Muscle
Muscle is a specialized type of animal tissue composed of elongated cells, known as muscle fibers or myocytes, that have the unique ability to contract and generate force, enabling movement, posture maintenance, and various physiological functions. These cells contain contractile proteins, primarily actin and myosin, which interact to produce shortening of the fibers through a process powered by adenosine triphosphate (ATP). Muscle tissue is highly vascularized and organized into bundles surrounded by connective tissue, allowing coordinated action across the body.[1][2] There are three primary types of muscle tissue in vertebrates: skeletal, cardiac, and smooth, distinguished by their structure, location, control mechanism, and appearance under a microscope. Skeletal muscle, which constitutes about 40% of body weight in humans, is striated (showing alternating light and dark bands), multinucleated, and under voluntary control via the somatic nervous system; it attaches to bones via tendons and is responsible for locomotion, manipulation of objects, and stabilizing joints. Cardiac muscle, found exclusively in the walls of the heart (myocardium), is also striated but features branching fibers connected by intercalated discs for synchronized contraction; it operates involuntarily through intrinsic pacemaker cells, pumping blood throughout the circulatory system. Smooth muscle lacks striations, has spindle-shaped cells with a single nucleus, and functions involuntarily; it lines the walls of hollow organs such as blood vessels, the gastrointestinal tract, and the urinary bladder, regulating processes like digestion, blood flow, and organ propulsion.[2][3][1] All muscle types share four fundamental properties: contractility, the capacity to shorten forcefully; excitability, the ability to respond to stimuli such as nerve impulses or hormones; extensibility, the potential to stretch without damage; and elasticity, the tendency to recoil to the original length after stretching or contraction. These properties underpin muscle's roles beyond movement, including heat production for thermoregulation (e.g., via shivering), communication through facial expressions and gestures, and maintenance of organ tone for functions like blood pressure regulation. Contraction in all muscles relies on calcium ions triggering interactions between actin and myosin filaments, though the regulatory mechanisms differ—neural input for skeletal muscle, autonomic nerves or hormones for smooth and cardiac.[4][2]Fundamentals
Definition and Role
Muscle tissue is a specialized form of excitable and contractile tissue composed primarily of elongated muscle cells, known as myocytes or muscle fibers, which are capable of generating force and motion through coordinated shortening.[5] These cells contain contractile proteins such as actin and myosin, arranged in structures that enable rapid response to stimuli and force production.[2] Muscle tissue is classified into three main types—skeletal, smooth, and cardiac—each adapted to specific functions within the body.[2] The primary physiological roles of muscle tissue include facilitating movement, such as locomotion and maintenance of posture; generating heat through thermogenesis to regulate body temperature; and supporting essential organ functions, for example, by pumping blood in the heart or propelling contents through digestive and vascular systems.[2] Skeletal muscles, in particular, enable voluntary movements and stabilize the body against gravity, while smooth and cardiac muscles handle involuntary processes critical for circulation and internal transport.[6] Additionally, muscle contraction contributes to overall energy metabolism, with skeletal muscle playing a key role in non-shivering thermogenesis during rest or activity.[7] Muscle tissue integrates closely with the musculoskeletal and nervous systems to achieve coordinated action, where neural signals trigger excitation-contraction coupling to produce precise force output.[2] In humans, muscles constitute approximately 40-50% of total body weight, predominantly as skeletal muscle, which significantly influences the basal metabolic rate by accounting for a major portion of resting energy expenditure.[8][7] This substantial mass underscores muscle's central role in metabolic homeostasis and physical performance.[9]Etymology and Terminology
The term "muscle" originates from the Latin word musculus, a diminutive form of mus meaning "mouse," reflecting the ancient observation that a contracting muscle resembles a small mouse moving beneath the skin.[10][11] This analogy dates back to classical antiquity, where the visual similarity of flexed biceps or other muscles to a scurrying rodent inspired the nomenclature.[12] The Greek equivalent root mŷs (or mys) similarly denoted both "mouse" and "muscle," serving as the basis for the common medical prefix myo-, which appears in terms related to muscular structures and functions. In ancient Greek medicine, Hippocrates (c. 460–370 BCE) described muscles using functional and locational descriptors, such as those based on their role in movement or position in the body, without a unified term like musculus.[13] Galen (129–c. 216 CE), building on Hippocratic ideas, expanded this by systematically referring to muscles as contractile tissues in his anatomical treatises, often employing Greek-derived terms that emphasized their fleshy, mover-like qualities.[13][14] The evolution toward modern standardization began in the 16th century with Andreas Vesalius, whose 1543 work De humani corporis fabrica introduced consistent Latin nomenclature for skeletal muscles, such as masseter (chewer) and rectus abdominis (straight abdominal), many of which remain in use today after refinements in the 19th century through efforts like the Basiliensia Nomina Anatomica (1895).[13][15] This shift reduced synonyms and descriptive variations prevalent in earlier eras, establishing a precise, internationally adopted system formalized in the Terminologia Anatomica (first edition 1998; second edition 2019), which in its latest edition includes 337 skeletal muscle entries.[13][16] Key terms in muscle biology include myocyte, derived from Greek *myo-* (muscle) and kytos (cell), referring to an individual muscle cell; myofibril, combining myo- with Latin fibrilla (small fiber), denoting the thread-like contractile components within a myocyte; and sarcomere, from Greek sarx (flesh) and meros (part), the fundamental repeating unit of striated muscle.[17][18] These terms emerged in the 19th and early 20th centuries amid advances in microscopy and histology, providing a precise vocabulary distinct from broader anatomical labels.[13] Terminology also distinguishes voluntary muscle (skeletal muscle under conscious neural control) from involuntary muscle (smooth and cardiac types regulated autonomically), a classification rooted in Galen's functional observations but formalized in 19th-century physiology to reflect differences in innervation and control.[19][20] A common misconception confuses muscles with tendons, where muscles are the contractile tissues that generate force for movement, while tendons are non-contractile collagenous bands that transmit that force from muscle to bone; this distinction is crucial in clinical contexts like injuries, as damage to one does not equate to the other.[21][22]Structure
Comparison of Types
Vertebrate muscle tissue is classified into three primary types—skeletal, smooth, and cardiac—distinguished by their locations, control mechanisms, and structural features, which enable specialized functions in movement, organ regulation, and circulation.[3] The following table summarizes key comparative aspects of these muscle types:| Feature | Skeletal Muscle | Smooth Muscle | Cardiac Muscle |
|---|---|---|---|
| Location | Attached to bones via tendons | Walls of hollow organs (e.g., intestines, blood vessels) | Walls of the heart |
| Control | Voluntary (conscious) | Involuntary (autonomic or hormonal) | Involuntary (autonomic) |
| Appearance | Striated (due to sarcomeres) | Non-striated | Striated (due to sarcomeres) |
| Innervation | Somatic nervous system (each fiber innervated) | Autonomic nervous system (varicose synapses, not all cells) | Autonomic nervous system (gap junctions propagate signals) |
| Contraction Speed | Fast | Slow and sustained | Fast and rhythmic |
| Fatigue Resistance | Low (prone to fatigue) | High (resistant) | High (highly resistant) |