Logan Steele
Muscle excitability is a well-studied phenomenon, with the excitability of human axons being reliably studied using the technique of threshold tracking. The central phenomenon of excitability is the action potential, which is a self-propagating wave of reversed membrane potential that passes along a nerve or muscle fibre. The permeability of an excitable membrane, such as that of a nerve or muscle fibre, to ions is controlled by the potential difference across it. The dominant ion in setting the resting membrane potential is potassium, which accounts for approximately 20% of the resting membrane conductance in skeletal muscle. The chloride conductance represented by ClC1 is the largest conductance at rest in a muscle fibre.
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What You'll Learn
The role of Cl- conductance in muscle fibre membranes
Muscle fibres, including those from fish, amphibians, reptiles, birds, and mammals, are characterised by high resting membrane permeability for Cl- ions. In fact, Cl- conductance plays a more important part in muscle fibre membranes than in the squid axon.
In resting human muscle, ClC-1 Cl- ion channels account for around 80% of the membrane conductance. This is because of the high membrane conductance and passive distribution of Cl- ions. ClC-1 dominates the inhibitory membrane current that counteracts action potential excitation in muscle.
The inverse relationship between ClC-1 function and muscle excitability is most clearly seen in myotonia congenita, a muscle disease resulting from loss-of-function mutations in the ClC-1 gene. In this disease, the muscle excitability is increased, leading to hyperexcitability. This is caused by reduced Cl- conductance.
In patients with myotonic dystrophy, the resting membrane potential is altered, with more expressed symptoms leading to more depolarized potentials. This altered potential is also caused by reduced Cl- conductance.
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The impact of K+ conductance on muscle resting potential
Muscle cells, like neurons, are excitable, meaning they can transition from a resting state to an excited state. The resting membrane potential of a cell is defined as the electrical potential difference across the plasma membrane when the cell is in a non-excited state. The dominant ion in setting the resting membrane potential is potassium, which accounts for approximately 20% of the resting membrane conductance in skeletal muscle.
The resting membrane potential is generated by the K+ that leaks from the inside of the cell to the outside via leak K+ channels, generating a negative charge inside the membrane compared to the outside. The membrane is permeable to K+ at rest because many channels are open. The K+ equilibrium potential is the major contributor to the resting membrane potential. However, since some sodium and other ions leak out of the cell at rest, the resting membrane potential is slightly more positive at -70 mV than the K+ equilibrium potential of -90 mV.
K+ conductance is one of the main factors in regulating the resting membrane potential. Inward rectifier channels are responsible for maintaining the membrane potential in the absence of an excitation electrical current. Delayed and inwardly rectifying K+ channels are the main candidates in regulating membrane excitability in human gastric corpus smooth muscle. In neurons, K+ conductance aids the removal of K+ from the extracellular fluid during activity.
The Na+/K+-dependent ATPase pump plays a large role in maintaining the ionic concentration gradient by exchanging 3 Na+ ions from inside the cell for every 2 K+ ions brought into the cell. While this pump does not significantly contribute to the charge of the membrane potential, it is crucial in maintaining the ionic gradients of Na+ and K+ across the membrane.
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Action potentials in muscle fibres
An action potential is a self-propagating wave of reversed membrane potential that passes along a nerve or muscle fibre. In muscle fibres, action potentials are triggered by the transmission of a motor nerve impulse from the brain to the lower motoneurones. The action potential then reaches the neuromuscular junction, which is the synapse between a motoneuron and a muscle fibre. This initiates the process of excitation-contraction coupling, which refers to the mechanism that converts action potentials in the muscle fibres into muscle fibre contraction.
The action potential generated by depolarizing the endplate membrane of muscle cells increases the calcium ion concentration in muscle plasma. This increase in calcium ions causes a conformational change in the troponin molecule, which is then transmitted to myosin. The conformation of myosin also changes, leading to the binding of actin and the transverse bridge. The binding of actin and myosin triggers the cross-bridge cycle, which causes fine muscle filaments to twist and dissociate, resulting in muscle contraction.
The major types of spontaneous activity in muscle fibres are fibrillation potentials, end-plate activity, positive sharp waves, fasciculation potentials, and repetitive discharges. Fibrillation potentials are muscle action potentials of short duration that result from the firing of single muscle fibres, either spontaneously or from mechanical irritation such as needle insertion. They are not visible at the skin surface but can be observed in an exposed muscle or on the tongue. Fibrillation potentials are most commonly associated with denervation, which can cause alterations in the sodium pump mechanism, leading to a decrease in the membrane resting potential.
