Unit 05 — Muscle Tissue · Question Bank

A skeletal muscle fibre is the textbook example of a cell whose architecture is dictated by its function. It has to generate fast, voluntary, short-burst force, transmit that force cleanly to a tendon, and propagate a single nerve impulse across a cell several centimetres long. Every feature you describe should be tied back to one of those three demands — the examiner is testing whether you can read structure as engineering.

Light-microscopic appearance

Each fibre is a long cylindrical syncytial cell, 10–100 µm thick and up to 40 mm long, formed in the embryo by fusion of many mononuclear myoblasts. Because of that origin it carries dozens of flattened nuclei pushed out to the periphery, lying just beneath the sarcolemma. The sarcoplasm is strongly acidophilic because it is packed with eosinophilic myofibrils running parallel to the long axis, and their precise register from one myofibril to the next produces distinct cross-striations — alternating dark A-bands and light I-bands resolvable at medium power. Skeletal fibres do not join end-to-end; there are no intercalated discs.

Myofibrils, sarcomeres and the banding pattern

Each myofibril is a longitudinal chain of repeating contractile units called sarcomeres, bounded by two adjacent Z-lines. From Z to Z each sarcomere contains a half I-band (thin filaments only), a full A-band (thick filaments, with thin-filament overlap at its edges), and another half I-band. Within the A-band, the central H-band is the zone of thick filaments alone, and a dark M-line in the middle of the H-band cross-links them. On contraction the thin filaments slide inwards: the A-band stays constant, the I- and H-bands narrow, and the Z-lines are pulled together — the sliding-filament principle, made visible by the banding.

Myofilaments

The thick filament is a bundle of myosin II molecules whose globular heads project outwards and carry both an ATPase site and an actin-binding site. The thin filament is a twisted F-actin polymer decorated with tropomyosin (covers the binding sites at rest) and the troponin complex (TnT–binds tropomyosin, TnI–inhibitory, TnC–binds Ca²⁺). When Ca²⁺ binds TnC the troponin tilts, tropomyosin slides aside, and the myosin heads can engage actin and execute the power stroke.

Sarcotubular system — the triad

To deliver the depolarisation to every sarcomere simultaneously, the sarcolemma invaginates as narrow T-tubules running transversely into the depth of the fibre, while the sarcoplasmic reticulum wraps each myofibril and widens into pancake-shaped terminal cisternae. At the A–I band junction a central T-tubule is flanked by two terminal cisternae — the triad, two per sarcomere. The T-tubule dihydropyridine receptor mechanically couples to the SR ryanodine receptor (RyR1), so an action potential triggers a millisecond burst of Ca²⁺ release. After contraction the SERCA pump returns Ca²⁺ to the SR for the next cycle.

Connective-tissue investments and regeneration

Each fibre is wrapped by a delicate reticular endomysium, which carries the capillaries; fibres are bundled into fascicles by the denser perimysium; and the whole muscle is sheathed by dense-irregular epimysium, continuous with the tendon — this nested arrangement transmits the sarcomeric force to bone. Between the sarcolemma and the basal lamina sit quiescent satellite cells, the resident myogenic stem cells responsible for regeneration after a tear or crush injury.

Clinical anchor

In Duchenne muscular dystrophy, loss of dystrophin uncouples the cortical actin cytoskeleton from the basal lamina; repeated contractions tear the sarcolemma and exhaust the satellite-cell pool, producing the progressive weakness and pseudohypertrophy that defines the disease. In myasthenia gravis, autoantibodies to the sarcolemmal ACh receptor at the motor end-plate produce fatigable weakness without touching the contractile machinery itself.

Cardiac muscle is engineered for a brief that is unique in the body — rhythmic, involuntary, fatigue-resistant contraction sustained for a lifetime, with the whole organ behaving as a single sheet rather than as a collection of independent cells. The histology delivers two distinctive features in service of that brief: the shape of the individual cardiomyocyte and the intercalated disc that joins one cell to the next. A good answer treats the disc not as a single junction but as a three-part composite whose components each solve one problem.

The cardiomyocyte

A cardiac muscle cell is a short, roughly cylindrical column about 15 µm thick and 100 µm long, and crucially it branches — in a good section you see Y-shaped profiles where one cell divides to abut two neighbours. Each cell has one (occasionally two) centrally placed nucleus, never the peripheral palisade of skeletal muscle. The sarcoplasm is cross-striated because cardiomyocytes also build sarcomeres, but the myofibrils are incompletely demarcated and tend to merge across the cell, so the striations look less crisp than in skeletal muscle. Mitochondria occupy roughly a third of the cell volume — the metabolic signature of a tissue that depends almost entirely on aerobic respiration.

