Logan Steele
Excitability is the ability of muscles to respond to a stimulus, which may be delivered from a motor neuron or a hormone. Muscle excitability is a result of the excessive electrical excitability of sarcolemma, which is the use of sodium channel blockers to dampen action potential firing. This excitability is a fundamental property of nerve and muscle cells (skeletal, cardiac and smooth) and certain other cell types, such as some endocrine cells. Individual muscles have lengthening, contractile, excitable, and recoil characteristics.
| Characteristics | Values |
|---|---|
| Definition | Excitability is the ability to respond to a stimulus, which may be delivered from a motor neuron or a hormone |
| Muscle Excitability | An excitable cell is one that, in response to certain environmental stimuli (electrical, chemical or mechanical), generates an all-or-none electrical signal or action potential (AP) |
| Muscle Types | Skeletal, cardiac and smooth muscles are excitable |
| Other Types | Some endocrine cells are excitable |
| Muscle Diseases | Myotonia congenita (MC), Paramyotonia congenita (PMC), and sodium channel myotonia are caused by excessive electrical excitability of sarcolemma |
| Treatment | Carbonic anhydrase inhibitors, such as acetazolamide and dichlorphenamide, can dampen sarcolemma excitability |
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What You'll Learn
Muscle excitability disorders
The normal contraction of skeletal muscle involves a series of steps where electrical signals originating from motor nerves are transmitted across the neuromuscular junction (NMJ), a specialised type of synapse, and disseminated along the muscle surface membrane. This ultimately leads to the interaction of actin and myosin, resulting in muscle contraction. However, if any one of these steps is disrupted, it can result in muscle excitability disorders, causing muscle weakness or paralysis, as observed in conditions like familial periodic paralysis.
One example of a muscle excitability disorder is myotonia, which is characterised by sustained muscle fibre discharge even after the external source of excitation has ended. Myotonia can be congenital, dystrophic, or paramyotonia congenita. It is often triggered by minimal voluntary contraction or mechanical stimulation, such as tapping on the muscle belly. Myotonic discharges can also be detected in other disorders, such as polymyositis and certain types of glycogen storage diseases.
Mutations in genes encoding voltage-gated ion channels, such as sodium (Na+), potassium (K+), and chloride (Cl-) channels, play a significant role in neuromuscular disorders. For instance, mutations in the chloride channel (ClC1) lead to decreased muscle chloride conductance, resulting in myotonic symptoms. Similarly, potassium ions (K+) flowing out of the cell during action potentials can accumulate in transverse tubules, causing depolarisation and impaired muscle relaxation.
Additionally, certain inherited muscle diseases, known as periodic paralyses, are characterised by intermittent episodes of skeletal muscle weakness or paralysis in individuals who otherwise appear normal. These episodes are often associated with changes in serum potassium (K+) concentration, which can increase or decrease consistently within a particular family.
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Muscle contractions
There are three types of muscles in mammals: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and provide the body with structure and strength. Cardiac muscles make up the heart's walls, enabling blood to be pumped through the vasculature. Smooth muscles are found throughout the body in blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder.
The physiological concept of muscle contraction is based on two variables: length and tension. Muscle shortening and contraction are not synonymous. Tension can be produced without changes in muscle length, such as when holding a dumbbell or a sleeping child in your arms. This is known as an isometric contraction, where muscle tension changes without any alteration in muscle length. Conversely, an isotonic contraction refers to a constant muscle tension despite changes in muscle length.
Concentric and eccentric contractions are two types of contractions that often occur together. A concentric contraction occurs when the muscle actively shortens and tightens to lift something heavy, generating tension. This happens when the force generated by the muscle exceeds the opposing load. An example is picking up a heavy box, where the arm muscles contract to hold the weight, and the leg muscles tighten to stand up with the additional weight.
On the other hand, an eccentric contraction is referred to as negative work, where the muscle lengthens to lower a heavy object. For instance, when lifting a dumbbell, the bicep muscle contracts to lift the weight, but when lowering it, the bicep remains contracted while lengthening.
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Muscle balance
The three main types of muscles are skeletal, smooth, and cardiac. The brain, nerves, and skeletal muscles work together to cause movement – this is collectively known as the neuromuscular system. The coordination of muscle and joint groups to provide skilled movement for meeting one’s daily demands occurs through the programming of this neuromuscular system, which starts soon after birth and builds on itself to ultimately provide a functional body.
Muscle imbalance occurs when there is a lack of parity between corresponding agonist and antagonist muscles. Muscular imbalance can also arise when a muscle performs outside of its normal physiological function. Classic symptoms of muscle imbalances are usually pain associated with the affected joint, small tissue damage, or lesions.
