What do you say a muscle is? This article compiled and put together by Ikpotokin Samuel tells us more especially in details what muscles are and are most probably made up of and most likely how they function.
Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.
Muscle tissues are derived from the mesodermal layer of embryonic germ cells in a process known as myogenesis. There are three types of muscle,
Skeletal or striated,
Cardiac, and
Smooth.
Muscle action can be classified as being either voluntary or involuntary. Cardiac and smooth muscles contract without conscious thought and are termed involuntary, whereas the skeletal muscles contract upon command.[citation needed] Skeletal muscles in turn can be divided into fast and slow twitch fibers.
Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads.[citation needed]
The term muscle is derived from the Latin musculus meaning "little mouse" perhaps because of the shape of certain muscles or because contracting muscles look like mice moving under the skin. (Alfred Carey Carpenter (2007). "Muscle". Anatomy Words. Retrieved October 3, 2014; Douglas Harper (2012). "Muscle". Online Etymology Dictionary. Retrieved October 3, 2012.)
| www.10tv.com |
Muscles are one of those things that most of us take completely for granted, but they are incredibly important for two key reasons:
Muscles are the "engine" that your body uses to propel itself. Although they work differently than a car engine or an electric motor, muscles do the same thing -- they turn energy into motion.
It would be impossible for you to do anything without your muscles. Absolutely everything that you conceive of with your brain is expressed as muscular motion. The only ways for you to express an idea are with the muscles of your larynx, mouth and tongue (spoken words), with the muscles of your fingers (written words or "talking with your hands") or with the skeletal muscles (body language, dancing, running, building or fighting, to name a few).
Because muscles are so crucial to any animal, they are incredibly sophisticated. They are efficient at turning fuel into motion, they are long-lasting, they are self-healing and they are able to grow stronger with practice. They do everything from allowing you to walk to keeping your blood flowing!
When most people think of "muscles," they think about the muscles that we can see. For example, most of us know about the biceps muscles in our arms. But there are three unique kinds of muscle in any mammal's body:
Skeletal muscle is the type of muscle that we can see and feel. When a body builder works out to increase muscle mass, skeletal muscle is what is being exercised. Skeletal muscles attach to the skeleton and come in pairs -- one muscle to move the bone in one direction and another to move it back the other way. These muscles usually contract voluntarily, meaning that you think about contracting them and your nervous system tells them to do so. They can do a short, single contraction (twitch) or a long, sustained contraction (tetanus).
Smooth muscle is found in your digestive system, blood vessels, bladder, airways and, in a female, the uterus. Smooth muscle has the ability to stretch and maintain tension for long periods of time. It contracts involuntarily, meaning that you do not have to think about contracting it because your nervous system controls it automatically. For example, your stomach and intestines do their muscular thing all day long, and, for the most part, you never know what's going on in there.
Cardiac muscle is found only in your heart, and its big features are endurance and consistency. It can stretch in a limited way, like smooth muscle, and contract with the force of a skeletal muscle. It is a twitch-muscle only and contracts involuntarily.
In this article, we will look at the different types of muscles in your body and the amazing technology that allows them to work so well. From here on, we will focus on skeletal muscle. The basic molecular processes are the same in all three types.
Animals use muscles to convert the chemical energy of ATP into mechanical work. Three different kinds of muscles are found in vertebrate animals.
Heart muscle — also called cardiac muscle — makes up the wall of the heart. Throughout our life, it contracts some 70 times per minute pumping about 5 liters of blood each minute.
Smooth muscle is found in the walls of all the hollow organs of the body (except the heart). Its contraction reduces the size of these structures. Thus it regulates the flow of blood in the arteries moves your breakfast along through your gastrointestinal tract expels urine from your urinary bladder sends babies out into the world from the uterus regulates the flow of air through the lungsThe contraction of smooth muscle is generally not under voluntary control. Skeletal muscle, as its name implies, is the muscle attached to the skeleton. It is also called striated muscle. The contraction of skeletal muscle is under voluntary control.
Anatomy of Skeletal Muscle
A single skeletal muscle, such as the triceps muscle, is attached at its
origin to a large area of bone; in this case, the humerus.
At its other end, the insertion, it tapers into a glistening white tendon which, in this case, is attached to the ulna, one of the bones of the lower arm.
As the triceps contracts, the insertion is pulled toward the origin and the arm is straightened or extended at the elbow. Thus the triceps is an extensor. Because skeletal muscle exerts force only when it contracts, a second muscle — a flexor — is needed to flex or bend the joint. The biceps muscle is the flexor of the lower arm. Together, the biceps and triceps make up an antagonistic pair of muscles. Similar pairs, working antagonistically across other joints, provide for almost all the movement of the skeleton.
