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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 (CNS) via afferent nerve fibres. This information can be processed by the brain as proprioception. The muscle spindle has both sensory and motor components. The sensory component detects muscle stretch and contributes to the sensation of angular position and movement of joints. The motor component regulates the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.
| Characteristics | Values |
|---|---|
| Description | Delicate sensory receptors |
| Location | Present in almost every muscle |
| Function | Inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching |
| Contribute to the sensation of angular position and movement of joints | |
| Help maintain posture and gait | |
| Regulate muscle stiffness | |
| Play a critical role in sensorimotor development | |
| Help compensate for fatigue | |
| Types of fibres | Intrafusal fibres and extrafusal fibres |
| Types of intrafusal fibres | Nuclear bag fibres and nuclear chain fibres |
| Types of afferent sensory fibres | Type Ia and Type II |
| Types of neurons | Alpha motor neurons, beta motor neurons, gamma motor neurons, interneurons |
| Diseases associated | Neuromuscular diseases, muscular dystrophy, basal ganglia disorder, spastic hypertonia |
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What You'll Learn
Muscle spindles mediate muscle stretch
Muscle spindles are specialised sensory organs found in almost every muscle. They are the most frequently found sense organs in skeletal muscles. They are made up of a bundle of specialised muscle fibres called intrafusal fibres, which are interspersed among the regular muscle fibres, or extrafusal fibres. The intrafusal fibres are of two types: nuclear bag fibres and nuclear chain fibres. Nuclear bag fibres are thicker and contain many nuclei that are centrally located, while nuclear chain fibres are shorter and thinner and have fewer nuclei in the central area of the fibre.
The responses of muscle spindles to changes in length also play a role in regulating muscle contraction. When a muscle is stretched, the muscle spindles activate motor neurons via the stretch reflex to resist the muscle stretch. This helps to prevent damage to the muscles from overstretching. The muscle spindles work together with the golgi tendon organs to regulate muscle stiffness and maintain robust locomotion.
Impaired muscle spindle function can lead to abnormal muscle tone, such as spasticity, and contribute to an unstable gait, frequent falls, and ataxic behaviour in patients with neuromuscular diseases. Therapeutic strategies for neuromuscular diseases should therefore aim to restore and maintain muscle spindle function.
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They detect changes in muscle length
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in muscle length. They are the most frequently found sense organs in skeletal muscles and are present in almost every muscle.
The mammalian muscle spindle comprises a small bundle of specialized muscle fibres (intrafusal fibres) and the sensory ending of a type Ia afferent fibre wrapped around them. The regular muscle fibres are called extrafusal fibres. Interspersed among these fibres are small encapsulated sensory receptors that have a fusiform or spindle shape. The entire apparatus is called the muscle spindle.
The central (equatorial) part of the intrafusal fibres is in intimate contact with afferent proprioceptive sensory neurons, termed primary "group Ia afferents" (forming the annulospiral endings) and (if present) secondary or "group II afferents". The polar regions of intrafusal fibres contain most of the contractile elements. The central region of the fibres does not contain contractile elements; this represents the sensory receptor area of the spindle.
When a muscle is stretched, the 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 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.
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They regulate muscle contraction
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They are delicate sensory receptors that inform 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, for maintaining posture, and for a stable gait.
The muscle spindle has both sensory and motor components. The sensory information conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The primary type Ia 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 fibres respond to muscle length changes 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 activation of these neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
The muscle spindle and the GTO (Golgi tendon organ) are two important proprioceptors that play a role in flexibility. They work reflexively to regulate muscle stiffness. The GTO can be considered the opposite of the muscle spindle, which serves to produce muscle contraction. When a GTO is stimulated, it causes its associated muscle to relax by interrupting its contraction.
The muscle spindles trigger the stretch reflex where an overstretched muscle spindle sends afferent signals through type Ia and type II sensory neurons to the spinal cord. Ia sensory neurons cause the contraction of the muscle and Ib causes the relaxation of the antagonist muscles. The GTO triggers the GTO reflex when a muscle is being over-contracted, sending afferent signals through type Ib afferent fibre to the spinal cord, which triggers the inhibition of the contracting muscle and contraction of the antagonist muscles.
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They play a role in sensorimotor development
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 (CNS) via afferent nerve fibres. This information can be processed by the brain as proprioception. They are the most frequently found sense organs in skeletal muscles and are present in almost every muscle.
The muscle spindle has both sensory and motor components. The sensory information conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The primary type Ia 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 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 alpha motor neurons then transmit via their efferent axons to the extrafusal fibres of the muscle, which generate force and thereby resist the stretch.
The muscle spindle and the Golgi tendon organ (GTO) are two important proprioceptors that play a role in flexibility. The GTO can be considered the opposite of the muscle spindle, which serves to produce muscle contraction. When a GTO is stimulated, it causes its associated muscle to relax by interrupting its contraction.
The muscle spindle plays a critical role in sensorimotor development. For example, after a stroke or spinal cord injury in humans, spastic hypertonia (spastic paralysis) often develops, whereby the stretch reflex in flexor muscles of the arms and extensor muscles of the legs is overly sensitive. This results in abnormal postures, stiffness and contractures. Hypertonia may be the result of over-sensitivity of alpha motor neurons and interneurons to the Ia and II afferent signals.
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They contribute to proprioception
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 (CNS) via afferent nerve fibres. This information can be processed by the brain as proprioception.
The mammalian muscle spindle comprises a small bundle of specialised muscle fibres (intrafusal fibres) and the sensory ending of a Ia afferent fibre wrapped around them. The regular muscle fibres are called extrafusal fibres. Interspersed among these fibres are small encapsulated sensory receptors that have a fusiform or spindle shape. The entire apparatus is called the muscle spindle.
The muscle spindle informs the CNS of the contractile state of the muscle by sending afferent impulses to the spinal cord when the muscle spindle is stretched. 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. The central region of the intrafusal muscle fibre contains the muscle fibre's nuclei, and the arrangement of the nuclei determines whether the intrafusal muscle fibres are considered nuclear bag fibres or nuclear chain fibres.
The muscle spindles contribute to proprioception by detecting muscle stretch. They stimulate flexor muscle spindles when a joint extends and flexor muscle fibres are stretched. The group Ia afferents are predominantly influenced by the dynamic aspect of stretch, so they mainly signal the angular velocity of joint movement. On the other hand, the group II afferents are mostly influenced by the static aspects of stretch, so they signal the angular joint position.
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Frequently asked questions
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They are the most frequently found sense organs in skeletal muscles and are present in almost every muscle.
Muscle spindles inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. This information is then used by the CNS 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 muscle spindle has both sensory and motor components. The sensory component detects changes in muscle length and velocity, while the motor component activates motor neurons to regulate muscle contraction and resist muscle stretch.
Impaired muscle spindles can lead to abnormal muscle tone, such as spasticity, and contribute to an unstable gait, frequent falls, and ataxic behavior. In neuromuscular diseases, therapeutic strategies should aim to restore and maintain muscle spindle function to improve proprioception and motor control.
