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A muscle is an organ which consists of many cells. Each individual muscle cell is called a muscle fiber.. The muscle fibers have multiple nuclei and contain between 1,000 to 2,000 threads of protien called myofibrils. Every myobril appears to be banded. The bands are Z shaped. The area between them is known as a sarcomere and it is the functional unit of any contraction. The sarcomere looks striated because of two proteins, myosin and actin. The thicker filaments of myosin have lateral extensions called cross-bridges. The actin filaments are attached to by the cross-bridges. Contractions begin when a nerve impulse reaches the muscle. The area that joins the muscle with a nerve branch is called a motor end plate. A number of closely arranged motor end plates form a motor unit. When a stimulus reaches the motor end plate the muscle fiber releases calcium into the muscle cytoplasm. The calcium breaks the chemical bond between the actin and the heads of the myosin cross-bridges. This triggers the breakdown of ATP in the cross-bridges, which releases energy. Using the ATP energy, the heads of the cross-bridges attach to new positions on the actin filaments. This causes the actin filaments to be toward the sarcomeres center. The heads then detach themselves and move back down the filament, attaching to new positions. This process is then repeated and because actin filaments are attached to the Z bands, the Z bands move closer to each other, which causes the whole sarcomere to contract. This synchronized shortening of sarcomeres along the entire muscle fiber causes the fiber and therefore the muscle to contract. This contraction process continues until calcium and ATP supplies are gone or until nerve stimulation stops. The relaxing of muscles requires energy too because the energy is needed for the heads of the cross-bridges to detach themselves and move back to thier original positions. When single muscle fibers are stimulated an all-or-none response occurs. Either the fiber contracts or it doesn't. The force of a contraction however, is controlled by the number of fibers that are stimulated. When more fibers are stimulated then the force of the contraction increases.
Muscles of Mastication
A group of four large muscles on each side moves the jaw for chewing and biting. The temporalis (in the temple) and the masseter (on the side of the jaw) can be felt when a person bites forcibly. The other muscles help provide the motions needed for grinding food between the molars.
Tongue, Swallowing, and Speech Muscles
The tongue, like the heart, is almost all muscle; unlike the routine and repetitive cardiac contractions, the tongue is capable of very precise, complicated, and elaborate movements. In eating, the tongue moves food around; in swallowing, it pushes food into the throat; and in talking, it articulates the sounds coming up from the larynx. Many bundles of fibers running in various directions produce all these complex movements. Once food enters the pharynx (the throat) it is moved down into the esophagus by waves of muscle contraction. Because of this humans can swallow water while standing on their heads. Special sets of muscles guard the larynx. Others alter the tension and position of the vocal cords, which are able to produce sound when the lungs force air between them.
Respiratory Muscles
Deep in the neck and between the ribs are respiratory muscles that lift and regulate the ribs during inspiration; they relax completely during expiration--a passive "collapse" of the bellows. Even more important to breathing is a dome-shaped horizontal partition--the diaphragm--below the lungs and above the abdominal cavity. Its contraction greatly enlarges the chest and draws air into the lungs with each inspiration.
Abdominal Wall Muscles
Layers of muscle surround the abdominal cavity like dynamic corsets; they enclose and can greatly compress the contents of the abdomen. This forceful compression is required to expel urine from the bladder, the contents of the colon and rectum during bowel movements, and the baby from a woman's uterus.
In each of these cases, smooth muscles in the walls of the hollow organs also take part in the expulsion of the contents. Further, each of the outlets is guarded by a ring of striated (voluntary) muscle that can be trained to prevent uncontrolled expulsions.
Back Muscles
The vertebral column has a great number of muscle bundles that help to position and move it. When caught off guard, these muscles can be strained, causing spasms and pain--a major cause of disability among working adults. Back problems currently rank second in North American industry as a cause of lost working days.
Shoulder and Hip Muscles
The scapula (shoulder blade) must be kept constantly in a proper position so that the upper limb and hand may be used. Large muscles radiate from it to the chest wall, spinal column, and skull. The shoulder joint is capable of many movements, because it has flexors, extensors, abductors (such as the huge deltoid muscle that covers the shoulder joint), adductors, and rotators.
The hip bones are much less mobile; the huge muscles at the hip are mostly concerned with moving and stabilizing the hip joint. This joint also requires a complete complement of muscles, the largest of which is the gluteus maximus ("biggest in the buttock"), which is important in climbing and running.
Muscles of the Arm, Forearm, Thigh, and Leg
The emphasis in these areas is on opposing groups of flexor and extensor muscles on the front and back of each limb. Almost all the muscles act on the joints just below them; some go on into the hand or foot to act as flexors and extensors of the digits.
