Dominic Stone
Muscle tone is the resistance of a muscle to active or passive stretch, or the overall stiffness of the muscle. It is largely controlled by the peripheral fusimotor system with input from the central nervous system (CNS). Reflexes function to counteract the active or passive stretch of the muscle tissue via monosynaptic connections from the spindles to the alpha motor neurons, and they work with the elastic components of the muscle to resist stretch. The response to muscle stretch is not fixed and can be adjusted according to the demands of the moment.
| Characteristics | Values |
|---|---|
| Definition | Muscle tone is the resistance of a muscle to active or passive stretch, or the overall stiffness of the muscle. |
| Function | Muscle tone helps to maintain posture and store energy. |
| Reflexes | Reflexes counteract the active or passive stretch of the muscle tissue via the monosynaptic connections from the spindles to the alpha motor neurons. |
| Abnormalities | Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. |
| Flexibility | Low muscle tone increases flexibility, while high muscle tone decreases it. |
| Strength | Low muscle tone decreases strength, while high muscle tone increases it. |
| Feedback Mechanisms | Negative feedback helps to counteract deviations from the desired muscle position. |
| Gain | The higher the gain, the larger the reflexive force of contraction by a muscle to counteract a change in length. |
| Loop Delay | The loop delay is the time between the detection of an error and the compensatory response by the muscle. |
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What You'll Learn
- Muscle tone is controlled by the peripheral fusimotor system with input from the CNS
- The monosynaptic stretch reflex is key to muscle tone genesis
- The muscle's overall stiffness or resistance to stretch is an important factor
- Muscle tone is maintained by interneurons and multiple descending fibre systems
- The muscle's ability to act as a spring and store energy is a function of muscle tone
Muscle tone is controlled by the peripheral fusimotor system with input from the CNS
Muscle tone is the continuous and passive partial contraction of muscles, which is often likened to the reflex tone described by Sherrington in decerebrate animals. It is generally believed that muscle tone is determined by the monosynaptic stretch reflex, and that tonic fusimotor activity is necessary for its production in normal humans.
The fusimotor system is a part of the peripheral nervous system (PNS), which is the part of the nervous system that lies outside the brain and spinal cord. The PNS plays a key role in sending information from different areas of the body to the brain, and in carrying out commands from the brain to various parts of the body. The fusimotor system, in particular, is involved in the regulation of spindle sensitivity, which in turn regulates central and peripheral coding of joint angles.
The muscle spindle is a unique sensory organ that has dual efferent and afferent innervations. Proprioceptive afferents from muscle spindles encode information about peripheral joint movements for the central nervous system (CNS). The sensitivity of muscle spindles is nonlinearly dependent on the activation of gamma (γ) motoneurons in the spinal cord, which receives input from the motor cortex. The CNS has the ability to control spindle sensitivity by fusimotor commands, and it may modulate spindle sensitivity by adjusting the central pattern of coding of joint angles.
While the fusimotor system is a part of the peripheral nervous system, it receives input from the CNS, and it is not entirely clear what information is carried in the fusimotor signal from the CNS to the muscle spindle. However, it is believed that the CNS informs the peripheral neuromuscular system about the desired joint position via a pathway separate from the α commands to the muscles.
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The monosynaptic stretch reflex is key to muscle tone genesis
Muscle tone genesis has traditionally been associated with the reflex tone described by Sherrington in decerebrate animals. This theory presumes that muscle tone is determined by the monosynaptic stretch reflex. The monosynaptic stretch reflex, also known as the muscle stretch reflex or deep tendon reflex, is a reflex arc that facilitates direct communication between sensory and motor neurons innervating the muscle. This reflex is initiated inside the muscle spindle, which detects both the magnitude and rate of muscle stretch.
The monosynaptic stretch reflex is the simplest and most well-known reflex in the human body. It is a direct response to muscle stretch involving one synapse in the spinal cord. When a muscle is subjected to a stretch stimulus, sensory impulses are transmitted from the muscle spindle through Ia afferent fibres to the dorsal root of the spinal cord. This is an example of a spinal reflex, resulting in a fast response that involves an afferent signal into the spinal cord and an efferent signal out to the muscle.
The monosynaptic stretch reflex is important for maintaining muscle tone and posture. When the muscle spindle is stretched, its nerve activity increases, leading to increased alpha motor neuron activity. This causes the muscle fibres to contract and resist the stretching force. Additionally, a secondary set of neurons causes the opposing muscle to relax, further contributing to posture maintenance. Gamma motoneurons also play a role in regulating the sensitivity of the stretch reflex by tightening or relaxing the fibres within the spindle.
The monosynaptic stretch reflex is often compared to the polysynaptic stretch reflex, which involves a single sensory stimulus synapsing on interneurons within the spinal cord's grey matter. This allows communication with multiple muscles for contraction or inhibition. While the monosynaptic reflex is typically an inducible action rather than a pathological condition, it can offer insights into underlying neurological dysfunction when present abnormally.
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The muscle's overall stiffness or resistance to stretch is an important factor
Muscle tone is the resistance of a muscle to active or passive stretch, or the overall stiffness of the muscle. The overall stiffness or resistance to stretch of a muscle is an important factor in maintaining posture and balance. For example, the muscle tone in the antigravity leg muscles helps to maintain posture by ensuring that the centre of gravity is aligned over the base of support.
