Muscle cells have these four characteristics: irritability, contractility,
extensibility, and elasticity. Irritability means that muscle cells can respond
to a stimulus. Contractibility means that muscle cells can make themselves
shorter. Extensibility means these cells can be made longer. Elasticity allows
muscle cells to contract and extend, and return unharmed to their original length.
There are three types of muscle: striated (striped), smooth and cardiac. Striated muscle makes up about 40% of the weight of the human body. It is the most common tissue composing our body. Striated muscle is composed of highly specialized cells known as muscle fibers. Each fiber is crossed by numerous alternately light and dark bands called microfibrils. Each fiber is surrounded by a thin layer of connective tissue called the sarcolemma.
The dark bands are called A bands and the light bands are called H bands. On either side of an A band is another light area known as the I band which is crossed by a very thin line of darker material known as the Z band. All that lies between two adjacent Z bands makes up one muscle fiber. A muscle fiber is called a sarcomere. The microfibrils that make up each sarcomere contain two kinds of even smaller myofilaments, thick and thin. The thick myofilaments are composed of the protein myosin. The thin myofilaments are composed of proteins called actin. Actin is composed of two proteins: tropomyosin and troponin. The striated (striped) appearance is the result of variations in actin and myosin content. A bands have both actin and myosin filaments. H bands contain only myosin filaments and I bands contain only actin filaments.
The actin filaments of one sarcomere attach to the actin filaments of another sarcomere at a structure known as the Z line. One sarcomere extends from one Z line to the next Z line.
Nerve fibers connect to muscle cells at a synapse called the neuromuscular junction. The neurotransmitter acetylcholine is released from the nerve fibers in response to an electrical impulse in that neuron. Acetylcholine diffuses across the synapse to the motor-end plate of the muscle fiber. This initiates an electrical impulse in the muscle fiber which spreads over that fiber causing it to contract. The acetylcholine is then destroyed by the enzyme cholinesterase. In the disease Myasthenia Gravis, acetylcholine is blocked at the neuromuscular junction by an accumulation of another chemical (possible lactic acid). This causes muscles to exhaust rapidly, resulting in muscular weakness. The eye muscles are frequently affected, causing drooping eyelids and double vision. There may also be difficulty smiling, chewing, speaking, swallowing, reaching, walking, or even breathing.
Curare, used by South American Indians as arrow poison, works by similarly blocking the action of acetylcholine at the neuromuscular junction. A chemical called neostigmine is used to treat both Myasthenia Gravis and curare poisoning. Neostigmine works by raising levels of acetylcholine.
Striated muscle tissue is 75% water, 20% protein and 5% minerals and various
organic substances. The most abundant minerals are potassium and sodium which
help cause muscle contraction. The most important organic compounds in muscles
are creatine phosphate, and the energy molecule adenosine triphosphate (ATP).
Muscle contraction requires energy from ATP. Creatine phosphate causes a more rapid production of ATP, and it assures a constant supply of ATP energy. The raw material for ATP energy is glucose, and glucose is stored in muscle tissue in the form of glycogen. Muscle cells require oxygen to make large amounts of ATP energy through aerobic respiration. If oxygen cannot be supplied to cells fast enough, muscle cells make ATP through anaerobic respiration. Anaerobic respiration produces smaller amounts of ATP energy than does aerobic respiration. Anaerobic respiration also causes lactic acid to accumulate in muscle cells. This lactic acid causes muscle cells to burn and hurt.
The number of striated muscle cells present in an infant is essentially the same as that found in the adult. Beyond infancy there is no reproduction of muscle cells. When muscle cells are used, they experience hypertrophy. Hypertrophy means to increase in size. If muscle cells are not used, they atrophy. Atrophy means to decrease in size. Any injury which causes us to not be able to use muscles will result in muscle atrophy.
If we exercise a muscle to the point where its cells are partially injured, when the muscle cells repair themselves they become larger. This is why people who lift weights gain muscle mass. However, exercise will never produce additional muscle cells.
Smooth muscle is also called involuntary muscle since its action, unlike that of striated muscle, is not consciously controlled. Smooth muscles are much shorter that striated muscle. The appearance of smooth muscle is also different from striated muscle in that they lack the cross-markings which make striated muscle stripped. Smooth muscle is found in the walls of the organs of the digestive tract, the walls of arteries and veins, and in the iris of the eye.
Cardiac muscle is found only in the heart. Cardiac muscle contracts rhythmically
to pump blood through the circulatory system. Unlike smooth muscles, cardiac
muscles show some striation, but are not as stripped as are striated muscles. The
individual cells of cardiac muscle cannot be clearly seen as they tend to
interlace with other cardiac muscle cells. Cardiac muscle is also distinguished
by the presence of disks which separate individual cells. Cardiac muscle has the
ability to contract without being stimulated by a nerve-born impulse. No other
type of muscle cell can do this. If the heart is removed from a freshly killed
animal, it will continue to beat rhythmically for a short period of time. If
striated or smooth muscle is removed from an animal they will not contract.