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There are more than 600 muscles in the human body, making up around 40% of body weight in healthy young adults. Muscles are pieces of soft tissue that help us move, breathe, swallow, and stay alive. There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. This text will focus on skeletal muscle and where it begins.
| Characteristics | Values |
|---|---|
| Definition | Muscles are pieces of soft tissue throughout your body that help you move, breathe, swallow and stay alive. |
| Number of muscles in the human body | More than 600 |
| Muscle composition | Skeletal muscle is composed of bundles of muscle fibres called myofibres which contain several myofibrils. |
| Muscle fibres | Skeletal muscle fibres are red and white and are usually between 10 and 100 micrometres in diameter and several centimetres long. |
| Muscle movement | Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. |
| Types of muscle tissue | There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. |
| Skeletal muscle | Skeletal muscle is the most common type of muscle in the body, comprising around 30-40% of total body mass. It is a voluntary muscle, controlled by the somatic nervous system. |
| Cardiac muscle | Cardiac muscle is found only in the walls of the heart and is an involuntary muscle controlled by the autonomic nervous system. |
| Smooth muscle | Smooth muscle is found within the walls of organs and structures such as the oesophagus, stomach, and intestines. It is also involuntary. |
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What You'll Learn
Skeletal muscle
Each skeletal muscle has three layers of connective tissue (called mysia) that enclose it, provide structure, and compartmentalise the muscle fibres within the muscle. The outermost layer is called the epimysium, which separates the muscle from other tissues and organs in the area, allowing the muscle to move independently. The middle layer is called the perimysium, which surrounds bundles of muscle fibres. The innermost layer is called the endomysium, which surrounds individual muscle fibres.
The strength of a skeletal muscle, in terms of force exerted on the skeleton, depends on several factors, including length, shortening speed, cross-sectional area, and neural activation of motor units. The strongest muscles in the body are typically those with the largest cross-sectional areas, such as the quadriceps femoris or the gluteus maximus.
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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 cardiac cycle consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, followed by a period of robust contraction and pumping of blood, called systole.
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Smooth muscle
The shape of smooth muscle is described as fusiform, which is round in the center and tapers at each end. Smooth muscle can tense and relax and has greater elastic properties than striated muscle. This elasticity is crucial in organ systems like the urinary bladder, where maintaining contractile tone is essential. Smooth muscle can be categorised into two functional types: single-unit and multi-unit. In single-unit smooth muscle, the entire bundle or sheet of smooth muscle cells contracts as a syncytium. In contrast, multi-unit smooth muscles contract and relax individually, and their contraction is confined to a particular muscle cell that receives direct stimulation.
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Voluntary vs involuntary muscles
The human body has two main types of muscles: voluntary and involuntary. Voluntary muscles, such as those in your arms and legs, are under your conscious control, allowing you to move and perform actions. They are typically striated, meaning they have a striped appearance under a microscope, and fatigue easily. Involuntary muscles, on the other hand, work autonomously, without conscious thought. They control essential functions like digestion, heartbeat, and breathing.
Voluntary muscles, also known as skeletal muscles, are the muscles that connect to your bones and allow you to perform a wide range of movements and functions. They make up 30% to 40% of your total body mass. Nerves in your somatic nervous system send signals to make them function. For example, if you reach for a book on a shelf, you are using the skeletal muscles in your neck, arm, and shoulder.
Voluntary muscles exhibit rapid contraction and relaxation compared to involuntary muscles. However, they also tire quickly and require regular intervals of rest. These muscles are essential as they are involved in the movement of body parts and locomotion of the body. Some examples of voluntary muscles include the biceps, triceps, quadriceps, diaphragm, pectoral muscles, abdominals, and hamstrings.
Involuntary muscles are the muscles that cannot be controlled by will or conscious thought and are often associated with organs that exhibit slow and regular contractions and relaxation. They are also termed smooth muscles or non-striated muscles as there are no striations when viewed under a microscope. These muscles are mostly found lining the walls of internal organs like the stomach, intestine, urinary bladder, and blood capillaries.
Involuntary muscles are involved in the movements of the internal organs and also aid in the passage of fluids and food in the digestive system. The diaphragm is a primary respiratory muscle that assists the process of breathing by increasing and decreasing the volume of the thoracic wall. It is a dome-shaped skeletal muscle present underneath the lungs and the heart. It separates the abdominal area from the chest. The cardiac muscle is another example of an involuntary muscle. It is composed of individual heart muscle cells called cardiomyocytes, which are joined together by intercalated discs.
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Muscle fibres
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions. Muscle fibres can adapt to changing demands by altering their size or fibre type composition. This plasticity is the basis for many physical therapy interventions designed to increase a patient's force development or endurance. Changes in fibre type composition may also be responsible for some of the impairments and disabilities seen in patients who are deconditioned due to prolonged inactivity, limb immobilisation, or muscle denervation.
Skeletal muscle fibres are 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 are further divided into subtypes: Type 2A, which can also use oxygen to generate energy but contain fewer mitochondria, making them lighter in appearance; and Type 2B, which don't use oxygen to generate energy but store it for short bursts of movement. Type 2B fibres have the fewest mitochondria and appear white. Skeletal muscle fibres are also called myofibers and are composed of myofibrils, which are made up of actin and myosin filaments or myofilaments. These filaments repeat in units called sarcomeres, which are the basic functional, contractile units necessary for muscle contraction.
Cardiac muscle fibres, on the other hand, are only found in the heart and have their own unique rhythm. They are branched and interconnected, and their contraction is controlled by special cells called pacemaker cells, which generate impulses that cause the cardiac muscle to contract in a constant or variable pace, facilitating the beating of the heart.
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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 move when you think about moving a part of your body. They are attached to bones and allow you to perform a wide range of movements. Cardiac muscle, or myocardium, makes up the middle layers of your heart. Smooth muscles are involuntary muscles that line the inside of some organs, such as the esophagus, stomach, and intestines.
Skeletal muscles make up between 30% to 40% of your total body mass. Examples of skeletal muscles include shoulder muscles, hamstring muscles, and abdominal muscles. The triceps brachii, for example, has four points of attachment: three bellies (origins) and one insertion. The bellies are attached to the scapula and humerus, and the insertion is attached to the ulna.
Cardiac muscle is an involuntary muscle that is controlled by the autonomic nervous system. It is found only in the walls of the heart. Smooth muscles are also involuntary and are found within the walls of organs and structures, such as the esophagus, stomach, intestines, and bladder. They play an important role in the female and male reproductive systems, as well as the urinary system.
