Wednesday, June 16, 2010

A little more about muscle tissue.

Botulinum Toxin is a bacterial toxin that blocks the release of acetyl choline from synaptic vesicles.  Botulinum toxin can be found in improperly canned foods.  A tiny amount can paralyze respiratory muscles and cause death.  Even though this can be deadly it is the primary ingredient in Botox.  It can help with strabismus, blepharospasm, spasms of the vocal cords that interfere with speech, cosmetic treatment and to alleviate chronic back pain due to muscle spasms. 

Curare is a plant poison that was used on arrows and blowgun darts.  It causes muscle paralysis by blocking acetyl choline receptors, thus inhibiting sodium ion channels.  Derivatives of curare are used during surgery to relax skeletal muscles. 
Anticholinesterase can slow the of Achase and removal of acetyl choline.  This chemical can strengthen weak muscle contractions.  It is treatment for myasthenia gravis, an antidote for curare poisoning and terminate the effects of curare after surgery. 

Muscle tension is controlled by the brain.  The axons of motor neurons are branched and innervate several muscle fibers within a muscle.  A motor unit is a single motor neuron and all the muscle fibers it innervates.  Recruitment occurs when contraction of more muscle fibers by stimulating more motor units in order to generate greater tension in a muscle.  2-3 muscle fibers per motor unit power our voice muscles.  10-20 muscle fibers per motor unit power eye movement.  2000-3000 muscle fibers move our arms and legs. 
A muscle twitch is a single contraction that lasts only a fraction of a second. 
Summation occurs when a muscle contraction is increased until maximal sustained contraction is reached. 
Tetanus is the maximal sustained contraction.
Tone is a continuous slight tension maintained by motor units that take turns contracting.  Muscle tone keeps skeletal muscles firm and keep the head from slumping forward on the chest. 

Skeletal Muscle fibers vary in their amount of myoglobin, mitochondria and capillaries.  Red muscle fibers have more myoglobin, capillaries and mitochondria than white muscle cells.  Contraction and relaxation speeds vary based on how fast myosin ATPase hydrolyzes ATP.  Resistance to fatigue depends on different metabolic reactions used to generate ATP. 
Slow oxidative muscle fibers are red in color and very resistant to fatigue, but they are the least powerful and smallest in diameter.  They make ATP aerobically. 
Fast oxidative muscle fibers are pinkish in color and they are glycolytic.  They have lots of mitochondria, myoglobin and blood vessels.  They split ATP very, very fast. 
Fast glycolytic muscle fibers are white in color.  It is most often used in anaerobic activity for a short duration.  They are the largest in diameter and make ATP via glycolysis. 

Most muscles contain a mixture of all fiber types, but remember that each motor unit only powers one type of cell.  Exercise can change the characteristics of skeletal muscles fibers to some degree, but not the number of cells. 

Cardiac muscles are striated, short, quadrangular-shaped, branching fibers.  They have a single centrally located nucleus.  Cells connected by intercarlated discs with gap junctions.  They have the same arrangement of thick and thin filaments as skeletal.  Cardiac muscles have more mitochondria compared to skeletal muscle fibers, contractions last 10-15 times longer due to prolonged delivery of calcium from the sarcoplasmic reticulum and the extracellular fluid.  They are involuntary and contract by themselves or from each other.  This continuous, rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.  Skeletal muscle depends on aerobic respiration to generate ATP.  

Anatomy of smooth muscles are usually involuntary.  The action potential spread through fibers by gap junctions.  Fibers are stimulated by neurotrasnmitters or hormones.  Myofilaments are randomly organized and have no sarcomeres.  They lack striations and transverse tubles.  Contractions of smooth muscles are initiated by calcium flow primarily from the interstitial fluid.  Calcium moves slowly out of the muscle for delaying muscle relaxation.  In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin. 

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