The intercalated disc — mechanical face

End-to-end, cardiomyocytes are linked by the intercalated disc, visible at light microscopy as a dark transverse step-like line. At the EM level the disc has two faces. On the transverse portion the disc carries the fascia adherens — an extensive adherens junction that anchors the actin filaments of the terminal sarcomere of each cell, so the systolic pull of one cell is transmitted directly into the next. Punctuating this surface are desmosomes (macula adherens), spot-welds that anchor desmin intermediate filaments and stop the cells from being torn apart under load. Together these two junctions make the disc a mechanical weld.

The intercalated disc — electrical face

On the longitudinal portion of the disc sit the gap junctions, formed by connexin channels that create low-resistance cytoplasmic continuity between adjacent cells. These let the wave of depolarisation jump from cell to cell without a synapse, so the entire myocardium contracts as a single functional syncytium. The geometry — mechanical junctions across, electrical junctions along — means the disc both holds the cells together and wires them in parallel.

Sarcotubular system and regeneration

The T-tubules in cardiac muscle are wider than in skeletal muscle and meet the SR at the Z-line, where each T-tubule is apposed to one SR terminal cisterna, forming a diad (one per sarcomere). The simpler coupling is enough because cardiac contraction also depends on Ca²⁺ entering directly through the T-tubule from extracellular fluid — the calcium-induced calcium release mechanism. Cardiac muscle has no satellite cells; once a cardiomyocyte dies it is not replaced, and the gap is filled by collagenous scar laid down by fibroblasts.

Clinical anchor

After a myocardial infarction, ischaemic cardiomyocytes lyse and release their contractile-protein cargo into the blood; the resulting rise in cardiac troponin I and T is the specific serum marker used to confirm the diagnosis. Because cardiac muscle cannot regenerate, the infarcted zone heals by non-contractile fibrous scar, which is prone to aneurysm and is the structural cause of post-infarct heart failure. Mutations in desmosomal proteins of the intercalated disc underlie arrhythmogenic right-ventricular cardiomyopathy, where mechanical failure of the disc lets cells pull apart under load and triggers lethal arrhythmias.

Smooth muscle is the involuntary muscle of every hollow viscus — gut, blood vessel, bladder, bronchus, uterus. Its design brief is the opposite of skeletal muscle: slow, sustained, energy-cheap contraction that can be held for hours rather than seconds. Almost every histological difference from striated muscle follows logically from that brief, and a strong answer makes that logic explicit rather than just listing absences.

Light-microscopic structure

Smooth muscle cells are spindle-shaped (fusiform), 5–20 µm at their widest and 60–100 µm long — though they stretch enormously, reaching over 500 µm in the term-pregnant uterus. Each cell carries a single, central, elongated cigar-shaped nucleus, often appearing corkscrewed when the cell is caught mid-contraction. The sarcoplasm is acidophilic and homogeneous, with no visible myofibrils and no cross-striations. There are no intercalated discs. In a sheet of gut wall the cells are staggered like overlapping fish scales, so a transverse section shows a mosaic of small round profiles, only some of which contain a nucleus.

Contractile apparatus

Actin and myosin are present, but they are not arranged in sarcomeres — that is why there are no striations. The thin filaments anchor instead at dense bodies: small electron-dense plaques scattered through the cytoplasm and attached to the inner face of the sarcolemma, the Z-line equivalent of smooth muscle. The dense bodies are linked by an intermediate-filament cytoskeleton of desmin and vimentin, so when contractile filaments shorten the whole cell purses inwards like a draw-string bag. There are no T-tubules; instead, small sarcolemmal invaginations called caveolae bring extracellular Ca²⁺ close to a modest peripheral SR.

Calcium handling and the control switch

Smooth muscle lacks troponin entirely. The Ca²⁺ that triggers contraction comes partly from the SR and partly from extracellular fluid (through voltage-gated and store-operated channels in the caveolae); it binds the cytoplasmic protein calmodulin, the Ca–calmodulin complex activates myosin light-chain kinase (MLCK), and MLCK phosphorylates the regulatory light chain of myosin, which only then can engage actin. The pathway is several steps longer than the troponin switch of striated muscle, which is exactly why smooth-muscle contractions develop over seconds and can be held tonically with very little ATP — the so-called latch state.

