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Muscle impulse, or muscle contraction, is the fundamental unit of muscle tissue, responsible for movement in the human body. The human body has over 600 muscles, which make up about half of a person's body weight. Each muscle is a discrete organ, consisting of skeletal muscle tissue, blood vessels, tendons, and nerves. Muscle contraction begins with an electrical nerve impulse that results in the release of calcium ions, which produce conformational changes that cause the muscle to contract. The strength of a muscle contraction depends on the number of motor units involved and the amount of stimulus from the nervous system.
| Characteristics | Values |
|---|---|
| Definition | A muscle impulse is an electrical nerve impulse that results in the contraction of muscles. |
| Muscles in the human body | Humans have over 600 individual muscles that make up about half of a person's body weight. |
| Types of muscles | There are three types of muscles: skeletal, cardiac, and smooth. |
| Muscle contraction | Muscle contraction is caused by the release of calcium ions, which produce conformational changes that result in the sliding of the threads through each other, shortening the muscle. |
| Strength of muscle contraction | The strength of a muscle contraction depends on the number of motor units involved in the contraction and the amount of stimulus from the nervous system. |
| Twitch contraction | A single nerve impulse of a motor neuron will cause a motor unit to contract briefly before relaxing, resulting in a small contraction known as a twitch contraction. |
| Temporal summation | If a motor neuron provides several signals within a short period, the strength and duration of the muscle contraction increase, known as temporal summation. |
| Tetanus | If a motor neuron provides many nerve impulses in rapid succession, the muscle may enter a state of tetanus, a complete and lasting contraction. |
| Isometric contractions | Isometric contractions are light contractions that increase muscle tension without causing movement. |
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What You'll Learn
Muscle contraction
The muscular system consists of over 600 individual muscles, comprising around half of a person's body weight. These muscles are constructed of skeletal muscle tissue, blood vessels, tendons, and nerves. Skeletal muscles are attached to bones and provide structure and strength to the body. They are responsible for movement and work in conjunction with the nervous system to produce a range of motions.
In addition to skeletal muscle, there are two other types: visceral (or smooth) muscle and cardiac muscle. Visceral muscle is the weakest type and is found inside organs like the stomach and intestines, as well as blood vessels. It is controlled by the unconscious part of the brain and is responsible for involuntary movements, such as pulmonary ventilation. Cardiac muscle, on the other hand, is found only in the heart and is responsible for pumping blood throughout the body. This type of muscle is also involuntary and is controlled by hormones and signals from the brain, which adjust the rate of contraction.
The process of muscle contraction can be further understood by examining the structure of striated muscles. These muscles are composed of individual muscle fibres, or myofibrils, which contain parallel thin and thick filaments. The thick filaments are made of the protein myosin, while the thin filaments consist of actin, tropomyosin, and troponin. During muscle contraction, an action potential causes depolarization in the myocyte membrane, leading to conformational changes and the eventual binding of myosin and actin. This complex process, known as excitation-contraction coupling, results in muscle fibres sliding past each other and producing the force necessary for movement.
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Types of muscle tissue
Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Each muscle type has unique cellular components, physiology, specific functions, and pathology.
Skeletal muscle is a specialised tissue attached to bones that allows movement. Together, skeletal muscles and bones are called the musculoskeletal system or locomotor system. Skeletal muscles are under our conscious control and are thus also known as voluntary muscles. They are broadly classified into two fibre types: type I (slow-twitch) and type II (fast-twitch). Type I, slow-twitch, or red muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red colour. It can carry more oxygen and sustain aerobic activity. Type II, fast-twitch muscle, has three major kinds, in order of increasing contraction speed: Type IIa, which is aerobic and rich in mitochondria and capillaries; Type IIx, which is less dense in mitochondria and myoglobin; and Type IIb, which is anaerobic, glycolytic, and "white" muscle that is even less dense in mitochondria and myoglobin. Skeletal muscle constitutes approximately 40% of the total human body weight.
Cardiac muscle, or myocardium, is an involuntary, striated muscle that makes up the middle layers of the heart. It is comprised of individual cardiomyocytes, which are structurally similar to skeletal muscle. Each cardiomyocyte contains cytoskeletal and contractile elements, all of which are connected through intercalated discs. These allow the cardiac muscle cells to receive rapid electrical transmission and contract as a single unit. The cardiac muscle also contains specialised cardiac pacemaker cells that allow for cardiac tissue to depolarize without external stimuli. The heart is the only organ that is also a muscle.
