Logan Steele
Muscle spindles are stretch receptors located within the belly of a skeletal muscle. They are fusiform (spindle-shaped) and are composed of contractile intrafusal bag and chain fibres. They are surrounded by a capsule of connective tissue and run parallel to the extrafusal muscle fibres. They inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. This information is then processed by the brain as proprioception.
| Characteristics | Values |
|---|---|
| Location | Within the belly of a skeletal muscle |
| Shape | Fusiform (spindle-shaped) |
| Composition | 5–14 muscle fibers, of which there are three types: dynamic nuclear bag fibers (bag1 fibers), static nuclear bag fibers (bag2 fibers), and nuclear chain fibers |
| Function | Stretch receptors that detect changes in muscle length and velocity |
| Sensory Information Conveyed By | Primary type Ia sensory fibers and secondary type II sensory fibers |
| Activation | Activation of muscle fibers by gamma motor neurons and, to a lesser extent, by one or two beta motor neurons |
| Density | Not uniform across the musculoskeletal system, with spindle abundance correlating to muscle fascicle length and fiber velocity during dynamic movement |
| Presence | Found in almost every muscle, but with low density, making them difficult to detect |
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What You'll Learn
Muscle spindles are located in skeletal muscles
Muscle spindles are stretch receptors located within the body of a skeletal muscle. They are fusiform (spindle-shaped) and are composed of specialised intrafusal muscle fibres. These intrafusal fibres are surrounded by a capsule of connective tissue and run parallel to the extrafusal muscle fibres.
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. This information is crucial for the CNS to compute the position and movement of our extremities in space, which is essential for motor control, maintaining posture, and achieving a stable gait.
The muscle spindle has both sensory and motor components. The sensory component involves the primary type Ia sensory fibres, which spiral around the muscle fibres within the spindle, and the secondary type II sensory fibres. These fibres respond to changes in muscle length and velocity, transmitting this information to the spinal cord.
The motor component involves the activation of muscle fibres within the spindle by gamma motor neurons, also known as fusimotor neurons. These neurons cause a contraction and stiffening of the end parts of the muscle spindle muscle fibres, regulating the sensitivity of the sensory afferents.
Muscle spindles are present in almost every muscle, making them the most frequently found sense organs in skeletal muscles. However, their density within the large muscle mass is low, making them challenging to detect. They play a crucial role in proprioception, providing sensory information about the contractile state and movement of muscles, muscle force, heaviness, stiffness, viscosity, and effort.
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They are embedded in muscle fibres
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in muscle length. They are composed of contractile intrafusal bag and chain fibres that lie parallel to the extrafusal muscle fibres. These intrafusal fibres are embedded in the muscle and are surrounded by a capsule of connective tissue.
The muscle spindle has a diameter of up to 100 μm and a length of several millimetres. It is composed of 5–14 muscle fibres, with each muscle spindle containing an average of 8–20 intrafusal fibres. These fibres are much thinner than extrafusal muscle fibres. The fibres are classified as dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres.
The intrafusal fibres are innervated by gamma motor neurons, which are located in the ventral horn of the spinal cord grey matter. These neurons activate the muscle fibres within the spindle, causing a contraction and stiffening of the end parts of the muscle spindle muscle fibres. The gamma motor neurons also regulate the sensitivity of the sensory afferents, which are located in the non-contractile central region of the intrafusal fibres.
The muscle spindle's function as a length sensor is due to its anatomical relationship with its parent muscle. When a muscle lengthens or stretches, it pulls on the spindle, causing it to stretch as well. This signals the muscle to contract, protecting it from being overstretched. This process is called the stretch reflex.
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They are fusiform (spindle-shaped)
Muscle spindles are fusiform, or spindle-shaped, encapsulated sensory organs located in skeletal muscles. They are composed of contractile intrafusal bag and chain fibres, which lie parallel to the fascicles of the regular, force-producing, extrafusal muscle fibres. The muscle spindle is surrounded by a capsule of connective tissue and is composed of 5–14 muscle fibres, of which there are three types: dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres. The intrafusal fibres are much thinner than extrafusal muscle fibres.
