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| Muscular System |
Muscular System : Introduction
The muscular system is the body's network of tissues that controls movement both of the body and within it (such as the heart's pumping action and the movement of food through the gut). Movement is generated through the contraction and relaxation of specific muscles.
The muscles of the body are divided into two main classes: skeletal (voluntary) and smooth (involuntary),
Skeletal muscles are attached to the skeleton and move various parts of the body. They are called voluntary because a person controls their use, such as in the flexing of an arm or the raising of a foot.
There are about 650 skeletal muscles in the whole human body. Smooth muscles are found in the stomach and intestinal walls, vein and artery walls, and in various internal organs. They are called involuntary muscles because a person generally cannot consciously control them. They are regulated by the autonomic nervous system (part of the nervous system that affects internal organs).
There are some others muscles which plays very important role in our body Cardiac Muscle (The heart is made up of the cardiac muscle), Facial Muscles (There are more than 30 muscles in the face) and Tongue (another unique muscle is the tongue, which is free at one end and only attached on the other end).
Another difference between skeletal and smooth muscles is that skeletal muscles are made of tissue fibers that are striated or striped. These alternating bands of light and dark result from the pattern of the filaments (threads) within each muscle cell. Smooth muscle fibers are not striated.
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| Human Muscle |
The cardiac or heart muscle (also called myocardium) is a unique type of muscle that does not fit clearly into either of the two classes of muscle. Like skeletal muscles, cardiac muscles are striated. But like smooth muscles, they are involuntary, controlled by the autonomic nervous system.
The longest muscle in the human body is the sartorius (pronounced sar-TOR-ee-us). It runs from the waist down across the front of thigh to the knee. Its purpose is to flex the hip and knee. The largest muscle in the body is the gluteus maximus (pronounced GLUE-tee-us MAX-si-mus; buttocks muscles). It moves the thighbone away from the body and straightens out the hip joint.
Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcomere. The best proposed model for understanding contraction is the sliding filament model of muscle contraction. Actin and myosin fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped heads that project toward the actin filaments.
Control : Muscular System
Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle. Acetylcholine, (a neurotransmitter used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the microscopic junction, called a synapse.
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| Muscle Control |
A group of chemical messengers cross the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum.
An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, tetanus. Also, a loss of function at the junction can produce paralysis.
Skeletal muscles are organized into hundreds of motor units, each of which involves a motor neuron, attached by a series of thin finger-like structures called axon terminals. These attach to and control discrete bundles of muscle fibers. A coordinated and fine tuned response to a specific circumstance will involve controlling the precise number of motor units used.
While individual muscle units contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit. Motor unit coordination, balance, and control frequently come under the direction of the cerebellum of the brain. This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process.
- Skeletal muscles : Muscular System
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| Skeletal |
Skeletal muscles are probably the most familiar type of muscle. They are the muscles that ache after strenuous work or exercise. Skeletal muscles make up about 40 percent of the body's mass or weight. They stabilize joints, help maintain posture, and give the body its general shape. They also use a great deal of oxygen and nutrients from the blood supply.
Skeletal muscles are attached to bones by tough, fibrous connective tissue called tendons. Tendons are rich in the protein collagen, which is arranged in a wavy way so that it can stretch and provide additional length at the muscle-bone junction.
Skeletal muscles act in pairs. The flexing (contracting) of one muscle is balanced by a lengthening (relaxation) of its paired muscle or a group of muscles. These antagonistic (opposite) muscles can open and close joints such as the elbow or knee. An example of antagonistic muscles are the biceps (muscles in the front of the upper arm) and the triceps (muscles in the back of the upper arm). When the biceps muscle flexes, the forearm bends in at the elbow toward the biceps; at the same time, the triceps muscle lengthens. When the forearm is bent back out in a straight-arm position, the biceps lengthens and the triceps flexes.
Muscles that contract and cause a joint to close, such as the biceps, are called flexor muscles. Those that contract and cause a joint to open, such as the triceps, are called extensors. Skeletal muscles that support the skull, backbone, and rib cage are called axial skeletal muscles. Skeletal muscles of the limbs (arms and legs) are called distal skeletal muscles.
Skeletal muscle fibers are stimulated to contract by electrical impulses from the nervous system. Nerves extend outward from the spinal cord to connect to muscle cells. The area where a muscle and a nerve connect is called the myoneural juncture. When instructed to do so, the nerve releases a chemical called a neurotransmitter that crosses the microscopic space between the nerve and the muscle and causes the muscle to contract.
Skeletal muscle fibers are characterized as fast or slow based on their activity patterns. Fast (also called white) muscle fibers contract
rapidly, have poor blood supply, operate without oxygen, and tire quickly. Slow (also called red) muscle fibers contract more slowly, have better blood supplies, operate with oxygen, and do not tire as easily. Slow muscle fibers are used in movements that are ongoing, such as maintaining posture.
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| Human Body |
- Smooth muscles : Muscular System
Smooth muscles are controlled directly by the autonomic nervous system and are involuntary, meaning that they are incapable of being moved by conscious thought. Functions such as heart beat and lungs (which are capable of being willingly controlled, be it to a limited extent) are involuntary muscles but are not smooth muscles.
Smooth muscle fibers line most of the internal hollow organs of the body, such as the intestines, stomach, and uterus (womb). They help move substances through tubular areas such as blood vessels and the small intestines. Smooth muscles contract automatically, spontaneously, and often rhythmically. They are slower to contract than skeletal muscles, but they can remain contracted longer.
Like skeletal muscles, smooth muscles contract in response to neurotransmitters released by nerves. Unlike skeletal muscles, some smooth muscles contract after being stimulated by hormones (chemicals secreted by glands). An example is oxytocin, a hormone released by the pituitary gland. It stimulates the smooth muscles of the uterus to contract during childbirth.
Smooth muscles are not as dependent on oxygen as skeletal muscles are. Smooth muscles use carbohydrates to generate much of their energy.
- Cardiac muscle : Muscular System
The heart is made up of the cardiac muscle, which is also referred to as the myocardium. These muscles are thick and contract in order to pump out the blood and then relax in order to allow more blood in. The cardiac muscle is an involuntary muscle, or the type that works without your volition. Special type of cells in the cardiac muscle, called the pacemaker, help in controlling the heartbeat.
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| Cardiac |
The cardiac muscle or myocardium contracts (beats) more than 2.5 billion times in an average lifetime. Like skeletal muscles, myocardium is striated. However, myocardial muscle fibers are smaller and shorter than skeletal muscle fibers.
The contractions of the myocardium are stimulated by an impulse sent out from a small clump (node) of specialized tissue in the upper right area of the heart. The impulse spreads across the upper area of the heart, causing this region to contract. This impulse also reaches another node, located near the lower right area of the heart. After receiving the initial impulse, the second node fires off its own impulse, causing the lower region of the heart to contract slightly after the upper region.
- Facial Muscles : Muscular System
There are more than 30 muscles in the face. Not all of the facial muscles are attached to bones, as is the case in the other parts of the body.
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| Facial Muscles |
Many of the facial muscles are attached to the underside of facial skin. The contractions of these muscles are what give the face its various expressions, such as frowning, laughter, surprise, sadness and so on.
- Tongue : Muscular System
Tongue is the unique muscles of musclar system, which is free at one end and only attached on the other end. The tongue actually comprises of a group of muscles, which work in unison, enabling you to chew and swallow food, and talk.
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| Tongue |
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