Innervation, coupling and regeneration

Innervation is autonomic and the neurotransmitter is released diffusely from varicosities, not from named end-plates. Adjacent cells are linked by gap junctions, so a wave of depolarisation spreads cell to cell across a sheet of gut or vessel wall, again as a functional syncytium. Smooth muscle uniquely retains the capacity to divide: cells undergo hyperplasia in the uterus during pregnancy and in the vessel wall during atherogenesis, and they can also secrete the surrounding matrix — a property neither striated muscle type has.

Differences from striated muscle

The list of absences therefore makes engineering sense: no sarcomeres (because the design favours sustained tone over speed), so no striations and no troponin; no T-tubules (because rapid signal delivery is not required), so no triads or diads; no satellite cells needed (because the cells themselves can divide). What smooth muscle has instead — dense bodies, caveolae, calmodulin–MLCK, gap junctions, mitotic capacity — is exactly the toolkit for a slow, tonic, regenerable squeezer.

Clinical anchor

Benign smooth-muscle tumours are called leiomyomas; uterine fibroids, present in roughly a quarter of women of reproductive age, are the commonest example. The malignant counterpart, leiomyosarcoma, is rare but aggressive. (The skeletal-muscle equivalents are the rhabdomyoma and the paediatric rhabdomyosarcoma.) In atherosclerosis, vascular smooth-muscle cells of the media migrate into the intima and secrete extracellular matrix, driving plaque growth — the proliferative and secretory capacities of smooth muscle are central to the disease.

This is the question TMU has put on essentially every Histology final, and the skill being tested is not memory of one feature but the ability to read three columns at once. The three muscle types are three engineering solutions to the same biological problem — how to build a cell that shortens reliably — and each histological feature you describe should be tied to the design brief it serves. A slide-room shortcut worth stating at the start: look at the nuclei first; many peripheral nuclei mean skeletal, one or two central nuclei in a branching cell mean cardiac, a single central cigar-shaped nucleus in a spindle cell means smooth.

Shape and nuclei

Skeletal fibres are long cylindrical syncytial cells, up to 40 mm long, with dozens of flattened nuclei displaced to the periphery beneath the sarcolemma — a direct consequence of their embryonic origin by myoblast fusion. Cardiac muscle cells are short, branching columns (~15 µm × ~100 µm) with one or two central nuclei; the branching outline is itself diagnostic. Smooth muscle cells are spindle-shaped and carry one central, elongated cigar-shaped nucleus. The nuclear pattern alone usually gives the diagnosis.

Striations and the contractile apparatus

Skeletal muscle shows distinct, sharp cross-striations because its sarcomeres are crisply demarcated and lie in perfect register across the fibre. Cardiac muscle is also striated but less distinctly: the myofibrils branch and merge, so the bands look fuzzier. Smooth muscle has no striations at all because its actin and myosin are anchored to scattered dense bodies rather than to a repeating sarcomeric lattice. Skeletal and cardiac muscle therefore have a sarcomere with A, I, H bands, Z and M lines; smooth muscle does not.

Membrane system — triad vs diad vs caveolae

Skeletal muscle has the classic triad (T-tubule + two SR terminal cisternae) at the A–I junction, two per sarcomere, coupled by DHPR–RyR1 for fast mechanical excitation–contraction coupling. Cardiac muscle has a diad (T-tubule + one SR cisterna) at the Z-line, one per sarcomere, and depends partly on extracellular Ca²⁺ entry (calcium-induced calcium release). Smooth muscle has no T-tubules; caveolae bring extracellular Ca²⁺ close to the membrane and the SR is modest.

Calcium switch and junctional coupling

In skeletal and cardiac muscle the Ca²⁺ sensor is troponin C, which displaces tropomyosin from actin in milliseconds. In smooth muscle there is no troponin; Ca²⁺ binds calmodulin, which activates myosin light-chain kinase (MLCK) to phosphorylate the myosin light chain — a slower switch that suits tonic contraction. Cell-to-cell coupling differs too: skeletal fibres receive an individual motor end-plate and are not electrically coupled; cardiac cells are joined end-to-end by the intercalated disc (fascia adherens + desmosomes for mechanical anchorage, gap junctions for electrical coupling); smooth muscle cells couple to one another by gap junctions alone, with autonomic varicosities providing diffuse innervation.

