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Muscle contraction is the activation of tension-generating sites within muscle cells. This process involves the nervous system sending messages to the muscular system, triggering chemical reactions that cause muscle fibres to reorganise themselves and shorten, resulting in muscle contraction. There are four types of striated muscle contractions: isometric, isotonic, concentric, and eccentric. Smooth and cardiac muscles, on the other hand, contract involuntarily without conscious intervention.
| Characteristics | Values |
|---|---|
| Definition | Muscle contraction is the activation of tension-generating sites within muscle cells. |
| Types | There are four types of striated muscle contractions: isometric, isotonic, concentric, and eccentric. |
| Isometric contraction | Change in muscle tension without a change in muscle length. |
| Isotonic contraction | Constant muscle tension with a change in muscle length. |
| Concentric contraction | Sufficient muscle tension to overcome the load, causing the muscle to contract and shorten. |
| Eccentric contraction | The muscle works to decelerate a joint at the end of a movement, acting as a braking force to protect joints from damage. |
| Muscle tissue types | There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. |
| Skeletal muscle | Skeletal muscle is voluntary muscle, anchored by tendons or aponeuroses to bones, and is responsible for skeletal movement and posture. |
| Cardiac muscle | Cardiac muscle is found in the walls of the heart and contracts involuntarily. |
| Smooth muscle | Smooth muscle is non-striated and involuntary, found within the walls of organs and structures such as the esophagus, stomach, and intestines. |
| Muscle fiber types | Skeletal muscle is classified into two fiber types: type I (slow-twitch) and type II (fast-twitch). |
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What You'll Learn
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
Skeletal muscle is the specialised tissue that is attached to bones and allows movement. Together, skeletal muscles and bones are called the musculoskeletal system (also known as the locomotor system). Skeletal muscle is broadly classified into two fibre types: type I (slow-twitch) and type II (fast-twitch). Type I, slow-twitch, is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red colour. Type II, fast-twitch muscle, has three major kinds that are, in order of increasing contractile speed: Type IIa, which is aerobic, rich in mitochondria and capillaries and appears red when deoxygenated; 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 is a highly oxygen-consuming tissue. Skeletal muscle constitutes approximately 40% of the total human body weight.
Cardiac Muscle
The heart is the only organ that is also a muscle. It is made of a special type of muscle tissue called cardiac muscle. Cardiac muscle or myocardium is an involuntary, striated muscle that encloses the chambers of the heart. It is composed of individual cardiomyocytes, which are similar in structure to skeletal muscle. Each cardiomyocyte contains cytoskeletal and contractile elements, all of which are connected through intercalated discs. These are highly adherent complexes that allow the cardiac muscle cells to receive rapid electrical transmission and contract as a single unit. The cells of cardiac muscle tissue are shorter than skeletal muscle tissue and form a network of many branches between the cells.
Smooth Muscle
Smooth muscle tissue is non-striated and involuntary. Smooth muscle 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 also commonly known as visceral muscle due to its lack of striations. Smooth muscle is present throughout the gastrointestinal, reproductive, urinary, vascular, and respiratory systems.
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Muscle contraction mechanisms
Muscle contraction is the activation of tension-generating sites within muscle cells. Muscle shortening and muscle contraction are not synonymous. Tension within the muscle can be produced without changes in muscle length, such as when holding a heavy weight in the same position.
There are four types of striated muscle contractions: isometric, isotonic, concentric, and eccentric. Isometric striated muscle contraction is characterised by a change in muscle tension without a change in muscle length. For example, when pushing against an immovable object or trying to lift a weight that is too heavy. Isotonic striated muscle contraction is characterised by constant muscle tension with a change in muscle length. This occurs when the contraction force matches the total load on a muscle, such as during activities like walking, running, or squatting.
Concentric striated muscle contraction occurs when there is sufficient muscle tension to overcome the load, and the muscle contracts and shortens. During this type of contraction, a muscle is stimulated to contract according to the sliding filament theory. An example of this is a biceps curl, where the arm bends at the elbow as the hand is moved from the leg to the shoulder.
Eccentric striated muscle contraction occurs when the muscle works to decelerate a joint at the end of a movement, as opposed to pulling a joint in the direction of the contraction. This can occur involuntarily, such as when attempting to lift a weight that is too heavy, or voluntarily, such as when resisting gravity during downhill walking. An example of an eccentric contraction is the triceps straightening the elbow during the bicep curl movement described above.
Cardiac muscle contraction occurs via excitation-contraction coupling (ECC), which utilises a mechanism called calcium-induced calcium release (CICR). ECC is the process of converting an electrical stimulus (AP) into a mechanical response (muscle contraction). CICR involves the conduction of calcium ions into the cardiomyocyte, leading to the further release of ions into the cytoplasm.
Smooth muscle does not contain the troponin complex required for skeletal muscle contraction and has a different mechanism for controlling contraction. This difference is characterised by how calcium enters the cell. Calcium can enter the cell through voltage-gated calcium channels activated by membrane depolarization, or through ligand-gated channels on the cell membrane opened by hormones or neurotransmitters.
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Muscle relaxation
Progressive muscle relaxation (PMR) is a widely used procedure and one of the simplest and easiest-to-learn techniques for relaxation. It was developed by Dr. Edmund Jacobson in the 1920s and was originally used to treat symptoms of anxiety. However, it has since been found to be effective for treating tension headaches, migraines, insomnia, and high blood pressure, among other ailments.
PMR is a two-step process to reduce stress and build awareness of sensations of tension and deep relaxation in various muscle groups. The first step is to create tension in specific muscle groups and begin to notice what tension feels like. The second step is to release this muscle tension and notice what a relaxed muscle feels like as the tension drains away. This process can be repeated several times in the same muscle groups, with diminishing degrees of tension, to deepen awareness and train the body to relax more deeply.
