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Muscle spindles are encapsulated mechanoreceptors found in most mammalian muscles. They are sensory receptors that inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. 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 response of muscle spindles to a certain muscle length or degree of stretch (static response) and the rate of change of muscle length or velocity of stretch (dynamic response) may be influenced separately by fusimotor fibres. Muscle spindle discharge refers to the activation of these muscle spindles, which can be influenced by factors such as temperature, muscle length, velocity of stretch, and disease states.
| Characteristics | Values |
|---|---|
| Definition | Muscle spindles are specialized sensory receptors consisting of fine muscle fibres that inform the central nervous system about the stretch and contraction state of a muscle. |
| Location | Muscle spindles are located in skeletal muscles and are rarely over 100 μm wide. |
| Composition | The muscle spindle is surrounded by a spindle-like capsule of connective tissue. Inside the capsule, thinner muscle fibres (intrafusal fibres) run along the long axis. |
| Function | Muscle spindles inform the CNS about changes in the length of individual muscles and the speed of stretching, enabling the computation of the position and movement of our extremities in space. |
| Types of Endings | Primary and Secondary. Primary endings have greater dynamic sensitivity and respond to changes in muscle length and velocity. Secondary endings respond to muscle length changes with a smaller velocity-sensitive component. |
| Temperature Sensitivity | Temperature affects the nervous outflow from muscle spindles. Warming activates group I afferent units, while cooling depresses them. |
| Clinical Significance | Dysfunction in muscle spindle signalling has been implicated in sensory neuropathies and coordination disorders. Increased resting discharge in muscle spindles may lead to increased muscle stiffness and degenerative events in patients with neuromuscular diseases. |
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What You'll Learn
Muscle spindle function in healthy and diseased muscle
Muscle spindles are delicate sensory receptors that inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. They are present in almost every muscle. When a muscle is stretched, the muscle spindles sense how much and how fast the muscle is being lengthened or shortened. This information is then used by the CNS to compute the position and movement of our extremities in space, which is essential for motor control, maintaining posture, and a stable gait.
The muscle spindle is a receptor located in skeletal muscles, and it is activated by the stretching of its sensory endings. It is made up of fine muscle fibres called intrafusal fibres, which lie parallel to the ordinary muscle fibres. The sensory endings of a primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile central portions of the intrafusal fibres. Gamma motor neurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch-sensitivity of the afferents. The muscle spindle can cause a reflexive muscle contraction to prevent the muscle from being stretched too far or too quickly.
In healthy muscle, the muscle spindle functions as described above, providing important information to the CNS and contributing to motor control and posture maintenance. However, in diseased muscle, the function of muscle spindles can be altered. For example, in muscular dystrophy, there is degeneration of skeletal muscle fibres, but research has shown that the proprioceptive function of muscle spindles is spared. Patients with muscular dystrophy have normal perception of passive movements and can respond with similar spatial and temporal movement characteristics as healthy individuals. Nevertheless, muscle spindle dysfunction can contribute to an unstable gait, frequent falls, and ataxic behaviour in patients with neuromuscular diseases.
Age-related changes in muscle spindle structure and function have also been observed. In aged humans, muscle spindles possess fewer intrafusal fibres and increased capsular thickness. Similar changes have been observed in old rats and mice, with a decrease in the spiral appearance of primary endings and an increase in the number of Ia afferents with large swellings. These structural changes are accompanied by a decline in function, as seen in electrophysiological studies where mature rat muscle spindles displayed a lower dynamic response compared to younger animals.
In summary, muscle spindles play a crucial role in healthy muscle function by providing sensory information to the CNS and contributing to motor control and posture maintenance. However, in diseased states, such as muscular dystrophy and advanced age, the structure and function of muscle spindles can be altered, leading to potential impairments in movement and stability.
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Effects of temperature on muscle spindle discharge
Muscle spindles are specialized sensory receptors that consist of fine muscle fibres that inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are present in almost every muscle.
The effects of temperature on muscle spindle discharge have been studied in cats, with single unit recordings made from afferents of muscle spindles during slow and fast temperature changes. The results suggest that the afferent outflow via thick myelinated fibres from a resting, moderately prestretched muscle strongly depends on temperature.
At raised intramuscular temperatures (about 42°C), the nervous outflow is characterized by increased activity in all of the Ia and many of the Ib afferents, while the majority of group II spindle afferents will be depressed. In contrast, in a cold muscle (about 29°C), the nervous outflow via afferents from primary spindle endings will be reduced, while the net activity from secondary spindle endings will be increased, and no marked changes are expected in the discharges of Ib fibres.
The effects of temperature on muscle spindle discharge have also been studied in mice and humans. In mice, single unit muscle spindle afferent responses to ramp-and-hold stretches and sinusoidal vibratory stimuli have been characterized. In humans, individual sensory afferent action potentials can be studied in vivo by intraneural microelectrodes inserted into accessible peripheral nerves.
In summary, the effects of temperature on muscle spindle discharge vary depending on the specific type of afferent and the temperature range studied. At warmer temperatures, there is generally an increase in activity, while at colder temperatures, activity may be reduced or increased depending on the specific afferent type.
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Muscle spindle discharge in normal and obstructed movements
Muscle spindles are specialised sensory receptors that inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are made up of fine muscle fibres that lie parallel to the ordinary (extrafusal) muscle fibres.
