Monday, June 14, 2010

Muscle tissue

As we have previously mentioned, there are three types of muscle tissue: skeletal, cardiac and smooth. When we refer to the muscle system, we generally speak specifically of the skeletal system.

As just a quick comparison of the three different types of muscle tissue...

Smooth muscle is spindle shaped and non-striated.  It surrounds our tubes and helps things move.  Smooth muscle in involuntary.  The smooth muscles contract and release slowly.

Cardiac muscle is also involuntary.  It is branched and striated and is only located on the heart.  The network of fibers contract as a unit and are self-exciting.

Skeletal muscle is striated and tubular.  It is usually attached to the skeleton and is almost completely voluntary.  Contraction and releasing occurs very quickly. 

All muscle tissue has several functions.  They move something.  They help to stabilize part of the body.  Muscles store and move substances in the body.  Maintenance of homeostasis.  

Muscle tissue also has several properties.  It is excitable (responds to chemicals released from nerve cells), conductive (ability to propagate electrical signals over membrane), contractile (ability to shorten and generate force), extensive (ability to be stretched without damaging the tissue... to a degree) and elastic (ability to return to original shape after being stretched). 

The muscular system is the voluntarily controlled muscles of the body.  Most skeletal muscles also are controlled subconsciously to some extent, such as the diaphragm.  Each muscle is an organ composed of skeletal muscle tissue and connective tissue.  Skeletal muscles are also called fibers or myofibers.

The connective tissue of muscles includes facia, perimysium, endomysium, tendons and aponeuroses. There are two types of facia, superficial (this is loose connective tissue and fat underlying the skin) and deep (dense irregular connective tissue around the muscle). Epimysium is the outermost connective tissue that surrounds the whole muscle.  It separates 10-100 fibers into fascicles.  The perimysium surrounds fascicles.  The endomysium separates individual muscle cells.  At the end of the muscle belly, a tendon is formed and attaches to the bone.  Aponeuroses are the connective tissue sheets attaching to bone or adjacent muscles. 

Intramuscular injections are used to give relatively large doses of drugs that can't be administered to the bloodstream.  If administered directly to the bloodstream, it could be extremely dangerous or fatal. 

Neurons that stimulate muscles to contract are called efferent neurons.  Muscle action depends on a rich blood supply to deliver nutrients and oxygen. 

The anatomy of a skeletal muscle consists of the whole muscle which is of the organ level.  The fascicle which is a single bundle of cells.  The muscle fiber is a single muscle cell.  You are born with all of the muscle cells you will ever have, the just get bigger when you exercise.  Muscle growth is called hypertrophy. 

The anatomy of a muscle fiber is a bit more.  The muscle cells contain most of the normal cell components.  Sarcolemma, T tubules and sarcoplasm are additional/different organelles.  The sarcomeres are the units of myofibrils.  Myofibrils are the bundles of myofilametns that contract.  There are two types of myofilaments: actin and myosin; their function and structure is wholly for muscle contractions.  Sarcoplasm reticulum is a modified endoplasmic reticulum that surrounds myofibrils and stores calcium.  A contraction of muscle is when the microfilaments slide past one another causing the muscle to shorten. 

Muscle proteins include contractile proteins and structural proteins.  Myosin is a thick filament as well as a contractile protein.  It resembles golf clubs or oars sticking off a boat.  Myosin converts ATP to energy of motion.  Actin is another contractile protein.  But instead of being thick like myosin, it is rather thin.  It provides a site where a myosin head can attach.  Tropomyosin covers the binding site for myosin on the actin strands.  Tropinin moves tropomyosin off the binding sites so that myosin can bond to it.  Structural proteins include titin which stabilizes the position of myosin at the M line and dystrophitin which links thin filaments to the sacrolemma. 

Physiology of muscle contractions:
motor neurons
action potential in muscle
role of calcium
tropopmyosin uncovers binding sites
cross bridging
role of ATP

Muscle Metabolism:
A huge amount of ATP is needed to power the contraction cycle and pump calcium into the sacroplasm.The ATP inside muscle fibers will only power contractions for a few seconds.  ATP must be produced by the muscle fiber after reserves are used up.  There are three ways that muscle fibers can do so: creatine phosphate, anaerobically and aerobically. 

Creatine phosphate is made by excess ATP.  Creatine phosphate transfers its high energy phosphate to ADP regenerating new ATP, but this is only useful for 8-15 seconds. 

Anaerobic respiration uses glucose to make ATP when creatine phosphate is used up.  The process gets glucose from the blood and glycogen from muscle fibers.  It makes 2 pyruvic acids and two ATP.  Pyruvic acid is converted to acetic acid and carried away by the blood.  This process is only good for about 30-40 seconds. 

Aerobic respiration occurs when an activity lasts longer than 30 seconds.  Aerobic respiration provides 90% of the needed ATP in activities lasting more than 10 minutes.  Pyruvic acid enters the mitochondria and is completely oxidized generating ATP, water, CO2 and heat.  Each molecule of pyruvic acid generates 36 ATP.  Muscle has two sources of oxygen, hemoglobin (from the blood) and myoglobin (from the muscle cell).  Myoglobin and hemoglobin are oxygen-binding proteins. 

Myoglobin stores some oxygen and reduces the muscle's constant need for blood supply during muscle contractions.  Muscle contractions compress blood vessels.  After exercise, heavy breathing continues to get oxygen back into the system.  Oxygen debt occurs when additional oxygen is used to restore muscle cells to resting level in three ways: the liver cells convert lactic acid into glycogen, to synthesize creatine phosphate into ATP and to replace the oxygen removed from myoglobin.

Fatigue occurs from a lowered pH, thanks to lactic acid.  Following exercise, muscle cramps are mostl ikely due to a temporary deficit of ATP.   Athletes experience less muscle fatigue because they produce less lactic acid. 

there is a little more, but that will way until after the test tomorrow.

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