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Muscle spindles are small, spindle-shaped sensory receptors located in skeletal muscle tissue. They are composed of several differentiated muscle fibres (intrafusal fibres) that are enclosed in a spindle-shaped connective tissue sac. The ends of the intrafusal fibres are contractile, but the central portion is non-contractile. Muscle spindles provide sensory information concerning changes in the length and tension of muscle fibres. Their main function is to respond to the stretch of a muscle and, through reflex action, to produce a stronger contraction to reduce the stretch.
| Characteristics | Values |
|---|---|
| Shape | Spindle-shaped |
| Type of Muscle Fiber | Skeletal muscle tissue |
| Function | Inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching |
| Composition | 5-14 muscle fibers |
| Types of Muscle Fibers | Dynamic nuclear bag fibers (bag1 fibers), static nuclear bag fibers (bag2 fibers), and nuclear chain fibers |
| Sensory Fibers | Primary type Ia sensory fibers (large diameter) and secondary type II sensory fibers (medium diameter) |
| Motor Neurons | Gamma motor neurons (fusimotor neurons) and beta motor neurons |
| Sensory Receptors | Group Ia and group II afferent fibers |
| Stretch Reflex | Myotatic reflex |
| Immunological Component | Muscle spindle macrophages (MSMPs) |
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What You'll Learn
Muscle spindles are small, spindle-shaped sensory receptors
The muscle spindle is attached in parallel to the extrafusal muscle fibres, which are the regular muscle fibres outside of the spindle. When a muscle is stretched, the spindle is also stretched, and this change in length is transmitted to the intrafusal fibres, which are subsequently stretched. This causes the intrafusal fibres to activate the extrafusal fibres, leading to a contraction of the muscle, thereby protecting it from being overstretched. This process is known as the stretch reflex.
The ends of the intrafusal fibres are contractile, while the central portion is non-contractile. The contractile portions of the intrafusal fibres are innervated by gamma motor neurons, which are responsible for modifying the sensitivity of the muscle spindle sensory afferents to stretch. When the gamma motor neurons are activated, they cause a contraction and stiffening of the end parts of the intrafusal fibres, elongating the central portions. This opens stretch-sensitive ion channels, increasing the probability of action potential firing and, thus, increasing the stretch-sensitivity of the muscle spindle afferents.
The muscle spindle density varies across the musculoskeletal system and is related to muscle fascicle length and fibre velocity during dynamic movement. Muscles that perform complex or precise movements have more spindles to help ensure exact control of their contractile activity. The muscle spindle is a delicate sensory receptor, and its dysfunction can contribute to an unstable gait, frequent falls, and ataxic behaviour in patients with neuromuscular diseases.
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They are located in skeletal muscle tissue
Muscle spindles are fusiform or spindle-shaped sensory receptors located in skeletal muscle tissue. They are small, and run parallel to the main muscle fibres (extrafusal fibres). The muscle spindle apparatus consists of a group of fine muscle fibres, called intrafusal muscle fibres, 4–10 mm long, whose central portions are not contractile. The ends of the intrafusal fibres are contractile.
The intrafusal fibres are enclosed in a spindle-shaped connective tissue capsule. Typically, this 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 nuclear bags actually come in two varieties: static bags and dynamic bags.
The muscle spindle's function is to inform the central nervous system (CNS) of the contractile state of the muscle by sending afferent impulses to the spinal cord when the muscle spindle is stretched. It does this by sensing how much and how fast a muscle is lengthened or shortened. When a muscle is stretched, the muscle spindle's length is also altered, and this change in length is transmitted to the intrafusal fibres, which are subsequently stretched. This signals the muscle to contract, to prevent it from being overstretched.
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons, also known as fusimotor 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.
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They consist of intrafusal muscle fibres
Muscle spindles are fusiform or spindle-shaped sensory receptors located in skeletal muscle tissue. They are small, and they run parallel to the main muscle fibres, which are called extrafusal fibres.
The muscle spindle consists of a group of fine muscle fibres, called intrafusal muscle fibres, which are 4–10 mm long. The ends of the intrafusal fibres are contractile, but the central portion is non-contractile. The connective tissue capsule that encloses the muscle spindle encloses two different types of intrafusal fibres.
