Box 106

Functional Considerations: Comparison of the Three Muscle Types

Cardiac muscle shares structural and functional characteristics with skeletal muscle and smooth muscle. In both cardiac and skeletal muscle, the contractile elements-thick and thin filaments-are organized into sarcomeres surrounded by sER and mitochondria. Both cardiac and smooth muscle cells retain their individuality, although both are in functional communication with their neighbors through gap junctions. In addition, cardiac and smooth muscle cells have a spontaneous beat that is regulated but not initiated by autonomic or hormonal stimuli. Both have centrally located nuclei and perinuclear organelles. These common characteristics suggest that cardiac muscle may have evolved in the direction of skeletal muscle from the smooth muscle of primitive circulatory systems. A summary of major characteristics of all three muscle types is provided in Table 10.1.

table 10.1. Comparison of the Three Muscle Types

Skeletal

Cardiac

Smooth

Cardiac

Structural features

Muscle cell

Location

Connective tissue components

Fiber

Striation Nucleus

T tubules

Cell-to-cell junctions

Special features

Functions

Type of innervation Efferent innervation Type of contraction

Regulation of contraction

Growth and regeneration

Mitosis

Response to demand Regeneration

Large, elongate cell, 10-100 ¡j,m in diameter, up to 100 cm in length (sartorius m. Muscles of skeleton visceral striated (e.g., tongue, esophagus, diaphragm)

Epimysium, perimysium, endomysium

Single skeletal muscle cell Present

Many peripheral

Present at A-l junction (triad: with two terminal cisternae), two T tubules/sarcomere

None

Well-developed sER and T tubules

Voluntary Somatic

"All or none" (red and white fibers)

By binding of Cato TnC, causes tropomyosin movement and exposes myosin-binding sites on actin filaments

None

Hypertrophy

Limited (satellite cells and myogenic cells from bone marrow)

Short, narrow cell, 10-15 ju.m in diameter, 80-100 /¿m in length

Heart, superior and inferior vena cava, pulmonary veins

Endomysium (subendocardial and subpericardial connective tissue)

Linear, branched arrangement of several cardiac muscle cells Present

Single central, surrounded by juxtanuclear region

Z lines (diad: with small terminal cisternae), one T tubule/sarcomere

Intercalated disks containing

1. Fasciae adherentes

2. Macula adherens (desmosome)

3. Gap junctions

Intercalated disks

Involuntary Autonomic

"All or none" rhythmic (pacemakers, conductive system of the heart)

By binding of Ca21 to TnC, causes tropomyosin movement and exposes myosin-binding sites on actin filaments

None (in normal condition) Hypertrophy

None (in normal condition)

Short, elongate, fusiform cell, 0.2-2 /im in diameter, 20-200 /xm in length

Vessels, organs, and viscera

Endomysium, sheaths and bundles

Single smooth muscle cell

None

Single central

Replaced by invagination and vesicle similar to caveolae

Gap junction (nexus)

Dense bodies, caveolae, and cytoplasmic vesicles

Involuntary Autonomic

Slow, partial, rhythmic, spontaneous contractions (pacemakers of stomach)

By phosphorylation of myosin light chain by myosin light chain kinase in the presence of Ca21-calmodulin complex

Present

Hypertrophy and hyperplasia Present

Fibroblasts in healing wounds may develop morphologic and functional characteristics of smooth muscle cells (myofibroblasts; see page 141). Epithelial cells in numerous locations, particularly sweat glands, mammary glands, salivary glands, and the iris of the eye, may acquire the characteristics of smooth muscle cells (myoepithelial cells). Myoid cells of the testis have a contractile function in the seminiferous tubules, and cells of the perineurium, a concentric layer of connective tissue that surrounds groups of nerve fibers and partitions peripheral nerves into distinct fascicles, function as contractile cells as well as transport barrier cells.

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