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Muscle spindles are stretch receptors found in most mammalian muscles that primarily detect changes in muscle length and velocity. They convey this information to the central nervous system (CNS) via afferent nerve fibres, which can be processed by the brain as proprioception. The two types of endings, primary and secondary, are both sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. The muscle spindle has both sensory and motor components. The sensory component is conveyed by primary type Ia sensory fibres, which spiral around the muscle fibres within the spindle, and secondary type II sensory fibres.
| Characteristics | Values |
|---|---|
| Type | Stretch receptors |
| Location | Within the body of a skeletal muscle |
| Function | Detect changes in the length of the muscle |
| Components | Sensory and motor components |
| Sensory Information Conveyed By | Primary type Ia sensory fibers, Secondary type II sensory fibers |
| Motor Neurons | Gamma motor neurons, Beta motor neurons |
| Muscle Spindle Density | Not uniform across the musculoskeletal system |
| Muscle Spindle Firing Rate | Related to the degree of muscle stretch |
| Muscle Spindles Play a Role In | Sensorimotor development |
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What You'll Learn
Muscle spindles measure muscle length and velocity
Muscle spindles are stretch receptors found in almost every muscle. They are mechanoreceptors that detect changes in muscle length and velocity, and they play a critical role in sensorimotor development. The two types of endings, primary and secondary, work together to provide the body with information about muscle length and velocity.
The primary endings, or type Ia sensory fibres, have a greater dynamic sensitivity and respond to both changes in muscle length and velocity. They spiral around all intrafusal muscle fibres, ending near the middle of each fibre. The secondary endings, or type II sensory fibres, sense muscle length and have a smaller velocity-sensitive component. They end adjacent to the central regions of the static bag and chain fibres.
Together, these fibres send information to the central nervous system (CNS) via afferent nerve fibres. The CNS can then compute the position and movement of our extremities in space, which is essential for motor control, maintaining posture, and a stable gait. For example, when a muscle is stretched, the muscle spindle is also stretched, sending signals through the primary and secondary afferents. This information is then processed by the brain as proprioception.
The muscle spindles' unique motor innervation, via gamma-motoneurons (or fusimotor neurons), also allows them to control their sensitivity to stretch. This is important for maintaining muscle spindle tautness and sensitivity over a wide range of muscle lengths.
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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. 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 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. These fibres send information by stretch-sensitive mechanically-gated ion channels of the axons. The primary endings transmit information regarding velocity and muscle length, while the secondary endings sense muscle length.
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons, also known as fusimotor neurons, and to a lesser extent, one or two beta motor neurons. These activate the muscle fibres within the spindle. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle. Activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
The CNS uses the information conveyed by muscle spindles to compute the position and movement of our extremities in space, which is a requirement for motor control, maintaining posture, and a stable gait.
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Spindles regulate the contraction of muscles
Muscle spindles are stretch receptors found in most mammalian muscles. They are responsible for detecting and conveying information about changes in muscle length and velocity to the central nervous system (CNS). This information is essential for the CNS to compute the position and movement of our limbs, enabling us to maintain posture and a stable gait.
The muscle spindle has both sensory and motor components. The sensory component consists of primary type Ia and secondary type II sensory fibres that spiral around and innervate the intrafusal muscle fibres. These fibres respond to changes in muscle length and velocity, transmitting this information to the spinal cord.
The motor component involves activation by motor neurons, particularly gamma motor neurons or fusimotor neurons. These neurons innervate the intrafusal fibres, causing a slight contraction that keeps the muscle spindle taut and sensitive to stretch. This activation is crucial for maintaining the spindle's sensitivity and responsiveness over a wide range of muscle lengths.
The regulation of muscle contraction by muscle spindles occurs through the stretch reflex. When a muscle is stretched, the muscle spindle stretches in parallel, activating the primary and secondary afferents. This activation triggers a response in the alpha motor neurons, which transmit signals to the extrafusal fibres, generating force and resisting the stretch. The alpha-gamma coactivation further ensures that the muscle spindles remain sensitive to stretch across different muscle lengths.
Additionally, the firing rate of muscle spindles during voluntary contraction provides insights into the activation of fusimotor neurons. A decrease in firing rate indicates unloading of the spindle due to muscle shortening, while a maintained or increased firing rate suggests the activation of fusimotor neurons, influencing the spindle's sensitivity.
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They are unique in possessing motor innervation
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 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 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 activation of muscle fibres within the spindle is achieved by up to a dozen gamma motor neurons and, to a lesser extent, by one or two beta motor neurons.
Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. The motor part of the spindle is provided by motor neurons. Up to a dozen gamma motor neurons, also known as fusimotor neurons, activate the muscle fibres within the spindle. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle.
The activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres. Fusimotor neurons are classified as static or dynamic according to the type of muscle fibres they innervate and their effects on the responses of the Ia and II sensory neurons innervating the central, or equatorial, region of the intrafusal fibres. The central region of the intrafusal fibres is in intimate contact with afferent proprioceptive sensory neurons, termed primary "group Ia afferents" and (if present) secondary or "group II afferents". In addition to the sensory neurons, intrafusal muscle fibres are innervated by efferent gamma motor neurons in both polar regions, where they form a cholinergic synapse.
The gamma motor neurons that innervate the spindles terminate on the polar contractile endings of the intrafusal myofibers, while the sensory terminals innervate the central domains of the intrafusal fibres, which mostly lack contractile elements. Alpha-gamma coactivation ensures that muscle spindles maintain sensitivity to stretch over a wide range of muscle lengths.
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Spindles play a critical role in sensorimotor development
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 CNS uses this information to compute the position and movement of our extremities in space, which is a requirement for motor control, maintaining posture, and a stable gait.
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 motor part of the spindle is provided by motor neurons, specifically gamma motor neurons (also known as fusimotor neurons) and, to a lesser extent, beta motor neurons.
The responses of muscle spindles to changes in length play an important role in regulating the contraction of muscles. For example, they can activate motor neurons via the stretch reflex to resist muscle stretch, resulting in a contraction and stiffening of the end parts of the muscle spindle muscle fibres. This is known as the monosynaptic stretch reflex, characterised by the specificity of monosynaptic connections between muscle spindle sensory afferents (Ia axons) and motoneurons (MNs). Ia sensory neurons establish strong monosynaptic connections with homonymous and synergistic MNs but not with functionally antagonistic or unrelated MNs.
Given the role of muscle spindles in conveying information about muscle length and stretch reflex, it is believed that muscle spindles play a critical role in sensorimotor development. For instance, studies have shown that abnormal muscle spindle development can result in reduced sensorimotor connections and impaired sensorimotor control. Furthermore, gain-of-function mutations in HRAS observed in Costello syndrome are associated with increased spindle number, providing insight into the genetic regulation of spindle density.
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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.
Muscle spindles convey length information to the central nervous system (CNS) via afferent nerve fibres. This information is then processed by the brain as proprioception.
Muscle spindles measure changes in muscle length and velocity. They are also believed to play a critical role in sensorimotor development.
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