3 Types of Muscle (review):

1. Skeletal Muscle — striated, voluntary, multinucleate (peripheral nuclei).
2. Cardiac Muscle — striated, involuntary, branched, single central nucleus, intercalated discs.
3. Smooth Muscle — non-striated, involuntary, spindle-shaped, single central nucleus.

Histological clues:
• Skeletal — long parallel cylindrical fibres + clear striations + nuclei at periphery.
• Cardiac — branched fibres + faint striations + intercalated discs (dark transverse lines) + central nucleus.
• Smooth — fusiform cells + no striations + central football-shaped nucleus.

MUSCLE TISSUE UNDER THE MICROSCOPE

Most muscle tissue is mesodermal in origin; the sphincter and dilator muscles of the iris are neuroectodermal exceptions and shares the property of contractility (via actin + myosin). Histology distinguishes the three types reliably.

I. SKELETAL MUSCLE:
• Cell = 'fibre' or 'myofibre' — long (up to 30 cm), cylindrical, multinucleate (nuclei at periphery, just under sarcolemma).
• Cross striations — alternating dark (A bands) and light (I bands), with Z-lines bisecting I bands; sarcomere = Z-to-Z.
• Sarcoplasm contains many myofibrils + sarcoplasmic reticulum (Ca²⁺ store) + T-tubules + glycogen + many mitochondria.
• Connective tissue coverings (3 levels): epimysium (whole muscle) > perimysium (fascicles) > endomysium (single fibre).
• Satellite cells — quiescent stem cells under basal lamina; activate to repair after injury.
• Fibre types:
– Type I (red, slow oxidative) — many mitochondria, lots of myoglobin, capillaries; fatigue-resistant. Postural muscles.
– Type IIa (intermediate).
– Type IIX (terminology and expression vary by species; adult horse muscle is described with IIX rather than a canonical IIB category) (white, fast glycolytic) — few mitochondria, big glycogen; fatigue quickly. Sprint muscles.

II. CARDIAC MUSCLE:
• Cells are short, BRANCHED, with usually ONE central nucleus.
• Striated like skeletal but with subtle differences.
• Intercalated discs — dark transverse lines where neighbouring cells meet end-to-end. Contain:
– Desmosomes + adherens junctions (mechanical anchorage).
– Gap junctions (electrical coupling — wave of depolarization).
• Auto-rhythmic: SA node 'pacemaker' cells initiate impulse → spread via gap junctions through the syncytium.
• Purkinje fibres — modified, larger conducting cardiomyocytes with few myofibrils, abundant glycogen — pale on H&E (looks 'empty').
• Regenerative capacity after myocardial infarction is extremely limited, so repair is predominantly by fibrous scar.

III. SMOOTH MUSCLE:
• Cells are spindle-shaped (fusiform), with single CENTRAL nucleus that becomes 'corkscrew' when contracted.
• NO striations visible (actin and myosin are not arranged in regular sarcomeres).
• Found in walls of viscera (gut, bladder, uterus, blood vessels, ducts), iris, ciliary body, hair follicle (arrector pili).
• Two functional types:
– Single-unit (visceral) — connected by many gap junctions; contracts as a sheet (gut, uterus, ureter).
– Multi-unit — each cell separately innervated; precise control (iris, large airways, ductus deferens).
• Slow, sustained tonic contraction; uses much less ATP per twitch.
• Hyperplasia (cell number ↑) + hypertrophy (cell size ↑) in pregnancy (uterus).

QUICK HISTOLOGY DIAGNOSIS RULES:
• Striations + peripheral nuclei = SKELETAL.
• Striations + branched + central nucleus + intercalated disc = CARDIAC.
• No striations + cigar-shaped nucleus = SMOOTH.