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Muscle excitability is the ability of muscles to respond to stimuli. Muscles are composed of specialized cells known as muscle cells or myocytes, commonly referred to as muscle fibers. These muscle fibers are stimulated by motor neurons, which cause the muscle fibers to contract. The contraction of muscle fibers is made possible by the presence of actin and myosin filaments, which slide past each other to produce contractions that move body parts. The stimulation of muscle fibers by motor neurons results in the release of ionic calcium, which interacts with the regulatory protein troponin to initiate contraction. This process, known as excitation-contraction coupling, is essential for muscle function and movement. Skeletal muscles, the most common type of muscle tissue in the body, are particularly susceptible to excitability due to their elongated structure and high number of nuclei.
| Characteristics | Values |
|---|---|
| Definition | Muscle is a tissue primarily composed of specialized cells/fibers which are capable of contracting in order to effect movement. |
| Composition | Muscle tissue is composed of specialized cells known as muscle cells or myocytes (commonly referred to as muscle fibers). |
| Contraction | Contraction is achieved by the muscle's structural unit, the muscle fiber, and by its functional unit, the motor unit. |
| Excitation-contraction coupling | This is the process by which a muscular action potential in the muscle fiber causes the myofibrils to contract. |
| Excitable | Muscles are excitable cells stimulated by motor neurons. |
| Extensibility | Ability of a muscle to be stretched without tearing. |
| Elasticity | Ability of a muscle to return to its normal shape. |
| Types | There are three types of vertebrate muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. |
| Functions | The functions of muscles include producing force and movement, maintaining body posture, controlling body temperature, stabilizing joints, and more. |
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What You'll Learn
Muscle excitability is evaluated by determining the Motor Threshold (MT)
Muscle excitability is the ability of muscles to respond to stimuli. Muscles are composed of excitable cells called muscle fibres, which are stimulated by motor neurons. The stimulation of these motor neurons causes the muscle fibres to contract. Cortical excitability in humans is typically evaluated by determining the Motor Threshold (MT). MT is defined as the stimulation intensity that elicits either muscle contraction or compound Motor Evoked Potentials (MEPs) measured by electromyography (EMG) on 50% of single-pulse TMS (spTMS) trials. MT can be assessed as “resting” MT (rMT) or “active” MT (aMT). rMT is when the target muscles are fully relaxed, while aMT involves moderate isometric contraction of target muscles, which generally lowers the threshold from rMT.
Transcranial Magnetic Stimulation (TMS) is a widely used investigative technique in motor cortical evaluation, particularly in studies evaluating lower-limb fatigue. The TMS intensity is determined by the resting and active motor thresholds and stimulus-response curves during muscular contraction. The TMS dosage is set relative to the minimum intensity required to elicit a response in a target muscle, i.e., the motor threshold (MT). MT has become the standard for determining TMS dosage due to safety concerns and its ability to stimulate the cortex effectively and reproducibly.
However, visually estimating MT by observing muscle twitches (OM-MT) can lead to overestimation of the MT and, consequently, unsafe TMS dosage. This is because OM-MT tends to yield significantly higher MT values than EMG-MT, which is based on electromyography recordings. Therefore, it is recommended to use EMG-MT to determine the MT accurately and avoid potential safety issues.
Furthermore, the accuracy of MT estimation can be improved by using a Bayesian adaptive method that incorporates prior MT knowledge and a stopping criterion based on MT precision estimation. This method has been shown to achieve similar accuracy to existing methods with fewer TMS pulses, making it a faster and more efficient way to determine MT.
In conclusion, muscle excitability is evaluated by determining the Motor Threshold (MT) through techniques such as TMS and EMG. Accurate determination of MT is crucial for safety and efficacy in therapeutic applications such as alleviating depression and understanding fatigue mechanisms.
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Electromyography (EMG) measures Motor Evoked Potentials (MEPs)
Motor Evoked Potentials (MEPs) are electrical signals produced when the motor regions of the brain or spinal cord are naturally stimulated. MEPs are measured by electromyography (EMG) surface electrodes applied over the muscle belly. The MEP can be used to estimate the excitability of the corticospinal tract, while the silent period which follows the MEP can be used to estimate corticospinal inhibition.
The Motor Threshold (MT) is defined as the stimulation intensity that elicits either overt muscle contraction or compound MEPs measured by EMG on 50% of single-pulse TMS trials. MT can be assessed as "resting" MT (rMT) or "active" MT (aMT). The latter generally lowers the threshold from rMT.
Transcranial magnetic stimulation (TMS) is a safe, non-invasive technique that delivers electromagnetic pulses to the cerebral cortex through a magnetic coil, inducing a focused electric field in the underlying brain tissue. When a single pulse of TMS is applied to the primary motor cortex with sufficient intensity, it depolarizes corticospinal neurons, eliciting a muscle contraction in the contralateral peripheral muscles, known as MEP. The MEP is linearly correlated with the number of activated corticospinal neurons and is considered valid to track corticospinal excitability.
