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Muscle excitability is the ability of a muscle cell to respond to a stimulus, which may be delivered from a motor neuron or a hormone. Muscle excitability is also defined as the ability of a muscle cell to change in membrane conductance in response to stimulation. Muscle excitability testing is a technique that provides in vivo information about membrane potential and ion channel function. It is mainly used in research but may have diagnostic uses, particularly in muscle channelopathies. Normal contraction of skeletal muscle requires that electrical signals originating in a motor nerve be transmitted across the neuromuscular junction, disseminated along the muscle surface membrane, and propagated into the fiber interior along the T-tubular system. Muscle excitability disorders can result in muscle weakness or paralysis.
| Characteristics | Values |
|---|---|
| Definition | Excitability is the ability of a muscle cell to change in membrane conductance in response to stimulation. |
| Muscle Contraction | Normal contraction of skeletal muscle requires that electrical signals originating in a motor nerve be transmitted across the neuromuscular junction. |
| Muscle Weakness | Failure at any one of the steps in the process of muscle contraction will result in muscle weakness or paralysis even in the presence of a normal contractile apparatus. |
| Muscle Excitability Testing | Muscle excitability testing provides in vivo information about membrane potential and ion channel function. |
| Supravitality | The human tissue survival period beyond resuscitation during which tissues are still sensitive to mechanical, electrical, or pharmacological stimuli. |
| Neurogenic Processes | Acute neurogenic processes show robust DMS responses but reduced or absent nerve-stimulated responses. |
| Hyperexcitability | Hyperexcitability is observed on the electromyogram or in muscle fiber intracellular recordings as the so-called myotonic discharges, which is an abnormal run of action potentials. |
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What You'll Learn
Muscle excitability testing
Muscle excitability is the ability of a muscle to respond to a stimulus, which may be delivered from a motor neuron or a hormone. Muscle excitability testing is an experimental technique that probes the properties of the muscle fibre membrane in vivo. It provides in vivo information about membrane potential and ion channel function.
The multi-fibre muscle velocity recovery cycle (MVRC) is an automated, fast, and simple application that has accelerated the use of muscle excitability testing. 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 repetitive stimulation protocol involves prolonged stimulation at 20 Hz to mimic short and long exercise tests.
Muscle excitability is affected by several non-pathological variables such as temperature, electrolytes, muscle fibre subtype, and patient age. It is also influenced by pathological variables such as neurogenic processes, nerve-evoked responses, and muscular atrophy.
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Disorders of muscle excitability
Muscle excitability is the ability of muscles to respond to a stimulus, which may be delivered from a motor neuron or a hormone. Disorders of muscle excitability can result in muscle weakness or paralysis.
Myotonia is a disorder of muscle excitability characterised by sustained muscle fibre discharge outlasting the external source of excitation. This can be triggered by a minimal voluntary contraction or mechanical stimulation. Myotonic discharges can be observed on an electromyogram. Myotonia congenita, myotonia dystrophica, and paramyotonia congenita are all diseases that share myotonia as the main symptom. Stiffness can affect various muscles, including handgrip myotonia, eye closure myotonia (lid lag), and tongue myotonia. Non-dystrophic myotonia is a rare neurological condition characterised by exacerbated sarcolemma excitability that manifests as delayed relaxation after contraction, leading to muscle stiffness.
In addition, mutations in the gene encoding the pore-forming subunit of voltage-gated Na+ channels, inward rectifier K+ channels, and chloride channels can cause neuromuscular disorders ranging from myotonia to paralysis. Mutations in chloride channels (ClC1) induce a decrease in muscle Cl- conductance, resulting in myotonic symptoms.
Acute neurogenic processes show robust DMS responses but reduced or absent nerve-stimulated responses. Studies have shown that the expected pattern of preserved muscle excitability, despite an impaired nerve-stimulated response, is best seen in acute stages, prior to the development of muscular atrophy.
Muscle excitability testing is a technique that provides in vivo information about membrane potential and ion channel function. It has been used mainly in research to reveal disease mechanisms across a broad range of neuromuscular disorders but may have additional diagnostic uses, particularly in muscle channelopathies.
Supravitality is the human tissue survival period beyond resuscitation during which tissues are still sensitive to mechanical, electrical, or pharmacological stimuli. This period can last up to 20 hours after death due to metabolic activity, mainly from anaerobic glycolysis and creatine kinase reaction.
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Muscle hypertrophy
Muscle excitability refers to the ability of muscles to respond to a stimulus, which may be delivered from a motor neuron or a hormone. Muscle excitability testing is a technique that provides in vivo information about membrane potential and ion channel function.