The muscle spindle is a stretch receptor that primarily detects changes in muscle length and velocity. It is activated by the stretching of its sensory endings and conveys length and velocity information to the central nervous system (CNS) via afferent nerve fibres. This information is then processed by the brain as proprioception, which is required for any coordinated movement, normal gait, and the maintenance of a stable posture.
The motor part of the 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. These neurons activate the muscle fibres within the spindle, causing a contraction and stiffening of the end parts of the muscle spindle muscle fibres. The gamma motor neurons control the sensitivity of the muscle spindle to stretch, which is particularly important during movement.
The muscle spindle has been studied in various animals, including cats, mice, and humans. In humans, muscle spindles are present in almost every muscle, with rough estimates suggesting approximately 50,000 muscle spindles in the entire human body. However, they are mostly absent in facial muscles.
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They are stretch receptors
Muscle spindles are stretch receptors located within the body of a skeletal muscle. They are fusiform (spindle-shaped) and are composed of specialised intrafusal muscle fibres. These intrafusal fibres are surrounded by a capsule of connective tissue and run parallel to the extrafusal muscle fibres.
The muscle spindle is a receptor that is excited by the stretching of its sensory endings. It detects changes in the length of the muscle and the speed of stretching. This information is conveyed to the central nervous system (CNS) via afferent nerve fibres, which can be processed by the brain as proprioception. The CNS then computes the position and movement of our extremities in space, which is essential for motor control, posture maintenance, 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 the muscle fibres within the spindle, and secondary type II sensory fibres. The Ia afferents respond to both changes in muscle length and velocity, while the II afferents serve as absolute length detectors. The activation of muscle fibres within the spindle is achieved through gamma motor neurons, and to a lesser extent, beta motor neurons.
The muscle spindle's function as a length sensor arises from its anatomical relationship with its parent muscle. Any length changes in the muscle result in the stretching of the intrafusal fibres, which is then detected by sensory receptors located on the equatorial and polar regions of the muscle spindle. This process is crucial for maintaining robust locomotion and protecting the muscle from being overstretched.
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They are absent in facial muscles
Muscle spindles are encapsulated sensory organs located within the belly of a skeletal muscle. They are fusiform (spindle-shaped) and are composed of 5–14 muscle fibres, with three types: dynamic nuclear bag fibres (bag1 fibres), static nuclear bag fibres (bag2 fibres), and nuclear chain fibres.
Muscle spindles are stretch receptors that detect changes in the length of a muscle and convey this information to the central nervous system (CNS). The CNS then computes the position and movement of our extremities in space, which is essential for motor control, maintaining posture, and a stable gait.
While muscle spindles are found throughout skeletal muscles, they are typically absent from the facial musculature. However, recent research has challenged this notion, reporting the presence of muscle spindles in the platysma muscle in both rhesus macaques and humans. These spindles were found to be robust in appearance.
Other studies have also identified muscle spindles in the zygomaticus major muscle of rhesus macaques and humans, as well as in non-primate species such as horses, white-tailed deer, coyotes, grey wolves, and domestic cats. The presence of muscle spindles in these facial muscles suggests that they may be more prevalent in mammalian facial musculature than previously thought.
It is important to note that the facial muscles have proprioception, which helps coordinate facial movements. This proprioception may be facilitated by other sensory organs that function in place of muscle spindles.
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Frequently asked questions
Muscle spindles are located in skeletal muscles, running parallel to the power-producing muscle fibres.
Muscle spindles are small, spindle-shaped sensory receptors. They are the most frequently found sense organs in skeletal muscles and are present in almost every muscle.
Muscle spindles are stretch receptors that detect changes in the length of the muscle. They convey this length information to the central nervous system, which then computes the position and movement of our extremities in space.
