Muscle tissue is responsible for most types of body movement.

I. Types of muscle:

A. Striated muscle - skeletal musculature, striated in appearance under microscope. Cells are unbranched and are multinucleate syncytia. These muscles are capable of voluntary, quick, forceful contractions.

B. Cardiac muscle - heart muscle, striated, cells may be branched. Cardiac muscle cells are either mono- or binucleate cells. They are connected with one and other by specialized junctional complexes called intercalated disks. Cardiac muscle is capable of involuntary, strong, rhythmic contractions

C. Smooth muscle - this muscle is not striated and is found in the walls of the visceral organs. Mononucleate cells. In addition to contraction, smooth muscle cells synthesize collagen, elastin, and proteoglycans (like fibroblasts). These muscle cells are capable of relatively slow contractions as compared to those of striated or cardiac muscle.

II. Basic units of muscle cell organization

A. sarcolemma - plasmalemma of muscle cells. External to this cell membrane is a well developed basement membrane.

B. sarcoplasm - cytoplasm of muscle cells excluding the myofibrils.

C. sarcoplasmic reticulum - smooth endoplasmic reticulum of muscle cells.

D. epimysium - thick layer of collagenous connective tissue that separates large bundles of muscle.

E. perimysium - collagenous connective tissue that separates smaller bundles of muscle cells called fascicles.

F. fascicle - bundle of muscle cells bounded by perimysium.

G. endomysium - thinner layer of connective tissue that separates individual muscle cells.

III. Skeletal muscle

A. Connective tissue in the form of epimysium, perimysium, and endomysium surrounds the components of striated muscle as described above.

B. Skeletal muscle is generally connected to bone via a piece of connective tissue called a tendon.

C. As mentioned above, individual muscle cells are syncytial (That is to say, each striated muscle cell contains multiple nuclei (multinucleate).

D. Each individual muscle cell is called a muscle fiber. Within the sarcoplasm of these cells are myofibrils composed of repeating sarcomere units (see below). These sarcomeres are the actual contractile apparatus.

E. The myofibrils are linear arrays of structures known as sarcomeres that are arranged in an end to end repeating pattern. The sarcomeres contain filaments of actin and myosin that interact to cause contraction of the muscle cells. See below for sarcomere structure.

F. If we look at the membranous surface of a skeletal, muscle cell we find multiple synapses from motor neurons in the central nervous system. These synapses are called myoneural junctions. Within the swollen ends of the axons that contact the muscle cell are small vesicles called synaptic vesicles that contain acetylcholine. When a nervous impulse (action potential) reaches the synapse, the contents of these vesicles are released into the gap between the axon and the muscle cell and are responsible for initiating an electric action potential in the sarcolemma that causes the muscle to contract.

IV. The Basic Sarcomere Unit of Striated Muscle


A. The basic unit of striated muscle, the sarcomere is diagramed above. It is composed of thin actin and thick myosin filaments (as well as a number of other molecules) arranged in an interdigitated linear array. These molecules form the sarcomere unit, that consists of,

A-band  (Anisotropic band) - area containing overlapping actin and myosin filaments accept in H-band region.

H-band - area of myosin filaments where there is no over lap of actin filaments.

I-band (Isotropic band) - thin actin filaments + Z-line.

Z-line - where actin filaments of adjacent sarcomeres are anchored.

M-line - formed by connections between the central portions of adjacent myosin filaments.

B. Let's consider at the muscle cell for a moment and consider how it contracts.

C. In skeletal muscle, contraction is under voluntary control. I decide to raise my arm for instance. Nervous stimuli in the form of electric action potentials are transmitted along motor nerves to the muscles concerned with raising the arm. These impulses cause the release of acetylcholine in synaptic vesicles at the myoneural junctions on the muscle cells. This causes an action potential in the sarcolemma that is transmitted along the surface of the muscle cell.

D. The skeletal muscle cell surface contains many tubular invaginations that extend through the cell sarcoplasm and around the individual myofibrils. These structures effectively carry the electrical impulse into the interior of the cell. In this way, the action potential reaches the sarcoplasmic reticulum of the cell much more quickly than it would in the electrical impulse had to pass from the surface of the cell into the cytoplasm to reach the sarcoplasmic reticulum.

E. The action potential causes the sarcoplasmic reticulum to release Ca+2 ions.

F. These bind to a site on the troponin molecules of the thin actin filaments. This causes the troponin molecules to change shape and expose a site on the actin that can bind the side chains of heavy meromysin on the thick myosin filaments. This binding catalyses a reaction that breaks down ATP that is bound to the myosin side chains, thus releasing energy that causes the heavy meromysin side chain to bend. As the numerous meromysin side chains bind to the actin filament and bend together in the same direction (the net bending after binding to actin is toward the M line), the actin and myosin filaments slide over each other. This movement lines up other actin and heavy meromyosin molecules which can bind and bend to move the filaments further. As the activity rapidly repeats itself, the overlap of filaments becomes complete and the I-bands and H-bands of the sarcomeres become thinner until the muscle relaxes.

