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Muscle excitability is a measure of a muscle's ability to contract. It is represented by rheobase (R(50)) and chronaxie (C(50)) values, with lower values indicating greater excitability. Muscle excitability testing is a technique that provides in vivo information about membrane potential and ion channel function. This technique is mainly used in research but may have diagnostic uses, particularly in muscle channelopathies. Excitability is important in the design of muscle tissue bioreactor systems, implantable muscle stimulators, and other systems that use electrical pulses to elicit muscle contractions.
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What You'll Learn
Muscle excitability testing
Muscle excitability is represented by rheobase (R(50), units = V/mm) and chronaxie (C(50), units = microseconds) values, with lower values for each indicating greater excitability. The assessment of muscle excitability is not a new concept, but more useful mechanisms of muscle excitability assessment are now employed in the research setting. For example, standard needle electromyography techniques do measure the presence of spontaneous activity, but this assessment is superficial and does not provide insight into the underlying cellular or electrochemical mechanisms.
Other common components of the muscle excitability assessment include the frequency ramp and repetitive stimulation protocols. The frequency ramp protocol measures changes in MFAP latency in response to trains of progressively increasing frequency conditioning stimuli up to 30 Hz. The MFAP latency obtained from the final stimulus in the train is compared to the MFAP latency from the initial stimulus to provide a further assessment of sarcolemmal supernormality, which is thought to be secondary to potassium accumulation within the T-tubule system. The repetitive stimulation protocol involves prolonged stimulation at 20 Hz to mimic short and long exercise tests.
It is important to note that muscle excitability is affected by several non-pathological variables such as temperature, electrolytes, muscle fibre subtype and patient age.
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Muscle contractions
The excitability of skeletal muscle fibres is critical for muscle contractions. Excitability is represented by rheobase (R(50)) and chronaxie (C(50)) values, with lower values indicating greater excitability. Muscle excitability testing provides valuable information about membrane potential and ion channel function, particularly in the context of muscle research and the diagnosis of neuromuscular disorders. The ClC-1 chloride (Cl-) ion channel plays a key role in regulating muscle excitability and activation. A significant rise in GCl, or muscle excitability, may compromise muscle function and prevent muscle fibre activation.
Additionally, the ability to sustain contractile function over time, which enables prolonged physical activity, varies between different muscles and individuals. Muscle fatigue, a state where contractile function becomes impaired and muscular power output declines, can be influenced by multiple factors, including the type of exercise and the type of muscle fibres recruited by the nervous system. The energy consumption of skeletal muscle increases significantly during exercise, and the metabolic state of the muscle can impact its performance. Excessive muscle fatigue is a recognised symptom in patients with metabolic myopathies, such as McArdle disease, where depletion of ATP limits membrane excitability.
Understanding the excitability of skeletal muscle during development, denervation, and tissue culture is an active area of research. For example, stimulated-denervated muscles retain excellent excitability when chronically electrically stimulated, and neonatal rat muscle excitability improves during the first six weeks of development, approaching adult muscle levels. The quantitative understanding of muscle excitability is also important for the design of muscle tissue bioreactor systems and implantable muscle stimulators, where electrical pulses are used to elicit muscle contractions.
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Muscle fatigue
The two main causes of muscle fatigue are neural fatigue and metabolic fatigue. Neural fatigue is the limitation of a nerve's ability to generate a sustained signal, while metabolic fatigue is caused by a shortage of fuel (substrates) within the muscle fibre, leading to a low ATP reservoir. Metabolic fatigue can also be caused by the accumulation of substances (metabolites) within the muscle fibre, which interfere with the release of calcium (Ca2+) or its ability to stimulate muscle contraction. These metabolites include chloride, potassium, lactic acid, ADP, magnesium (Mg2+), reactive oxygen species, and inorganic phosphate.
