Dominic Stone
Muscle tetanus, also known as 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. During a muscle tetanus, the contractile elements are maintained, allowing them to shorten enough to extend the elastic elements fully and reach maximum tension. The frequency at which this occurs is typically 50-100 Hz, and it is characterized by the absence of a discernable relaxation phase between twitches. This phenomenon is distinct from the disease called tetanus and is a normal physiological process, often observed during voluntary actions such as holding a heavy box or maintaining a crouching position.
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What You'll Learn
Fused tetanus
Muscle tetanus, or a tetanic contraction, is a sustained muscle contraction that occurs when a motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. A tetanic contraction can be either unfused (incomplete) or fused (complete).
The rate of stimulation that produces fused tetanus is called the fusion frequency. During tetanic contractions, muscles can shorten, lengthen, or remain the same length. This type of contraction is usually normal, such as when holding up a heavy box or maintaining a crouching position.
Tetanic contraction can exist in a variety of states, including isotonic and isometric forms. For example, lifting a heavy box off the floor is isotonic, while holding it at an elevated position is isometric.
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Unfused tetanus
Muscle tetanus, or a tetanic contraction, is a sustained muscle contraction that occurs when a motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. Each stimulus causes a twitch, and if the stimuli are delivered at a high enough frequency, the twitches will overlap, resulting in a tetanic contraction.
A tetanic contraction can be either unfused (incomplete) or fused (complete). An unfused tetanus occurs when muscle fibres do not completely relax before the next stimulus because they are being stimulated at a fast rate. The mechanical response to repeated stimulation depends on the rate of stimulation. If pulses are very close together, there will be no response from the muscle. If the pulses are further apart, the muscle will be relaxing when the second pulse is given, and the tension will appear in waves in phase with the stimulation, causing an unfused tetanus.
The unfused tetanus of fast and slow motor units has been studied in rat muscle. The shapes of successive contractions are variable and differ from individual twitches. As the pulse number increases, the parameters of the contractions change differently for slow and fast motor units. For slow motor units, the maximal forces and time parameters increase considerably up to the fourth response, after which they remain nearly constant or show only a slight increase. For fast motor units, the maximal forces and durations also increase, while the remaining time parameters initially increase and then remain constant or decrease.
Linear relaxation has also been studied in isometric unfused tetanus of the human flexor digitorum sublimis muscle. With a decrease in force level, the shoulder on the relaxation curve shifted to the right. The length of the linear portion weakly depended on the activation level. With increased tetanus duration, linear relaxation becomes longer and slower at all force levels.
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Tetanus vs tetany
Tetanus and tetany are two distinct medical terms that sound similar but differ in meaning. Tetanus is an infectious disease that affects the central nervous system. It is caused by the bacterium Clostridium tetani, which enters the body through skin wounds contaminated with soil containing bacterial spores. This bacterium secretes a neurotoxin called tetanospasmin, which acts on the synapses and causes disinhibition of neuronal activity, resulting in muscular spasms and neuromuscular junction blockade.
On the other hand, tetany is a clinical manifestation or symptom that can occur in various medical conditions. It is characterised by muscular spasms, usually with intervening periods of recovery. Tetany is often associated with electrolyte imbalances, particularly low calcium levels (hypocalcemia), magnesium deficiency, or low potassium levels. It can also be caused by toxins or underlying medical conditions such as kidney failure or parathyroid disease.
The key distinction between the two lies in their nature: tetanus is an infectious disease with a specific bacterial cause, while tetany is a symptom or clinical sign that can manifest in different clinical conditions. Tetany can even occur as a result of tetanus infection, indicating the relationship between the two terms.
While tetanus is typically associated with flexor muscle spasms, tetany can present with muscular spasms of varying intensities and frequencies. Tetany is considered a medical emergency requiring immediate medical attention, as it can be a sign of mild to severe electrolyte imbalances or other underlying health conditions.
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Causes of tetanus
Muscle tetanus, or tetanic contraction, is an uncommon but serious infection caused by bacteria found in the environment. The disease is caused by Clostridium tetani, a type of bacteria that is commonly found in the form of spores in the environment, including in soil, dust, and manure. Tetanus is not contagious, and infection occurs when the spores enter the body through broken skin, usually through injuries.
Wounds contaminated with dirt, feces, or saliva, as well as puncture wounds caused by objects like nails or needles, are particularly susceptible to tetanus infection. Injuries with dead tissue, such as burns, crush injuries, or frostbite, are also at risk due to the potential for bacteria growth. In addition, tetanus may occur in individuals who are unable to recall a specific wound or injury.
Neonatal tetanus is a form of the disease that affects newborns and can be caused by non-sterile instruments used during birth or contaminated materials used to cover the umbilical stump. Deliveries performed by individuals with unclean hands or on contaminated surfaces are also risk factors for neonatal tetanus. In 2018, approximately 25,000 newborns died from neonatal tetanus, a significant reduction from 1988 when it claimed the lives of an estimated 787,000 newborns within their first month of life.
The best way to prevent tetanus is through immunization with tetanus-toxoid-containing vaccines (TTCV). The World Health Organization (WHO) recommends a total of 6 doses of TTCV for lifelong protection, consisting of 3 primary doses followed by 3 booster doses. Routine immunization during antenatal care and childhood, as well as strengthening clean delivery practices and neonatal tetanus surveillance, are crucial to preventing tetanus and protecting individuals from this serious infection.
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Muscle tone and tetanic contractions
Muscle tone is a healthy form of involuntary sustained partial contraction. It is important for maintaining posture. For example, when in a crouching position, some muscles require sustained contraction to hold the position.
A tetanic contraction is a sustained muscle contraction that occurs when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. This can also be referred to as a tetanized state, tetanus, or physiologic tetanus (to differentiate it from the disease called tetanus). Each stimulus causes a twitch, and if the stimuli are delivered at a high enough frequency, the twitches will overlap, resulting in a tetanic contraction.
Tetanic contractions can be either unfused (incomplete) or fused (complete). An unfused tetanus occurs when the muscle fibres do not completely relax between stimuli because they are being stimulated at a fast rate. A fused tetanus occurs when there is no relaxation of the muscle fibres between stimuli, and it occurs during a high rate of stimulation. This is the strongest single-unit twitch in contraction.
The mechanical response to repeated stimulation depends on the rate of stimulation. If the pulses are 300 milliseconds apart, the muscle will be relaxing when the second pulse is given, and the tension will appear in waves in phase with the stimulation, causing an unfused tetanus. However, if the stimulation rate is increased, it is possible to stimulate the muscle at a frequency where the tension developed by the muscle remains constant. This latter type of contraction is called a fused tetanus, and the rate of stimulation that produces it is called the fusion frequency.
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Frequently asked questions
Muscle tetanus, or a tetanic contraction, is a sustained muscle contraction that occurs when a motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. This results in a fused or unfused tetanus.
A fused tetanus occurs when there is no relaxation of the muscle fibres between stimuli, resulting in the strongest single-unit twitch in contraction. This is the maximal possible contraction.
An unfused tetanus occurs when the muscle fibres are stimulated at a fast rate and do not completely relax before the next stimulus. This results in an incomplete tetanus.
Muscle tetanus can be caused by various factors, including low concentrations of ionized serum calcium, hyperventilation, tapping the peripheral nerves, or blood supply occlusion to the limbs.
