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Muscle contraction is the activation of tension-generating sites within muscle cells. This process is controlled by various factors, including the type of muscle, its structure, and the nervous system. There are three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are under voluntary control, while cardiac and smooth muscles are under involuntary control by the body's autonomic nervous system. The structure of muscle fibres, particularly the arrangement of thin and thick filaments, also plays a role in controlling contraction. Additionally, neural stimulation, calcium ions, and proteins like actin, tropomyosin, and troponin are involved in the complex process of muscle contraction and relaxation.
| Characteristics | Values |
|---|---|
| Muscle contraction | The activation of tension-generating sites within muscle cells |
| Muscle relaxation | The return of muscle fibres to a low-tension state |
| Muscle types | Skeletal, cardiac, and smooth |
| Skeletal muscle contraction | Voluntary |
| Cardiac muscle contraction | Involuntary |
| Smooth muscle contraction | Involuntary |
| Smooth muscle types | Single-unit and multi-unit |
| Striated muscle fibres | Contain actin and myosin filaments |
| Smooth muscle fibres | Do not contain sarcomeres |
| Calcium entry into cells | Voltage-gated Ca channels, ligand-gated channels, increase in intracellular inositol triphosphate |
| Contraction strength | Dependent on cross-sectional area of muscle fibre and frequency of neural stimulation |
| Contraction types | Isometric, isotonic, concentric, eccentric |
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What You'll Learn
Skeletal muscle contraction
Skeletal muscles are attached to bones and provide the body with structure and strength. They work in conjunction with the bones of the skeleton to enable body movement. They are also associated with the diaphragmatic, oesophageal, and eye muscles. Skeletal muscles also help maintain the body's posture, store amino acids, and maintain core body temperature via shivering.
Excitation-contraction coupling refers to the mechanism that converts the action potentials mentioned above in the muscle fibres into muscle fibre contraction. The action potentials at the muscle cell membrane surrounding the myofibrils travel into the T-tubules, which are responsible for propagating the action potentials into the sarcoplasmic reticulum (SR). The SR then releases calcium ions (Ca2+) into the sarcoplasm (cytoplasm of muscle cells).
The calcium ions bind to troponin, a protein complex located along the tropomyosin filaments, exposing the myosin-binding sites on the actin filaments. The myosin heads are then attracted to actin, and myosin binds actin at its binding site, forming the cross-bridge. The myosin heads then pull the actin at the binding sites, detach, re-cock, and attach to more binding sites, repeating this process, which is known as the cross-bridge cycle. This process requires energy, which is provided by ATP.
The amount of tension produced during skeletal muscle contraction depends on the cross-sectional area of the muscle fibre and the frequency of neural stimulation. Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere.
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Calcium ions
The myosin heads can then bind to these exposed sites on the actin filament, forming cross-bridges. This is followed by a power stroke, where the myosin heads pivot and pull the actin filaments towards the centre of the sarcomere, the functional unit of muscle fibres. This sliding of actin and myosin filaments past each other shortens the sarcomere, causing the muscle to contract. Calcium ions are essential for muscle contraction as they trigger the exposure of myosin-binding sites on actin, allowing the formation of cross-bridges and the sliding of filaments that leads to muscle contraction.
In smooth muscle tissue, calcium enters the cell through three mechanisms that increase intracellular concentration. The first mechanism involves voltage-gated Ca channels that are activated by membrane depolarization, allowing Ca to enter the cell. The second mechanism involves hormones or neurotransmitters opening ligand-gated channels on the cell membrane. The third mechanism involves hormones and neurotransmitters such as norepinephrine and angiotensin II, which can cause an increase in intracellular inositol triphosphate (IP3) via the phospholipase-C (PLC) pathway. IP3 can then bind to receptors on the SR and cause Ca to be released.
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Muscle fibre types
Skeletal muscle is one of the three types of vertebrate muscle tissue, the others being cardiac and smooth muscle. Skeletal muscle is attached to bones and gives the body structure and strength. Cardiac muscle makes up the walls of the heart, while smooth muscle is found in blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder.
Skeletal muscle can be classified into three types of muscle fibres: slow oxidative, fast oxidative, and fast glycolytic. Slow oxidative fibres (also called slow twitch or Type I) contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue.
Fast oxidative fibres (also called fast twitch or Type IIa) have relatively fast contractions and primarily use aerobic respiration to generate ATP. They use aerobic metabolism to produce ATP and generate higher-tension contractions than slow oxidative fibres.
