Muscle Histology and Ultrastructure

General Characteristics

Muscle fiber = muscle cell: specialized for conductivity and contractility in the long axis

Myofibrils, myofilaments = acto-myosin macro molecules

Summary of Muscle Types

Muscle Type	Function	Structure	# Nuclei	Nuclei Regenerate?	Repair
Smooth	Involuntary	Non-striated	Uninucleated	Yes	Cell division
Cardiac	Involuntary	Striated	Uninucleated	Only progenitor cells	No
Skeletal	Voluntary	Striated	Multinucleated	No	Cell death and replacement by progenitor cells

Smooth Muscle

Distribution: Blood vessels, intestinal tract, tubular structures of containment, ie organs of involuntary movement, except the heart; skin spleen, eye, penis, broad ligament, visceral pleura

General appearance: Elongate tappering spindle fibers cigar shaped nuclei; length (18-200u); fusiform cells –staggered conformation

Grouping:

Fascicle bundle of fibers packed together – staggered

Isolated bundles – erector pili muscles

Aggregated bundles – irregular spongework (prostrate, penis)
Layers are apparent in intestine and duodenum; fibers are intertwined in stomach, bladder and uterus

Intercellular relation: Nexus or communicating junction – gap or tight junction allows electrical coupling

breaks continuity of basement membrane – reticular fibers produced by smooth muscle cell, collagen and elastin also possible

Vasculature: moderately supplied, capillaries end in connective tissue

Internal Cellular Features: Nucleus and peri-nuclear cytoplasmic cap (rER and free ribosomes)= basophilic material

Golgi complex and mito are also localized here

Sarcolemma contain cytoplasm, contacts nerve and other muscle cells, fibroblast CT matrix and fibers
Cytoplasim = extremely eosinophilic due to sparse ribosomes and abundance of fillaments

Structural Basis of Contraction: Thin filament surrounded by thick filaments arrangement

Troponin and tropomyosin present but contraction is myosin regulated

Dense bodies contain alpha-actin – comparable to Z-lines of skeletal muscle
caveolae= indentation of cell membrane. Along with filaments attached to membrane, likely involved in uptake and release of Ca⁺⁺ during contraction

Skeletal Muscle

General Appearance: large, straight unbranched,multinucleated, syncytial muscle fibers organized into fasicles (-CT: binds muscle, tranfers force to bone)

Intercellular relations: No gap junction communication – myo-neural junction is only communication

completely surrounded by basement membrane which merges with CT

Structural Plan of Skeletal Muscle as Organ – Fibers (cells) are grouped into fasicles which make up the muscle

fibers can attach directly to fasicle on one end, both or neither; limit of extension of a fiber is 5mm
# of fibers per muscle varries 260K in biceps; 1.5K stapedius

Epimysium = fibrous sheath around the whole muscle; Lymphatics

Perimysium = fibrous connective tissue around each fasicle (creats septal compartments of muscle). Lymphatics are here.

continuous with endomysium; septa carry large branches of the arteries
large, strong fibers merged with connective tissue

Endomysium = delicate connective tissue between muscle fibers

containing collagenous, elastic, and reticular fibers; there are capillaries that supply the fibers

nerve fibers are innervated at the motor end plate

Hierarchy of Organization

(1) fascicle

(2) fiber

(3) myofibril (sarcomere) – A-band, Z-line, I-band

(4) myofilaments – myosin, actin, tropomyosin, troponin

Internal features: RER, free ribosomes, mito, and golgi all near nuclei.

T-tubules = invagination of sarcolemma at distinct point (A-I junctions in humans) investing myofibril and mito. Communicates with Sarcoplasmic Reticulum at the terminal end (cisternae) – involveld in Ca⁺⁺ release/uptake

Structural basis of Contraction: Sarcomere=Z-line to Z-line; Thick filament = myosin, Thin filament = actin, tropomyosin, and troponin

Cardiac Muscle

General features

in myocardium and major blood vessles to heart
syncytium without fused cells
twice as many endomysial capilaries as skeletal muscle

General Appearance: Striated but larger than SkM and branched with one central nuclei per cell

Internal features: Same as SkM except Striation less pronounced and T-tubules enter at Z-line of each sarcomere

Intercellular Relations: Intercalated Discs include gap, tight junctions and desmosomes – propagate contraction implulse