The Muscle Fiber
Skeletal muscle is made up of thousands of cylindrical muscle fibers often running all the way from origin to insertion. The fibers are bound together by connective tissue through which run blood vessels and nerves.
Each muscle fibers contains:
an array of myofibrils that are stacked lengthwise and run the entire length of the fiber;
mitochondria;
an extensive smooth endoplasmic reticulum (SER);
many nuclei (thus each skeletal muscle fiber is a syncytium).
The multiple nuclei arise from the fact that each muscle fiber develops from the fusion of many cells (called myoblasts).
The number of fibers is probably fixed early in life. This is regulated by myostatin, a cytokine that is synthesized in muscle cells (and circulates as a hormone later in life). Myostatin suppresses skeletal muscle development. (Cytokines secreted by a cell type that inhibit proliferation of that same type of cell are called chalones.) Cattle and mice with inactivating mutations in their myostatin genes develop much larger muscles. Some athletes and other remarkably strong people have been found to carry one mutant myostatin gene. These discoveries have already led to the growth of an illicit market in drugs supposedly able to suppress myostatin.
In adults, increased muscle mass comes about through an increase in the thickness of the individual fibers and increase in the amount of connective tissue. In the mouse, at least, fibers increase in size by attracting more myoblasts to fuse with them. The fibers attract more myoblasts by releasing the cytokine interleukin 4 (IL-4). Anything that lowers the level of myostatin also leads to an increase in fiber size.
Because a muscle fiber is not a single cell, its parts are often given special names such as
sarcolemma for plasma membrane
sarcoplasmic reticulum for endoplasmic reticulum
sarcosomes for mitochondria
sarcoplasm for cytoplasm
although this tends to obscure the essential similarity in structure and function of these structures and those found in other cells.
The nuclei and mitochondria are located just beneath the plasma membrane. The endoplasmic reticulum extends between the myofibrils.
Seen from the side under the microscope, skeletal muscle fibers show a pattern of cross banding, which gives rise to the other name: striated muscle.
The striated appearance of the muscle fiber is created by a pattern of alternating
dark A bands and
light I bands.
The A bands are bisected by the H zone running through the center of which is the M line.
The I bands are bisected by the Z disk.
Each myofibril is made up of arrays of parallel filaments.
The thick filaments have a diameter of about 15 nm. They are composed of the protein myosin.
The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins:
troponin and
tropomyosin.
The anatomy of a sarcomere
The entire array of thick and thin filaments between the Z disks is called a sarcomere.
The thick filaments produce the dark A band.
The thin filaments extend in each direction from the Z disk. Where they do not overlap the thick filaments, they create the light I band.
The H zone is that portion of the A band where the thick and thin filaments do not overlap.
The M line runs through the exact center of the sarcomere. Molecules of the giant protein, titin, extend from the M line to the Z disk. One of its functions is to provide elasticity to the muscle. It also provides a scaffold for the assembly of a precise number of myosin molecules in the thick filament (294 in one case). It may also dictate the number of actin molecules in the thin filaments.
Shortening of the sarcomeres in a myofibril produces the shortening of the myofibril and, in turn, of the muscle fiber of which it is a part. [This electron micrograph of a single sarcomere was kindly provided by Dr. H. E. Huxley.]
Activation of Skeletal Muscle
The contraction of skeletal muscle is controlled by the nervous system. The Dying Lioness (an Assyrian relief dating from about 650 B.C. and supplied through the courtesy of The Trustees of the British Museum) shows this vividly. Injury to the spinal cord has paralyzed the otherwise undamaged hind legs.
In this respect, skeletal muscle differs from smooth and cardiac muscle. Both cardiac and smooth muscle can contract without being stimulated by the nervous system. Nerves of the autonomic branch of the nervous system lead to both smooth and cardiac muscle, but their effect is one of moderating the rate and/or strength of contraction.
The Neuromuscular Junction
Nerve impulses (action potentials) traveling down the motor neurons of the sensory-somatic branch of the nervous system cause the skeletal muscle fibers at which they terminate to contract. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. (The neuromuscular junction is also called the myoneural junction.)
The terminals of motor axons contain thousands of vesicles filled with acetylcholine (ACh).
Many of these can be seen in the electron micrograph on the left (courtesy of Prof. B. Katz).
When an action potential reaches the axon terminal, hundreds of these vesicles discharge their ACh onto a specialized area of postsynaptic membrane on the muscle fiber (the folded membrane running diagonally upward from the lower left). This area contains a cluster of transmembrane channels that are opened by ACh and let sodium ions (Na+) diffuse in.
The interior of a resting muscle fiber has a resting potential of about −95 mV. The influx of sodium ions reduces the charge, creating an end plate potential. If the end plate potential reaches the threshold voltage (approximately −50 mV), sodium ions flow in with a rush and an action potential is created in the fiber. The action potential sweeps down the length of the fiber just as it does in an axon.