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Muscle tone is the continuous and passive partial contraction of muscles, which helps maintain posture and balance. It is important for generating reflexes and controlling the proper function of other organ systems. Muscle tone is controlled by the sensory muscle spindle, which measures muscle stretch. Smooth and cardiac muscles do not have specialized muscle spindles and maintain their tone through autonomous feedback from the muscle fibres, neurons, and associated tissues. Disorders of muscle tone can arise from dysfunction in neural circuits in the brain, spinal cord, and muscle spindle, manifesting as hypertonia or hypotonia.
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What You'll Learn
Muscle tone is controlled by the sensory muscle spindle
Muscle tone refers to the continuous and passive partial contraction of muscles, which is important for generating reflexes, maintaining posture and balance, and controlling the proper function of organ systems. Muscle tone is not limited to skeletal muscles, as cardiac and smooth muscles also exhibit this property.
The maintenance of muscle tone is controlled by the sensory muscle spindle, which measures muscle stretch. Muscle spindles are small sensory organs with an elongated shape, found within the belly of a skeletal muscle. They are composed of contractile intrafusal bag and chain fibres, which allow them to stretch with the muscle. The muscle spindle has both sensory and motor components. The sensory component is responsible for detecting and conveying information about muscle stretch, while the motor component enables the activation of muscle fibres within the spindle.
The sensory information is conveyed by primary type Ia sensory fibres, which spiral around the muscle fibres within the spindle, and secondary type II sensory fibres, which end adjacent to the central regions of the static bag and chain fibres. These fibres respond to changes in muscle length and velocity, transmitting this information to the spinal cord. The muscle spindle's sensitivity to stretch can be modified by the activation of gamma motor neurons, which regulate the sensitivity of the sensory afferents.
To maintain muscle tone, the muscle spindles operate a feedback loop by directly triggering motor neurons linked to their associated muscles. This activation of motor neurons can lead to a contraction and stiffening of the end parts of the muscle spindle muscle fibres, generating a reflexive muscle contraction known as the stretch or myotatic reflex. This reflexive contraction can help prevent the muscle from being stretched too far or too quickly.
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Tone is not limited to skeletal muscles
Muscle tone is traditionally defined as the tension in a relaxed muscle or the resistance felt during the passive stretching of a joint when the muscles are at rest. It is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. While muscle tone is often associated with skeletal muscles, it is also a property of cardiac and smooth muscles.
Smooth and cardiac muscles do not have specialized muscle spindles, which are sensory units associated with muscle tissue that help maintain muscle tone. Instead, tone in these muscle types is maintained through autonomous feedback from the muscle fibres, neurons, and associated tissues. This feedback mechanism ensures that these muscles can contract and relax appropriately, contributing to overall muscle tone.
The maintenance of muscle tone is a complex process involving various physiological components. It is regulated by neural circuits in the brain, spinal cord, and muscle spindle, with input from sensory neurons. Muscle spindles, composed of nuclear bag fibres and nuclear chain fibres, detect muscle stretch and activate impulses to the spinal cord and cerebral cortex, generating immediate reflexes and providing information about the degree of stretch. This activation also triggers a feedback loop that directly stimulates motor neurons linked to the associated muscles, helping to maintain tone.
Additionally, muscle tone can be influenced by efferent signals from the spinal cord, resulting in the contraction of extrafusal muscle fibres and contributing to static stretch reflex responses. The body actively maintains a balance between the tone of flexor and extensor muscle groups, and disruptions in this balance can lead to muscle cramps. In certain cases, muscle relaxants or quinine may be warranted to alleviate these cramps.
Abnormalities in muscle tone can manifest as hypertonia or hypotonia, resulting from disorders affecting the neural circuits involved in its regulation. Hypertonia, associated with upper motor neuron diseases, can present as spasticity or rigidity, while hypotonia, associated with lower motor neuron diseases, may exhibit as muscle flaccidity with decreased stretch reflex responses. These conditions highlight the importance of maintaining proper muscle tone for overall physical function and well-being.
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Smooth and cardiac muscles do not have specialised muscle spindles
Muscle tone is the maintenance of a partial contraction of a muscle, which is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. Muscle tone is controlled by the sensory muscle spindle, which measures muscle stretch. Muscle spindles are composed of nuclear bag fibres and nuclear chain fibres, which contain sensory neurons. When stretched, muscle spindles become activated, triggering impulses to the spinal cord that can generate an immediate reflex.