During voluntary movements, the discharge activity of muscle spindle endings located in the tail and hind limb muscles was recorded in cats. During active shortening of the receptor-bearing muscles, both primary and secondary endings tended to fall silent, with this effect being more pronounced the higher the rate of muscle shortening. In unobstructed movements where muscle velocities exceed 0.2 resting lengths per second (lr/sec), the firing patterns of spindle afferents are dominated by their responses to the length variations. At velocities lower than 0.2 lr/sec, fusimotor action may predominate.
When active muscle shortening is unexpectedly halted, both primary and secondary endings resume firing, but the increases in discharge rate are not as abrupt as expected if there were strong co-activation of fusimotor and skeletomotor neurones. This suggests that in unobstructed movements, the firing patterns of spindle afferents are dominated by their responses to length variations in the muscle.
In cats, the response of spindle endings to muscle length or degree of stretch (static response) and the rate of change of muscle length or velocity of stretch (dynamic response) can be influenced separately by fusimotor fibres. Most muscle spindles contain two nuclear-bag fibres that are functionally different and are independently controlled by a static fusimotor (gamma) neurone and a dynamic (beta or gamma) neurone.
In humans, individual sensory afferent ("single unit") action potentials can be studied in vivo by intraneural microelectrodes inserted into accessible peripheral nerves (microneurography). In mice, single-unit muscle spindle afferent responses to ramp-and-hold stretches and sinusoidal vibratory stimuli have been well characterised.
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Muscle spindle structure and development
Muscle spindles are encapsulated sensory organs found in most muscles. They are composed of 5–14 muscle fibres, with two types of afferent sensory fibre endings. The first type is large myelinated nerves called Ia or primary afferents, which spiral around all intrafusal muscle fibres, ending near the middle of each fibre. The second type is the secondary type II sensory fibres, which have a medium diameter and end adjacent to the central regions of the static bag and chain fibres. The muscle spindle is fusiform (spindle-shaped), and the fibres that make it up are called intrafusal muscle fibres. The regular muscle fibres outside of the spindle are called extrafusal muscle fibres.
The intrafusal fibres are innervated by motor nerves (the gamma motor neurons) and two different types of sensory fibres: the type Ia stretch receptors and the type II afferent sensory receptors. The type Ia sensory fibres convey sensory information, while the type II sensory fibres are stretch-sensitive and sense the rate at which the muscle is stretched. The type II afferents are slowly adapting and inform the CNS about the static stretch of the muscle.
The muscle spindle is located within the belly of a skeletal muscle and has a capsule of connective tissue. It runs parallel to the extrafusal muscle fibres, unlike the Golgi tendon organs, which are oriented in series. The intrafusal fibres connect to the shell of the spindle from the inside, while the extrafusal fibres have elastic connections to the outside of the spindle shell. The muscle spindle is, therefore, deformed in the direction of the length change of the extrafusal fibres.
The muscle spindle is an important proprioceptor, and its development is essential for motor control, maintaining posture, and a stable gait. Studies have shown that muscle spindles play a critical role in sensorimotor development. For example, the postnatal development of mouse muscle spindles involves the transformation of the "web-like" appearance of the sensory nerve terminal into the typical annulospiral ending.
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Muscle spindle discharge and motor control
Muscle spindles are specialised sensory receptors that consist of fine muscle fibres that inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are found in most mammalian muscles and play a critical role in sensorimotor development. The CNS uses the information from the muscle spindles to compute the position and movement of our extremities in space, which is essential for motor control, maintaining posture, and a stable gait.
The muscle spindle is a receptor located in skeletal muscles and is excited by the stretching of its sensory endings. It is surrounded by a spindle-like capsule of connective tissue, with thinner muscle fibres (intrafusal fibres) running along its long axis. The spindle's centre contains large groups of nuclei but has little contractility due to the low number of myofibrils present. The sensory afferents of the muscle spindle have two types of endings: primary and secondary. Both types are sensitive to changes in muscle length and velocity, with the primary endings exhibiting a greater dynamic sensitivity.
The primary type Ia sensory fibres spiral around all intrafusal muscle fibres, ending near the middle of each fibre. These fibres respond to both changes in muscle length and velocity and transmit this information to the spinal cord in the form of changes in the rate of action potentials. The secondary type II sensory fibres end adjacent to the central regions of the static bag and chain fibres. They respond to muscle length changes with a smaller velocity-sensitive component and transmit this information to the spinal cord as well. The activation of these sensory fibres causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
The discharge of muscle spindles is influenced by the fusimotor system, which includes alpha and beta fibres, and gamma efferent fibres. The fusimotor system provides a background discharge to the muscle spindles, allowing them to detect irregularities in movement and initiate appropriate reflex contractions. The response of the muscle spindles to a certain muscle length or degree of stretch (static response) and the rate of change of muscle length or velocity of stretch (dynamic response) can be separately influenced by fusimotor fibres. The dynamic response is particularly important during unobstructed movements when muscle velocities exceed 0.2 resting lengths per second, where the firing patterns of spindle afferents are dominated by their responses to length variations.
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Frequently asked questions
A muscle spindle is a specialised sensory receptor consisting of fine muscle fibres that inform the central nervous system about the stretch and contraction state of a muscle.
Muscle spindle afferents are found in muscle fascicles of peripheral nerves and supply a specific muscle. They are often spontaneously active at rest, with the mean frequency of the tonic discharge increasing during the stretch of the receptor-bearing muscle.
The central nervous system computes the position and movement of our extremities in space, which is a requirement for motor control, maintaining posture, and a stable gait.
The primary type Ia sensory fibres spiral around all intrafusal muscle fibres, ending near the middle of each fibre. When a muscle is stretched, these fibres respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials.
