Dominic Stone
Muscle spindles are specialised sensory organs that detect changes in the length and tension of muscle fibres. They are stretch-responsive sensory receptors that lie in parallel with 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. Muscle spindles also contribute to the sensation of angular position and movement of joints.
| Characteristics | Values |
|---|---|
| Main function | To respond to the stretch of a muscle and, through reflex action, to produce a stronger contraction to reduce the stretch |
| Shape | Fusiform |
| Position | Attached in parallel to the regular or extrafusal fibres of the muscle |
| Number | There are more spindles in muscles that perform complex movements |
| Cells | Two specialised cells within the spindle, called intrafusal fibres |
| Afferents | Two sensory afferents and one motor efferent innervating the intrafusal fibres |
| Neurons | Cell bodies of proprioceptive afferent fibres constitute 5-10% of all neurons in the dorsal root ganglion |
| Alert | Alert the brain that nearby joints and soft tissues are in danger of being stretched too far |
| Muscle activation | Indicate the degree to which the muscle must be activated in order to overcome a given resistance |
| Nerve signals | Emit low-level sensory nerve signals that assist in the maintenance of muscle tonus and postural adjustments |
| Response | Respond to stretch by an antagonistic muscle, to gravity, or to a load being applied to the muscle |
| Receptor | A stretch-responsive sensory receptor that lies in parallel with muscle fibres |
| Motor innervation | Has a motor innervation (the fusimotor system), through which its responsiveness can be controlled by the brain |
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What You'll Learn
Muscle spindles are stretch-responsive sensory receptors
The main function of muscle spindles is to respond to the stretch of a muscle and, through reflex action, to produce a stronger contraction to reduce the stretch. They emit low-level sensory nerve signals that assist in the maintenance of muscle tonus and postural adjustments. Muscle tonus is a state of low-level muscle contraction at rest.
Muscle spindles are also important for body awareness, also known as proprioception and kinesthetic awareness. They indicate the degree to which the muscle must be activated in order to overcome a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle.
Muscle spindles contribute to the sensation of angular position and movement of joints that are moved by the relevant muscles. For example, if a joint extends and flexor muscle fibres are stretched, flexor muscle spindles are stimulated.
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They lie in parallel with muscle fibres
Muscle spindles are fusiform-shaped sensory receptors that lie in parallel with muscle fibres. They are shortened by muscle contraction and stimulated by stretch. 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.
Muscle spindles are attached in parallel to the regular or extrafusal fibres of the muscle. Consequently, when the muscle is stretched, so is the spindle. There are two specialised cells within the spindle, called intrafusal fibres. There are two sensory afferents and one motor efferent innervating the intrafusal fibres. The muscle spindle is an unusual receptor because it has a motor innervation (the fusimotor system), through which its responsiveness can be controlled by the brain.
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). Spindles thus indicate the degree to which the muscle must be activated in order to overcome a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle.
Muscle spindles also respond to stretch by an antagonistic muscle, to gravity, or to a load being applied to the muscle. The head jerk is an example of the response to gravity, but other muscles such as the back extensors and quadriceps function the same way for unconscious postural adjustments.
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They are shortened by muscle contraction
Muscle spindles are fusiform in shape and are attached in parallel to the regular or extrafusal fibres of the muscle. They are shortened by muscle contraction and stimulated by stretch. They provide sensory information about 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.
Muscle spindles are specialised sensory organs that contribute to the muscle sense by detecting muscle stretch. They are composed of two specialised cells, called intrafusal fibres, which are innervated by two sensory afferents and one motor efferent. These afferent fibres constitute 5–10% of all neurons in the dorsal root ganglion. The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far, which is important for body awareness and proprioception.
As a load increases, the muscle is stretched to a greater extent, and the engagement of muscle spindles results in greater activation of the muscle. Muscle spindles respond not only to stretch but also to gravity and load stimuli. For example, when a person is standing with their elbows at 90 degrees and palms facing up, the placement of a 10-lb weight in their hands will stimulate the muscle spindles.
The muscle spindle is an unusual receptor because it has a motor innervation (the fusimotor system), which allows its responsiveness to be controlled by the brain. Afferents from muscle spindle endings project to the spinal cord, where they have extensive reflex connections, and also up the spinal cord to the cerebellum and brain. These connections are involved in programming and updating the command for movement and in the conscious appreciation of where the limb is in space.
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They are stimulated by stretch
Muscle spindles are sensory organs that detect changes in the length and tension of muscle fibres. They are stimulated by stretch and respond to this through reflex action, producing a stronger contraction to reduce the stretch.
The muscle spindle is fusiform in shape and is attached in parallel to the regular or extrafusal fibres of the muscle. When the muscle is stretched, so is the spindle. There are two specialised cells within the spindle, called intrafusal fibres, and two sensory afferents and one motor efferent innervating the intrafusal fibres.
The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle. This is important for understanding body awareness, also known as proprioception and kinesthetic awareness.
The muscle spindle is an unusual receptor because it has a motor innervation (the fusimotor system), through which its responsiveness can be controlled by the brain. Afferents from muscle spindle endings project to the spinal cord, where they have extensive reflex connections, and also up the spinal cord to the cerebellum and brain, where they are involved in programming and updating the command for movement and in the conscious appreciation of where the limb is in space.
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They are involved in programming and updating the command for movement
Muscle spindles are specialised sensory organs that detect changes in the length and tension of muscle fibres. They are fusiform in shape and are attached in parallel to the regular or extrafusal fibres of the muscle. When the muscle is stretched, so is the spindle.
The muscle spindle functions to alert the brain that nearby joints and soft tissues are in danger of being stretched too far. This is important for understanding body awareness, also known as proprioception and kinesthetic awareness. Spindles indicate the degree to which the muscle must be activated in order to overcome a given resistance. As a load increases, the muscle is stretched to a greater extent, and engagement of muscle spindles results in greater activation of the muscle.
Muscle spindles also respond to stretch by an antagonistic muscle, to gravity, or to a load being applied to the muscle. For example, if a joint extends and flexor muscle fibres are stretched, flexor muscle spindles are stimulated.
Afferents from muscle spindle endings project to the spinal cord, where they have extensive reflex connections, and also up the spinal cord to the cerebellum and brain, where they are involved in programming and updating the command for movement and in the conscious appreciation of where the limb is in space. The muscle spindle is an unusual receptor because it has a motor innervation (the fusimotor system), through which its responsiveness can be controlled by the brain.
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Frequently asked questions
Muscle spindles are specialised sensory organs that detect changes in the length and tension of muscle fibres. They are also known as proprioceptive afferent fibres.
Muscle spindles emit low-level sensory nerve signals that assist in the maintenance of muscle tonus and postural adjustments. They respond to stretch by an antagonistic muscle, to gravity, or to a load being applied to the muscle.
Muscle spindles alert the brain that nearby joints and soft tissues are in danger of being stretched too far. They indicate the degree to which the muscle must be activated in order to overcome a given resistance.