Fast glycolytic fibres (also called fast twitch or Type IIx) have fast contractions and primarily use anaerobic glycolysis to generate ATP. They have a large diameter and large volumes of glycogen, which is used to generate ATP quickly. These fibres fatigue quickly and are only used for short periods, but they can produce rapid, forceful contractions associated with quick, powerful movements.
The speed of contraction depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. The amount of tension produced depends on the cross-sectional area of the muscle fibre and the frequency of neural stimulation. Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere.
Skeletal muscle contraction begins at the neuromuscular junction, the synapse between a motoneuron and a muscle fibre. The propagation of action potentials to the motoneuron results in the opening of voltage-gated calcium channels, allowing the flow of calcium ions into the cell. This initiates a series of events that lead to muscle fibre contraction.
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Multiunit smooth muscle cells
Smooth muscles do not contain sarcomeres, unlike striated muscles. They use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body. These muscles are under involuntary control by reflexes and the body's autonomic nervous system (ANS).
There are two types of smooth muscle cells: single-unit and multiunit. Single-unit smooth muscle cells are found in the gut and blood vessels. They are linked together via gap junctions, allowing for contraction as a functional syncytium. Multiunit smooth muscle cells are found in the muscles of the eye and at the base of the hair follicles. They contract by being separately stimulated by nerves of the ANS and allow for fine control and gradual response.
Stimuli for contraction can come from autonomic nerves or hormones, but each cell reacts separately. For example, in the intestines, single-unit smooth muscle allows for the coordinated movement of food along the digestive tract, while in the eye, multiunit smooth muscle can adjust the size of the pupil independently without affecting the adjacent muscle cells.
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Muscle relaxation
PMR is a two-step process. Firstly, individuals tense specific muscle groups and focus on what tension feels like in that body part. Then, they release the tension and observe the feeling of relaxation as it spreads through the muscle group. This process is repeated for various muscle groups, often with diminishing degrees of tension, to deepen awareness and achieve a state of deep relaxation. The recommended practice time for PMR is 10 to 20 minutes in a quiet, comfortable area, either sitting or lying down.
PMR has been found to be effective in reducing stress and anxiety, and symptoms of depression. It can also help with insomnia and provide relief from certain types of chronic pain, including headaches, cancer pain, and neck pain. Additionally, it has been shown to be beneficial for conditions such as high blood pressure, digestive issues, and bipolar disorder.
Another important aspect of muscle relaxation is understanding the physiological process of muscle contraction. Muscle contraction and relaxation are based on two variables: length and tension. When a muscle contracts, it shortens, and upon termination of the contraction, muscle relaxation occurs, returning the muscle fibres to a low-tension state. Calcium plays a crucial role in muscle contraction, with intracellular calcium concentration increases leading to contraction.
Overall, muscle relaxation techniques like PMR offer a simple and effective way to manage stress, improve sleep, and promote overall well-being by helping individuals recognise and release muscle tension.
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Frequently asked questions
Muscle contraction is the activation of tension-generating sites within muscle cells. It is important to note that muscle shortening and contraction are not the same thing. Tension within a muscle can be produced without a change in its length, such as when holding something heavy in the same position.
Mammals have three types of muscles: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and are under voluntary control. Cardiac muscles make up the heart's walls and are under involuntary control. Smooth muscles are found in blood vessels, the gastrointestinal tract, bronchioles, uterus, and bladder, and they are also under involuntary control.
Skeletal muscle contraction begins at the neuromuscular junction, which is the synapse between a motoneuron and a muscle fiber. The propagation of action potentials to the motoneuron results in the opening of voltage-gated calcium channels, allowing calcium to enter the cell. Acetylcholine (ACh) is then released, diffusing to the postsynaptic membrane. ACh binds to receptors, initiating action potentials in the muscle fiber, which lead to contraction.
Smooth muscle contraction is influenced by multiple factors, including spontaneous electrical activity, neural and hormonal inputs, and local chemical changes. Calcium plays a key role in smooth muscle contraction, entering the cell through voltage-gated channels, ligand-gated channels, or via the phospholipase-C (PLC) pathway.
Muscle tension is the force exerted by a muscle on a bone or another object. The amount of tension produced depends on the cross-sectional area of the muscle fiber and the frequency of neural stimulation. Greater tension is achieved when more motor neurons are stimulated, leading to more myofiber contractions.
