Logan Steele
Muscle spindles are specialized sensory receptors that inform the central nervous system about the stretch and contraction state of a muscle. They are found within the belly of a skeletal muscle and are composed of 5–14 muscle fibres, with three types: dynamic nuclear bag fibres, static nuclear bag fibres, and nuclear chain fibres. The muscle spindle functions as a stretch detector, sensing how much and how fast a muscle is lengthened or shortened. This information is conveyed to the central nervous system, which then computes the position and movement of our extremities in space, contributing to motor control, posture maintenance, and a stable gait.
| Characteristics | Values |
|---|---|
| Description | Delicate sensory receptors that inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching |
| Location | Within the body of a skeletal muscle |
| Shape | Fusiform (spindle-shaped) |
| Composition | 5-14 muscle fibres, of which there are three types: dynamic nuclear bag fibres, static nuclear bag fibres, and nuclear chain fibres |
| Function | Detect changes in muscle length and speed of stretching, and regulate muscle contraction and stiffness |
| Sensory Components | Primary type Ia sensory fibres, secondary type II sensory fibres |
| Motor Components | Motor neurons, including gamma motor neurons (fusimotor neurons) and beta motor neurons |
| Proprioception | Plays a role in proprioception, helping to compute the position and movement of body extremities |
| Reflexes | Triggers the stretch reflex to prevent overstretching of the muscle |
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What You'll Learn
Muscle spindles are stretch receptors
Muscle spindles are the most frequently found sense organs in skeletal muscles and are present in almost every muscle. They are small sensory organs with an elongated shape. The muscle spindle has both sensory and motor components. The sensory information is conveyed by primary type Ia sensory fibres, which spiral around muscle fibres within the spindle, and secondary type II sensory fibres. The motor part of the spindle is provided by motor neurons, including gamma motor neurons, also known as fusimotor neurons.
When a muscle is stretched, the change in length is transmitted to the spindles and their intrafusal fibres, which are subsequently stretched. This may signal the muscle to contract to prevent it from stretching too far, too quickly. The stimulation of a reflexive muscle contraction is known as the stretch or myotatic reflex. The muscle spindle thus functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far.
The muscle spindle signals muscle length and velocity to the CNS through two types of specialised sensory fibres that innervate the intrafusal fibres. These sensory fibres have stretch receptors that open and close as a function of the length of the intrafusal fibre. The activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres.
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They are located in skeletal muscles
Muscle spindles are specialised sensory receptors that inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are located in skeletal muscles and are present in almost every muscle. The muscle spindle has both sensory and motor components.
The muscle spindle is a receptor located in skeletal muscles and is excited by the stretching of its sensory endings. It may be several millimetres in length and rarely exceeds 100 μm in width. It is surrounded by a spindle-like capsule of connective tissue. The muscle spindle lies in parallel with the ordinary (extrafusal) muscle fibres. Inside the capsule, thinner muscle fibres (intrafusal fibres) run along the long axis.
The intrafusal fibres are up to 8 mm long in humans and about 400 μm long in mice. They are oriented parallel to the surrounding (extrafusal) muscle fibres. Each muscle spindle contains an average of 3–5 (mouse) or 8–20 (human) intrafusal fibres. With a diameter of 8 to 25 μm, intrafusal muscle fibres are much thinner than extrafusal muscle fibres.
The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. These are important concepts in understanding body awareness (also known as proprioception and kinesthetic awareness). As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle.
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They have a unique fusiform shape
Muscle spindles are fusiform (spindle-shaped) sensory receptors that lie within the belly of a skeletal muscle. They are composed of specialised intrafusal muscle fibres that run parallel to the surrounding ordinary (extrafusal) muscle fibres. Intrafusal fibres are thinner than extrafusal fibres, with a diameter of 8 to 25 μm. They are also much shorter, measuring up to 8 mm in humans and about 400 μm in mice.
The muscle spindle's unique fusiform shape is defined by its specialised fibres, which are classified into three types: dynamic nuclear bag fibres, static nuclear bag fibres, and nuclear chain fibres. Dynamic nuclear bag fibres primarily signal information about the rate of change (velocity) of muscle length. Static nuclear bag fibres, on the other hand, signal information about the static length of a muscle. Lastly, nuclear chain fibres derive their name from their unique structure, with their nuclei aligned in a single row (chain) in the centre of the fibre. These fibres also signal information about the static length of the muscle.
