1. Striated (skeletal and cardiac) - Two zones of cross bands are clearly seen in longitudinal H&E preparations, a dark zone and a light zone. When viewed through crossed polaroid filters, the dark band appears brighter than the light band, indicating a higher degree of organization in the dark or "A" band (anisotropic with respect to polarized light). The other band ("I" band) is said to be isotopic. A dark "Z" line can be discerned in the middle of the "I" band, and the unit structure taken from one Z line to the next is called the sarcomere (sarcomere = ½ I + A + ½ I). This banding pattern is produced by a highly organized array of microfilaments.

2. Non-striated (smooth muscle) - These cells also contain microfilaments capable of producing contraction, but they are not organized to the extent that repeating patterns occur in these cells.

1. Voluntary - (skeletal) is controlled by alpha motor neurons. Conscious action is voluntary - Tone is automatic and is controlled by extra-pyramidal fibers acting on motor neurons.

2. Involuntary - (cardiac, smooth) although still classified as involuntary, biofeedback experiments have demonstrated that contraction or activity of these muscles can be consciously modulated through the autonomic nervous system that reaches these muscles.

1. Epimysium - Variably dense irregular connective tissue surrounds the entire muscle.

2. Perimysium - loose connective tissue that contains larger blood vessels and nerves, and that surrounds fascicles (bundles of myofibers), permitting the fascicles to move relative to one another. There may be several orders of fascicles (fascicles within fascicles). Usually fascicles are easily seen because they shrink away from each other, but this is not always true.

3. Endomysium - fine reticular fibers with capillaries surrounding individual myofibers binding them together in fascicles. The reticular fibers anchor in to the external lamina, an amorphous layer (like basal lamina) surrounding the muscle fiber and that contains fibronectin and type V collagen. Nerve terminals also traverse this sheath. The term sarcolemma is reserved for the muscle cell plasma membrane only.

4. Muscle soreness after unusual exercise is due to injury of endomysium and perimysium. Inflammatory cells move into those regions of hyperemia and edema. (Experentia 37:1311, 1981.) These cells probably release growth factors that stimulate repair and growth.

1. Nuclei - each cylinder (myofiber) is a multinucleate syncytium with the flattened oval nuclei positioned just under the sarcolemma.

2. Sarcoplasm. Contains the following features

Skeletal muscle has a limited capacity for repair; injuries being largely repaired by the formation of scar. But some new muscle cells can form either from non-differentiated satellite cells or after de-differentiation of fragments of the injured muscle cells.

Hypertrophy is a well known consequence of regular exercise. The type of exercise greatly influences the type of changes that will occur. Weight lifting causes an increase in myofibrils, but endurance exercise increases capillarity, myoglobin, and mitochondria. Staron et al. J. Histochem. Cytochem. 32:146-152, 1984.

For each skeletal muscle fiber, there is one neuromuscular junction that is highly specialized and involves both the axon terminal and the sarcolemma. Bulbous expansions of the axon terminal filled with synaptic vesicles and a few mitochondria are associated with a specialized depression of the sarcolemma. This post-synaptic membrane is thrown into numerous junctional folds, increasing the surface area where acetylcholine receptors and acetylcholine esterase are located. Capping over the exposed surface of the axon is a thin sheet of Schwann cell cytoplasm.

A motor unit consists of one alpha motor neuron, plus all of the skeletal muscle fibers it supplies. The number of muscle fibers in a unit of a postural muscle may be several hundred, but less than 50 in an extraocular muscle. Several different motor units contribute to each fascicle. Look for motor end plates on a special slide that is teased muscle fibers stained with gold chloride.

Besides pain nerve endings there are two kinds of propiroceptive nerve endings, muscle spindles and Golgi tendon organs. The muscle spindle consists of special small (intrafusal) skeletal muscle fibers enclosed in a spindle-shaped, connective tissue sheath and innervated by small gamma motor nerve fibers and sensory nerve endings. The spindle is located in the perimysium and is parallel with the extrafusal fibers so that it functions as a stretch receptor for example as in the knee jerk reflex. Golgi tendon organs respond to potentially excessive stretch to inhibit muscle contraction.

The connective tissue in cardiac muscle corresponding to the endomysium of skeletal muscle is a reticular network with some collagen fibers, characterized by an exceptionally high concentration of capillaries.

1. Nuclei - usually described as mononucleated with the cylindrical nucleus centrally placed but it is often binucleate. Nuclear length is about 4 times the diameter.

2. Sarcoplasm contains:

Impulse conduction occurs from cell to cell through the intercalated disk, a specialized junction between the ends of the cells. It traverses the fiber in a step or dovetail fashion through the region of Z lines and provides both a physical and an electrical junction. Physical attachment is provided by desmosomes in the transverse portions of the disk where thin filaments attach to a fascia adherens as in Z lines. Gap junctions located in the longitudinal portions of the intercalated disk afford electrical continuity. Cardiac muscle cells have an intrinsic rhythmicity of contraction that is usually slower than that of the nodal cells.

Smooth muscle frequently occurs in bundles separated by c.t. septae. In arteries, especially elastic arteries, layers of smooth muscle are separated by elastic lamina. The elastin in these sites is probably produced by the smooth muscle cells.

Microscopically, smooth muscle consists of elongate tapered mononucleate cells 4 to 10 micrometers max diameter. The elongate nucleus (nuclear length may be 10 times the diameter) is positioned eccentrically in the widest portion of the cell. When seen in longitudinal section, the nucleus provides a useful index of contraction since it becomes pleated or folded when contraction occurs.

Myofilaments make up the myofibrils (not cross striated) that predominate in the cytoplasm of smooth muscle. Subsarcolemmal caveolae serve as calcium reservoirs needed to induce contraction. There are no T-tubules. Both actin and myosin are present and contraction is induced by an elevation of calcium ions which, when bound to calmodulin, activates myosin light-chain kinase. This phosphorylates myosin allowing it to bind to actin. The myofibrils are bound to dense bodies, corresponding to Z lines, that link the myofibrils to the plasma membrane at one end and intermediate filaments at the other. Other cytoplasmic organelles are few. The few mitochondria tend to be clustered near the nucleus. Glycolysis is undoubtedly the major source of energy for smooth muscle.

Two types of smooth muscle innervation have been described. The most common is the visceral (unitary) type in which a few nerve fibers end on or near a few muscle cells. The impulse to contract is then passed from cell to cell via gap junctions (nexus) that are low impedance connections between cells. Much less common is the direct (multiunit) innervation of each individual muscle fiber. Such an arrangement exists in the muscle coat of the vas deferens. Smooth muscle is also influenced by circulating hormones - epinephrine, norepinephrine and prostaglandins. Special receptors for these are located on the cytoplasmic membrane but their morphological distribution is unknown.