Logan Steele
Muscle tetanus, also known as tetanic contraction, is a sustained muscle contraction that occurs when a motor nerve stimulates a skeletal muscle to emit action potentials at a high rate. This results in a constant tension in the muscle, leading to its maximal possible contraction. During tetanic contractions, muscles can shorten, lengthen, or maintain a constant length. The driving factor behind muscle contractions is the need to produce energy through metabolic processes, which break down compounds such as ATP, creatine phosphate, glycogen, and carbohydrates. Inorganic phosphate (Pi) plays a significant role in muscle fatigue, as it increases during fatigue and can combine with calcium to form calcium phosphate, impacting muscle performance.
| Characteristics | Values |
|---|---|
| Muscle tetanic contraction | A sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate |
| Tetanus | A fused tetanus is when there is no relaxation of the muscle fibres between stimuli and it occurs during a high rate of stimulation |
| Muscle fatigue | Caused by a decline in the performance of muscles when they are used at near their maximum capacity |
| Inorganic phosphate | Substantially increases during fatigue and may enter the sarcoplasmic reticulum (SR) |
| Muscle metabolism | The driving factor is the need to produce energy to support muscular contractions |
| Muscle tone | A healthy form of involuntary sustained partial contraction |
| Muscle cramps | Caused by the disease tetanus due to a lack of inhibition to the neurons that supply muscles |
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What You'll Learn
The role of phosphate and calcium stores in muscle fatigue
Muscle fatigue is a decline in performance caused by the effects of metabolic changes on the contractile machinery or the activation processes. It is characterised by reduced force production, decreased velocity of shortening, and slowed relaxation. Intensive muscle activity causes a substantial increase in the concentration of inorganic phosphate (Pi) in the myoplasm ([Pi]myo), which affects both the myofibrillar proteins and the activation processes.
The increase in [Pi]myo contributes to muscle fatigue by reducing the release of Ca2+ from the sarcoplasmic reticulum (SR). This occurs when [Pi]myo enters the SR and binds to Ca2+, forming an insoluble precipitate of calcium phosphate (CaPi). This reduces the amount of free Ca2+ available for release and contributes to the reduced force production associated with muscle fatigue.
Intracellular calcium release declines during muscle fatigue and contributes to the reduction in force. This decline in calcium release can be explained by the calcium phosphate precipitation" hypothesis, which states that the formation of CaPi reduces the amount of free Ca2+ available for release. This hypothesis is supported by experiments showing that increased intracellular phosphate results in a decrease in Ca2+ release from the SR.
In addition to the role of phosphate, calcium stores in the SR also play a role in muscle fatigue. The store of releasable Ca2+ in the SR declines during fatigue, mirroring the decline in SR Ca2+ release. This decline in calcium stores may be due to the formation of CaPi, which reduces the amount of free Ca2+ available for release.
Furthermore, muscle fatigue can also be caused by the breakdown of glycogen to lactic acid, which leads to an increase in H+ ions and a decrease in pH, causing acidosis and impairing muscle contraction. However, the role of reduced pH as a major cause of muscle fatigue is being challenged by recent studies suggesting that it may have little effect on contraction in mammalian muscle at physiological temperatures.
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The formation of ATP
Adenosine triphosphate (ATP) is a nucleoside triphosphate consisting of a nitrogenous base (adenine), a ribose sugar, and three serially bonded phosphate groups. ATP is commonly referred to as the "energy currency" of the cell, as it provides readily releasable energy in the bond between the second and third phosphate groups. The body requires energy to maintain proper functioning, and ATP is the source of energy for use and storage at the cellular level.
ATP is biosynthesized in several ways, including photophosphorylation, which is specific to plants and cyanobacteria. Photophosphorylation is the creation of ATP from ADP using energy from sunlight and occurs during photosynthesis. Another method of ATP formation is through cellular respiration in the mitochondria of a cell, which can occur through either aerobic or anaerobic respiration. Aerobic respiration produces ATP from glucose and oxygen, along with carbon dioxide and water as byproducts. In contrast, anaerobic respiration uses chemicals other than oxygen and is primarily used by archaea and bacteria that live in anaerobic environments.
ATP can also be produced through fermentation, which does not require oxygen. Fermentation is different from anaerobic respiration because it does not use an electron transport chain. Yeast and bacteria are examples of organisms that use fermentation to generate ATP. Additionally, ATP can be synthesized through catabolic mechanisms such as cellular respiration, beta-oxidation, and ketosis. The majority of ATP synthesis occurs in cellular respiration within the mitochondrial matrix, generating approximately 32 ATP molecules per molecule of glucose that is oxidized.
ATP is essential for various cellular processes, including muscle contraction, nerve impulse propagation, substrate phosphorylation, and chemical synthesis. During muscle contraction, ATP provides energy by cleaving its third phosphate group, releasing energy to form ADP (adenosine diphosphate). The breakdown of ATP through hydrolysis also serves a broad range of cell functions, including signaling and DNA/RNA synthesis. To meet the high demand for ATP, cells within the human body depend on the hydrolysis of 100 to 150 moles of ATP per day to ensure proper functioning.
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The impact of oxygen availability on muscle contractions
The availability of oxygen has a significant impact on muscle contractions. During exercise, the body's cardiovascular system undergoes adjustments to deliver oxygen and nutrients to the tissues that need them, including the heart, respiratory muscles, and contracting skeletal muscles. This process is known as exercise hyperemia. The largest increase in blood flow occurs in the exercising skeletal muscles due to their mass relative to other tissues.
Oxygen is essential for the production of adenosine triphosphate (ATP), which provides the energy required for muscle contractions. In the absence of oxygen, a process called anaerobic glycolysis occurs, where a molecule of glucose is broken down into lactic acid, producing only two ATP molecules. This process is energetically inefficient and can only sustain moderate physical activity for about a minute.
