a bag-like structure in the centre of the intrafusal fibre.
The muscle spindle is activated when the muscle is stretched, which in turn 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. This impulse tells the muscle to contract with greater force in order to decrease the speed at which the muscle is being stretched. The strength or degree of the muscle's response is determined by the speed at which the stretch occurs; where the stretch occurs more rapidly, the spindle stimulates a greater firing frequency of the motor neuron, and the more forceful the contraction of the muscle is in response. This response is primarily protective, to avoid the potential damage that could occur when a muscle is rapidly stretched beyond its limit.
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They convey length information to the central nervous system
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 information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception.
The muscle spindle has both sensory and motor components. The sensory information is conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The muscle spindle is the central, encapsulated region of a group of modified muscle fibres, the intrafusal fibres. Spindle afferents terminate in this region, enclosed by a connective tissue capsule.
The intrafusal fibres are usually about 10 in number in humans and are less than a centimetre long. They lie in parallel with the surrounding muscle fibres. The spindles are distributed throughout flexor and extensor muscles, more densely in small muscles serving fine movement. In most muscles, the spindles are distributed throughout the belly of the muscle with their greatest concentration near the main intramuscular nerves.
The activation of muscle fibres within the spindle by gamma motor neurons and, to a lesser extent, by beta motor neurons, plays an important role in regulating the contraction of muscles. For example, by activating motor neurons via the stretch reflex to resist muscle stretch. This response is primarily protective, to avoid the potential damage that could occur when a muscle is rapidly stretched beyond its limit.
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They regulate muscle contraction
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 (CNS) via afferent nerve fibres. This information can be processed by the brain as proprioception.
The muscle spindles' responses to changes in length play an important role in regulating the contraction of muscles. For example, they activate motor neurons via the stretch reflex to resist muscle stretch. This is known as the myotatic reflex. When a muscle is stretched, the muscle spindles are activated, sending 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. The strength or degree of the muscle's response is determined by the speed at which the stretch occurs. When the stretch occurs more rapidly, the spindle stimulates a greater firing frequency of the motor neuron, and the muscle contracts more forcefully in response.
This response is primarily protective, to avoid potential damage when a muscle is stretched beyond its limit. For instance, when someone accidentally steps into a pothole and twists their ankle, the rapid stretch of the ankle and spindles within the stretched muscle cause the spindles to fire rapid impulses to the nervous system, resulting in a rapid contraction of the stretched muscles. This protective response can prevent a completely torn muscle or broken ankle, usually resulting in nothing worse than a minor sprain.
The muscle spindle is the central, encapsulated region of a group of modified muscle fibres, the intrafusal fibres. Each muscle spindle is less than a centimetre long and consists of 2–12 intrafusal fibres (usually about 10 in humans) in an envelope of connective tissue, lying in parallel with the surrounding muscle fibres. The ends of the intrafusal fibres are contractile, but the central portion is non-contractile and innervated by special neurons called gamma motor neurons. The function of these gamma motor neurons is to modify the sensitivity of the muscle spindle sensory afferents to stretch.
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They are fusiform structures
Muscle spindles are fusiform structures that are around 0.5–3.0 mm in length. They are found longitudinally oriented at the edge of muscle fasciculi. Each muscle spindle consists of 2–12 intrafusal fibres, usually about 10 in humans, in an envelope of connective tissue. The intrafusal fibres lie in parallel with the surrounding muscle fibres.
The muscle spindle is the central, encapsulated region of a group of modified muscle fibres, the intrafusal fibres. Spindle afferents terminate in this region, where they are enclosed by a connective tissue capsule. The capsule is thick and fibrous and contains multiple small variably sized intrafusal muscle fibres, nerve fibres, specialised nerve endings, and blood vessels.
The muscle spindle has both sensory and motor components. The sensory component involves the detection of changes in muscle length and the speed of change in muscle length. This information is conveyed to the central nervous system via afferent nerve fibres and processed as proprioception. The motor component involves the activation of motor neurons via the stretch reflex to resist muscle stretch.
The function of the muscle spindle is to maintain muscle tone by responding to stretch. When a muscle is stretched, the spindles are activated and send an impulse to the spinal cord, resulting in the activation of more motor neurons. This leads to a greater contraction of the muscle to resist the stretch. The strength of the muscle response depends on the speed of the stretch, with faster stretches resulting in a higher firing frequency of the motor neuron and a more forceful contraction.
The muscle spindle is an important structure involved in proprioception and motor control. It helps regulate muscle stiffness and plays a role in reflex muscle stiffness, implicit motor adaptation, and segmental stretch reflexes.
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They are found in jaw-closing muscles
Muscle spindles are sensory receptors that are located in muscles. They detect changes in muscle length and the speed of change in muscle length. They are fusiform structures, 0.5–3.0 mm in length, found longitudinally oriented at the edge of muscle fasciculi. Each muscle spindle is less than a centimetre long and consists of 2–12 intrafusal fibres.
The muscle spindles in the jaw-closing muscles have some sensory nerve (Ia and II) fibres that have their cell bodies in the brain and form the trigeminal mesencephalic nucleus. This is in contrast to spindles in limb muscles, where the sensory cell bodies are in spinal ganglia. The trigeminal mesencephalic nucleus records physiological responses of primary and secondary endings of the muscle spindles in the jaw-closing musculature. The primary ending shows a transient dynamic response followed by a static response when the ending is stretched during the act of jaw-opening.
The muscle spindles in jaw-closing muscles also play a role in the jaw-closing reflex. When the jaw-closing muscles are stretched by a rapid downward tap on the chin, the muscle spindles are also stretched. This causes a burst of action potentials to travel along the group Ia primary afferent nerve fibres coming from the primary endings within the spindles. The primary afferents then cause the activation of the alpha-motoneurons of the same jaw-closing muscle, leading to a fast contraction of the same jaw-closing muscle. This reflex assists in preventing the jaw from flopping up and down during running.
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Frequently asked questions
Almost every muscle contains muscle spindles.
Muscle spindles are fusiform structures, 0.5-3.0 mm in length, found longitudinally oriented at the edge of muscle fasciculi. They are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle.
Muscle spindles inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. This information is then used by the CNS to compute the position and movement of our extremities in space, which is essential for maintaining posture, motor control, and a stable gait.
Each muscle spindle consists of 2-12 intrafusal fibres (usually about 10 in humans) in an envelope of connective tissue, lying in parallel with the surrounding muscle fibres. They have a thick surrounding fibrous capsule and contain multiple small variably sized intrafusal muscle fibres, nerve fibres, specialised nerve endings, and blood vessels.
When a muscle lengthens, the spindles are stretched. This stretch activates the muscle spindle, which sends 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.
