Logan Steele
The human body is made up of over 600 muscles, which help us do everything from moving our bodies to breathing and staying alive. Skeletal muscles, which are voluntary, make up 30-40% of our total body mass and are responsible for most of the movements our bodies make. These muscles are made up of thousands of small fibres that contract and relax to move our bones and, in turn, our bodies. This process is called muscle contraction and it is triggered by a message from the nervous system, which causes chemical reactions in the muscles. During muscle contraction, muscles release calcium ions, which initiate muscle contractions and create force and movement.
| Characteristics | Values |
|---|---|
| Chemicals released during muscle contraction | Acetylcholine, Calcium ions, Sodium ions, Potassium ions |
| Neurotransmitters | Acetylcholine, Epinephrine, Norepinephrine |
| Neurotransmitter receptors | Muscarinic acetylcholine receptors (mAChRs), Adrenergic receptors, Ryanodine receptors, Dihydropyridine receptors |
| Neurotransmitter functions | Excitation, Inhibition, Relaxation |
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What You'll Learn
Calcium ions
Calcium release through ryanodine receptors in the SR is triggered differently in various muscles. In cardiac and smooth muscle, an electrical impulse (action potential) triggers calcium ions to enter the cell through an L-type calcium channel. These calcium ions then bind to and activate the ryanodine receptor, resulting in a larger increase in intracellular calcium. On the other hand, skeletal muscle has a direct connection between the L-type calcium channel and the ryanodine receptor. Therefore, activation of the L-type calcium channel directly activates the ryanodine receptor, causing the release of calcium ions.
The release of calcium ions from the SR is known as a calcium spark. This calcium-induced calcium release (CICR) mechanism is essential for muscle contraction. Calcium ions activate the ryanodine receptor (RyR1 in muscle cells), which releases more calcium stored inside the SR into the cytoplasm of the cell. The calcium diffusing in the cytoplasm between the myosin and actin filaments of the muscle fibrils causes the filaments to slide into each other, triggering the contraction of the entire muscle fibre.
The calcium signalling apparatus, or calcium cycle, includes the ryanodine receptor, the troponin protein complex, the calcium pump, and calsequestrin. The troponin protein complex mediates the calcium effect on the myofibrillar structures, leading to contraction. Calcium binds to troponin, changing its shape and removing tropomyosin from the actin binding sites. This allows the formation of cross-bridges between actin and myosin, triggering contraction.
Calcium ion levels are tightly regulated, as too much calcium within cells can lead to hardening (calcification) and cell death. Calcium pumps, such as Sarco(endo)plasmic reticulum Ca2+ ATPases (SERCA), are responsible for pumping calcium ions back into the SR, allowing the muscle cell to relax.
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Acetylcholine
In the peripheral nervous system, acetylcholine is released into the neuromuscular junction, where nerves meet muscle cells. Acetylcholine is involved in the activation of muscles in the body. It binds to nicotinic ion-channel receptors on the muscle cell membrane, causing the ion channels to open. Sodium ions then flow into the muscle cell, initiating a sequence of steps that finally produce muscle contraction. Acetylcholine is the chief neurotransmitter of the parasympathetic nervous system, which contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate.
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Energy
Adenosine triphosphate (ATP) is the source of energy for all muscle contractions. Energy is released when ATP is broken down into adenosine diphosphate (ADP) and inorganic phosphate (Pi).
ATP is not stored in large amounts in skeletal muscle, so maintaining its availability for muscle contraction is a limiting factor. The body uses both anaerobic (without oxygen) and aerobic (with oxygen) means to resynthesise ATP from other sources, including creatine phosphate (CP) and muscle glycogen.
During intense exercise, the rate of ATP demand can increase by up to 1,000-fold compared to that at rest. The body responds with a coordinated metabolic response, with all energy systems contributing to different degrees depending on the intensity and duration of the exercise.
The two main anaerobic sources of ATP are from phosphocreatine (PCr) and anaerobic glycolysis. PCr is used for rapid, high-intensity contractions but is depleted in less than 30 seconds and takes several minutes to replenish. Anaerobic glycolysis produces a build-up of lactic acid in the cells, causing the 'burn' sensation felt during intense exercise.
