Victor Forge
The human body is a complex system of organs and tissues, with muscles being a vital component. Muscles are what enable our bodies to move, from voluntary actions like sprinting to involuntary movements such as breathing. There are three types of muscles: skeletal, cardiac, and smooth muscle. Skeletal muscles, which are attached to bones by tendons, are under our conscious control and enable a wide range of movements. Cardiac muscles, on the other hand, are involuntary and constitute the heart, which beats without our conscious input. Smooth muscles line the insides of some organs, aiding in essential functions like digestion and breathing. This diverse array of muscles, working in harmony with other organs, keeps our bodies functioning optimally.
| Characteristics | Values |
|---|---|
| Definition | Organs that are made of muscle tissue |
| Types | Skeletal, cardiac, and smooth muscle |
| Skeletal Muscle Composition | Skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue |
| Skeletal Muscle Function | Allow for a wide range of movements and functions, including posture, movement, and breathing |
| Skeletal Muscle Characteristics | Vary in size, shape, and arrangement of fibers; may be broad or narrow in shape; fibers may be parallel to the long axis of the muscle, converge to a narrow attachment, or be oblique |
| Skeletal Muscle Control | Voluntary |
| Cardiac Muscle Function | Make up the middle layers of the heart |
| Cardiac Muscle Control | Involuntary |
| Smooth Muscle Function | Line the inside of some organs, including the intestines and lungs; play a role in the female and male reproductive systems, urinary system, respiratory system, and digestive system |
| Smooth Muscle Control | Involuntary |
| Muscle Groups | Often classified by location (e.g., chest, leg, back) or type of movement (e.g., abductors, flexors, extensors) |
| Muscle Communication | Communicate with other organs such as adipose tissue, liver, pancreas, bones, and brain through the release of cytokines, peptides, and myokines |
| Muscle Injuries | Strains (pulled muscles) are the most common injury, occurring when muscle fibers are stretched beyond their limit |
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What You'll Learn
Skeletal muscle as an organ
Skeletal muscle is considered an organ of the muscular system. It is the most common type of muscle in the human body, comprising 30% to 40% of total body mass. Skeletal muscles are attached to bones by tendons and allow for a wide range of voluntary movements and functions. They are responsible for posture, movement, and breathing, which are made possible by muscle fibre contractions.
Skeletal muscles vary in size, shape, and arrangement of fibres. Each skeletal muscle fibre is a single cylindrical muscle cell, and each muscle is made up of hundreds or thousands of these fibres bundled together and wrapped in a connective tissue covering called the epimysium. The connective tissue provides support and protection for the muscle cells and allows them to withstand the forces of contraction. It also forms pathways for blood vessels and nerves. Skeletal muscles have an abundant supply of blood vessels and nerves, which is directly related to their primary function of contraction.
In addition to their mechanical functions, skeletal muscles have recently been identified as secretory organs. They produce, express, and release cytokines and other peptides, now referred to as "myokines," which exert autocrine, paracrine, or endocrine effects. This discovery has provided new insights into how skeletal muscles communicate with other organs, such as adipose tissue, liver, pancreas, bones, and brain.
The health benefits of exercise are also linked to skeletal muscle as an endocrine organ. Myokines produced by skeletal muscle may contribute to the protective effects of exercise, particularly in relation to diseases associated with physical inactivity.
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Cardiac muscle
The heart is the only organ in the human body that is also a muscle. This muscle, known as the cardiac muscle or myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle.
Cardiomyocytes are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. Myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
The primary function of the 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 coordinated contraction of cardiomyocytes allows the ventricle to squeeze in several directions simultaneously, maximising the amount of blood squeezed out of the heart with each heartbeat.
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Smooth muscle
In the urinary system, smooth muscle helps to rid the body of toxins and maintain electrolyte balance. Smooth muscle is also involved in the regulation of passage widths within the body, such as in the sphincters of the bladder and anus, where it stays flexed and only relaxes when needed. Smooth muscle is particularly important in the female reproductive system, where it facilitates contractions during labour. Smooth muscle is vulnerable to various conditions, including injuries and paralysis, similar to other types of muscles.
