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Muscle spindles are sensory structures that inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. They are made up of a group of small muscle fibres called intrafusal muscle fibres, which are typically 4–8 mm long. The intrafusal fibres are surrounded by regular muscle fibres called extrafusal fibres. When a muscle is stretched, the muscle spindle senses how much and how fast it is lengthened or shortened, and signals the muscle to contract to prevent it from overstretching. This process is called the stretch reflex.
| Characteristics | Values |
|---|---|
| Definition | Muscle spindles are sophisticated structures that house many sensory endings sensitive to muscle length and velocity. |
| Discovery | Muscle spindles were first discovered in 1862 by Kölliker and named ‘Muskelspindeln’ (i.e. muscle spindles in German) by Kühne in 1863. |
| Function | Muscle spindles inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. |
| Structure | 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. |
| Types of Fibres | Nuclear chain intrafusal fibres and nuclear bag fibres. |
| Types of Afferent Sensory Fibres | Primary (group Ia) afferent and secondary (group II) afferent. |
| Motor Neurons | Gamma motor neurons, also known as fusimotor neurons. |
| Density | There are approximately 50,000 muscle spindles in the human body. |
| Role | Muscle spindles play a critical role in sensorimotor development and maintaining locomotion. |
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What You'll Learn
- Muscle spindles are a group of specialised muscle fibres within a muscle
- They are stretch receptors with their own motor supply
- They inform the central nervous system about changes in muscle length and speed of stretching
- They are made up of intrafusal muscle fibres, which are up to 8mm long in humans
- They play a critical role in sensorimotor development and maintaining locomotion
Muscle spindles are a group of specialised muscle fibres within a muscle
Muscle spindles are made up of intrafusal muscle fibres, which are typically 4–8 mm long and are oriented parallel to the regular, power-producing extrafusal muscle fibres. The intrafusal fibres are enclosed within a connective tissue capsule and consist of two types: nuclear chain intrafusal fibres and nuclear bag fibres. The nuclear chain intrafusal fibres have a set of aligned nuclei in the centre, while the nuclear bag fibres have a random arrangement of nuclei in a bag-like structure.
The sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibres, which spiral around the central, non-contractile portions of the intrafusal fibres. These sensory fibres respond to changes in muscle length and velocity and transmit this information to the spinal cord. The group Ia afferents have a large diameter, while the group II afferents have a medium diameter.
In addition to the sensory neurons, the intrafusal fibres are also innervated by efferent gamma motor neurons, which regulate the sensitivity of the sensory afferents. Activation of these neurons causes a contraction of the end portions of the intrafusal fibres, elongating the central portions and increasing the stretch-sensitivity of the muscle spindle.
Muscle spindles play a critical role in sensorimotor development and maintaining robust locomotion. They are involved in proprioception, which is the sense of movement and body position. Dysfunction in muscle spindle signalling has been implicated in sensory neuropathies, coordination disorders, and neuromuscular diseases.
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They are stretch receptors with their own motor supply
Muscle spindles are stretch receptors with their own motor supply. They are considered the most famous proprioceptor in the human body. They are small sensory organs with an elongated shape, consisting of a group of small, specialised muscle fibres within a muscle. These fibres are called intrafusal muscle fibres and are oriented parallel to the regular, power-producing extrafusal muscle fibres.
The intrafusal fibres are enclosed in a sheath of connective tissue and are typically 4–8 mm long. Each muscle spindle contains an average of 8–20 intrafusal fibres. These fibres are innervated by two kinds of neurons: afferent sensory neurons and efferent motoneurons. The sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibres, which spiral around the non-contractile central portions of the intrafusal fibres.
The motor part of the spindle is provided by motor neurons, specifically gamma motor neurons, also known as fusimotor 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 function of these gamma motor neurons is to modify the sensitivity of the muscle spindle sensory afferents to stretch. When a muscle is stretched, the primary type Ia sensory fibres of the muscle spindle respond to changes in muscle length and velocity, transmitting this information to the spinal cord.
The muscle spindle is a sophisticated structure that plays a critical role in sensorimotor development and proprioception. It keeps the central nervous system updated about the position and movement of body parts, which is essential for motor control, maintaining posture, and a stable gait.
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They inform the central nervous system about changes in muscle length and speed of stretching
Muscle spindles are sophisticated structures that house many sensory endings sensitive to muscle length and velocity. They are stretch receptors with their own motor supply consisting of several intrafusal muscle fibres. The intrafusal fibres are up to 8mm long in humans and are oriented parallel to the regular, power-producing extrafusal muscle fibres.