Smooth muscle is non-striated and involuntary. It is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin that control the erection of body hair. Smooth muscle is present throughout the gastrointestinal, reproductive, urinary, vascular, and respiratory systems.
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Muscle movement
The process of muscle contraction is initiated by electrical nerve impulses, which result in the release of calcium ions into the myosin-actin structure. This release of ions causes conformational changes that lead to the sliding of the threads through each other, ultimately resulting in the shortening of the muscle fibres and the production of movement. The strength of a muscle contraction can be influenced by two primary factors: the number of motor units involved and the amount of stimulus from the nervous system. When a motor neuron sends a single impulse, a brief contraction known as a twitch contraction occurs. However, if the motor neuron transmits multiple signals in quick succession, the muscle contraction becomes stronger and more sustained, a phenomenon known as temporal summation.
In addition to skeletal muscles, there are two other types of muscle tissue: visceral (or smooth) muscle and cardiac muscle. Visceral muscle is found in organs like the stomach, intestines, and blood vessels, and it is responsible for moving substances through these organs. Cardiac muscle, on the other hand, is unique to the heart and is responsible for pumping blood throughout the body. Unlike skeletal muscles, both visceral and cardiac muscles are considered involuntary muscles, meaning they cannot be directly controlled by the conscious mind.
The structure of muscle fibres also varies between different types of muscles. Striated muscles, for example, derive their name from their appearance under a microscope, characterised by light and dark stripes. These muscles are composed of individual muscle fibres that contain smaller units called myofibrils, which are made up of parallel thin and thick filaments. The thin filaments consist of actin, tropomyosin, and troponin, while the thick filaments are composed of the protein myosin. The interaction between these filaments is integral to the process of muscle contraction, known as excitation-contraction coupling.
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Muscle relaxation
Progressive muscle relaxation (PMR) is a technique that can be used to relax muscles and relieve stress, anxiety, insomnia, and chronic pain. PMR was developed by American physician Edmund Jacobson in the 1920s or 1930s based on the theory that physical relaxation can promote mental calmness.
PMR involves tightening and relaxing muscle groups one at a time in a specific order. The process starts by sitting or lying down, relaxing the entire body, and taking five deep breaths. Then, starting with the lower extremities, each muscle group is tensed for 5–10 seconds and then relaxed for 10–20 seconds before moving on to the next muscle group. During the tensing phase, it is recommended to inhale, and during the relaxation phase, to exhale and focus on the changes in the body. This technique can be practised for 15–20 minutes in a quiet, comfortable area, wearing loose, lightweight clothing.
PMR has been found to have therapeutic benefits for various conditions, including headaches, cancer pain, high blood pressure, digestive disturbances, and dental patients. It has also been shown to reduce symptoms of depression, anxiety, and stress while improving feelings of well-being and quality of life.
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Muscle fibres
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions. Skeletal muscle fibres can be further classified as Type 1 or Type 2. Type 1 fibres utilise oxygen to generate energy for movement and have a higher density of mitochondria, making them appear darker. Type 2 fibres can be further divided into subtypes 2A and 2B. Type 2A fibres can use oxygen to generate energy, but they contain fewer mitochondria, making them lighter in colour. Type 2B fibres don’t use oxygen to generate energy; instead, they store energy for short bursts of movement and appear white.
The speed of contraction of a muscle fibre depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP about twice as rapidly as slow fibres, resulting in faster cross-bridge cycling. The number of slow and fast-twitch fibres in the body varies between individuals and is determined by genetics. People who excel at endurance sports tend to have more slow-twitch fibres, while those better at sprinting tend to have more fast-twitch fibres.
Training can influence both types of fibres. Sprint training can improve the power generated by slow-twitch fibres, while endurance training can increase the endurance level of fast-twitch fibres. However, training cannot make slow-twitch fibres as powerful as fast-twitch, nor can it make fast-twitch fibres as fatigue-resistant as slow-twitch. Muscle fibres can also adapt to changing demands by changing size or fibre type composition, which serves as the basis for physical therapy interventions aimed at increasing a patient's force development or endurance.
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Frequently asked questions
A muscle impulse is an electrical nerve impulse that causes the release of calcium ions into the myosin-actin structure, resulting in muscle contraction.
Calcium ions produce conformational changes that result in the sliding of the threads through each other, shortening the myosin-actin structure and causing the muscle to contract.
The strength of a muscle contraction depends on two factors: the number of motor units involved and the amount of stimulus from the nervous system. If a motor neuron sends several signals in a short period, the muscle contraction's strength and duration increase.
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