There are two types of muscle hypertrophy: myofibrillar and sarcoplasmic. Myofibrillar hypertrophy refers to an increase in the number of myofibrils, resulting in greater muscle strength and density. Sarcoplasmic hypertrophy, on the other hand, involves increased muscle glycogen storage, providing the body with more sustained energy for endurance.
To promote muscle hypertrophy, individuals can engage in strength training, perform a variety of exercises, and ensure adequate recovery time. Training variables such as frequency, intensity, and total volume directly impact hypertrophy, with a gradual increase in these variables yielding better results. Additionally, a positive energy balance, where more calories are consumed than burned, supports muscle growth.
It is important to note that muscular hypertrophy is influenced by biological factors such as DNA and sex, with testosterone playing a significant role in muscle growth. Myostatin-related muscular hypertrophy is a rare genetic condition characterized by reduced body fat and increased muscle size and strength.
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Supravitality
Muscle excitability refers to the ability of muscles to respond to stimuli, such as those delivered from motor neurons or hormones. This process involves the transmission of electrical signals from motor nerves across the neuromuscular junction, along the muscle surface membrane, and into the fibre interior, ultimately resulting in muscle contraction.
Supravital reactions are influenced by various factors, including antemortem health conditions, medical treatments, drug consumption, temperature changes, and climate. These reactions are easily observable in undamaged, unburnt cadavers, making them valuable tools for estimating the post-mortem interval (PMI) during early decomposition.
Muscle excitability testing is a technique used primarily in research to study disease mechanisms in neuromuscular disorders, particularly muscle channelopathies. It provides valuable in vivo information about membrane potential and ion channel function, which can aid in understanding and diagnosing muscle disorders.
The development of automated, fast, and simple applications, such as multi-fibre muscle velocity recovery cycles (MVRC), has enhanced the use of muscle excitability testing. These methods have been instrumental in advancing our understanding of neuromuscular disorders and may also hold diagnostic value, especially in muscle channelopathies.
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Muscle fibre membrane properties
Muscle excitability refers to the ability of a muscle cell to respond to a stimulus, which may be delivered from a motor neuron or a hormone.
The membrane of a muscle fibre plays a crucial role in the excitability of the muscle. It has unique properties that facilitate the transmission of electrical signals and the coordination of muscle contractions.
The muscle fibre membrane, also known as the sarcolemma, is a thin semipermeable membrane that surrounds each muscle fibre. It is composed of phospholipids and proteins, forming a lipid bilayer that separates the intracellular fluid from the extracellular environment. The membrane is essential for maintaining the integrity and function of the muscle fibre, as it acts as a barrier and regulates the movement of ions, molecules, and nutrients into and out of the cell.
One of the key properties of the muscle fibre membrane is its excitability, which is the ability to respond to electrical stimuli. This excitability is due to the presence of voltage-gated ion channels embedded in the membrane. These ion channels can detect changes in the membrane's electrical potential and open or close in response, allowing the movement of ions across the membrane. This movement of ions creates an electrical current, which propagates along the membrane and triggers a chain of events leading to muscle contraction.
The muscle fibre membrane also exhibits a property called membrane potential, which is the difference in electrical potential across the membrane. This membrane potential is critical for maintaining the resting state of the muscle fibre and for generating action potentials during muscle excitability. At rest, the membrane potential is typically negative inside the cell compared to the outside, and this potential difference is maintained by the selective permeability of the membrane to specific ions.
In addition to membrane potential, the muscle fibre membrane also possesses ion channel function. Ion channels are transmembrane proteins that form pores in the membrane, allowing the passage of specific ions while blocking others. These ion channels are selective, allowing only certain types of ions to pass through, such as sodium, potassium, calcium, or chloride ions. The function of these ion channels is crucial for maintaining the membrane potential and for generating the electrical currents necessary for muscle contraction.
The properties of the muscle fibre membrane, including its excitability, ion channel function, and membrane potential, can be studied through muscle excitability testing. These tests provide valuable in vivo information about the membrane's behaviour and its role in neuromuscular disorders and muscle channelopathies.
Overall, the muscle fibre membrane plays a critical role in muscle excitability by transmitting electrical signals, regulating ion movement, and coordinating the complex process of muscle contraction. Its unique properties contribute to our understanding of muscle function and provide insights into the diagnosis and treatment of muscle-related disorders.
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Frequently asked questions
Muscle excitability is the ability of a muscle cell to respond to a stimulus, such as a change in membrane potential, by contracting.
Muscle excitability testing provides in vivo information about membrane potential and ion channel function.
Muscle excitability testing is used mainly in research but it may have diagnostic uses, particularly in muscle channelopathies.
Some disorders of muscle excitability include denervation, myotonic disorders, myositis, and myopathy.
Hyperexcitability is observed as myotonic discharges, which is an abnormal run of action potentials. This can lead to muscle stiffness and hypertrophy.