The experimental method of exciting a muscle cell typically involves using a stimulating electrode to mimic the brief pulse of local circuit current generated by an action potential. However, if a depolarizing current is allowed to flow continuously, the muscle cell membrane does not fire repetitively. Instead, the sodium gates open and then become inactivated, while the potassium gates open repetitively, opposing excitation. This effect is similar to a continuous absolute refractory period.
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Muscle excitability testing methods
Muscle excitability testing is a technique used to obtain in vivo information about the properties of the muscle fibre membrane, such as membrane potential and ion channel function. This technique is mainly used in research to reveal disease mechanisms across a range of neuromuscular disorders, but it may also have diagnostic applications, especially in muscle channelopathies.
Conventional electrophysiological methods such as nerve conduction studies and electromyography are commonly used for the diagnosis of neuromuscular disorders. However, they offer limited insights into muscle fibre membrane properties and underlying disease mechanisms. In contrast, muscle excitability testing provides a more detailed assessment of the muscle fibre membrane, making it a valuable tool for understanding muscle function and disease processes.
One common method of muscle excitability testing is the multi-fibre muscle velocity recovery cycle (MVRC), which was introduced in 2009. MVRC is an automated, fast, and simple technique that has increased the use of muscle excitability testing. It involves measuring changes in MFAP (muscle fibre action potential) latency in response to trains of progressively increasing frequency conditioning stimuli up to 30 Hz. The comparison of MFAP latencies from the initial and final stimuli provides an assessment of sarcolemmal supernormality, which is associated with potassium accumulation within the T-tubule system.
Other muscle excitability testing protocols include frequency ramp and repetitive stimulation. The frequency ramp protocol further assesses sarcolemmal supernormality, while the repetitive stimulation protocol involves prolonged stimulation at 20 Hz to mimic short and long exercise tests. This repetitive stimulation protocol is particularly useful in the evaluation of channelopathies.
It is important to note that muscle excitability is influenced by several non-pathological variables, including temperature, electrolytes, muscle fibre subtype, and patient age. Therefore, these factors must be considered when interpreting the results of muscle excitability tests.
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The effect of hormones on muscle excitability
The human body contains many different hormones, which have a variety of effects on muscle excitability. For example, the hormone epinephrine helps muscles produce force. Testosterone, a hormone produced primarily in the testes of men and the adrenal glands of women, also has a significant impact on muscle excitability. Testosterone binds to androgen receptors inside muscle cells, signalling the cell's nucleus to increase protein synthesis and thereby increasing muscle fibre size. Testosterone replacement in women with low levels can lead to increased lean muscle mass and improved muscle strength.
Additionally, resistance exercise naturally increases the concentration of anabolic hormones in the blood, such as testosterone and growth hormone (GH), which stimulates muscle protein synthesis. The effect of this is twofold: firstly, it promotes the growth and repair of muscle tissues, and secondly, it increases muscle strength. This increase in muscle strength is due to the increase in the number of neurotransmitters at the end of the motor neuron at the neuromuscular junction.
The release of GH from the pituitary gland is also stimulated by muscular force production. GH is the primary hormone in a superfamily of various types and forms of the primary hormone. It is a 191 amino acid hormone, with shorter amino acid variants, aggregates, and binding proteins, all with known and unknown biological functions.
Furthermore, ovarian hormones such as progesterone and oestrogen also have an impact on muscle excitability. Progesterone has muscle-relaxing properties, especially during pregnancy, where it helps prevent uterine contractions. Oestrogen, on the other hand, helps to preserve muscle mass and maintain bone health, contributing to overall musculoskeletal health.
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Frequently asked questions
Yes, muscle cells have excitable membranes. The permeability of an excitable membrane to ions is controlled by the potential difference across it.
All animal cells have a resting potential, ion pumps, and a membrane that acts like an RC circuit. What distinguishes neurons (and to a lesser extent muscle and endocrine cells) from other cells is their excitability. Excitability is the ability of a cell to generate and propagate a large, rapid potential change in response to a relatively small trigger stimulus.
The normal experimental method of exciting a muscle cell is to mimic, with a stimulating electrode, the brief pulse of local circuit current that is generated by an action potential.
Evoked response amplitude and muscle fiber conduction velocity are measured to study muscle excitability. The magnitude of the response depends on the extent and speed of needle movement.