To enable sustained muscular activity, oxygen is necessary for a process called oxidative metabolism or aerobic respiration. During aerobic respiration, mitochondria break down a molecule of glucose into carbon dioxide and water, generating 36 molecules of ATP, a significantly higher yield compared to anaerobic glycolysis.
The transition from rest to exercise requires adjustments in the cardiovascular system to meet the increased oxygen and nutrient demands of the active skeletal muscles. This includes an increase in heart rate and cardiac contractility, enhanced blood flow to respiratory muscles, vasodilation, and increased blood flow to the contracting skeletal muscles. These alterations are coordinated by the sympathetic nervous system to maintain cardiovascular stability during intense exercise.
Prior contractions can also impact muscle oxygen pressure during subsequent contractions. Studies have shown that prior exercise can speed up pulmonary oxygen uptake kinetics, potentially due to increased muscle oxygen delivery or a more rapid elevation of oxidative phosphorylation. This suggests that priming muscles with prior exercise can enhance oxygen uptake during subsequent exercise.
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The function of motor units in tetanic contractions
A tetanic contraction is a sustained muscle contraction that occurs when a motor nerve that innervates a skeletal muscle emits action potentials at a high rate. This results in a motor unit being maximally stimulated by its motor neuron and remaining in that state for an extended period.
Motor units are fundamental contractile units comprising a motor nerve fibre and the muscle fibres it supplies. When a nerve fibre fires, all the muscle fibres connected to it contract simultaneously. During a tetanic contraction, the motor unit is stimulated by multiple impulses at a high frequency, causing a series of twitches that overlap and result in a sustained contraction. This is known as a fused tetanus, where there is no relaxation of the muscle fibres between stimuli, resulting in the strongest single-unit twitch in contraction. The contracting tension in the muscle remains constant, and the muscle can shorten, lengthen, or maintain its length.
Tetanic contractions can also be unfused, where the muscle fibres do not completely relax before the next stimulus because they are being stimulated at a fast rate. In this case, the tension developed by the muscle remains constant, and the contraction is called a fused tetanus. The rate of stimulation that produces a fused tetanus is called the fusion frequency.
The resistance to fatigue of fast motor units has been studied in the rat medial gastrocnemius muscle, where the boost and sag effects in unfused tetanic contractions were observed at constant stimulation frequencies. However, during voluntary movements, the intervals between successive discharges of motoneurons vary, and the extra-efficient force production at the onset of contraction (boost) was also observed during stimulation with variable intervals.
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The differentiation between tetanic contractions and tetany
Tetanic contractions and tetany are two distinct concepts, with key differences in their causes, nature, and effects.
Tetanic Contractions
Tetanic contractions, also known as physiologic tetanus, refer to sustained muscle contractions that occur when a motor nerve stimulates a skeletal muscle at a very high rate. This results in a state where the motor unit is maximally stimulated and remains that way for a period. Each stimulus causes a twitch, and when these stimuli are delivered rapidly, the twitches overlap, leading to a tetanic contraction. This can be further classified into unfused and fused tetanus. An unfused tetanus occurs when the muscle fibres do not completely relax before the next stimulus due to the fast rate of stimulation. In contrast, a fused tetanus happens when there is no relaxation of the muscle fibres between stimuli, resulting in the strongest single-unit twitch contraction.
Tetanic contractions are typically normal and are often observed in everyday activities such as holding a heavy box or maintaining a crouching position. These contractions contribute to muscle tone and help maintain posture.
Tetany
Tetany, on the other hand, is a symptom characterised by involuntary muscle contractions and overly stimulated peripheral nerves. It is caused by electrolyte imbalances, most commonly low blood calcium levels (hypocalcemia). Other types of electrolyte imbalances that can lead to tetany include hypomagnesemia, which is when magnesium levels in the blood are too low, and respiratory alkalosis, which occurs due to breathing too fast or too deeply, resulting in reduced carbon dioxide levels in the blood.
Tetany typically involves uncontrolled muscle contractions but does not involve an uncontrolled surge of electrical activity in the brain, which distinguishes it from seizures. However, severe cases of tetany can result in seizures.
In summary, the key differentiation between tetanic contractions and tetany lies in their nature and causes. Tetanic contractions are normal, sustained muscle contractions resulting from rapid stimulation of motor nerves. On the other hand, tetany is a symptom involving involuntary and uncontrolled muscle contractions caused by electrolyte imbalances, most often low blood calcium levels.
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Frequently asked questions
A tetanic contraction is a sustained muscle contraction that occurs when a motor nerve that innervates a skeletal muscle emits action potentials at a high rate.
Unfused tetanus occurs when the muscle fibres do not completely relax before the next stimulus because they are being stimulated at a fast rate. Fused tetanus, on the other hand, occurs when there is no relaxation of the muscle fibres between stimuli and it happens during a high rate of stimulation.
Inorganic phosphate (Pi) plays a significant role in muscle contractions and fatigue. During fatigue, Pi increases and can enter the sarcoplasmic reticulum (SR), combining with Ca2+ to form calcium phosphate (CaPi). This leads to reduced SR Ca2+ release and subsequent muscle fatigue.
ATP provides energy for muscle contractions by cleaving its third phosphate group and releasing energy to form ADP (adenosine diphosphate). During rapid and intense contractions, phosphagens can be used to rapidly rebuild ATP and maintain its level.
Tetanus contractions are typically associated with a deficiency of calcium. Calcium ions interact with the exterior surface of sodium channels in nerve cells, influencing the voltage level required to open voltage-gated sodium channels. Hypocalcemia, or low calcium levels, can increase the possibility of action potentials and involuntary muscle contractions.
