Dominic Stone
Muscle physiology is a branch of physiology that studies the mechanism behind muscle contraction. The human body has over 600 muscles, which are made up of thousands of small fibres woven together. These fibres work together to move our organs and body. There are three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and help with body movement and posture. Smooth muscles line the walls of hollow organs like the stomach and intestines, and cardiac muscles are found in the heart. To learn muscle physiology, one can start with understanding the anatomy and functions of muscle fibres, their contraction, and the role of neurons and neurotransmitters in this process.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of muscle tissue | Skeletal, smooth, and cardiac |
| Muscle composition | Thousands of small muscle fibers |
| Muscle movement | Voluntary and involuntary |
| Muscle functions | Movement, posture, stabilization of joints, metabolism, and generation of heat |
| Muscle contraction | Shortening of muscle fibers due to action potentials in motor neurons |
| Muscle relaxation | Reversal of chemical processes in muscle fibers |
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What You'll Learn
Skeletal muscle anatomy
Skeletal muscles are the most common type of muscles in the body, comprising 30% to 40% of total body mass. They are attached to bones, enabling movement of the skeleton, and are also involved in maintaining body temperature, storing nutrients, and stabilising joints. These muscles are under voluntary control, meaning that you can consciously control how and when they move. For example, when you reach for a book on a shelf, you are using the skeletal muscles in your neck, arm, and shoulder.
Skeletal muscles are made up of bundles of muscle fibres called myofibers, which contain several myofibrils. Each myofibril represents a muscle cell with its basic cellular unit, the sarcomere. Myofibrils are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance.
Each muscle fibre is surrounded by a layer of connective tissue called the endomysium. The endomysium, along with the perimysium and epimysium, bundle together thousands of muscle fibres, providing structure and strength to the muscle. These three layers of connective tissue extend beyond the muscle belly, forming a tendon or aponeurosis that attaches the muscle to the bone.
Skeletal muscles vary in size, shape, and arrangement of fibres. They can range from extremely tiny strands, such as the stapedium muscle of the middle ear, to large masses like the thigh muscles. The shape of skeletal muscles can be broad or narrow, and the fibres can be arranged in different directions relative to the long axis of the muscle.
Keeping your skeletal muscles strong and healthy is important for overall health. Talk to your healthcare provider about exercises, movements, or physical activities that can help maintain and build muscle strength.
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Muscle contraction physiology
The human body has more than 600 muscles, which are made up of thousands of small fibres woven together. These fibres are what allow us to move our bodies and our internal organs. There are three types of muscle tissue: skeletal, smooth, and cardiac.
Skeletal muscles are attached to bones and enable the movement of the skeleton, as well as helping to maintain posture and stabilise joints. They are also involved in breathing, swallowing, and maintaining body temperature through the generation of heat. Skeletal muscle tissue is a striated muscle fibre under voluntary control. Each skeletal muscle is made up of muscle fibres, which are in turn composed of smaller units called myofibrils. These myofibrils are surrounded by the muscle cell membrane (sarcolemma), which form deep invaginations called transverse tubules (T-tubules) within the myofibril. The myofibrils contain both thick and thin filaments. The thick filaments are composed of actin and myosin, which power contraction and are organised into repeating arrays called sarcomeres. The thin filaments are composed of actin, tropomyosin, and troponin. Tropomyosin prevents actin and myosin from interacting when the muscle is at rest, consequently preventing muscle contraction. Troponin is a three-protein complex: Troponin T, which facilitates the binding of troponin to tropomyosin; Troponin I, which stops the actin-myosin interaction by blocking the myosin-binding sites; and Troponin C, which binds calcium to initiate muscle contraction.
Cardiac muscle tissue is found only in the heart and is responsible for the cardiac contraction that pumps blood throughout the body. It is a striated muscle fibre under involuntary control by the body's autonomic nervous system (ANS).
Smooth muscle fibres are found throughout the blood vessels, gastrointestinal (GI) 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. These fibres are under involuntary control by reflexes and the body's ANS.
Muscle contraction begins when the nervous system generates a signal, which travels through a type of nerve cell called a motor neuron. When the nervous system signal reaches the neuromuscular junction, a chemical message is released by the motor neuron. This chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fibre, starting a chemical reaction within the muscle. When acetylcholine reaches receptors on the membranes of muscle fibres, membrane channels open and allow an influx of sodium ions into the cytoplasm of the muscle fibre. This triggers the release of stored calcium ions, which diffuse into the muscle fibre. The relationship between the chains of proteins within the muscle cells changes, leading to the contraction. When the stimulation of the motor neuron providing the impulse to the muscle fibres stops, the chemical reaction that causes the rearrangement of the muscle fibres' proteins is stopped, and the muscle relaxes.
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Types of muscle movement
The human body has over 600 muscles, which can be grouped by their location or the type of movement they perform. The three main types of muscles are skeletal, smooth, and cardiac.
Skeletal muscles are attached to bones via tendons and are found throughout the body, including in the tongue, diaphragm, eye socket, and upper oesophagus. They are also present in the limbs and are grouped into opposing pairs, such as the biceps and triceps. Skeletal muscles are under conscious control and are therefore also known as voluntary muscles. They are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilising joints.
Smooth muscles, on the other hand, are located in various internal structures, including the digestive tract, uterus, and blood vessels. They are arranged in layered sheets that contract in waves along the length of the structure. Smooth muscles are not under conscious control, and their motion occurs without our awareness, making them involuntary muscles.