The muscle spindle's fusiform structure allows it to effectively transmit information about muscle length and velocity to the central nervous system (CNS). This is achieved through two types of specialised sensory fibres that innervate the intrafusal fibres: Type Ia and Type II sensory fibres. Type Ia fibres, also known as primary afferents, wrap around the central portion of all three types of intrafusal fibres in an annulospiral manner. They respond to muscle length changes and transmit signals to the CNS about velocity and muscle length. Type II fibres, or secondary afferents, branch out and attach to the ends of the central region of the nuclear chain fibres. They respond to muscle length changes as well, but with a smaller velocity-sensitive component, and transmit signals about muscle length to the spinal cord.
The muscle spindle's fusiform shape also enables it to contract and stiffen its end parts through the activation of gamma motor neurons, also known as fusimotor neurons. These neurons specifically innervate the intrafusal muscle fibres, causing them to contract and modify the sensitivity of the muscle spindle sensory afferents to stretch. This contraction opens stretch-sensitive ion channels in the sensory endings, increasing the probability of action potential firing and, consequently, the stretch sensitivity of the muscle spindle afferents.
The fusiform structure of the muscle spindle, with its intrafusal fibres, is essential for its function as a stretch receptor. It allows the muscle spindle to detect changes in muscle length and velocity, transmit this information to the CNS, and regulate muscle contraction and stiffness. This unique shape ensures precise control of contractile activity and contributes to our understanding of body awareness, or proprioception.
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They inform the CNS about muscle length and speed of stretching
Muscle spindles are specialised sensory receptors that inform the central nervous system (CNS) about the stretch and contraction state of a muscle. They are found within the belly of a skeletal muscle and are present in almost every muscle. They are composed of 5–14 muscle fibres, of which there are three types: dynamic nuclear bag fibres, static nuclear bag fibres, and nuclear chain fibres.
The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. They sense how much and how fast a muscle is lengthened or shortened, and this information is transmitted to the CNS via afferent nerve fibres. The CNS then uses this information to compute the position and movement of our extremities in space, which is essential for motor control, maintaining posture, and a stable gait.
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. These fibres send information to the CNS about the stretch and length of the muscle via stretch-sensitive mechanically-gated ion channels of the axons. The motor component 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 cause the contraction and stiffening of the end parts of the muscle spindle muscle fibres.
The muscle spindle plays an important role in regulating the contraction of muscles. When a muscle is stretched, the change in length is transmitted to the spindles and their intrafusal fibres, which are subsequently stretched. This can trigger a reflexive muscle contraction, known as the stretch or myotatic reflex, to prevent the muscle from being stretched too far or too quickly. This contraction is achieved by activating motor neurons via the stretch reflex, which resists the muscle 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 muscle length and convey this information to the central nervous system (CNS) via afferent nerve fibres. This information is then processed by the brain as proprioception.
The muscle spindle has both sensory and motor components. The sensory component is conveyed 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: up to a dozen gamma motor neurons (also known as fusimotor neurons) and, to a lesser extent, one or two beta motor neurons.
The stimulation of a reflexive muscle contraction is known as the stretch or myotatic reflex. When a muscle is stretched, the change in length is transmitted to the spindles and their intrafusal fibres, which are subsequently stretched. This may signal the muscle to contract to prevent it from being stretched too far, too quickly. The muscle spindle thus indicates the degree to which the muscle must be activated to overcome a given resistance.
The activation of neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibres. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle. The contraction of intrafusal muscle fibres activates the muscle spindle, which then sends afferent signals through type Ia and type II sensory neurons to the spinal cord. Type Ia sensory neurons cause the contraction of the muscle, while type Ib causes the relaxation of the antagonist muscles.
Muscles that perform precise movements have many spindles per unit of mass to help ensure exact control of their contractile activity.
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Frequently asked questions
A muscle spindle is a specialised sensory receptor that informs the central nervous system about the stretch and contraction state of a muscle.
Muscle spindles detect changes in the length of a muscle and convey this information to the central nervous system. This helps the CNS compute the position and movement of our extremities in space, which is important for motor control, maintaining posture and a stable gait.
Muscle spindles are composed of specialised fibres called intrafusal muscle fibres. These fibres are thinner than regular muscle fibres and are surrounded by a capsule of connective tissue.
Muscle spindles are found within skeletal muscles and are present in almost every muscle. They lie in parallel with the ordinary (extrafusal) muscle fibres.
