Lab: Muscle Tissue Histology

Lab: Muscle Tissue Histology

Skeletal Muscle Histology

Individual skeletal muscle cells (fibres) are large, multinucleated, elongated cells. In longitudinal section, clear striation (A and I bands) is visible. In cross section, the fibre outlines are polygonal; peripherally placed nuclei are a distinguishing feature (compare with cardiac muscle: centrally placed nuclei). Each fibre is surrounded by endomysium (delicate connective tissue containing capillaries and fibroblasts). Fibres are grouped into fascicles by perimysium; the entire muscle is enclosed by epimysium.

The sarcomere hierarchy: muscle fibre (= cell) → myofibrils (parallel contractile units) → sarcomeres (from Z-line to Z-line) → myofilaments (thick myosin + thin actin). In cross section, not all myofibrils look identical because they are cut at different sarcomere intervals — some at the A-band, others at the I-band or H-zone.

Fibre type differentiation with NADH-TR stain: dark staining = slow oxidative fibres (Type I) — more oxidative enzymes, more mitochondria, more myoglobin. Light staining = fast glycolytic fibres (Type IIB) — fewer oxidative enzymes. A mosaic pattern of light and dark fibres reflects mixed fibre composition of most skeletal muscles.

Smooth Muscle Histology

Smooth (visceral) muscle lacks the organised sarcomere structure — it is non-striated. Contractile proteins (actin and myosin) are present but not regularly arranged into myofibrils. Cells are spindle-shaped with a single central nucleus. Found throughout visceral structures: GI tract (muscularis mucosae, circular, and longitudinal layers), urinary tract, respiratory tree, reproductive organs, blood vessel walls.

Muscle Spindles and Golgi Tendon Organs

Muscle spindles are encapsulated mechanoreceptors within skeletal muscle. They contain intrafusal muscle fibres (nuclear bag and chain fibres), which are distinct from the main extrafusal fibres. The spindle is supplied by sensory axons (Ia primary endings, II secondary endings) and gamma motor neurons (maintain spindle sensitivity). They respond to muscle stretch (length and velocity). Golgi tendon organs (GTOs) are encapsulated mechanoreceptors in tendons; they are sensitive to tension alone (not stretch) and send Ib afferent signals that produce autogenic inhibition.

Neuromuscular Junction Histology

Motor endplates (MEPs) are the specialised NMJ structures between the axon of a motor neuron and a skeletal muscle fibre. Axon terminal bouton (B): contains neurotransmitter vesicles and mitochondria; sits in a depression on the sarcolemma. Post-junctional folds: infoldings of the sarcolemma beneath the bouton that increase surface area and contain nAChR. Schwann cell processes ensheath the bouton; fibroblast processes (A) are also present. Immunohistochemistry (antibody with attached dye/fluorophore) is used to detect specific molecular structures like nAChR.