Cardiac muscle comprises the walls of the heart, allowing blood to be pumped through the vasculature.
Muscle movements can be further categorised into types of contractions and types of joint movements.
Types of Contractions
- Isotonic contraction: This occurs when there is constant muscle tension with a change in muscle length. It is seen during activities like walking, running, or squatting.
- Concentric contraction: This happens when there is sufficient muscle tension to overcome the load, causing the muscle to contract and shorten. Examples include a biceps curl or standing from a squatting position.
- Eccentric contraction: This occurs when the muscle works to decelerate a joint at the end of a movement, such as when lowering a weight.
Types of Joint Movements
- Flexion and extension: These are usually movements forward and backward from the body, such as nodding the head. Flexion decreases the angle between two bones (bending), while extension increases the angle (straightening a bend).
- Abduction and adduction: These are typically side-to-side movements, such as moving the arm laterally when doing jumping jacks. Abduction is moving away from the body's midline, while adduction is moving towards it.
- Supination and pronation: These refer to rotating the forearm so the palm faces up or down, respectively.
- Elevation and depression: These are up-and-down movements, such as chewing or shrugging the shoulders.
- Superior and inferior rotation: These are movements of the scapula, defined by the direction of movement of the glenoid cavity. Superior rotation is required for full abduction of the upper limb.
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Muscle health and injuries
Muscle health is important for everyone, not just athletes or those who look muscular. There are more than 600 muscles in the human body, and they are all essential for movement and keeping the body alive. Skeletal muscles, for example, help you maintain your posture and stabilize joints, while the cardiac muscle in your heart pumps blood throughout your body. Smooth muscles, meanwhile, are found in the walls of hollow organs like the stomach and intestines, helping to move substances through them.
Muscles are made of thousands of small fibres woven together, and these fibres can sometimes be overstretched or pressed together too much, leading to injury. Muscle injuries are very common, especially in athletes, and they can be caused by a single traumatic event or by overuse. Acute injuries are usually the result of a direct impact or compressive force, like a blow or collision in contact sports, while overuse injuries are related to unusual, prolonged, or very intense physical activity.
There are several types of muscle injuries, including lacerations, contusions, degenerative diseases, and strains. Lacerations occur when an external object cuts the muscle, while contusions are caused by compressive forces that don't break the skin but damage blood vessels and cause internal bleeding, also known as bruises. Strains happen when muscle fibres cannot withstand excessive tensile forces and are placed in a lengthened state, usually across two joints.
If you suspect a muscle injury, it's important to follow the RICE (Rest, Ice, Compression, and Elevation) principle to minimise bleeding and the extent of the injury. Clinical examination and imaging diagnostics can then be used to assess the severity and determine the best course of treatment. Most muscle injuries are managed conservatively, and surgery is usually only needed for larger tears. Rehabilitation exercises are important to optimise healing and recovery, but an early return to activity can increase the risk of reinjury.
To maintain muscle health and prevent injuries, it's important to stay active and perform exercises or physical activities that are suitable for your body. Consult a healthcare provider for guidance on the best ways to support your muscle health and what to do in case of an injury.
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Muscle cell structure
The muscular system is made up of three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that attach to bones, enabling movement of the skeleton and maintaining body temperature and posture. They can also attach to the skin, such as those in our face that control facial expressions. Smooth muscles, on the other hand, lie mainly in the walls of hollow organs like the stomach and intestines, helping to propel substances through these organs. Lastly, cardiac muscle is only present in the heart and is responsible for pumping blood throughout the body through cardiac contractions.
Skeletal muscles are composed of bundles of muscle fibres called myofibers, each containing several myofibrils. These myofibrils are contractile units within the muscle cell, consisting of an arrangement of longitudinal myofilaments, including thin actin filaments and thick myosin filaments. The thick filaments are composed of myosin, a protein polypeptide, while the thin filaments contain actin, tropomyosin, and troponin. The sarcomere is the functional contractile unit of the myofibril of a striated muscle.
Each skeletal muscle fibre is a single cylindrical muscle cell or myocyte. Myocytes are long cylindrical cells with multiple nuclei located just below the cell membrane, called the sarcolemma. The sarcolemma is a tubular sheath that encases and defines each muscle fibre, forming a barrier between extracellular and intracellular compartments. It also makes tiny tunnels that project downwards from the surface into the centre of the muscle fibre, known as transverse tubules or T tubules, which serve as a major location for ion exchange.
Myocytes are derived from the fusion of myoblasts, or early muscle cells with a single nucleus. During myogenesis, myoblasts combine to form myocytes (multinucleated muscle cells). Myocytes are surrounded by a connective tissue sheath called the endomysium. Bundles of myofibers form fascicles, and these fascicles are surrounded by another layer of connective tissue called the perimysium. Together, the endomysium, perimysium, and epimysium bundle together thousands of muscle fibres, providing structure and strength to the muscle. These connective tissues also extend beyond the muscle belly, forming a tendon or aponeurosis that attaches the muscle to the bone.
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Frequently asked questions
Muscle physiology is a branch of physiology that studies the mechanism behind muscle contraction.
Muscle contraction is the shortening of muscle fibres. This is initiated by action potentials in motor neurons, which cause the release of neurotransmitters from synaptic vesicles.
There are three types of muscle tissue: skeletal, smooth, and cardiac.
The biceps brachii in your upper arm is a skeletal muscle.
The beating of your heart is an involuntary muscle movement.