The intrinsic resistance of a muscle to stretch results from the elastic properties of the tendons, connective tissue, and the muscle tissue itself. This means that a muscle behaves much like a spring. The stiffer the muscle, the more force it can exert to counteract a given change in length. This is important for maintaining balance and posture, as the muscle can store energy and release it at a later time.
The stiffness of a muscle is not fixed and can be adjusted according to the demands of the moment. The response to muscle stretch is mediated by the stretch reflex, which can be either dynamic or static. The stretch reflex functions continuously to keep the muscle position as close as possible to the length chosen by the central nervous system (CNS). The CNS regulates muscle length by sending signals to the motor neuron pool in the spinal cord, which then stimulates the muscle to contract.
The gain, or sensitivity of the muscle spindles, determines the reflexive force of contraction. The gain can be adjusted by the overall level of fusimotor activity, by presynaptic modulation of excitatory and inhibitory interneurons, and by direct connections to the motor neuron pool. The loop delay, or time between the detection of an error and the compensatory response by the muscle, is also an important factor in accurately regulating movement around a joint, especially during rapid or extremely precise movements.
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Muscle tone is maintained by interneurons and multiple descending fibre systems
Muscle tone is the continuous and passive partial contraction of muscles. It is often likened to the reflex tone described by Sherrington in decerebrate animals, which assumes that muscle tone is entirely determined by the monosynaptic stretch reflex. However, data suggests that non-reflex, mechanical mechanisms are involved in maintaining resting muscle tone. This includes the role of interneurons and multiple descending fibre systems.
Interneurons are a type of neuron that are located in the spinal cord, which is the first level of the motor hierarchy. The spinal cord is also the site of many complex neural circuits that perform the basic processing of motor control. These circuits execute the low-level commands that generate the proper forces on individual muscles and muscle groups to enable adaptive movements. The spinal cord contains circuitry for rhythmic behaviours such as walking.
Interneurons receive inputs from a range of peripheral sensory receptors and descending fibre systems. These descending fibre systems include fast-twitch and slow-twitch muscle fibres. Most muscles contain both types of fibres, but in different proportions. Fast-twitch fibres are recruited when the input onto motor neurons is large enough, and they generate more force than slow-twitch fibres. However, they cannot maintain this force for as long as slow-twitch fibres. On the other hand, slow-twitch fibres are used to maintain balance and posture.
The muscle spindle, composed of intrafusal fibres, is essential for maintaining muscle tone. When a resting muscle is stretched, the muscle spindle stretches in parallel, sending signals through the primary and secondary afferents. Activation of gamma motor neurons causes a weak contraction of the intrafusal fibres, keeping the muscle spindle taut and sensitive to stretch. This process is crucial for maintaining muscle tone and enabling adaptive movements.
In summary, muscle tone is maintained by the complex interaction of interneurons, descending fibre systems, and various physiological processes. These components work together to generate the proper forces in muscles and enable adaptive movements, contributing to overall muscle tone.
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The muscle's ability to act as a spring and store energy is a function of muscle tone
Muscle tone is often likened to the reflex tone described by Sherrington in decerebrate animals. Traditionally, muscle tone is believed to be fully determined by the monosynaptic stretch reflex, which is necessary for its production in normal humans. However, data suggests that non-reflex, mechanical mechanisms are also involved in maintaining resting muscle tone.
The muscles that power vertebrate locomotion are associated with springy tissues, such as tendons, which are within the muscles and in connective tissue. These springs have a simple action: they stretch and store elastic strain energy when a force is applied to them and then recoil to release energy when the force decays. This elastic action serves several functions, including metabolic energy conservation, amplification of muscle power output, attenuation of muscle power input, and rapid mechanical feedback that aids stability.
Tendons, for example, can reduce the metabolic rate of muscle activity by decreasing the volume of the muscle that is active in producing force. This allows the muscle-tendon system to absorb energy at a rate beyond the muscle's maximum capacity. Tendons can also store energy from muscle contraction and then release it to increase the mechanical energy of the body or a body segment. This can result in a net gain of energy, as seen in the active lengthening of muscle fibres.
Therefore, the muscles' ability to act as a spring and store energy is indeed a function of muscle tone. This elastic mechanism is essential for the effective function of the muscle motors that power movement in vertebrates, including humans.
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Frequently asked questions
Muscle tone is the resistance of a muscle to active or passive stretch, or the overall stiffness of the muscle. It is largely controlled by the peripheral fusimotor system with input from the central nervous system (CNS).
Reflexes function to counteract the active or passive stretch of the muscle tissue via monosynaptic connections from the spindles to the alpha motor neurons. The stretch reflex can be of two types: dynamic and static.
A dynamic stretch reflex is a sudden contraction of the muscle due to an efferent signal from the cord (alpha motor neuron) to the extrafusal fibers. On the other hand, a static stretch reflex is a mild sustained contraction of the extrafusal muscle fibers due to asynchronous contraction when the muscle is stretched.
Muscle tone is regulated by the CNS, which chooses the overall muscle length. The muscle spindles detect deviations in the intended position and relay this information back to the motor neuron pool, resulting in an increase or decrease in the force of contraction to counteract the change in length.
