Logan Steele
The human body has more than 600 muscles, which help us do everything from moving our bodies to breathing and staying alive. The three types of muscles in the body are skeletal, cardiac, and smooth muscle. Skeletal muscles are the most common type, attached to bones and responsible for producing movement. They contract in response to a stimulus, and their contraction is triggered by messages from the nervous system. Cardiac muscle comprises the walls of the heart, allowing blood to be pumped through the vasculature, and its contraction occurs via excitation-contraction coupling. Smooth muscle is found throughout the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder, and is controlled by the autonomic nervous system.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Made of thousands of small fibers woven together |
| Muscle function | Help the body move, breathe, and stay alive |
| Types of movements | Voluntary and involuntary |
| Muscle contraction | Skeletal muscles contract and relax to move the body |
| Muscle relaxation | Reversal of chemical processes in muscle fibers |
| Muscle disorders | Muscular dystrophies, congenital myopathies, inflammatory disorders, etc. |
| Muscle atrophy | Caused by disuse, denervation, systemic illness, chronic glucocorticoid use, malnourishment, etc. |
| Muscle mass | Varies with age, gender, height, and weight |
| Types of muscles | Skeletal, cardiac, and smooth |
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Muscle contraction
The mechanism of muscle contraction can be summarised in three steps. Firstly, a message travels from the nervous system to the muscular system, triggering chemical reactions. Secondly, the chemical reactions lead to the muscle fibres reorganising themselves in a way that shortens the muscle—that is the contraction. Finally, when the nervous system signal is no longer present, the chemical process reverses, and the muscle fibres rearrange again, and the muscle relaxes.
The process of muscle contraction is complex and can be described in terms of two variables: length and tension. In natural movements, muscle contractions are multifaceted as they can produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is likely to remain the same in muscles that contract during locomotion.
There are several types of muscle contractions, including concentric, eccentric, isometric, and isotonic contractions. A concentric contraction occurs when muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts. An eccentric contraction happens when the muscle is actively lengthened during normal activity, such as when walking or lowering something heavy. An isometric contraction occurs when the muscle is actively held at a set length, such as when carrying something in your arms in front of you. An isotonic contraction is when the tension in the muscle remains constant despite a change in muscle length, such as when the muscle's force of contraction matches the total load on the muscle.
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Muscle relaxation
The human body has over 600 muscles, which help us do everything from moving our bodies to keeping us alive. Skeletal muscles contract and relax to mechanically move the body. This process is called the mechanism of muscle contraction.
The process of muscle relaxation can be described in a few steps. Firstly, the motor neuron stops releasing its chemical signal, acetylcholine (ACh), into the synapse at the neuromuscular junction (NMJ). This causes the muscle fibre to repolarize, which closes the gates in the sarcoplasmic reticulum (SR) where calcium (Ca++) ions were being released.
ATP-driven pumps then move the Ca++ ions out of the sarcoplasm back into the SR. This results in the "reshielding" of the actin-binding sites on the thin filaments. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fibre loses its tension and relaxes.
The number of skeletal muscle fibres in a given muscle is genetically determined and does not change. Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fibre. Factors such as hormones and stress can increase the production of sarcomeres and myofibrils within the muscle fibres, resulting in increased muscle mass and bulk.
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Muscle disorders
Muscular dystrophies are inherited disorders that cause weakness and the wasting away of muscle tissue. Neuromuscular disorders, such as Lambert-Eaton Myasthenic Syndrome (LEMS), also result in muscle weakness and can be inherited or caused by a spontaneous gene mutation or immune system disorder. LEMS is often associated with small cell lung cancer and can lead to respiratory failure in the late stages.
Congenital myopathies are another type of muscle disorder that can be caused by genetic mutations or disruptions in muscle structure and function during fetal development. These disorders can affect the ability of muscles to contract and relax, leading to movement and posture abnormalities.
Inflammatory muscle disorders, such as myositis, are characterized by muscle inflammation and can cause muscle pain, weakness, and fatigue. These disorders can be triggered by infections, medications, or autoimmune conditions.