Muscle length and strength between opposing muscle groups need to be in balance for normal movement and function. The muscles on each side of the body should be symmetrical in size and strength. When a muscle on one side of the body is larger, smaller, stronger, or weaker than the corresponding muscle on the other side, a muscle imbalance occurs. Joint muscular imbalance occurs when the muscles surrounding a joint do not work together with opposing force to keep the bones of the joint centered for optimum movement. If one or more of these muscles becomes weaker, stronger, looser, or tighter than normal, a muscle imbalance occurs, and joint movement can be limited.
To avoid or fix muscle imbalances, it is important to ensure that your exercise form is proper. One way to avoid exercise-induced muscle imbalance is to focus on function and the whole body. Avoid trying to build huge muscles in one area. For example, when lifting weights or performing lunges, always do the same number of reps on both sides of the body. For proper movement, opposing muscle groups must coordinate with each other, and this coordination is dependent on these opposing muscle groups being in balance.
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Muscle extensibility
The passive extensibility of skeletal muscles is defined as the ability of skeletal muscles to lengthen without muscle activation. It is an important component of total muscle function as it allows for the maximal length of both non-activated and activated muscles. Maximal muscle length contributes to the maximal joint range of motion, which is believed to influence functional activities and athletic performance.
Therapeutic interventions designed to increase the passive extensibility of muscles are employed as an important component of physical rehabilitation and sports. Studies have suggested that optimal muscle function is probably achieved by increasing muscle length, length extensibility, passive elastic stiffness, mass and strength. However, additional studies are needed to investigate these relationships, particularly for aged muscles and for muscles affected by clinical disorders, disease and injury.
Various theories have been proposed to explain the increases in muscle extensibility observed after intermittent stretching. Most of these theories advocate a mechanical increase in length, including viscoelastic deformation, plastic deformation, increased sarcomeres in series, and neuromuscular relaxation. However, more recent sensory theories suggest that increases in muscle extensibility are due to a modification of sensation only.
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Intracellular negativity
The concept of intracellular negativity is integral to understanding the excitability of muscles and nerve cells. Intracellular negativity refers to the electrochemical equilibrium of ions, specifically potassium (K+) and sodium (+) ions, across the cell membrane. This phenomenon is crucial for the generation of action potentials in excitable cells, which include nerve and muscle cells.
Every cell in the body has a membrane potential, but only excitable cells like nerves and muscles can modify this potential to generate an action potential. The resting membrane potential of a cell is influenced by the concentration of ions on both sides of the cell membrane. The cell membrane, or plasma membrane, is selectively permeable, allowing specific ions to pass through and contribute to the electrochemical gradient.
At rest, the cell membrane has a higher permeability to potassium ions (K+). This results in a larger efflux of potassium ions compared to the influx of sodium ions, leading to a higher concentration of potassium ions inside the cell and a higher concentration of sodium ions outside. This concentration gradient contributes to maintaining the negativity of the intracellular space, as the cytosol (intracellular fluid) becomes more electronegative than the extracellular fluid.
The Na(+)-K(+)-ATPase pump plays a crucial role in establishing and maintaining this concentration gradient. This pump uses energy to exchange three sodium ions from inside the cell for two potassium ions from the extracellular space. The resulting concentration gradient contributes to the generation of an action potential. During an action potential, the membrane potential temporarily changes, allowing a large influx of sodium ions, which reduces the negativity of the intracellular space and brings it closer to the threshold for an action potential.
In summary, intracellular negativity is a fundamental property of excitable cells, such as nerve and muscle cells, and it is maintained by the selective permeability of the cell membrane and the active transport of ions. This negativity is essential for the generation of action potentials, which are integral to the proper functioning of these excitable cells.
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Frequently asked questions
Excitability is the ability of muscles to respond to a stimulus, which may be delivered from a motor neuron or a hormone.
An example of a stimulus could be a needle electrode inserted into the muscle, which causes a brief burst of electrical discharges.
Excitability is a fundamental property of nerve and muscle cells (skeletal, cardiac and smooth), as well as certain other cell types, such as some endocrine cells. An excitable cell is one that, in response to certain environmental stimuli (electrical, chemical or mechanical), generates an all-or-none electrical signal or action potential (AP).
An action potential (AP) is an electrical signal triggered by a depolarization of the membrane, which is produced by the applied stimulus.
Some examples of muscle excitability disorders include Myotonia Congenita (MC), Paramyotonia Congenita (PMC), and sodium channel myotonia. These diseases are caused by mutations in the voltage-gated chloride channel ClC-1 or gain-of-function of the voltage-gated sodium channel Nav1.4.