The MEP test measures electrical signaling through the motor pathways of the nervous system. It does this by stimulating the area of the brain that controls movement, called the motor cortex. When the motor cortex is stimulated, it makes an electrical signal that can be detected in muscles at other points in the body. An MEP test gives information on electrical signals moving from the brain to a target muscle group.
MEPs were originally reported following electrical stimulation of the motor cortex. Subsequently, magnetic stimuli were introduced to evoke MEPs. The latter method, TMS, is largely preferred since magnetic fields pass unattenuated through the skull and scalp, without nociceptive activation, and penetrate easily into the brain, generating an electrical current that activates the neural tissue.
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Muscle contraction is caused by muscle fibres
A skeletal muscle contains multiple fascicles, or bundles of muscle fibres. Each individual fibre and muscle is surrounded by a type of connective tissue layer of fascia. Muscle fibres are formed from the fusion of developmental myoblasts in a process known as myogenesis, resulting in long multinucleated cells. In these cells, the nuclei, or myonuclei, are located along the inside of the cell membrane.
Muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments. These are repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fibre necessary for muscle contraction. The myosin heads pull on the actin filaments, causing the sarcomere and the muscle fibre to contract. This motion of the myosin heads is similar to the oars when an individual rows a boat.
The nervous system generates a signal, an impulse called an action potential, which travels through a type of nerve cell called a motor neuron. When the nervous system signal reaches the neuromuscular junction, a chemical message is released by the motor neuron. This chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fibre. That starts a chemical reaction within the muscle.
The complex process leading to muscle contraction, called excitation-contraction coupling, begins when an action potential causes depolarization in the myocyte membrane. The depolarization is spread via the transverse (T) tubules, which help spread depolarization signals to the entire muscle fibre.
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Muscle fibre composition
Slow oxidative (SO) fibres contract slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They are commonly found in muscles that require endurance, such as the soleus muscle in the leg. On the other hand, fast oxidative (FO) and fast glycolytic (FG) fibres contract more quickly and are used in muscles that require more explosive movements. For example, the extraocular muscles that position the eyes have a high proportion of fast-twitch fibres.
The composition of muscle fibre types is influenced by both genetic and environmental factors. The number of slow and fast-twitch fibres in an individual is determined by their genetics. However, the composition can also adapt to changing demands, such as physical training or different environmental conditions. For instance, endurance training can increase the endurance level of fast-twitch fibres, while sprint training can improve the power generated by slow-twitch fibres. Additionally, environmental factors, such as water temperature in fish, can influence the expression of different fibre types.
Muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments. These myofilaments are arranged in repeated units called sarcomeres, which are the basic functional units necessary for muscle contraction. The contraction of muscle fibres is stimulated by motor neurons, which release the neurotransmitter acetylcholine at the neuromuscular junctions. This process, known as excitation-contraction coupling, involves the release of ionic calcium from the sarcoplasmic reticulum, which interacts with regulatory proteins to enable muscle contraction.
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Muscle excitability abnormalities
One example of a muscle excitability abnormality is non-dystrophic myotonia, a rare neurological condition characterised by exacerbated sarcolemma excitability. This condition leads to delayed relaxation after contraction, resulting in muscle stiffness and pain. It can affect various muscles, including handgrip, eye closure, and tongue muscles. Myotonia congenita (MC) is a form of non-dystrophic myotonia caused by loss-of-function mutations in the voltage-gated chloride channel ClC-1.
Machado-Joseph disease (MJD) is another condition associated with muscle excitability abnormalities. MJD patients often experience muscle cramps and fasciculations, which can be disabling and affect various parts of the body, including the lower limbs, face, and abdominal muscles.
Additionally, mutations in the gene encoding voltage-gated ion channels can cause neuromuscular disorders such as myotonia and paralysis. These mutations disrupt the normal functioning of ion channels, leading to sustained muscle discharges and impaired relaxation.
The study of muscle excitability abnormalities has implications for understanding the pathophysiology of various neurological and neuromuscular disorders. Techniques such as electromyography (EMG) and needle electrode insertion can provide important measures of muscle excitability, helping to characterise and treat these abnormalities effectively.
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Frequently asked questions
Muscles are tissues composed of specialized cells or fibres that are capable of contracting in order to effect movement. They are part of the voluntary muscular system and are attached to bones by tendons.
Excitable muscles are those that can respond to stimuli, which may be delivered from a motor neuron or a hormone. They are characterised by their ability to contract, be stretched, and return to their original shape.
Skeletal muscles are the most common type of excitable muscle tissue in the body. They are attached to the bones of the skeleton and enable movement. Other examples include cardiac muscle and smooth muscle.
Excitable muscles work in groups and patterns of movement. They have lengthening, contractile, extensibility, and recoil characteristics. Contractility allows a muscle to shorten with force, to lengthen passively, and to move. Excitation-contraction coupling is the process by which a muscular action potential causes the myofibrils to contract.
