Logan Steele
Chapter 5, titled 'The Muscular System', provides an overview of the structure and functions of the muscular system. It compares the three major types of muscle tissue—skeletal, smooth, and cardiac—and explains how muscles contract according to the sliding filament theory. The chapter also delves into the relationship between physical exercise and health, and discusses various musculoskeletal and neuromuscular disorders. With almost 650 skeletal muscles in the human body, this chapter is a comprehensive guide to understanding the role of muscles in our bodies, from enabling voluntary movements like running to involuntary actions like breathing.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of movements | Voluntary, Involuntary |
| Types of muscle tissue | Skeletal, Cardiac, Smooth |
| Skeletal muscle function | Contract in response to a stimulus, Produce movement, Sustain body posture and position, Maintain body temperature, Store nutrients, Stabilize joints |
| Cardiac muscle function | Makes up the walls of the heart, Responsible for rhythmic contractions |
| Smooth muscle function | Control movement in internal organs and structures |
| Muscle contraction | Increase in tension or decrease in length of a muscle |
| Isometric muscle contraction | Muscle tension changes but muscle length remains the same |
| Isotonic muscle contraction | Muscle length changes but muscle tension remains the same |
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What You'll Learn
Skeletal muscles
The biceps brachii, for example, acts as the prime mover when lifting a cup, while the brachialis serves as a synergist, assisting in the movement. Antagonists, such as the hamstrings, play a crucial role in muscle function by maintaining body or limb positions and controlling rapid movements. Skeletal muscles are also involved in producing facial expressions, forming smiles or frowns, and enabling speech.
Some disorders and conditions can affect skeletal muscles, such as Duchenne Muscular Dystrophy (DMD), which is characterised by progressive muscle wasting and weakness due to the absence of the dystrophin protein. DMD leads to skeletal and cardiac muscle degeneration and typically affects approximately 1 in 3,600 male births worldwide.
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Smooth muscles
Smooth muscle is one of the three types of muscle in the human body, the other two being skeletal and cardiac muscle. Unlike skeletal muscle, smooth muscle is involuntary and non-striated. It is capable of maintaining tone for extended periods and often contracts involuntarily. Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern.
Smooth muscle is found throughout the body and serves a variety of functions. It is present in the stomach and intestines, where it helps with digestion and nutrient collection. It is also found in the urinary system, where it helps to rid the body of toxins and maintain electrolyte balance. Smooth muscle is also present in the female and male reproductive systems, the respiratory system, and the digestive system. It plays a crucial role in the regulation of blood pressure and tissue oxygenation.
Smooth muscle cells are spindle-shaped and have a single nucleus. They range in size from 30 to 200 μm. Smooth muscle cells contain actin and myosin, which are the main proteins involved in muscle contraction. These proteins interact to generate tension and cause the muscle to contract. The actin filaments attach to dense bodies that are spread throughout the cell. The dense bodies are analogous to the Z-discs of skeletal muscle and provide an internal framework for the contractile proteins to work against.
Smooth muscle action potentials are unique as they are initiated or modulated by membrane potential. They are slower than skeletal action potentials and can last up to fifty times longer. This is due to the slower opening of calcium channels in smooth muscle cells, which leads to slow repolarization. Smooth muscle contraction relies on the presence of calcium ions, which enter the cell through calcium channels in the sarcolemma. The influx of calcium ions triggers the activation of an enzyme called myosin (light chain) kinase, which then attaches to actin-binding sites and pulls on the thin filaments, resulting in muscle contraction.
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Cardiac muscles
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle, also called myocardium, is an involuntary muscle that makes up the thick middle layer of the heart. It is surrounded by a thin outer layer called the epicardium (or visceral pericardium) and an inner endocardium. The endocardium is not cardiac muscle and forms the inner lining of the heart chambers and valves.
Cardiac muscle cells appear striated or striped under a microscope. These stripes occur due to alternating filaments that comprise myosin and actin proteins. The dark stripes indicate thick filaments that comprise myosin proteins, while the thin, lighter filaments contain actin. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, causing the cell to shrink. The cell uses ATP to power this contraction.
Cardiac muscle cells contain mitochondria, which many refer to as the powerhouse of the cells. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma of cardiac muscle cells contains voltage-gated calcium channels, specialized ion channels that skeletal muscle does not possess. Cardiac muscle cells contain branched fibres connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
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Voluntary vs involuntary movements
The human body has more than 600 muscles that help us do everything from breathing to moving our bodies. The muscular system includes skeletal muscles, cardiac muscles, and smooth muscles.
Skeletal muscles are attached to bones and enable voluntary body movements. They work with bones, tendons, and ligaments to support our weight and move us. We consciously control these movements through our nervous system. For example, flicking your thumb to scroll through an article on your phone and sprinting around a track are both voluntary movements.
Smooth and cardiac muscles, on the other hand, are under involuntary control. Smooth muscles control movement in other internal organs and structures. They are found in the walls of blood vessels and structures such as the urinary bladder, intestines, and stomach. Cardiac muscles make up the middle layers of the heart and are responsible for its rhythmic contractions. The involuntary movements of these muscles keep our bodies working properly. For instance, our heart beating and the muscles in our chest and back moving our ribs when we breathe are involuntary movements.
While skeletal muscles are typically associated with voluntary movements, they can also produce involuntary movements. For example, in the rare condition of cervical dystonia, the muscles in the neck contract involuntarily, causing the person's head to twist to one side.
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Muscle injuries and pain
The healing process for injured skeletal muscles goes through overlapping phases of degeneration, inflammation, regeneration, and fibrosis. During the inflammatory phase, significant inflammation and swelling occur, followed by the regeneration of muscle fibers. However, scar tissue formation also takes place, which can make the muscle more susceptible to future injuries.
To manage muscle injuries, the POLICE principle (an update to the RICE principle) is recommended within the first 24 to 48 hours to minimise swelling and relieve pain. This includes Protection, Optimal Loading, Ice, Compression, and Elevation. While the RICE principle has shown effectiveness in individual components, the overall concept is yet to be proven in randomised clinical trials. More recently, the PEACE and LOVE principles have been proposed to optimise soft tissue recovery and provide a comprehensive approach to injury management.
For acute injuries, offloading the muscle can be beneficial during the initial stages. Crutches may be useful for hamstring strains, for example. Anti-inflammatory medication can help control pain, but there is conflicting evidence regarding the use of NSAIDs (nonsteroidal anti-inflammatory drugs) in improving healing or reducing recovery time. In some cases, they may even impede the healing process.
In the case of large hematomas, aspiration, or the removal of blood through a needle, can help manage pain and potentially decrease healing time. While the use of corticosteroid injections in acute muscle injuries is controversial, they can provide temporary pain relief. Additionally, platelet-rich plasma (PRP) has gained popularity, but its effectiveness in significantly reducing healing time is still uncertain.
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Frequently asked questions
There are over 600 muscles in the human body, accounting for about half of a person's weight.
The three major types of muscle tissue are skeletal, cardiac, and smooth muscle tissue.
The muscular system enables movement in the body, helps maintain posture, and supports the functioning of various bodily systems such as the respiratory and digestive systems.
Holding a book in the same position is an example of an isometric muscle contraction, where muscle tension changes but muscle length remains the same.