In addition to these disorders, rhabdomyolysis is a condition that involves the breakdown of skeletal muscle, releasing products that can damage the kidneys and lead to acute renal failure. Atrophy of skeletal muscle can also occur due to disuse, denervation, systemic illness, chronic glucocorticoid use, or malnutrition, resulting in reduced muscle mass and strength.
It is important to consult a healthcare provider for proper diagnosis and treatment of muscle disorders, as they can vary in severity and impact on an individual's quality of life.
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Muscle types
The human body has more than 600 muscles that help us do everything from moving our bodies to breathing and keeping us alive. There are three types of muscles in the human body: skeletal, smooth, and cardiac.
Skeletal muscles are attached to the skeleton and are under voluntary control. They contract in response to a stimulus and are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. Skeletal muscles also contain fast-twitch and slow-twitch muscle fibres. Fast-twitch muscle fibres contract quickly and use short bursts of energy, while slow-twitch muscle fibres move slowly and help with posture. Skeletal muscles make up approximately 40% of the human body weight and contain 50 to 75% of all body proteins.
Smooth muscles, on the other hand, are involuntary muscles that line the inside of some organs. They are found in the walls of hollow visceral organs such as the liver, pancreas, and intestines. Smooth muscle cells are spindle-shaped and contract slowly and rhythmically.
Lastly, cardiac muscle cells are located in the walls of the heart and are under involuntary control. The heart beats on its own without any input from our nervous system. Cardiac muscle cells are rectangular in shape and are striated, appearing striped. The contraction of cardiac muscle is involuntary, strong, and rhythmical, allowing blood to be pumped through the cardiovascular system.
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Muscle fibres
The human body contains more than 600 muscles, which are made up of thousands of small fibres woven together. These fibres stretch and press together to enable movement. The muscular system is composed of specialised cells called muscle fibres, which have the predominant function of contractibility.
There are three types of muscles in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are the most common type of muscle in the body, and they are attached to bones to give the body structure and strength. They are also attached to internal organs and blood vessels. Skeletal muscles are voluntary muscles, meaning an individual can control how and when they move and work. They make up between 30% and 40% of total body mass. Cardiac and smooth muscle are involuntary muscles, controlled by the autonomic nervous system.
Skeletal muscles produce movement, sustain body posture and position, maintain body temperature, store nutrients, and stabilise joints. Skeletal muscle contraction is under voluntary control, receiving neural inputs that allow for conscious control of muscles. Skeletal muscle disorders typically manifest as muscle weakness.
The mechanism of muscle contraction can be summarised in three steps. Firstly, a message travels from the nervous system to the muscular system, triggering chemical reactions. Secondly, the chemical reactions lead to the muscle fibres reorganising themselves in a way that shortens the muscle—this is the contraction. Finally, when the nervous system signal is no longer present, the chemical process reverses, and the muscle fibres rearrange again, causing the muscle to relax.
The complex process leading to muscle contraction, called excitation-contraction coupling, begins when an action potential causes depolarisation in the myocyte membrane. The depolarisation is spread via the transverse (T) tubules, which help spread depolarisation signals to the entire muscle fibre. When acetylcholine binds to receptors on the muscle fibre membrane, membrane channels open and the process that contracts relaxed muscle fibres begins.
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Frequently asked questions
Muscles are pieces of soft tissue throughout your body that help you do everything from moving your body to breathing and staying alive. There are more than 600 muscles in the human body.
There are three types of muscles in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are the most common type, comprising between 30% and 40% of your total body mass. Cardiac and smooth muscles are involuntary muscles controlled by the autonomic nervous system.
Muscle contractions can be categorised as isotonic, concentric, or eccentric. Isotonic contractions occur when the contraction force matches the total load on a muscle, as in walking or running. Concentric contractions occur when there is sufficient muscle tension to overcome the load, as in a bicep curl. Eccentric contractions occur when the muscle works to decelerate a joint at the end of a movement, as in lowering a heavy weight.
Muscle contractions occur when muscles fibres shorten due to chemical reactions triggered by signals from the nervous system. When a signal is sent from the nervous system, it triggers a series of chemical reactions that lead to the reorganisation of muscle fibres, resulting in contraction. When the signal stops, the chemical reactions reverse, the muscle fibres rearrange, and the muscle relaxes.