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Voluntary vs involuntary movements
Human movements can be broadly classified into two types: voluntary and involuntary movements. Voluntary movements are self-generated, willed actions performed as a result of cognitive processes. They are conscious actions that require the mover to have the intention to move. For example, running and dancing are voluntary movements. On the other hand, involuntary movements are non-intentional and occur without conscious control. They are automatic muscle responses to particular stimuli and do not involve the brain or conscious attention. Reflexes, such as the knee-jerk response, are examples of involuntary movements.
Voluntary movements are controlled by the motor cortex, a zone of the cerebrum located behind the frontal lobe. The motor cortex sends neural messages that move through the brain stem along the spinal cord and into the neural network to the muscle being commanded. This allows for the well-controlled fluid movement of skeletal muscles that attach to bones, such as the muscles in the arms, legs, neck, back, and trunk.
Involuntary movements, on the other hand, are controlled by other regions of the brain, such as the hypothalamus. They are produced by a non-intentional, pathological activation anywhere within the final common pathways or the two main loops (basal ganglia loop and cerebellar loop). Involuntary movements can be divided into four major groups: tremor, myoclonus, ballism/chorea, and dystonia/athetosis. Tremors are characterized by abnormal oscillations that produce abnormal movements. Myoclonus refers to sudden, brief, shock-like involuntary movements that can arise anywhere from the cortex to the muscle. Ballism/chorea involves suddenly appearing, irregular, phasic movements, while dystonia/athetosis involves sustained, long-duration muscle contractions sometimes associated with torsion components.
While voluntary movements may get more attention, involuntary movements play a crucial role in everyday life. Reflexes, for example, help maintain balance and safety. Additionally, involuntary muscles control automatic internal processes necessary for survival, such as those involved in the functioning of the heart, lungs, and digestive system.
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Muscle contractions
A muscle contraction is the tightening, shortening, or lengthening of a muscle when an individual engages in some activity. Muscle contractions can occur when an individual holds, picks up, stretches, or exercises with weights. Following muscle contraction, muscle relaxation occurs, allowing the contracted muscles to return to their normal state.
Muscles are organs that move the body and perform two types of movements: voluntary and involuntary. Voluntary movements are actions that an individual controls, such as sprinting around a track or scrolling through an article on a phone. Involuntary movements occur automatically without conscious thought, such as the movement of organs that keep the body functioning properly.
There are three types of muscles in mammals: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. They are voluntary muscles that move when an individual thinks about moving a specific body part. Cardiac muscle, or myocardium, makes up the middle layers of the heart and is an involuntary muscle. Smooth muscle is involuntary and lines the inside of some organs, performing essential jobs such as moving waste through intestines and helping lungs expand during breathing.
The physiological concept of muscle contraction is based on two variables: length and tension. Muscle shortening and contraction are not synonymous, as tension within a muscle can be produced without changes in its length. For example, holding a dumbbell in the same position or holding a sleeping child in your arms involves muscle contraction without changes in muscle length.
The complex process of muscle contraction, known as excitation-contraction coupling, begins with an action potential that causes depolarization in the myocyte membrane. This depolarization spreads via transverse (T) tubules, leading to a conformational change in dihydropyridine receptors and the opening of nearby ryanodine receptors on the sarcoplasmic reticulum (SR). The release of calcium from the SR initiates a series of events, including the binding of calcium to troponin C, a conformational change, and the shifting of tropomyosin, allowing the attachment of myosin heads to actin filaments. Cross-bridge cycling is initiated when ATP binds to an ATP-binding domain on the myosin head. The subsequent dissociation of myosin from actin breaks the cross-bridge, leading to the hydrolysis of ATP and changes in the conformation of myosin heads. The release of phosphate and ADP causes the myosin to return to its original position, pulling on the actin filament and resulting in the contraction of the sarcomere and the muscle fiber.
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Frequently asked questions
An organ is a group of different tissues that work together to perform a specific function or group of functions.
A muscle is a type of tissue. There are three types of muscle tissue in the body: skeletal, smooth, and cardiac.
The heart is the only organ that is also a muscle. It is made of a special type of muscle tissue called cardiac muscle. Skeletal muscles are also sometimes considered organs of the muscular system.