While muscle tone is not limited to skeletal muscles, smooth and cardiac muscles do not have specialised muscle spindles. Smooth muscle is short, spindle-shaped, and has no evident striation, with a single nucleus in each fibre. It is responsible for involuntary movements, such as moving food through the digestive system, controlling respiration, and regulating blood flow in arteries by contraction. Smooth muscle is found on the walls of major organs and passageways.
Cardiac muscle, on the other hand, is short, branched, appears striated, and has a single central nucleus. It is only found in the heart, where it contracts to pump blood. Smooth and cardiac muscles maintain their tone through autonomous feedback from the muscle fibres, neurons, and associated tissues. This feedback mechanism allows these muscles to control their contraction and relaxation, ensuring proper functioning.
It is important to note that while smooth and cardiac muscles lack specialised muscle spindles, they still possess other specialised structures and functions that enable them to carry out their unique roles in the body effectively.
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Physical disorders can result in abnormally low or high muscle tone
Muscle tone is the continuous and passive partial contraction of a muscle, which helps maintain posture, generate reflexes, and maintain balance. It is controlled by the sensory muscle spindle, which measures muscle stretch. Disorders of muscle tone can arise from dysfunction in these pathways and manifest as hypertonia or hypotonia.
Hypotonia
Hypotonia is a condition characterised by poor muscle tone, resulting in floppiness. It is caused by abnormally decreased resistance encountered with passive movement of a joint. It is often challenging for clinicians to determine the underlying cause of hypotonia. Benign congenital hypotonia is a nonprogressive disorder of the neuromuscular system. Muscle tone improves with age, but patients often have delayed milestone achievement. Routine blood tests, including muscle enzyme results, are normal. It is associated with the risk of joint dislocations later in life.
Hypertonia
The loss of supraspinal control mechanisms gives rise to hypertonia, resulting in spasticity or rigidity. Spasticity is characterised by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex. It is one component of upper motor neuron syndrome. Spasticity may be reduced or abolished by administering oral medications such as baclofen, gabapentin, diazepam, or tizanidine in conjunction with physical therapy.
Other Disorders
Dystonia and paratonia are also abnormalities of muscle tone, arising from network dysfunction between the basal ganglia and the thalamo-cerebello-cortical connections. Paratonia is associated with advanced glycation end products (AGEs), which increase tissue stiffness by forming cross-links in muscle collagen, impairing skeletal muscle function. Malnutrition and starvation can also lead to decreased muscle tone, along with weight loss, muscle wasting, and reduced immunity.
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Muscle tone is a complex and dynamic state
The simple definition of tone as the resistance to passive stretch is physiologically a complex interlaced network encompassing neural circuits in the brain, spinal cord, and muscle spindle. The muscle spindle is a sensory unit associated with muscle tissue that is responsible for maintaining muscle tone. When stretched, muscle spindles become activated, triggering impulses to the spinal cord that can generate an immediate reflex. Spindles can also trigger impulses to the cerebral cortex, providing information about the degree of stretch within the muscle. To maintain tone, spindles also operate a feedback loop by directly triggering motor neurons linked to their associated muscles.
Smooth and cardiac muscles do not have specialized muscle spindles. Tone is maintained through autonomous feedback from the muscle fibers, neurons, and associated tissues. Muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons. It is important to assess several areas before deciding if a person has high, low, or normal muscle tone. Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone.
Bernstein, in his hierarchical model of movement construction, postulated that muscle tone is an adaptive function of the neuromotor apparatus that responds adequately to commands by fine-tuning the excitability of the sensory and motor cells for the tasks of active postural or movement control. This definition makes muscle tone an active contributor to movement and postural tasks.
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Frequently asked questions
Muscle tone is the maintenance of partial contraction of a muscle, which is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems.
Muscle tone is controlled by the sensory muscle spindle, which measures muscle stretch. The muscle spindle operates a feedback loop by directly triggering motor neurons linked to their associated muscles.
Muscles with high tone are perceived as "tight, light, strong", whereas low tone muscles are perceived as "lax, flabby, floppy, mushy, dead weight". In general, low tone increases flexibility and decreases strength, and high tone decreases flexibility and increases strength.
Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Hypotonia is seen in lower motor neuron diseases like poliomyelitis, and hypertonia is seen in upper motor neuron diseases like lesions in the pyramidal tract and extrapyramidal tract.
