Athena Vale
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 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 component is provided by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The motor component is provided by motor neurons, including gamma motor neurons, also known as fusimotor neurons, which activate the muscle fibres within the spindle.
| Characteristics | Values |
|---|---|
| Definition | Stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. |
| Location | Found within the belly of a skeletal muscle. |
| Function | Convey length information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception. |
| Components | Sensory and motor components. |
| Sensory Information Conveyed By | Primary type Ia sensory fibres and secondary type II sensory fibres. |
| Activation | Activation of up to a dozen gamma motor neurons and, to a lesser extent, by one or two beta motor neurons. |
| Response | When stretched, the change in length is transmitted to the spindles and their intrafusal fibres, which are subsequently stretched. This signals the muscle to contract to prevent it from being overstretched. |
| Role in Locomotion | Muscle spindles play a pivotal role in maintaining robust locomotor robustness in both mice and humans. |
| Problems | An upper motor neuron lesion can lead to a loss of supraspinal inhibition. A basal ganglia disorder can cause excessive supraspinal activation. |
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What You'll Learn
Muscle spindle structure
Muscle spindles are small sensory organs with an elongated shape, found within fascicles of muscle fibres. They are oriented parallel to the power-producing (extrafusal) muscle fibres and connected to those fibres with strands of connective tissue.
The overall structure of muscle spindles consists of contractile fibres in the two polar regions flanking an encapsulated equatorial (i.e. central) non-contractile zone. The sensory terminals in the equatorial region are responsible for signal transduction. Contraction of the polar regions adjusts the length of the muscle spindle and, therefore, the sensitivity of sensory afferents.
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. Typically, the connective tissue 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 randomly arranged nuclei in a bag-like structure in the centre of the intrafusal fibre.
Nuclear bag fibres 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 primary type Ia sensory fibres (large diameter) spiral around all intrafusal muscle fibres, ending near the middle of each fibre. Secondary type II sensory fibres (medium diameter) end adjacent to the central regions of the static bag and chain fibres.
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Muscle spindle function
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 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 muscle contraction, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.
The muscle spindle has both sensory and motor components. The afferent terminals, along with efferent motor neuron terminals, and the intrafusal muscle fibres are encapsulated, and the entire structure is referred to as the muscle spindle. Most spindles consist of several intrafusal muscle fibres and are subject to efferent motor control from dynamic and static gamma Motor Neurons (γMN). The γMNs that innervate the spindles terminate on the contractile endings of the intrafusal myofibers, while the sensory terminals innervate the central domains of the intrafusal fibres, which mostly lack contractile elements.
The two MS afferent subtypes are both activated by stretch of the intrafusal muscle fibres, either caused by passive movement or when induced by the activity of γMNs. The function of γMNs is to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholine by the active γMN, the end portions of the intrafusal muscle fibres contract, thus elongating the non-contractile central portions. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents.
The muscle spindle density is not uniform across the musculoskeletal system. Recent biomechanical modelling suggests that spindle abundance correlates with muscle fascicle length and fibre velocity during dynamic movement, emphasizing the relationship between muscle structure and proprioceptive requirements. Muscle spindles are important for maintaining robust locomotion in both mice and humans.
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Muscle spindle sensory afferents
Muscle spindles are stretch receptors located within the body of a skeletal muscle. They primarily detect changes in muscle length and convey this information to the central nervous system via afferent nerve fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components.
The afferent terminals, along with efferent motor neuron terminals, and the intrafusal muscle fibres are encapsulated, and the entire structure is referred to as the muscle spindle. A typical spindle is innervated by two different classes of muscle spindle afferents: group Ia (or primary) and group II (or secondary) muscle spindle afferents. The two MS afferent subtypes are both activated by a stretch of the intrafusal muscle fibres, either caused by passive movement or when induced by the activity of gamma motor neurons.
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 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 end adjacent to the central regions of the static bag and chain fibres. They respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord.
The sensitivity of the spindle endings is controlled by the activity of the gamma motor fibres that innervate the intrafusal muscle fibres. The gamma motor neurons do not supplement the force of muscle contraction provided by the extrafusal fibres, but instead modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibres contract, thus elongating the non-contractile central portions. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents.
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Muscle spindle stretch reflex
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 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 stretch reflex is the most basic reflex pathway in the body. It is an involuntary, unlearned, repeatable, automatic reaction to a specific stimulus that does not require input from the brain. The muscle stretch reflex is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought to be maintaining the muscle at a constant length, but the response is often coordinated across multiple muscles and even joints.
When a muscle is stretched, primary type Ia sensory fibres of the muscle spindle 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. Likewise, secondary type II sensory fibres respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Ia afferent signals are transmitted monosynaptically to many alpha motor neurons of the receptor-bearing muscle. The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibres of the muscle, which generate force and thereby resist the stretch.
The sensitivity of the reflex is regulated by gamma motor neurons. These lead to tightening or relaxing of muscle fibres within the muscle spindle. It is thought that this takes place to allow preservation of the stretch reflex when muscles are contracted, although not much is known about it.
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Muscle spindle and motor neurons
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 via afferent nerve fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components.
The afferent terminals, along with efferent motor neuron terminals, and the intrafusal muscle fibres are encapsulated, and the entire structure is referred to as the muscle spindle. The muscle spindle consists of several intrafusal muscle fibres and is subject to efferent motor control from dynamic and static gamma motor neurons. 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.
The two types of muscle spindle afferents are group Ia (or primary) and group II (or secondary) muscle spindle afferents. The primary type Ia sensory fibres spiral around all intrafusal muscle fibres, ending near the middle of each fibre. The secondary type II sensory fibres end adjacent to the central regions of the static bag and chain fibres. These fibres send information by stretch-sensitive mechanically-gated ion channels of the axons. The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons also known as fusimotor neurons.
The function of the gamma motor neurons is not to supplement the force of muscle contraction provided by the extrafusal fibres, but to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon the release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibres contract, thus elongating the non-contractile central portions. This opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents.
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Frequently asked questions
A muscle spindle is a stretch receptor within the body of a skeletal muscle that primarily detects changes in the length of the muscle.
Muscle spindles have both sensory and motor components. The sensory component detects changes in muscle length and velocity and transmits this information to the spinal cord in the form of action potentials. The motor component then causes the muscle to contract and resist the stretch.
The primary ending (type Ia sensory fiber) detects changes in muscle length and velocity, while the secondary ending (type II sensory fiber) primarily detects changes in muscle length.
Gamma motor neurons modify the sensitivity of the muscle spindle sensory afferents to stretch. They do not supplement the force of muscle contraction but instead activate the muscle fibres within the spindle.
Muscle spindles play a pivotal role in maintaining robust locomotion in both mice and humans. They help to modulate kinematics and muscle activity in response to perturbations, ensuring accurate and functional locomotion.
