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The human body has over 600 muscles, making up about half of a person's body weight. These muscles are responsible for everything from pumping blood around the body to helping you lift a heavy backpack. There are three types of muscle tissue: visceral, cardiac, and skeletal. Visceral muscle, also known as smooth muscle, is found inside organs like the stomach and intestines. Cardiac muscle is responsible for pumping blood throughout the body and is found only in the heart. Skeletal muscle, also known as striated muscle, is the most common type of muscle in the body and is responsible for a wide range of movements and functions.
| Characteristics | Values |
|---|---|
| Total number of muscles in the human body | 600+ |
| Muscle tissue types | Visceral, cardiac, and skeletal |
| Most common muscle type | Skeletal |
| Muscle that is hard-working and beats thousands of times a day | Heart |
| Percentage of total body mass that skeletal muscles make up | 30-40% |
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What You'll Learn
Skeletal muscle
The individual contractile cells within skeletal muscles are called muscle fibres or myocytes. These fibres are typically 2-3 cm long and 100 μm in diameter, and they are surrounded by a connective tissue layer called the endomysium. The skeletal muscle cells are much longer than those found in cardiac or smooth muscle tissue. They receive signals from nerves in the somatic nervous system, allowing for conscious control of movement.
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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. The heart may use anaerobic processes in a limited capacity. The cardiac action potential lasts approximately 200 ms and is divided into five phases: resting, upstroke, early repolarization, plateau, and final repolarization.
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Smooth muscle
Unlike skeletal muscles, which respond to conscious control, smooth muscles contract involuntarily. This means that their movement happens without conscious thought. Smooth muscles are also involved in passage width control, with some muscles managing the width of passages inside the body. For example, smooth muscles in sphincters like the bladder stay flexed and only relax when needed.
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Serratus anterior muscle
The serratus anterior muscle is a fan-shaped muscle that covers the top of the ribcage, just below the armpits. It originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax and inserts along the superior angle, medial border, and inferior angle of the scapula. Its main part lies deep under the scapula and the pectoral muscles.
The serratus anterior muscle is divided into three parts: the upper, middle, and lower sections. The upper section, or superior part, involves the first to second ribs attaching to the superior angle of the scapula. The middle section, or intermediate part, involves the second to third ribs attaching to the medial border of the scapula. The lower section, or inferior part, involves the fourth to ninth rib attaching to the medial border and inferior angle of the scapula. The inferior part is the most powerful and prominent part of the three.
The serratus anterior is also known as the "big swing muscle" or the "boxer's muscle" because it is responsible for the protraction of the scapula, or the pulling of the scapula forward and around the rib cage. This movement occurs, for example, when someone throws a punch. The serratus anterior also acts with the upper and lower fibres of the trapezius muscle to sustain the upward rotation of the scapula, which allows for overhead lifting.
The serratus anterior muscle is innervated by the long thoracic nerve, a branch of the brachial plexus. The long thoracic nerve travels inferiorly on the surface of the serratus anterior muscle. Damage to this nerve is the most common cause of winged scapula.
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Intermediate back muscles
The back muscles are organised into three groups: superficial, intermediate, and intrinsic. The superficial group is associated with the movement of the shoulder, the intermediate group is associated with the movement of the thoracic cage, and the deep or intrinsic group is associated with the movement of the vertebral column.
The intermediate back muscles are located just above and below the ribcage and help with breathing by moving the ribs. They are made up of two muscles: the serratus posterior superior and the serratus posterior inferior. These muscles run from the vertebral column to the rib cage and assist with elevating and depressing the ribs. The serratus posterior superior is thin and rectangular, originating from the lower part of the ligamentum nuchae, and the cervical and thoracic spines (usually C7-T3). Its fibres pass in an inferolateral direction, attaching to ribs 2-5. The serratus posterior inferior, on the other hand, is broad and strong. It lies underneath the latissimus dorsi and originates from the thoracic and lumbar spines (usually T11-L3). Its fibres pass in a superolateral direction, attaching to ribs 9-12.
The intermediate back muscles are covered superficially by the superficial back muscles and are classified as extrinsic muscles, as they do not develop in the back.
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Frequently asked questions
The pectoralis minor is situated below the pectoralis major, which is the biggest and strongest chest muscle. The pectoralis minor is integral to shoulder movement and is prone to painful tightness.
The eye muscles are referred to as "external" or "extrinsic" muscles. There are six muscles that control eye movement, four of which are rectus muscles. These are the superior rectus, inferior rectus, medial rectus, and lateral rectus.
The suboccipital muscles are just below the occipital bone at the base of the skull. They help extend your head in different directions. There are about 30 muscles in the neck that allow for a variety of movements and support the head and upper back.
The intercostal muscles are within the rib cage and consist of three layers: external, internal, and innermost. They combine to fill the space between the ribs and help expand the chest wall during inhalation.
The serratus anterior is a fan-shaped muscle that extends from the lower shoulder to the ribs. It is known as the "boxer muscle" because it enables a long and strong reach.
