MUSCLE TISSUE Pg 1 There are three types of muscle skeletal cardiac and smooth All three types together account for 50 of your body s weight Which two are striated Which is voluntary We will focus primarily on skeletal muscle with characteristics of the others pointed out also Muscle has several functions 1 movement of the body skeletal of blood cardiac smooth skeletal of foods and wastes digestive tract smooth muscle of air diaphragm a skeletal muscle urine bladder ureters and urethra are all of smooth muscle 2 posture and balance skeletal muscle is wired with gyroscopes that sense too much deviation from upright posture and correct for it skeletal muscle also holds your joints nice and tight 3 control body openings and passages sphincters close your eye mouth anus and other internal and external passages 4 protection between your xiphoid process and pubic symphysis is a wide expanse covered only by muscle 5 communication talking gesturing conveying emotions simply by using your facial muscles 6 heat production 85 of the body s heat is produced by muscle contractions only 25 40 of a muscle s activity generates a contraction the rest is released as heat There are five important physiologic capabilities significant to skeletal muscle 1 responsiveness excitability to produce a contraction they must respond to neurotransmitters as well as to stretching 2 conductivity the electrical stimulation begun by a nerve impulse must be carried throughout the muscle 3 contractility muscles must be able to shorten 4 extensibility muscles must be able to stretch to become somewhat longer than their normal resting length 5 elasticity muscles must be able to return to their original length after being stretched or shortened ARCHITECTURE OF MUSCLE Skeletal muscle is built of small microscopic elements linked together to form the full muscle From smallest to largest they are 1 filaments there are two types each made of a different protein thin filament its core protein is actin wrapped with two other proteins troponin and tropomyosin thick filament its core protein is myosin a b c elastic filament it is made of a protein called titin its job is to return the muscle to length after contracting 2 3 fibril a fibril is simply the grouping of a lot of filaments fiber a fiber is a grouping of numerous fibrils this is also called a myofiber myo and sarco indicate muscle it is the basic muscle cell its cytoplasm is called sarcoplasm its cell membrane is the sarcolemma each myofiber is further wrapped in connective tissue called endomysium 4 fascicle a fascicle is a bundle of myofibers each fascicle is wrapped in connective tissue called perimysium 5 muscle finally the fascicles group together to form the muscle the muscle is wrapped in epimysium the muscle is further wrapped in fascia which separates the muscle from other surrounding structures The epimysium blends with the fascia to create the tendon which attaches the muscle to the bone pg 2 These connective tissue wrappings are called series elastic elements for this reason This will be further explained shortly but for now understand that the shortening of a muscle is caused by the thick filament grabbing and pulling on the thin filament creating what is called internal active tension That internal pull must be transferred to the bone and is done so by pulling on the S E E in sequence creating the tug that pulls a bone That external pull is called external passive tension and is never as strong as internal tension The difference between the two the loss of energy is what escapes as heat Some of the heat is generated by friction in the S E E but most comes from burning the fuel required to produce the energy needed Muscles are attached to bone in three different but similar manners 1 direct attachment often the epimysium and fascia blend directly with the periosteum of a bone as in the example of some facial muscles and the intercostal muscles indirect attachment this is when a muscle attaches to a bone by using a tendon cord like structure 2 3 aponeurosis at times a muscle s epimysium blends in to that of another muscle by a thin broad sheet identical to a tendon except for its shape flat vs cylindrical as in some facial or skull muscles or the pectoralis major attaching at one point to an abdominal muscle Let s now dig in to the guts of a skeletal muscle The microscopic filaments are arranged in a specific manner seen below Z disc thin filament actin H or Bare zone z o o l l l zone of overlap l runs the full length of the myosin filament thick filament myosin Z disc l I band l A band l I band l This architectural marvel is called a sarcomere There are millions of these in each muscle It is arranged as follows The thick and thin filaments must overlap at the zone of overlap The bare zone is the area between two thin filaments You ll see shortly why it and the zone of overlap are so important The A band extends from one end of a thick filament to the other end and appears dark under a microscope dArk The I band extends from the end of a thick filament to the end of the thick filament in the adjoining sarcomere and appears light under a microscope lIght It is these alternating dark and light bands that give skeletal muscle its striations A tiny additional filament made of titin runs through the myosin and attaches it to the Z disc Like a spring it helps the sarcomere recoil after a contraction It will greatly behoove you to be able to draw and label a sarcomere pg 3 Looking further at the filaments you see two more proteins on the thin filament They are there for this reason To make a muscle contract the myosin must grab onto the actin and pull it into the bare zone and is always ready and eager to do so But if it attached anytime it wanted to your muscle would always be in a state of contraction These other two proteins block the sites on the actin that the myosin attaches to preventing it from attaching until you decide to make it do so thus the voluntary nature of skeletal muscle These proteins are called tropomyosin which sits directly over the actin and troponin which perches like a bead on the tropomyosin and are referred to as regulatory proteins when in place they regulate when the actin and myosin filaments can work together to contract The actin and myosin are referred to as contractile proteins because they attach to one another to cause a contraction The thick filament is thick because it is made of numerous myosin protein molecules