Did you know that:
You have about 500 muscles in your body?
The muscles in the calf of your leg weigh in the neighborhood of one kilogram (more than 2 pounds)?
The tiny muscle that tightens your eardrum weighs about as much as a match head?
Muscle is heavier than water?
LET'S take a look at the kind of muscle that we can, for the most part, control voluntarily, called skeletal muscle, and leave other kinds, like those of the heart and the intestines, for another time.
All movement in the body results from contraction of muscle. Most of the skeletal muscle can be controlled voluntarily. When the brain decides on a specific movement, signals are passed from brain cell to brain cell until they arrive at a certain area called the motor area. Here an action-transmitting nerve cell (motor neuron) sends a wave of chemical influence down to the anterior part of the spinal cord where it connects with a spinal cord nerve cell and induces it to "fire." The signal then travels down the long transmitting fiber (called an axon) of the spinal cord nerve cell to a specialized receiving structure, termed the motor end-plate, which is attached to part of the muscle to be used. The muscle then contracts. Its action may be slow or fast, gentle or forcible; the variations are controlled by the brain's movement-control tower, the cerebellum (see figure 1).
Muscles usually act in groups. When you decide to walk, muscles all the way from the soles of your feet to your shoulders begin to act, all coordinated into a pattern. When you swing your foot for ward, muscles in the back of your leg and thigh contract. When you swing your foot back, those muscles relax and the opposite set contracts, all under control from the brain. Such alternation or rhythmic activity involves what is called reciprocal innervation, a sort of nerve-function teeter-totter.
Coordination of muscles and muscle groups is effortless and natural in most of us, but the baby learning to walk or the child learning to write must slowly initiate the proper muscle movements. Repetition makes the movements more and more familiar and automatic until whole complex patterns of movement are habitual.
Some muscles are directly controlled by reflex arcs. These fascinating units consist of a muscle, the nerve of feeling that conveys information from the muscle to the spinal cord, and the nerve of motion that orders action from the cord back to the muscle. The familiar knee-jerk is an example. When the ten don just below the kneecap, or patella, is struck, the muscle in the front of the thigh is stretched a bit. The automatic response is a signal to the spinal cord and an instant return message, which says "Contract," to the muscle. Although we cannot activate reflex arcs voluntarily, we may inhibit them. Indeed, when the control area of the brain is damaged, the reflex muscles' contraction is greater. Muscles are enclosed in a thin fibroustissue sheath called fascia. Along with the fascia go arteries, ending as capillaries to feed the muscle. While resting muscle uses only a moderate amount of blood, during strenuous muscular effort these arteries open up to furnish about ten times as much blood as during muscle rest.
A Closer Look
So far we have described only the naked-eye or gross appearance of muscle. The microscope reveals beautiful details far beyond the powers of the unaided eye. The muscle is found to be made of fibers, about 1/20-millimeter (five hundred fibers to an inch!) wide, and indefinitely long—often as long as the whole muscle. On the surface of each fiber are many nuclei, dark-staining cell headquarters, distributed along its length. The fiber is contained in a sheath of the most gossamer delicacy, about 1/1000 mm. thick, called sarcolemma.
The microscope also reveals dark stripes, or striations, crosswise of the fiber, spaced about two to three micrometers (2-3/1,000 mm.) apart. When one cuts across the muscle fibers, they appear as circles, or, if cut extremely carefully, as hexagons neatly fitted together. Each fiber is made up of many fibrils (little fibers) of about one to two micrometers diameter (see figures 2 and 3).
The ordinary microscope, magnifying the muscle 1,000 times, can reveal no further detail than we have so far described. But the electron microscope unveils a fascinating picture, delicate and intricate beyond imagination. The cross-striations are now seen to be sub divided into five separate kinds of lines, and the fibrils are seen to consist of even finer filaments of about 1/25- micrometer diameter (1/600,000 inch).