The aerobic system, on the other hand, predominates during low-intensity, longer-duration activities such as walking and jogging, when oxygen is available. It is also the energy system most active at rest, producing more ATP molecules compared to anaerobic systems. During these activities, glucose can be broken down completely through glycolysis, the Krebs cycle, and the Electron Transport Chain, avoiding the build-up of lactic acid.
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Movement
The human body has over 600 muscles that work together to enable movement. Muscles perform two types of movements: voluntary and involuntary. Voluntary movements are actions that you consciously control, such as walking or lifting an object. Involuntary movements occur without conscious thought, like digesting food or pumping blood through the heart.
Skeletal muscles, which are part of the musculoskeletal system, are responsible for most of the body's movements. They contract and relax to mechanically move the body. When a message from the nervous system reaches the muscle, it triggers a chemical reaction that leads to the reorganisation of muscle fibres, resulting in muscle contraction and subsequent movement. This process is known as the mechanism of muscle contraction.
The direction and type of movement depend on the arrangement and function of the muscle fibres. For example, muscles with fewer muscle fibres, such as those in the eyes or fingers, are capable of fine movements. On the other hand, muscles that require significant strength, like leg or arm muscles, possess a higher number of muscle fibres.
Additionally, muscles can be classified by the type of movement they facilitate. For instance, abductors, flexors, and extensors are grouped based on the specific movements they enable. Furthermore, skeletal muscles rarely work alone to achieve movement. Fixator muscles assist in movement by stabilising the body, allowing for balanced and controlled actions like lifting heavy objects.
In terms of the mechanics of movement, there are two types of muscle contractions: isometric and isotonic. Isometric contractions increase muscle tension without causing movement, such as when holding an object still or maintaining posture. Isotonic contractions, on the other hand, produce movement and are necessary for developing muscle mass through weightlifting. During isotonic contractions, muscles can either shorten, resulting in a concentric contraction, or lengthen, leading to an eccentric contraction.
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Relaxation
The human body is composed of more than 600 muscles that help us move, breathe, and perform a wide range of functions. Skeletal muscles, which make up 30% to 40% of our total body mass, are voluntary muscles that we can control consciously. They are responsible for various movements, from holding our bodies still to running marathons.
For our muscles to relax, they must first contract. This contraction is initiated by a signal from the nervous system, specifically a motor neuron, which releases a neurotransmitter called acetylcholine (ACh) into the synapse at the neuromuscular junction. This chemical message binds to receptors on the outside of the muscle fiber, starting a chemical reaction within the muscle. The influx of sodium ions (Na+) triggers an action potential, which leads to the release of calcium ions (Ca2+) from storage in the sarcoplasmic reticulum (SR). The release of calcium ions initiates muscle contractions, causing the muscle fibers to reorganise and shorten.
When the stimulation from the motor neuron stops, the chemical reaction that caused the rearrangement of muscle fibers is halted. This triggers a reversal of the chemical processes, leading to muscle relaxation. The muscle fiber repolarises, closing the channels through which Ca2+ was released. ATP-driven pumps then work to move Ca2+ out of the sarcoplasm and back into the SR. This process is known as "reshielding," where the actin-binding sites on the thin filaments are restored. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses its tension and relaxes.
It is important to note that muscle relaxation can also be influenced by factors such as hormones, stress, and anabolic steroids, which can lead to hypertrophy, or increased muscle mass. Additionally, relaxation can be achieved through various pathways, including the binding of Ca2+ with cytoplasmic proteins, allowing for quick relaxation in fast-twitch muscles.
In summary, muscle relaxation occurs through a complex series of chemical and physical processes that involve the nervous system, motor neurons, and the interaction of various ions and proteins within the muscle fibers.
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Frequently asked questions
Muscles release calcium ions when they contract. This process begins when a motor neuron releases a chemical signal, which causes a muscle fibre to depolarise and release calcium ions from storage.
When muscles relax, they stop releasing the chemical signal, acetylcholine (ACh), and the calcium ions are moved out of the muscle fibre.
Skeletal muscles release calcium ions during contraction, just like other muscle types. They are also responsible for releasing energy for metabolism.
Muscles release calcium ions, which initiate contraction, leading to movement. This movement is achieved through the shortening and relaxing of muscle fibres.