The first type of intrafusal fibres are the nuclear chain fibres, which have a set of aligned nuclei in the centre. The second type are the nuclear bag fibres, which have a clump of nuclei randomly arranged in a bag-like structure in the centre of the intrafusal fibre. These nuclear bags come in two varieties: static bags and dynamic bags. The static nuclear bag and nuclear chain fibres receive a second kind of innervation, classified as II afferents.
The muscle spindle's function is to inform the central nervous system (CNS) of the contractile state of the muscle. It does this by sending afferent impulses to the spinal cord when the muscle spindle is stretched. This is called the stretch reflex. When a muscle spindle’s associated muscle is stretched, it pulls on the spindle, causing it to stretch and lose its spiral shape. This signals the muscle to contract, protecting it from being overstretched.
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They are activated by gamma motor neurons
Muscle spindles are small, spindle-shaped sensory receptors located within skeletal muscle tissue. They are a group of fine muscle fibres, known as intrafusal muscle fibres, which are enclosed in a spindle-shaped capsule of connective tissue. The intrafusal fibres are innervated by gamma motor neurons.
The ends of intrafusal fibres are contractile, but the central portion is non-contractile. The gamma motor neurons innervate these contractile portions, which are the striated ends of the spindle. The activation of these contractile mechanisms by the gamma motor neurons is similar to the activation of ordinary motor neurons.
The role of the gamma motor neurons is to control the length of the intrafusal fibres. By activating the contractile mechanism, the gamma motor neurons maintain the sensitivity of the muscle spindle when muscles contract. This allows the muscle spindle to maintain peak operation at all muscle lengths.
The muscle spindle, as a whole, is responsible for informing the central nervous system (CNS) of the contractile state of the muscle. When the muscle spindle is stretched, it sends afferent impulses to the spinal cord. This provides the CNS with important information about muscle length and its changes.
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They are stretch detectors
Muscle spindles are small, spindle-shaped sensory receptors located in skeletal muscle tissue. They are composed of specialised intrafusal muscle fibres, which are enclosed in a fusiform or spindle-shaped capsule of connective tissue. This entire apparatus is called the muscle spindle.
The muscle spindle's main function is to act as a stretch detector, sensing how much and how fast a muscle is lengthened or shortened. When a muscle is stretched, the spindle is also stretched, as it is attached in parallel to the regular muscle fibres. This activates the muscle spindle, which then signals the muscle to contract to prevent it from being overstretched.
The muscle spindle achieves this through its intrafusal fibres, which have contractile ends and a non-contractile central portion. When the muscle spindle is stretched, the ends of the intrafusal fibres contract, elongating the central portion. This opens stretch-sensitive ion channels of the sensory endings, increasing the probability of action potential firing.
The muscle spindle then sends afferent impulses to the spinal cord, transmitting information about changes in muscle length and velocity. This information is crucial for the central nervous system to compute the position and movement of our extremities, which is essential for motor control, maintaining posture, and a stable gait.
The gamma motor neurons also play a role in modifying the sensitivity of the muscle spindle sensory afferents to stretch. They do not supplement the force of muscle contraction but instead increase the stretch-sensitivity of the muscle spindle afferents. This helps the muscle spindle maintain its sensitivity when muscles contract.
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Frequently asked questions
A muscle spindle is a small, spindle-shaped sensory receptor located in skeletal muscle tissue. It informs the central nervous system (CNS) of the contractile state of the muscle by sending afferent impulses to the spinal cord when the muscle spindle is stretched.
Muscle spindles are 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.
Muscle spindles provide sensory information concerning changes in the length and tension of muscle fibres. They respond to the stretch of a muscle and, through reflex action, produce a stronger contraction to reduce the stretch.
When a muscle spindle is stretched, it pulls on the spiral shape of the spindle, causing it to stretch as well. This signals the muscle to contract, protecting it from being overstretched.
A simple example of muscle spindle activity is the knee-jerk reflex, where a sharp tap on the patellar tendon stretches the muscle spindle fibres, causing a sudden kicking movement of the lower leg.