A system of tiny tubes (tubules) full of a gelatinous material is spread over the surface of each muscle fiber, with rootlets penetrating into the interior. This system of tubules helps to convey nerve impulses to the contracting parts of the muscle. Since the electron microscope can magnify up to 250,000 times, it makes all this detail very distinct.
Now, a little more about the cross striations. The ordinary microscope shows two—"A bands" that appear bright, and "I bands" dark, in polarized light (see figure 3). But with the electron micro scope, the A-band is divided by a central "H-band," which, in turn, is divided by a narrow "M-band." The I-band has a "Z-band" in its middle. The portion of a filament between two Z-bands (about 2 to 3 micrometers long) is called a sarcomere, or basic unit of muscle contraction (see figure 4).
We are not finished reciting complexities. We never are when describing God's handiwork.
The filaments contain two kinds of protein, myosin and actin (see figure 4). Myosin comes in tiny rods, thick in the middle and thin at the ends, 1 micro meter long and 100 times thinner in diameter. Between myosin rods are cross-connections to keep them in position. The actin is also in tiny threads (actually several smaller threads inter twined) about half as thick as the myosin rods. The actin threads are placed between the myosin rods and can slide lengthwise among them. This sliding action constitutes the shortening of muscle that is called contraction.
Even though we can see all this fine detail at magnifications of 150,000 or so, we still cannot distinguish the actual molecules, which are even smaller than anything described above, although proteins are often 50,000 times larger than water molecules.
How Muscles Contract
Now, let's assume that an order to contract has arrived at the receiving area of the muscle. In a millisecond or so it conveys the necessary electrical message to cause the actin threads to ratchet their way between the myosin rods and shorten the muscle. The time required to do this can be adapted to slow movements of rowing a boat or the lightning movements of a trained boxer.
Many muscles are always slightly contracted. Such constant tension in the muscle is called tonus. We are ordinarily unconscious of tonus, but if you lift your friend's relaxed arm and then lift an arm of a paralyzed person, the difference in muscle tone is obvious.
Muscles have position-sense. A person whose nervous system is healthy can walk downstairs in the dark and not miss a step. But when the nerves of position-sense are interrupted, the stair-walker does not know where his legs and feet are, and he falls.
Muscles first appear early in the development of a baby in the womb and grow with him. Their size is partly determined by sex hormones. Men usually have heavier muscles than women do. Exercise also contributes to muscle size, causing individual muscle fibers to en large. Infants have as many muscle fibers as he-man loggers, but each fiber is much smaller.
Lack of exercise makes muscles shrink. If muscle effort is lessened, the flow of blood is meager, muscle nutrition lags, and muscle size dwindles.
Our muscles are afflicted by a variety of diseases. We have just mentioned one— disuse shrinkage, or atrophy. If a nerve to a muscle is cut, or if the spinal cord or the motor area in the brain is damaged, certain muscles are paralyzed, and they shrink. Indeed, some victims of poliomyelitis (polio) suffer such shrinkage of the paralyzed muscles that the whole muscle disappears and is replaced by fat.
Muscles are attached to bones and other structures by very strong fibrous tissue. So strong is this tissue and so tenacious its attachment to the bone, that an unusually violent muscle con traction, as from a powerful electric shock, may break a large bone or crush vertebrae.
Some children are born with faulty muscle chemistry, leading to a variety of diseases called muscular dystrophies. The muscle fibers may be normal in size but be very weak. Another some what related disease is progressive muscular atrophy, resulting from slow dis appearance of the nerve cells in the spinal cord. Another and mysterious muscle disease is my astheniagravis, which means "severe weakening of muscle." People who have this disease may suffer such weakness that they cannot swallow or even keep their eyes open, and without help they cannot breathe.
Marvelously complex in design, intricate and harmonious in function, and instantly responsive to chemical and electrical signals detailing our need, muscle opens to the seeing eye more of the wisdom, power, and love of our Creator. We are, indeed, "fearfully and wonderfully made."
