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Muscle tone is the state of partial contraction of muscles at rest, which is essential for maintaining posture and joint stability. Muscle tone is maintained by the coordinated activation of motor neurons and muscle receptors, including muscle spindles and Golgi tendon organs. Muscle spindles are stretch receptors located within skeletal muscles that detect and respond to changes in muscle length and velocity, thereby regulating muscle contraction and resistance to stretch. They consist of intrafusal muscle fibres that are activated by gamma motor neurons, which adjust their firing rate and stretch sensitivity. Golgi tendon organs, on the other hand, are located between the muscle and tendon, and they detect muscle tension and changes in tension, sending signals to the brain to prevent injury from excessive tension.
| Characteristics | Values |
|---|---|
| Type | Muscle spindle |
| Location | Within the body of a skeletal muscle |
| Composition | 6-8 specialized muscle fibres |
| Function | Detect changes in the length of the muscle |
| Sensory information | Conveyed by primary type Ia sensory fibres |
| Motor neurons | Activated by up to a dozen gamma motor neurons |
| Secondary sensory fibres | Secondary type II sensory fibres |
| Muscle force | Controlled by alpha-gamma coactivation |
| Muscle relaxation | Occurs when muscle fibres return to a low-tension state |
| Muscle contraction | Occurs when calcium ions are released |
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What You'll Learn
Muscle spindles are stretch receptors
Each muscle spindle contains an average of 8–20 intrafusal fibres, which are much thinner than extrafusal muscle fibres. The intrafusal fibres are oriented parallel to the regular, power-producing extrafusal muscle fibres. The extrafusal fibres are the typical muscle fibres that allow the muscle to do work.
The muscle spindle is activated when the muscle is stretched, causing the spindle to stretch and lose its spiral shape. This signals the muscle to contract, which then triggers the spiral shape to be regained. This process is called the stretch reflex.
The muscle spindle is innervated by motor nerves (the gamma motor neurons) and two different types of sensory fibres: the type Ia stretch receptors and the type II afferent sensory receptors. The gamma motor neurons activate the intrafusal muscle fibres, changing the resting firing rate and stretch sensitivity of the afferents.
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Alpha-gamma coactivation
Muscle spindles are a collection of specialized muscle fibres located within the muscle mass itself. They are stretch receptors that allow communication with the spinal cord and brain, providing information on the body's position in space (proprioception) and the velocity of body limbs in relation to space.
The firing of gamma motor neurons in sync with alpha motor neurons pulls muscle spindles from the polar ends of the fibres, keeping the spindles taut and sensitive to changes in muscle length. This process is essential for maintaining muscle tone and preventing muscle spasms or spasticity.
The gamma loop, or alpha-gamma loop, is a feedback loop in the nervous system that regulates muscle tension. It is important because when gamma motor neurons become overactive, or when the firing of alpha and gamma motor neurons becomes imbalanced, muscle tension increases, and spasticity or rigidity can occur. This can lead to sore and painful muscles, misalignment of the skeleton, compressed joints, and elevated blood pressure.
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Intrafusal fibres
The muscle spindle is a proprioceptive organ that detects and mediates static and dynamic information about skeletal muscle fibre length and stretch. Sensory information is communicated to the central nervous system, which mediates the appropriate motor response. The muscle spindle is richly innervated by mechanosensitive nerve endings that terminate in the non-contractile central portion of the fibres. These nerve endings are highly sensitive to changes in muscle fibre length, particularly the rate of change of length.
Intrafusal muscle fibres are not to be confused with extrafusal muscle fibres, which contract, generating skeletal movement and are innervated by alpha motor neurons. There are two types of intrafusal muscle fibres: nuclear bag fibres and nuclear chain fibres. They bear two types of sensory endings, known as annulospiral and flower-spray endings. Both ends of these fibres contract, but the central region only stretches and does not contract.
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Golgi tendon organs
The Golgi tendon organ (GTO) is a proprioceptor or sensory receptor that senses changes in muscle tension. It is located at the interface between a muscle and its tendon, known as the musculotendinous or myotendinous junction. GTOs are nearly as common in most muscles as muscle spindles.
The GTO was discovered by Italian physician Camillo Golgi, for whom it is eponymously named. It is a fusiform receptor enclosed within a thin perineural capsule containing packed tendinous collagen, with several muscle fibres attached to one pole. The collagenous fibres within the capsule are penetrated by fibres of sensory neurons, the terminal branches of which intertwine with the collagenous fibres. Each capsule is about 1 mm long, has a diameter of about 0.1 mm, and is perforated by one or more afferent type Ib sensory nerve fibres (Aɑ fibre), which are thinner than Ia fibres.
The GTO is activated by stretch or active contraction of a muscle and transmits information about muscle tension. When the muscle generates force, the sensory terminals are compressed, deforming the terminals of the Ib afferent axon. This opens stretch-sensitive cation channels, causing the axon to depolarize and fire nerve impulses that are propagated to the spinal cord. The action potential frequency signals the force being developed by 10-20 extrafusal muscle fibres in the muscle.
GTOs are sensitive to changes in tension and rate of tension and, because they are located in the musculotendinous junctions, they are responsible for sending information to the brain as soon as they sense an overload. If there is too much muscle tension, the GTO will inhibit the muscle from creating any force, thus protecting the muscle from injury. This works in concert with muscle spindles that monitor muscle length.
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Gamma motor neurons
Muscle spindles are collections of specialised muscle fibres located within the muscle mass itself. They are stretch receptors that signal the length and rate of change of length (velocity) of the muscle. The muscle spindle is made up of intrafusal fibres that are oriented parallel to the regular, power-producing extrafusal muscle fibres.
The central nervous system (CNS) controls muscle spindle sensitivity through the fusimotor system, which includes gamma motor neurons (also called fusimotor neurons). When the CNS sends signals to alpha motor neurons to contract, it simultaneously instructs gamma motor neurons to contract the intrafusal fibres. This coordinated process, known as alpha-gamma coactivation, maintains the tautness of muscle spindles and their sensitivity to changes in muscle length.
Overactive input from gamma motor neurons can lead to abnormal muscle tone, such as spasticity. This typically occurs due to cortical damage and a loss of inhibitory impulses.
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Frequently asked questions
Muscle spindles are stretch receptors that maintain muscle tone. They are collections of specialised muscle fibres that detect changes in the length of the muscle and convey this information to the central nervous system.
Muscle spindles are made of intrafusal muscle fibres, which are oriented parallel to the regular, power-producing extrafusal muscle fibres.
Impaired muscle spindles can lead to abnormal muscle tone, such as spasticity, which is a velocity-dependent increase in resistance to passive stretch. This can cause exaggerated tendon reflexes, abnormal postures, stiffness and contractures.
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