When we see parts of our body moving, we understand that a muscle has to contract and relax in order for this movement to occur. However, the process by which this contraction and relaxation of muscles occur is complex and requires millions of tiny fibers to slide in a coordinated manner. Humans and other vertebrates have three distinct kinds of muscles – skeletal muscle, smooth muscle, and cardiac muscle.
Each of these muscle types is composed of different types of muscle cells (also known as myocytes). Skeletal muscle can be contracted voluntarily, whereas contraction of smooth muscle and cardiac muscle is involuntary. In all three types of muscles, the contractile property of the muscle is due to the presence of actin and myosin proteins within the myocytes. The proteins form filaments that can slide to cause muscles to contract.
The source of energy used for muscle contraction is ATP, which is generated through both aerobic and anaerobic metabolic processes. Muscle cells are rich in mitochondria which are the power houses of cells. The energy demands of muscles are high as the process by which muscle contractions occurs involves energy-demanding sliding of millions of tiny fibers simultaneously.
How Muscles Work
In order to understand the process of muscle contraction, it is important to first understand how the contractile proteins (actin and myosin) are placed within the myocytes. When examined under a microscope, the skeletal and cardiac muscles have a striated appearance. This striated appearance is due to the arrangement of actin and myosin protein filaments into contractile units known as sarcomeres.
Each sarcomere appears to have alternating dark and light bands that are formed by the arrangement of actin and myosin strands in stacks that can slide between each other. Myosin strands are located in the centre of each sarcomere and appear as thick filaments. Actin strands appear as thin filaments that are located on either side of the central stack of myosin filaments.
During muscle contraction, the sarcomere shortens due to the sliding of the thin actin filaments between the thick myosin filaments. The cytoplasm of each muscle cell has multiple myofibrils that are composed of repeated sarcomere sections. The structure is highly efficient for sudden muscle contractions, rapid and repetitive contractions as well as forceful contractions when and where necessary.
The entire process by which muscles work is largely the same across all types of muscles. From the tiny muscles that are in the wall of arteries to the large gluteal muscles, the sliding of these tiny filaments ensures that muscles cannot only move parts of the body but also cause narrowing of cavities and push substances along tracts like the gut.
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Mechanism of Skeletal Muscle Contraction
The signal for the contraction of skeletal muscle comes from the nerves that innervate the muscle. To begin the process of skeletal muscle contraction, the endings of the nerves release a neurotransmitter chemical named acetylcholine. Acetylcholine receptors that are present on the surface of the skeletal muscle cells bind to this neurotransmitter and initiate the process of muscle contraction.
This step of converting a neural signal into a muscle contraction response is referred to as excitation-contraction coupling. Once acetylcholine binds to the receptor on the surface of the muscle cell, a cascade of events happen that cause flooding of the cytoplasm with calcium ions. In a resting muscle cell, this calcium is stored within an organelle known as the sarcoplasmic reticulum.
Once the calcium ions are released into the cytoplasm, they bind to the actin filament. Binding of the calcium to actin filament results in a conformational change which allows myosin to attach to actin and pull it. This causes the actin filament to slide in between the myosin filaments. Myosin uses energy from ATP breakdown to power this movement of millions of filaments simultaneously.
Repeated binding of myosin with calcium ions and ATP causes the sarcomere to shorten and the muscle cell to contract. Once the calcium in the cytoplasm is sequestered back into the sarcoplasmic reticulum, myosin releases actin and the muscle relaxes. The intricate movements that parts of the body are able to do is a result of the joints and the various muscles that work together or against each other.
Mechanism of Smooth Muscle Contraction
Just like skeletal muscles, smooth muscles also contract through sliding of actin filaments between layers of myosin filaments. However, unlike in skeletal muscle, cytoplasmic calcium activates myosin through phosphorylation rather than by binding actin filaments.These muscles ares not under voluntary control although it can be indirectly stimulated through various factors.
.Muscles are stimulated by neurotransmitters that are secreted by the end of a nerve. This occurs when an electrical impulse travels across a nerve. However, the contraction of smooth muscles can also be initiated through other factors such as hormones, tissue stretching, local changes in chemical environment, and spontaneous electrical activity in the smooth muscle cells.
Mechanism of Cardiac Muscle Contraction
Cardiac muscles are stimulated to contract by electrical signals that are generated in the sinoatrial node of the heart. When the electrical impulse from the sinoatrial node reaches the membrane of the cardiac myocytes, they depolarize. This leads to an influx of extracellular calcium into the cardiac myocytes. Influx of extracellular calcium into the cardiac myocyte stimulates the release of calcium from sarcoplasmic reticulum.
The process of the calcium spike in cardiac myocytes is referred to as calcium-induced calcium release. Once the calcium concentration in the cytoplasm of cardiac myocyte increases, contraction of sarcomere occurs in a manner similar to that in the skeletal muscle. Another unusual feature is the conduction system in the heart which delays an impulse to ensure that the atria contracts and then the ventricules contract a short while later.
This contraction and relaxation occurs rhythmically throughout life to ensure that the heart keeps pumping blood. It is essential to circulate oxygen and nutrients which sustains life. Cardiac muscles are therefore among the hardest working muscles in the body as its repeated contractions and short duration of relaxation never stops.