The impact of muscle fatigue can range from a minor decrease in performance to more severe cases that indicate a serious disorder. Rest and recovery are typically recommended for muscle fatigue, but in more severe or persistent cases, medical attention may be required. Treatment options can include hot and cold therapy, anti-inflammatory medications, or physical therapy to improve mobility and speed up recovery.
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Muscle tissue types
Muscle tissue is composed of specialised cells, or muscle fibres, that are capable of contracting to effect movement. There are three types of muscle tissue in the human body: skeletal, cardiac, and smooth muscle tissue. Each type has a unique structure and a specific role.
Skeletal muscle tissue forms skeletal muscles, which attach to bones or skin and control locomotion and any movement that can be consciously controlled. Skeletal muscles are long and cylindrical in appearance, and when viewed under a microscope, they have a striped or striated appearance. The striations are caused by the regular arrangement of contractile proteins (actin and myosin). Skeletal muscle tissue is composed of long cells called muscle fibres, which are organised into bundles supplied by blood vessels and innervated by motor neurons. Skeletal muscle is the most common type of muscle tissue found in the body and consists of highly elongated, multinucleate, non-branching cells that are arranged in a parallel manner.
Cardiac muscle tissue is only found in the heart, and cardiac contractions pump blood throughout the body and maintain blood pressure. Like skeletal muscle, cardiac muscle is striated and is composed of similar contractile proteins. However, cardiac muscle cells are much shorter and broader and are branched at their ends. They usually have one nucleus each, but sometimes they may have two. The striations in cardiac muscle are not as distinct as those in skeletal muscle due to the presence of large amounts of mitochondria and other organelles in the cell.
Smooth muscle tissue is found in the walls of hollow organs throughout the body, including the intestines, stomach, urinary bladder, uterus, and respiratory tract. It is also found in blood vessels and the eye. Smooth muscle contractions are involuntary movements triggered by impulses from the autonomic nervous system. The arrangement of cells within smooth muscle tissue allows for contraction and relaxation with great elasticity. Smooth muscle tissue has no striations and has only one nucleus per cell.
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Muscle development
Muscle excitability refers to the ability of muscles to be stimulated to contract. This is particularly relevant to skeletal muscle, which is responsible for locomotion and any movement that can be consciously controlled. Skeletal muscle tissue has a striped or striated appearance under a microscope, caused by the regular arrangement of contractile proteins actin and myosin. Actin interacts with myosin for muscle contraction.
The excitability of skeletal muscle tissue is important for the design of muscle tissue bioreactor systems, implantable muscle stimulators, and other systems that use electrical pulses to cause muscle contractions. Muscle excitability testing provides in vivo information about membrane potential and ion channel function. It is mainly used in research, but it may have diagnostic uses, especially in muscle channelopathies.
The excitability of skeletal muscle is represented by rheobase (R50) and chronaxie (C50) values, with lower values indicating greater excitability. Adult skeletal muscle has the highest excitability, while chronically denervated whole muscles and muscle tissue engineered in vitro have exceptionally low excitability. Muscle excitability is regulated by cellular signaling systems that control the function of ion channels that determine the resting membrane conductance (Gm).
In fast-twitch muscle, prolonged firing of action potentials triggers an increase in Gm, reducing muscle fibre excitability and causing action potential failure. A significant rise in GCl (the chloride ion channel) may even compromise muscle excitability and prevent muscle fibre activation. This is a well-known symptom in patients with metabolic myopathies, such as McArdle disease, where the breakdown of glycogen is impaired.
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Frequently asked questions
Excitability in muscles refers to the ability of muscle fibres to be excited and to propagate action potentials along the muscle fibres. This process is required for muscle contractions.
Muscle excitability testing provides in vivo information about membrane potential and ion channel function. This technique is mainly used in research but may have diagnostic uses, particularly in muscle channelopathies.
Adult skeletal muscle is the most excitable, while chronically denervated whole muscles and muscle engineered in vitro from cell lines have exceptionally low excitability.
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