The muscle spindle informs the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes 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 is, therefore, critical for sensorimotor development.
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. The primary type Ia sensory 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. The secondary type II sensory fibres respond to muscle length changes and transmit this signal to the spinal cord with a smaller velocity-sensitive component.
The muscle spindle's response to changes in muscle fibre length is mediated by two kinds of neurons: afferent sensory neurons and efferent motoneurons. The gamma motor neurons, or fusimotor neurons, are a type of efferent motoneuron that activates the muscle fibres within the spindle. They do not supplement the force of muscle contraction but modify the sensitivity of the muscle spindle sensory afferents to stretch.
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They are made up of intrafusal muscle fibres, which are up to 8mm long in humans
Muscle spindles are sophisticated structures that house many sensory endings sensitive to muscle length and velocity. They are small sensory organs with an elongated shape, involved in proprioception. They are made up of intrafusal muscle fibres, which are up to 8mm long in humans. These intrafusal fibres are muscle fibres located inside the spindles. They connect to the shell of the spindle from the inside, while extrafusal fibres have elastic connections to the outside of the spindle shell.
The intrafusal fibres are oriented parallel to the regular, power-producing extrafusal muscle fibres. They are called intrafusal because they are 'fused inside' the muscle spindle. These fibres have no connections to the muscle tendon, and their contraction has no visible external mechanical effect. The central portions of these fibres are non-contractile.
The muscle spindle consists of a group of fine muscle fibres, typically enclosed within a connective tissue capsule. Each muscle spindle contains on average 8–20 (human) intrafusal fibres. There are three types of intrafusal fibres: nuclear bag1, nuclear bag2, and nuclear chain 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 a stretch receptor with its own motor supply. 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. These fibres send information by stretch-sensitive mechanically-gated ion channels of the axons.
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They play a critical role in sensorimotor development and maintaining locomotion
Muscle spindles are sophisticated structures that house many sensory endings sensitive to muscle length and velocity. They are small sensory organs with an elongated shape, found in almost every muscle. These spindles are made up of contractile fibres in two polar regions, with a non-contractile central zone. The central zone is where signal transduction occurs.
The muscle spindle is a stretch receptor that informs the central nervous system (CNS) of the contractile state of the muscle. When a muscle is stretched, the spindle is activated and stretched, causing the muscle to contract. This process is called the stretch reflex and it protects the muscle from being overstretched. The muscle spindle's role in the stretch reflex is critical for maintaining locomotion.
The CNS relies on the muscle spindle to compute the position and movement of body parts, which is essential for motor control, maintaining posture, and a stable gait. The muscle spindle's ability to sense and respond to muscle length and velocity changes ensures that the body can maintain balance and coordination during movement. This is particularly important for activities that require quick adjustments, such as sports or dynamic movements.
Additionally, the muscle spindle plays a critical role in sensorimotor development. It contributes to the concept of 'muscular sense' or 'kinaesthesis', which is our sense of movement. Dysfunction in muscle spindle signalling has been linked to sensory neuropathies and coordination disorders. Understanding the genetic mutations affecting spindle development can inform targeted therapies for proprioceptive deficits and neuromuscular diseases, which often result in unstable gait and frequent falls.
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Frequently asked questions
A muscle spindle is a group of specialised muscle fibres within a muscle. They are sensory structures that inform the central nervous system (CNS) about the position and movement of body parts.
Muscle spindles are stretch receptors that detect how much and how fast a muscle is being lengthened or shortened. This information is used by the CNS to compute the position and movement of our extremities in space, which is necessary for motor control, maintaining posture, and a stable gait.
Muscle spindles consist of intrafusal muscle fibres, which are up to 8-10 mm long. Each muscle spindle contains an average of 8-20 intrafusal fibres. The intrafusal fibres are of two types: nuclear chain fibres and nuclear bag fibres. The nuclear bag fibres are further classified as static or dynamic.
Muscle spindles are innervated by afferent sensory neurons and efferent motoneurons. The sensory innervation arises from group Ia and group II afferent fibres, which spiral around the intrafusal fibres. When the muscle is stretched, these fibres respond to changes in muscle length and velocity and transmit this information to the spinal cord. This process is known as the stretch reflex and helps protect the muscle from being overstretched.
