I. Muscle is capable of transforming chemical energy into force resulting in contraction
II. Striated muscle– includes skeletal muscle and cardiac muscle
III. Smooth muscle – visceral muscle found in the wall of tubular organs
IV. Overall functions of skeletal muscle
-movement
-guards entrances/exits
-posture & support
-heat production
V. Skeletal Muscle features
A. Overall description –anchored to the skeleton via connective tissue tendons; comprised of muscle cells called muscle fibers, well vascularized and well innervated; skeletal muscle is voluntary
B. Cells are striated with light and dark bands; multinucleated cells, no mitosis in adult
C. Myofibrils –longitudinally running cylinders within myofiber and consisting of proteins called myofilaments
D. Sarcoplasmic Reticulum– the myofiber’s version of the endoplasmic reticulum which is quite extensive
E. Sarcomeres– the repeating unit of myofilaments that serves as the functional unit of skeletal muscle contraction. Sarcomeres consist of the filaments from one Z line to the next.
F. Thin Myofilaments –includes actin which is anchored to the Z line
G. Thick Myofilaments –Myosin which interdigitates with the thin filaments
H. Striations –result from repeating myosin and actin such that A band is the dark region containing thick & thin filaments and the I band is the light region containing thin filaments and the H zone is the region between two opposing thin filaments containing only thick filament
I. Actin
1. Comprised of two intertwined strands of a protein called F actin –each F actin is like two strands of a bead necklace twirled together
2. G actin is the subunit with an active site that binds to the head of myosin –G actin is like the individual beads on the necklace
J. Tropomyosin– also a thin filament that wraps around the F actin – like a ribbon wrapping the bead necklace and physically blocks the active sites of actin so that it cannot bind to myosin.
K. Troponin– also a thin filament that has one binding site for tropomyosin and the other for calcium – like the spots of glue holding the tropomyosin in place
L. Myosin– thick myofilament – shaped like golf clubs with two chains intertwined to form a tail and a double globular head projecting from it at an angle. Myosin has a binding site for actin and a binding site for ATP.
VI. Mechanism of Contraction, Sliding Filament Mechanism –filaments do not change length, they merely slide over each other based on the moving of the myosin head that has formed a cross bridge with actin
A. Cross bridge –forms between the myosin heads and the actin binding site once tropomyosin has moved out of the way
B. Power stroke –myosin heads also have a binding site for ATP and this provides the energy for the myosin heads to swivel towards the center of the sarcomere, pulling actin along with it.
C. ATP is needed again to disengage the actin and the myosin in order for relaxation to occur
VII. Regulation of Muscle Contraction
A. Tropomyosin –prevents interaction between actin and myosin
B. Troponin –keeps the tropomyosin “glued” in place
C. Calcium– binds to troponin and changes its shape resulting in the movement of the tropomyosin out of the way so that actin and myosin can bind to each other. When calcium is removed from the environment, tropomyosin again blocks contraction
1. Calcium is stored in the sarcoplasmic reticulum (especially its lateral sacs or terminal cisterns) –AP travels into the cell via extensions of the muscle cell membrane, termed T tubules, which extend to the SR resulting in increased permeability to calcium. Ca2+ then diffuses into the cytosol and leads to contraction
2. After contraction, the SR has decreased permeability to releasing Ca2+, and the calcium is pumped back into the SR via active transport and with lower calcium levels, relaxation takes place
VIII. Excitation – Contraction Coupling – term used for the sequence of events by which the AP on the sarcolemma (muscle cell membrane) leads to the power stroke happening with the cross bridges.
A. AP of the sarcolemma travels into the cells’ interior via the T-tubules which travel close to the sarcoplasmic reticulum
B. AP leads to a conformational change in receptors thus opening calcium- release channels of the SR membrane
IX. Muscle Energy Metabolism
A. ATP –the currency for energy for the muscle and numerous molecules are broken down for contraction
B. ATP releases energy as it is hydrolyzed to ADP and inorganic phosphate.
C. ATP is needed for power stroke and active transport of calcium and muscle relaxation
D. ATP stored in muscle –only provides a few seconds of energy
E. Phosphocreatine-provides a few more seconds of energy
F. Aerobic metabolism –requires oxygen, is very efficient but slow
G. Anaerobic metabolism –provides ATP fairly quickly but is inefficient; as exercise continues, lactic acid builds up
H. Muscle fatigue –lack of ATP
I. Repaying oxygen and glycogen debts after exercise
X. Muscle Fiber Types
A. Type I, slow oxidative, slow-twitch fibers
1. Abundant mitochondria
2. Abundant myoglobin –stores oxygen
3. Abundant blood capillaries –results in dark red color
4. Geared towards aerobic metabolism
5. Fatigue resistant
6. e.g. calf and postural back muscles
B. Type II (b) Fast Twitch, Fast Glycolytic Fibers
1. Quick & fatigable
2. Geared towards anaerobic metabolism
3. Poor in mitochondria and myoglobin
4. Poor in blood capillaries –resulting in white color
5. Eye movement muscles and biceps brachii are examples
C. Type II (a), Fast twitch oxidative fibers
1. Intermediate fibers
2. Combine fast twitch responses with aerobic fatigue-resistant metabolism
3. Relatively rare, except in some endurance-trained athletes