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The human body has more than 600 muscles, which help us do everything from breathing and digesting food to running marathons. There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles, meaning we control how and when they move and work. Skeletal muscles make up about 30% to 40% of our total body mass and are responsible for most of the movements of the body. Cardiac and smooth muscles are involuntary muscles, controlled by the autonomic nervous system. Cardiac muscles are found in the walls of the heart, while smooth muscles are found in the walls of organs and structures such as the esophagus, stomach, and intestines.
| Characteristics | Values |
|---|---|
| Number of muscles in the body | More than 600 |
| Muscle composition | Blood vessels, lymphatics, contractile muscle fibres, and connective tissue sheaths |
| Types of muscle tissue | Skeletal, cardiac, and smooth |
| Types of skeletal muscle fibres | Type I (slow-twitch) and Type II (fast-twitch) |
| Types of muscle movement | Voluntary and involuntary |
| Muscle functions | Movement, maintaining body temperature, storing nutrients, and stabilising joints |
| Muscle contractions | Controlled by the number of motor units involved and the stimulus from the nervous system |
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Skeletal muscle
The fibres are surrounded by different types of connective tissue sheaths, including the epimysium, perimysium, and endomysium. The epimysium forms the outermost layer, while the perimysium surrounds bundles of muscle fibres, and the endomysium is the innermost layer surrounding individual fibres. Skeletal muscle fibres are red and white and have a striped appearance due to the arrangement of sarcomeres, leading to them also being called striated muscles.
The skeletal muscle receives its blood supply from a primary artery that runs parallel to the muscle fibre. This artery gives rise to smaller branches, including feed arteries, primary arterioles, and eventually, terminal arterioles. The terminal arterioles perfuse capillaries within the endomysium, forming a microvascular unit, which is the smallest unit in skeletal muscle where blood flow can be regulated. Lymph capillaries also originate in the microvascular unit and drain tissue fluid, forming lymphatic vessels that depend on muscle movement for drainage.
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Cardiac muscle
The human body has three types of muscles: skeletal, cardiac, and smooth muscle. Cardiac muscle, also called myocardium or heart muscle, is an involuntary, striated muscle that constitutes the main tissue of the heart wall. The heart is made up of three layers, with the myocardium forming a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The heart may use anaerobic processes in a limited capacity, but aerobic production is the core utilization process.
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Smooth muscle
The shape of smooth muscle cells is fusiform, with a round centre and tapered ends. These cells contain thick and thin filaments arranged in a non-striated pattern, resulting in a homogeneous appearance. Actin and myosin, the main proteins involved in muscle contraction, are found in large amounts in smooth muscle cytoplasm. The actin filaments attach to dense bodies spread throughout the cell, and the calcium-containing sarcoplasmic reticulum aids in sustaining contraction.
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Muscle fibres
There are three types of muscle tissue in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles, meaning an individual can control how and when they move. These muscles are attached to the bones of the skeleton and consist of flexible muscle fibres that range from less than half an inch to just over 3 inches in diameter. Skeletal muscle fibres are further classified into two types: type 1 and type 2. Type 1 fibres utilise oxygen to generate energy for movement and have a higher density of mitochondria, making them appear dark. Type 2 fibres can be further subdivided into subtypes based on their oxygen usage and mitochondria content.
Cardiac muscles are striated and are only found in the heart. These muscles have their own rhythm and are responsible for the contraction of the heart. Smooth muscles, on the other hand, are involuntary and do not have a striated appearance. They are found in the urinary system and help rid the body of waste and toxins.
It is important to note that the number of muscle fibres cannot be increased through exercise. Instead, muscles grow larger through muscle cell growth and the addition of new protein filaments. The power generated by muscle fibres can be influenced by training, with endurance sports athletes tending to have a higher number of slow-twitch fibres and sprinters having a higher number of fast-twitch fibres.
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Muscle contractions
There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles, meaning you control how and when they move and work. They make up between 30% and 40% of your total body mass and are attached to bones, allowing you to perform a wide range of movements and functions. Skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons. A single motor neuron can innervate multiple muscle fibres, causing them to contract simultaneously. Once innervated, the protein filaments within each skeletal muscle fibre slide past each other to produce a contraction, known as the sliding filament theory.
Cardiac and smooth muscles are involuntary muscles controlled by the autonomic nervous system. Cardiac muscles comprise the walls of the heart, allowing blood to be pumped through the vasculature. Smooth muscles are found throughout the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder. They do not contain sarcomeres but use actin and myosin contraction to constrict blood vessels and move the contents of hollow organs in the body.
Muscles serve several purposes in the body. They offer stability to joints and connective tissues, produce heat to maintain body temperature, and help maintain posture. Muscle contractions can be described as isometric if muscle tension changes but the muscle length remains the same. In contrast, a muscle contraction is isotonic if muscle tension remains the same throughout the contraction. If the muscle length shortens, the contraction is concentric, and if the muscle length lengthens, the contraction is eccentric.
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Frequently asked questions
There are three types of muscles in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles that you can control, while cardiac and smooth muscles are involuntary.
Skeletal muscle is made of thousands of small fibres woven together. These fibres are called actin and myosin, and they contract to allow movement. Skeletal muscle also contains tendons and perimysium.
Muscles are responsible for movement, including voluntary and involuntary actions. They also help to maintain body temperature, store nutrients, and stabilise joints.
The outermost layer of connective tissue surrounding a muscle is called the epimysium.
