The Striated Musculature
Microscopically both the skeletal and cardiac musculature have a “striated” appearance due to their densely packed myofibrils. Therefore they are referred to as striated muscle tissue. However they differ partly in their histology and physiology.
Structure of the skeletal muscle
Muscle fibers and connective tissue layers make up the skeletal muscle. A skeletal muscle fiber is around 20-100 µm thick and up to 20 cm long. Embryologically it develops by the chain-like fusion of myoblasts. About 200-250 muscle fibers are surrounded by endomysium forming the functional unit of the muscle, the primary bundle. Groups of primary bundles are wrapped by the perimysium building secondary bundles (“meat fibers”). The entire muscle is finally enclosed by epimysium and lies within its fascia, a tight connective tissue that separates the muscle from the surrounding structures.
Structure of the skeletal muscle fiber
Histologically noticeable are the spindle shaped nuclei located peripherally. The eosinophilic sarcoplasm (=cytoplasm) comprises almost completely of myofibrils. Along the myofibrils runs the longitudinal (L-) system (=sarcoplasmic reticulum). The sarcolemma (=cell membrane) is surrounded by a basal lamina and deeply invaginates into the sarcoplasm forming the transverse (T-) tubules. The bright bands are referred to as isotropic (I-) bands, the dark bands as anisotropic (A-) bands. In the center of the A-bands runs the M-line where the myosin filaments are anchored. The actin filaments are attached to the Z-line which is found in the middle of the I-bands. The area between two Z-lines makes up a functional unit, the sarcomere. The myosin filaments bind to the actin filaments through cross bridges. The contraction of the muscle results from the ATP-dependent rowing motion of the myosin heads causing a shift of the actin filaments.
Function and innervation
The skeletal musculature is part of the locomotor system and has the task of moving and stabilizing the skeleton. Therefore they are attached to the bones by collagen-rich tendons. But also other organs such as the tongue, mimic musculature and the diaphragm consist of skeletal musculature. The innervation is carried out by the somatic nervous system so that (almost) all skeletal muscles may be controlled voluntarily. A motor neuron and its associated muscle fibers make up a motor unit. Fine muscles (e.g. outer eye muscles) have small motor units and therefore can be controlled more precisely in comparison to gross muscles (e.g. back muscles).
Structure of the cardiac muscle and fiber
A cardiac muscle cell (cardiomyocyte) is about 10-20 µm thick and 50-100 µm long. The cytoplasm contains myofibrils and densely packed mitochondria. The fibrils do not run strictly parallel to each other but rather branch in a complex pattern. The cardiac muscle cell has one centrally located nucleus. The structure of the sarcomere resembles the one of the skeletal muscle cell. The T-tubules are larger and branched whereas the L-system is smaller. Characteristic are the intercalated discs which connect the cardiac cells moth mechanically and electrically.
Function and innervation
Cardiac muscle cells are specialized striated muscle cells found only in the heart. Their main task is the heart contraction. In addition they produce the atrial natriuretic peptide (ANP) in the atria. The ANP stimulates the diuresis and thus lowers the blood pressure. The innervation is carried out autonomously by special cardiac muscle cells mainly found subendocardially. The heart conduction system is made up of the sinoatrial (SA) node – known as the “pacemaker” – the atrioventricular (AV) node, the bundle of His, the left and right bundle branches (Tawara branches) and the Purkinje fibers.