Intrinsic Muscles of the Hand and Foot
In addition to the long muscles that come from above and provide force, the digits have many small local muscles that produce fine movements. The combined functions of the two groups of muscles provide both power grip and precision manipulation in the hand.
The human foot lost most of these functions when it became highly specialized for bipedal posture and walking, which are actually much more human traits than manipulation. Although apes can manipulate with their feet as well as with their hands, they have some difficulty standing upright, a task that the human foot performs superbly and with a great economy of muscle power.
All muscles have basically the same structure, from the tiny stapedius that moves and steadies the little stapes bone in the middle ear to the huge quadriceps femoris located in the front of the thigh. Each muscle has an attachment at both ends, called the origin and the insertion, and a fleshy contractile part, the muscle belly. Origins and insertions are usually made of noncontractile tendons; however, muscle arrangement and attachments vary with the mechanical needs at specific JOINTS. For example, many muscles in the limbs have bundles of muscle fibers arranged like the barbs of a feather and springing from the edges of a tendon, called a pennate architecture. This form greatly increases muscular force but reduces the distance through which the insertion may be pulled. Having many bundles of fibers in the muscle belly gives greater strength, while longer bundles give a greater range of motion.
Motor Units
The nerve control of the muscle fibers is organized in an economical fashion. Each motor nerve fiber running in a motor nerve to a muscle supplies a group of muscle fibers, which twitch each time an impulse is sent down the nerve fiber from the spinal cord. The frequency of these twitches can be increased and the number of motor units involved can be increased until all units are twitching rapidly (up to 50 times per second). The blending of all the twitches results in a smooth contraction of the entire muscle. If the twitches fail to blend, coming in bursts, an obvious tremor results. The disease tetanus occurs when the frequency of pulses is such that the fibers remain tense continuously.
Mechanics
A muscle produces movements of a joint and can only act on the joints it crosses; muscles also steady joints, preventing movements in the direction opposite to those intended. For instance, the triceps brachii, which makes up the muscle mass on the back of the arm, not only is a powerful extensor straightener of the elbow joint; it also contracts strongly to prevent the elbow from bending when pushing with the hand. In many muscles of the lower limbs it is gravity that must be counteracted; for example, the muscles are just as important to keep a person erect as they are in raising the heels off the ground.
The optimum angle of pull of a muscle is achieved when the muscle is acting more or less across a right angle of a joint; however, muscles are usually working across angles that are necessarily less advantageous. The inefficiency is only of a mechanical nature, though, because the extra heat energy generated helps warm-blooded animals maintain their temperature for optimum functioning of the body.
In mechanical terms, the skeleton provides leverage, the muscles are the motors that act on the levers, and the joints provide fulcrum. The body employs all the typical leverage forms known to engineers. Most levers in the human body are levers of the first class; these are exemplified by the familiar see-saw and the crow-bar. Levers of the third class are also common in the body; the ordinary hinged door is an example, the handle being the point at which the force is applied. In the body, this force is exerted by a tendon attached to a bone; almost always the tendon is attached near a hinge in order to provide proper mechanical advantage.
Actions
The brain needs to be specifically trained in order to contract individual muscles. Animals, including human beings, "will" movements of joints and limbs. Individuals do not consider the amount of contraction necessary in biceps brachii, brachialis, and brachioradialis muscles to develop the force and speed required to move an elbow. Voluntary movement takes learning, orifices wide; others narrow or close them. Human facial muscles consist of many wisps and bundles, all with Latin names that indicate their location or function; for example, levator anguli oris means the "elevator of the angle of the mouth." All the muscles of the face are supplied by branches of two main nerves, the right and left facial nerves, which arise from the brain stem. When one of those nerves is injured or inflamed, half of the face is paralyzed in a condition (usually temporary) known as Bell's palsy.
Skeletal muscles are subject to a variety of diseases, usually categorized as forms of paralysis. Dermatomyositis and polymyositis may cause inflammation of muscles. These diseases are caused by bacteria or viruses. They are commonly localized and often the result of an injury. Staphylococcus organisms are normally responsible. In addition to bacteria and viruses, however, the muscles may also be invaded by animal organisms, including protozoa, helminths, or worms. Among the best known such infestation is trichinosis, infection with the roundworm Trichinella spiralis.
Poliomyelitis and polyneuritis result in paralysis and muscular wasting. Bell's palsy affects the facial nerve, resulting in paralysis of all the muscles on one side of the face. Muscular dystrophies are hereditary diseases characterized by progressive muscular weakness and wasting. Myasthenia gravis is a condition in which transmission of nerve impulses is incomplete. Severe cases may cause paralysis.
