Dominic Stone
The human body is made up of more than 600 muscles, which are used to do everything from moving the body to keeping us alive. There are three types of muscle tissue in vertebrates: skeletal, smooth, and cardiac. Skeletal muscles are the most common type of muscle in the body and are attached to bones, allowing for a wide range of movements and functions. They are voluntary muscles, meaning that you control how and when they move. Cardiac and smooth muscles are involuntary and are controlled by the autonomic nervous system. Cardiac muscle is found only in the heart and is responsible for pumping blood throughout the body. Smooth muscle is found within the walls of organs and structures such as the esophagus, stomach, and intestines.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of small fibres woven together |
| Types of muscle tissue | Skeletal, cardiac, and smooth |
| Skeletal muscle composition | Actin, myosin, and support proteins |
| Skeletal muscle function | Contraction in response to a stimulus |
| Skeletal muscle mass in men | 42% of total body mass |
| Skeletal muscle mass in women | 36% of total body mass |
| Smooth muscle location | Walls of organs and structures like the oesophagus, stomach, intestines, etc. |
| Cardiac muscle location | Middle layers of the heart |
| Cardiac muscle function | Responsible for rhythmic contractions of the heart |
| Muscle energy sources | Aerobic respiration (low to moderate force) and lactic acid fermentation (high force) |
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Skeletal muscle
The function of skeletal muscle is to contract in response to a stimulus, allowing for movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. Skeletal muscle disorders typically manifest as muscle weakness and can include muscular dystrophies, congenital myopathies, inflammatory disorders, and diseases affecting the neuromuscular junction.
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Cardiac muscle
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. Aerobic production is the core utilization process; however, the heart may use anaerobic processes in a limited capacity. The cardiac action potential lasts approximately 200 ms and is divided into 5 phases: resting, upstroke, early repolarization, plateau, and final repolarization.
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Smooth muscle
In the digestive system, smooth muscle helps with digestion and nutrient collection. In the urinary system, it aids in toxin removal and electrolyte balance. Smooth muscle in the arteries and veins regulates blood pressure and tissue oxygenation. Additionally, smooth muscle in the skin causes hairs to stand on end, resulting in goosebumps.
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Muscle contraction
On the other hand, an isotonic contraction refers to a muscle contraction where muscle tension remains the same, but the length of the muscle changes. During an isotonic contraction, the muscle can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction. A concentric contraction occurs when muscle tension is sufficient to overcome the load, resulting in the muscle shortening as it contracts. An example of this is picking up a heavy box, where your arm muscles contract to hold the weight, and your leg muscles tighten as you stand up with the additional weight.
Eccentric contractions occur when the muscle lengthens during normal activities such as walking or lowering a heavy object. During walking, your quadriceps muscles actively lengthen as your heel touches the ground and your knee bends or straightens in stride.
The complex process of muscle contraction, known as excitation-contraction coupling, begins with an action potential causing depolarization in the myocyte membrane. This depolarization spreads via the transverse (T) tubules, which are invaginations of the muscle cell membrane that help transmit signals to the entire muscle fiber. The T tubules then undergo a conformational change, causing nearby ryanodine receptors on the sarcoplasmic reticulum (SR) to open. The SR is the storage site for calcium within muscle cells, and when calcium is released, it binds to troponin C, leading to a shift in tropomyosin. This shift allows the myosin heads to attach to the actin filaments, forming cross-bridges.
The strength of a muscle contraction is influenced by two factors: the number of motor units involved and the amount of stimulus from the nervous system. A single nerve impulse from a motor neuron causes a brief contraction known as a twitch contraction. However, if the motor neuron fires multiple signals in quick succession, the muscle contraction becomes stronger and more sustained, known as temporal summation. If the motor neuron fires rapidly and repeatedly, the muscle may enter a state of tetanus, characterized by prolonged and intense contraction.
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Muscle disorders
There are many types of muscle disorders, including those that affect the musculoskeletal system and those that affect the neuromuscular system. Skeletal muscle disorders typically manifest as muscle weakness and include muscular dystrophies, congenital myopathies, inflammatory disorders, and diseases affecting the neuromuscular junction. Muscular dystrophy causes weakness and the wasting away of muscle tissue. Congenital myopathies are present at birth and can be caused by genetic mutations or disruptions in muscle development in the womb. Skeletal muscle atrophy can result from disuse, denervation, systemic illness, chronic glucocorticoid use, and malnourishment. It is caused by increased protein breakdown and decreased protein synthesis, leading to a reduction in muscle mass and individual muscle fibre diameter.
Neuromuscular disorders affect the nerves that control voluntary muscles and the nerves that communicate sensory information to the brain. When these nerves become unhealthy or die, communication between the nervous system and muscles breaks down, leading to muscle weakness and atrophy. Some neuromuscular disorders have symptoms that begin in infancy, while others may appear in childhood or adulthood. There is currently no cure for neuromuscular disorders, but treatments such as medications, physical therapy, occupational therapy, and surgery can help manage symptoms and delay disease progression.
Other muscle disorders include myoclonus, which affects the normal contraction of muscles, and mitochondrial disorders. In some cases, the cause of muscle disorders may be unknown.
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Frequently asked questions
There are three types of muscles in the human body: skeletal, smooth, and cardiac.
Skeletal muscles are the most common type of muscle in the human body, comprising 30% to 40% of the total body mass. They are attached to bones and allow us to perform various movements and functions. These muscles are under voluntary control.
Smooth and cardiac muscles are involuntary muscles. Smooth muscles are found within the walls of organs and structures like the esophagus, stomach, intestines, and blood vessels. Cardiac muscles make up the heart and are responsible for its rhythmic contractions.