PMR involves alternately tightening and relaxing 14 different muscle groups, one at a time, in a specific pattern. The goal is to release tension from the muscles while helping you recognize what that tension feels like. It is recommended to tense each muscle group and hold for 5 seconds, then exhale and relax for 10 to 20 seconds before moving on to the next muscle group.
- Lie down or sit in a quiet, comfortable area.
- Take five deep, slow breaths.
- Lift your toes upward, hold, then let go.
- Pull your toes downward, hold, then let go.
- Tense your calf muscles, hold, then relax.
- Move your knees toward each other, hold, then relax.
- Squeeze your thigh muscles, hold, then relax.
- Clench your hands, pause, then relax.
- Tense your arms, hold, then relax.
- Squeeze your buttocks, pause, then relax.
- Contract your abdominal muscles, pause, then relax.
- Inhale and tighten your chest, hold, then exhale and relax.
- Raise your shoulders to your ears, pause, then relax.
- Purse your lips together, hold, then release.
- Open your mouth wide.
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Striated muscle
Skeletal muscle is the most common type of muscle in the human body. Skeletal muscles are attached to some component of the skeleton, and they consist of flexible muscle fibres that range from less than half an inch to just over 3 inches in diameter. These fibres contract, allowing the muscles to move bones so that the body can perform different movements. Skeletal muscles are voluntary muscles, meaning that a person controls how and when they move and work. Nerves in the somatic nervous system send signals to make them function.
Cardiac muscle contractions occur due to a myogenic response of the heart's pacemaker cells. An action potential (AP) is induced by the pacemaker cells in the sinoatrial (SA) and atrioventricular (AV) nodes and is conducted to contractile cardiomyocytes through gap junctions. As the AP travels between sarcomeres, it activates the Ca channels in the T tubules, leading to an influx of Ca ions into the cardiomyocyte.
There are four types of striated muscle contractions: isometric, isotonic, concentric, and eccentric. An isometric contraction of a muscle generates tension without changing length. An isotonic contraction involves constant muscle tension with a change in muscle length. This occurs when the contraction force matches the total load on a muscle. In a concentric contraction, muscle tension is sufficient to overcome the load, and the muscle contracts and shortens. An eccentric contraction occurs when the muscle works to decelerate a joint at the end of a movement, as opposed to pulling a joint in the direction of the contraction.
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Calcium's role in muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. Calcium (Ca) plays a crucial role in the contraction and relaxation of muscles. Calcium ions (Ca2+) are the main regulatory and signalling molecule in muscle fibres, and their variable expression in proteins involved in Ca2+ signalling and handling determines the contractile properties of muscle fibres.
Calcium ions enter the cell through three mechanisms that increase intracellular concentration. Firstly, voltage-gated Ca channels are activated by membrane depolarization, allowing Ca to enter the cell. Secondly, hormones or neurotransmitters such as norepinephrine and angiotensin II can open ligand-gated channels on the cell membrane. Finally, hormones and neurotransmitters can, via the phospholipase-C (PLC) pathway, increase intracellular inositol triphosphate (IP3). IP3 can then bind to receptors on the SR and cause Ca to be released.
The Ca2+ signalling apparatus includes the ryanodine receptor, the sarcoplasmic reticulum Ca2+ release channel, the troponin protein complex, the Ca2+ pump responsible for Ca2+ reuptake into the sarcoplasmic reticulum, and calsequestrin, the Ca2+-storage protein in the sarcoplasmic reticulum.
In cardiac muscle contraction, an action potential (AP) activates the Ca channels in the T tubules, leading to an influx of Ca ions into the cardiomyocyte. The Ca in the cytoplasm then binds to cardiac troponin C, which moves the troponin complex away from the actin-binding site. Removal of the troponin complex frees actin, which becomes bound by myosin and initiates contraction. Intracellular Ca is then removed by the SR, dropping the concentration of intracellular Ca. This decrease in intracellular Ca concentration returns the troponin complex to its inhibiting position on the active site of actin, ending contraction as the actin filaments return to their initial position, relaxing the muscle.
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Frequently asked questions
Muscle contraction is the activation of tension-generating sites within muscle cells. This process does not always result in muscle shortening, as muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position.
There are four types of striated muscle contractions: isometric, isotonic, concentric, and eccentric. Isometric contractions are when muscle tension changes without a change in muscle length, for example, when pushing against an immovable object. Isotonic contractions are when muscle tension remains constant but the muscle length changes, such as during walking or running. Concentric contractions occur when muscle tension overcomes the load, causing the muscle to contract and shorten, such as during a bicep curl. Eccentric contractions act as a braking force to concentric contractions, protecting the joints from damage.
The process of muscle contraction can be summarised in three steps. First, a message is sent from the nervous system to the muscular system, triggering chemical reactions. Second, these chemical reactions lead to muscle fibres reorganising themselves in a way that shortens the muscle, resulting in contraction. Third, when the nervous system signal stops, the chemical process reverses, the muscle fibres rearrange, and the muscle relaxes.
Calcium plays a crucial role in muscle contraction, especially in cardiac muscle. An increase in intracellular calcium concentration activates calcium channels in the T tubules, allowing calcium ions to enter the cardiomyocyte. The calcium then binds to cardiac troponin C, moving the troponin complex away from the actin-binding site. This initiates contraction as actin becomes bound by myosin. When calcium concentration decreases, the troponin complex returns to its inhibiting position, ending contraction and relaxing the muscle.
