Structure of Skeletal Muscle
எலும்புத்தசையின் அமைப்பு
The A band never changes length during contraction — the I band and H zone shrink. If a statement says 'A band widens', it is false.
The idea
Each organised skeletal muscle in the body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia. Each bundle contains a number of muscle fibres. Each muscle fibre is lined by the plasma membrane called sarcolemma enclosing the sarcoplasm; the muscle fibre is a syncytium as the sarcoplasm contains many nuclei. The endoplasmic reticulum, i.e. the sarcoplasmic reticulum, of muscle fibres is the store house of calcium ions. A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils. Each myofibril has alternate dark and light bands: the light bands contain actin and are called I-bands or isotropic bands, whereas the dark bands, called A-bands or anisotropic bands, contain myosin. In the centre of each I-band is an elastic fibre called the Z line which bisects it. The thin filaments are firmly attached to the Z line; the thick filaments in the A band are held together in the middle by a thin fibrous membrane called the M line. The portion of the myofibril between two successive Z lines is the sarcomere, the functional unit of contraction. In a resting state the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments, leaving the central part of the thick filaments — the H zone. Each actin (thin) filament is made of two F (filamentous) actins helically wound to each other, each a polymer of monomeric G (globular) actins; two filaments of another protein, tropomyosin, run close to the F actins, and a complex protein, troponin, is distributed at regular intervals on the tropomyosin; in the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments. Each myosin (thick) filament is a polymer of monomeric proteins called meromyosins; each meromyosin has a globular head with a short arm (heavy meromyosin, HMM) and a tail (light meromyosin, LMM). The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
Why it matters
Thousands of identical sarcomeres in series let a muscle shorten smoothly and strongly; precise overlap of thin and thick filaments is what generates force.
NEET lens
Must-know: fascia = collagenous connective tissue holding muscle bundles; sarcoplasmic reticulum stores Ca2+; I band = actin (isotropic), A band = myosin (anisotropic); Z line bisects I band; M line holds thick filaments; sarcomere = between two Z lines; troponin masks myosin-binding sites on actin; myosin head = ATPase with ATP and actin binding sites; HMM = head + short arm, LMM = tail.
Clinical significance
Red muscle fibres are rich in myoglobin and mitochondria and suit endurance; white fibres have less myoglobin, depend on anaerobic metabolism and suit short bursts — the reason sprinters and marathon runners train differently.
Introduction
A skeletal muscle is built from bundles of long multinucleate fibres, each packed with myofibrils made of repeating sarcomeres of actin and myosin.
Core concept
Muscle → fascicles (held by fascia, collagenous connective tissue) → muscle fibres (sarcolemma, sarcoplasm, many nuclei = syncytium; sarcoplasmic reticulum stores Ca2+) → myofibrils. Bands: I band (light, actin, isotropic) bisected by Z line; A band (dark, myosin, anisotropic) with M line in the middle; H zone = central part of thick filament not overlapped by thin filaments. Sarcomere = between two successive Z lines (functional unit). Actin filament: 2 F-actins (polymers of G-actin) helically wound + 2 tropomyosin filaments + troponin at intervals (masks myosin-binding sites at rest). Myosin filament: polymer of meromyosins; each = globular head with short arm (HMM) + tail (LMM); head = ATPase, binding sites for ATP and active sites for actin; heads project outward as cross arms.
Go deeper
Red fibres: high myoglobin, many mitochondria, aerobic. White fibres: little myoglobin, fewer mitochondria, more sarcoplasmic reticulum, anaerobic. Muscle bundles are held together by collagenous connective tissue, not by elastic tissue.
From Class 10
You know muscles are made of fibres. NEET goes inside the fibre to the sarcomere — the actual unit that shortens.