G. This sort of action in thousands of repeating sarcomeres of each myofibril in the cell causes considerable decrease in the muscle cells length.

H. The ATP for all this activity comes from numerous mitochondria (called sarcosomes) that are associated with the myofibrils. During contraction, energy is initially derived from ATP and phosphocreatine from stores present in the sarcoplasm. As activity continues, the mitochondria metabolise glucose derived from glycogen molecules that are stored in the sarcoplasm in order to provide more ATP.

I. Oxygen is obviously necessary for mitochondrial metabolism of glucose. In order to provide enough oxygen, the muscle cells also contain myoglobulin which is an oxygen binding protein similar to hemoglobulin and has a high affinity for oxygen. This makes it possible for the muscle cells to have a ready store of oxygen on hand for their activity. It also allows them to pull oxygen out of the blood at a high rate so that the high metabolic rates can be maintained.

V. Cardiac muscle

A. Unlike skeletal muscle, cardiac muscle cells are not a syncytium for the most part, though some cells may have two nuclei.

B. The structure of cardiac muscle cells is similar to that of striated skeletal muscle in that myofibrils and sarcomeres are present with activity mediated by release of Ca+2 from sarcoplasmic reticulum.

C. The major differences between these two muscle types are that the cardiac muscle cells are branched and are held together by intercalated disks. They are also shorter in length than skeletal muscle cells.

1. Intercalated disks are specialized junctional complexes that bind cardiac muscle cells together. These so-called disks are interdigitating regions of the plasmalemma of adjacent cardiac muscle cells that hold the cells together. The intercalated disks form the irregular, jagged, dark lines that are characteristic seen in appropriately stained cardiac muscle sections.

2. This junctional complex is composed of a number of structures that are organized along adjacent muscle cell plasmalemmas in a repeating array,

a. desmosomes (macula adherens) - previously described structures that hold cells together. Located between adjacent myofilaments.

b. fascia (zonula) adherens - where the myofilaments of the sarcomeres at the ends of myofibrils adhere to the sarcolemma. Located where myofilaments end at the muscle cell plasmalemma.

c. gap junctions are present - connectiong between muscle cells that allow transfer of ions between them. This allows the cells to coordinate their activities. Action potentials can spread quickly between the sarcoplasmic reticulum of cardiac muscle cells via gap junctions. Thus, cardiac muscle cells can coordinate their movements. The gap junctions are located along the plasmalemmas of adjacent muscle cells in the regions between myofilaments.

D. Cardiac muscle cells also have a transverse tubule system present, but it is not as regular as the sarcolemma system of skeletal muscle and there are fewer T-tubules.

VI. Smooth muscle

A. This sort of muscle consists of long, overlapping, spindle shaped cells that have some characteristics that are similar to those of fibroblasts.

B. Smooth muscle cells similarity to fibroblasts is evident in that they are able to synthesize collagen, elastin, and proteoglycans

C. There are no sarcomere structures, but filaments of actin and a type of myosin are present. Thus, contraction is much less organized and occurs more slowly than it does in striated or cardiac muscle. Another reason for this slower contraction is that smooth muscle cells do not contain a transverse tubule system. (Also see G. below)

D. Since the actin and myosin filaments are not constrained by a sarcomere/myofilament arrangement, the actin and myosin filaments are able to achieve a greater degree of overlap when they contract resulting in a greater degree of contraction.

E. While smooth muscle cells are slow to contract, they have the ability to remain contracted for long periods of time.

F. The bundles of smooth muscle are organized as fascicles similar to what is seen in striated and cardiac muscle, Thus, a perimysium with endomysium between cells and epimysium deliniating bundles of fascicles can be identified. However, in sectioned tissue, this arrangement is often not very evident, presumably because the regions of connective tissue are much thinner than those of cardiac and skeletal muscle.

G. The contaction of smooth muscle cells is involuntary and the neuromuscular junctions controlling contractile rhythms may be on the surrounding epimysium rather than directly on muscle cells. As a result, neurotransmitters have to diffuse across this connective tissue layer and onto the plasmalemma of the smooth muscle cells in order to initiate the action potential that causes contraction. This is another reason for the slower contraction of smooth muscle cells.

H. Smooth muscles exhibit spontaneous contractile activity (doesn't require nervous stimulation). Thus, the innervation that is present acts to modify the contractile activity rather than initiate it.