Control and regeneration

Skeletal muscle is voluntary, the other two involuntary. Regenerative capacity follows a clear ranking: skeletal muscle regenerates well via satellite cells beneath the basal lamina; cardiac muscle is essentially non-regenerating and heals by fibrosis — the structural basis of post-MI heart failure; smooth muscle retains the capacity to divide outright and can also secrete its surrounding matrix (relevant in pregnancy and atherogenesis).

Clinical anchor

The clinical contrasts mirror the histology. Skeletal muscle disease typically targets either the membrane (Duchenne — dystrophin) or the neuromuscular junction (myasthenia gravis — anti-AChR). Cardiac muscle disease is dominated by the irreversibility of cell loss: troponin release confirms an MI, and the resulting fibrous scar drives later heart failure. Smooth-muscle pathology is dominated by its retained proliferative capacity — uterine fibroids (leiomyomas) and the intimal proliferation of atherosclerosis are both expressions of that property.

The sliding-filament theory is the bridge between histology and physiology — it explains why a sarcomere looks the way it does on the EM and why the I- and H-bands narrow on contraction while the A-band stays constant. A strong answer first describes the two filament types, then the trigger that releases Ca²⁺, then the cross-bridge cycle that does the mechanical work, and finally the band-length changes that prove the theory is true.

The thick filament

Each thick filament is a bipolar bundle of about 300 myosin II molecules. The long rod-like tails interdigitate at the centre of the filament to form the bare zone (which sits at the M-line), and the globular heads project outwards in a helical array. Every head carries two functional sites: an ATPase and an actin-binding pocket. The lever-arm conformation of the head changes with the nucleotide state, and that change is what drives the power stroke.

The thin filament

The thin filament is a twisted double-helical polymer of F-actin, with two regulatory proteins draped along the groove. Tropomyosin is a long fibrous coiled-coil that physically covers the myosin-binding sites on actin at rest. Troponin is a three-subunit complex bound to tropomyosin at every seventh actin: TnT binds tropomyosin, TnI is the inhibitory subunit that locks tropomyosin over the binding site, and TnC binds the Ca²⁺ ion that throws the switch.

Trigger — excitation–contraction coupling

The motor neuron releases ACh onto the motor end-plate; the sarcolemma depolarises and the wave runs down every T-tubule. Where the T-tubule meets the SR at the triad (A–I junction), the voltage-sensing dihydropyridine receptor mechanically pulls open the ryanodine receptor (RyR1) in the SR terminal cisterna, and stored Ca²⁺ floods the sarcoplasm. Ca²⁺ binds TnC; troponin tilts; tropomyosin slides off the myosin-binding sites. The contractile apparatus is now armed.

Cross-bridge cycle

Each myosin head, already cocked in the high-energy ADP·Pi state, binds actin and releases Pi — this is the power stroke, swivelling the head and dragging the thin filament about 10 nm toward the M-line. ADP leaves, and a fresh ATP binds the head, which causes it to release actin. The head then hydrolyses ATP back to ADP·Pi, re-cocking itself for the next cycle. Each filament has hundreds of heads cycling asynchronously, so the macroscopic effect is smooth sliding rather than ratchety jerks. The cycle continues for as long as Ca²⁺ and ATP are available.

Visible consequences — band-length changes

Because the thin filaments slide inward over the thick filaments but the thick filaments do not change length, the A-band remains constant, the I-band and H-band narrow, and the Z-lines are drawn together. A maximally shortened sarcomere may have no resolvable H-band at all. This pattern of band-length change is the original evidence on which Huxley and Huxley founded the sliding-filament theory in 1954, and it is the histological answer the examiner is looking for.

Termination and clinical anchor

When the impulse stops, the SR SERCA pump sucks Ca²⁺ back into the cisternae against a roughly 10 000-fold gradient. Sarcoplasmic Ca²⁺ falls, TnC lets go, tropomyosin re-covers the binding sites, and the fibre relaxes. If ATP cannot be supplied — as after death — myosin heads cannot detach from actin and the body goes into rigor mortis. In malignant hyperthermia, a mutation in RyR1 makes the channel hypersensitive to volatile anaesthetics; uncontrolled SR Ca²⁺ release locks the muscle into sustained contraction, generating a runaway rise in body temperature unless dantrolene (a RyR1 blocker) is given.