Three Types of Muscle Exist: Skeletal, Cardiac, & Smooth
Muscle is a highly specialized tissue configured to convert the chemical energy of ATP potential into mechanical energy on a mass, macroscopic scale. Three types of muscle are found in vertebrates. Skeletal and cardiac muscle display a striated appearance, a consequence of the parallel alignment of the fibrils of their contractile apparatus. Smooth muscle is devoid of striations, a consequence of the (more) random orientation of its contractile fibrils. Mechanically, these differences in orientation mean that cardiac and skeletal muscle contract and exert force in only one dimension, like a coil spring, while contracting smooth muscle contracts exerts force and shortens in all directions, like the polymer skin of an inflated balloon. Skeletal muscle also is under conscious or voluntary nervous control, while both cardiac and smooth muscle work in an unconscious, involuntary manner.
The Sarcomere Is the Functional Unit of Muscle
Striated muscle is composed of multinucleated muscle fiber cells, which may extend the entire length of the muscle, sur rounded by an electrically excitable plasma membrane, the sarcolemma. Within each individual muscle fiber cell oriented longitudinally along its length are bundles of parallel myofibrils, consisting of interdigitated, overlapping thick and thin filaments, embedded in intracellular fluid termed sarcoplasm. When a myofibril is examined by electron microscopy, alternating dark and light bands can be observed (Figure 1).
Fig1. The structure of voluntary muscle. The sarcomere is the region between the Z lines. (Reproduced with permission from Bloom W, Fawcett DW: A Textbook of Histology, 11th ed. Philadelphia, PA: Saunders; 1986. Drawing by Sylvia Colard Keene.)
These bands are referred to as A and I bands, respectively. In the A band, the thin filaments (dark bands) are arranged around the thick filament (myosin) as a secondary hexagonal array. Each thin filament lies symmetrically between three thick filaments (Figure 2, center, mid cross section), and each thick filament is surrounded symmetrically by six thin filaments. The central region of the A band, a seemingly less dense region known as the H band, consists entirely of thick filaments. The I band (light bands) corresponds to a zone containing only thin filaments, and is bisected by the dense and narrow Z line, which consists of a complex network of polypeptides anchoring the thin filaments together (see Figure2).
Fig2. Arrangement of filaments in striated muscle. (A) Extended. The positions of the I, A, and H bands in the extended state are shown. The thin filaments partly overlap the ends of the thick filaments, and the thin filaments are shown anchored in the Z lines (often called Z disks). In the lower part of Figure 2A, “arrowheads,” pointing in opposite directions, are shown emanating from the myosin (thick) filaments. Four actin (thin) filaments are shown attached to two Z lines via α-actinin. The central region of the three myosin filaments, free of arrowheads, is called the M band (not labeled). Cross sections through the M bands, through an area where myosin and actin filaments overlap and through an area in which solely actin filaments are present, are shown. (B) Contracted. The actin filaments are seen to have slipped along the sides of the myosin fibers toward each other. The lengths of the thick filaments (indicated by the A bands) and the thin filaments (distance between Z lines and the adjacent edges of the H bands) have not changed. However, the lengths of the sarcomeres have been reduced (from 2300 to 1500 nm), and the lengths of the H and I bands are also reduced because of the overlap between the thick and thin filaments. These morphologic observations provided part of the basis for the sliding filament model of muscle contraction.
The sarcomere is defined as the region between two Z lines (see Figures 1 and 2) and is repeated along the axis of a fibril at distances of 1500 to 2300 nm depending on the state of contraction. Most muscle fiber cells are aligned so that their sarcomeres are in parallel register (see Figure 1).
Thin & Thick Filaments Slide Past Each Other During Contraction
The sliding filament model was largely based on careful morphologic observations on resting, extended, and contracting muscle. When muscle contracts, the H zones and the I bands shorten (see legend to Figure 2). Given that the thin and thick filaments remain intact, it was concluded that the inter digitated filaments slide past one another during contraction. As the tension developed during muscle contraction is proportionate to the degree to which the filaments overlapped, it was apparent that contraction involved a dynamic interaction between the filaments.
