1. Contraction of Muscle

2. Sliding Filament Mechanism During muscle contraction, myosin cross bridges pull on the thin filaments, causing them to slide inward toward the “H” zone. During muscle contraction, myosin cross bridges pull on the thin filaments, causing them to slide inward toward the “H” zone. “Z” discs come towards each other and the sarcomere shortens, but the thick and thin filaments do not change in length. The sliding of filaments and shortening of sarcomeres causes the shortening of the whole muscle fiber and ultimately the entire muscle. “Z” discs come towards each other and the sarcomere shortens, but the thick and thin filaments do not change in length. The sliding of filaments and shortening of sarcomeres causes the shortening of the whole muscle fiber and ultimately the entire muscle.

3. Sliding Filament Mechanism When a nerve impulse (nerve action potential) reaches an axon terminal, the synaptic vesicles of the terminal release the chemical acetylcholine (Ach) which ultimately initiates a muscle action potential in the muscle fiber sarcolemma that then travels into transverse tubules and causes some of its stored Ca +2 to be released into the sarcoplasm. When a nerve impulse (nerve action potential) reaches an axon terminal, the synaptic vesicles of the terminal release the chemical acetylcholine (Ach) which ultimately initiates a muscle action potential in the muscle fiber sarcolemma that then travels into transverse tubules and causes some of its stored Ca +2 to be released into the sarcoplasm.

4. Sliding Filament Mechanism The muscle action potential releases calcium ions that combine with troponin, causing it to pull on tropomyosin to change its orientation, thus exposing myosin-binding sites on actin. The muscle action potential releases calcium ions that combine with troponin, causing it to pull on tropomyosin to change its orientation, thus exposing myosin-binding sites on actin. The immediate, direct source of energy for muscle contraction is ATP. ATPase splits ATP into ADP and +P and the released energy activates (energizes) myosin cross bridges. The immediate, direct source of energy for muscle contraction is ATP. ATPase splits ATP into ADP and +P and the released energy activates (energizes) myosin cross bridges.

5. Sliding Filament Mechanism Activated cross bridges attach to actin and a change in the orientation of the cross bridges occurs. This is called a “power stroke.” Activated cross bridges attach to actin and a change in the orientation of the cross bridges occurs. This is called a “power stroke.” This movement results in the sliding of thin filaments. This movement results in the sliding of thin filaments.

6. Sliding Filament Mechanism Once the power stroke is complete, ATP again combines with the ATP-binding sites on the myosin cross bridges; as ATP binds, the myosin head detaches from actin and the cycle may be reinitiated repeatedly. Once the power stroke is complete, ATP again combines with the ATP-binding sites on the myosin cross bridges; as ATP binds, the myosin head detaches from actin and the cycle may be reinitiated repeatedly. Relaxation is brought about when Ach is broken down by the enzyme acetycholinesterase (Ache) and Ca +2 is moved from the sarcoplasm back into the sarcoplasmic reticulum Relaxation is brought about when Ach is broken down by the enzyme acetycholinesterase (Ache) and Ca +2 is moved from the sarcoplasm back into the sarcoplasmic reticulum Calcium removal is accomplished by active transport pumps and a calcium-binding protein called calsequestrin. Calcium removal is accomplished by active transport pumps and a calcium-binding protein called calsequestrin.

7. Muscle Tone A sustained partial contraction of portions of a relaxed skeletal muscle results in a firmness known as “muscle tone.” A sustained partial contraction of portions of a relaxed skeletal muscle results in a firmness known as “muscle tone.” At any given moment, a few muscle fibers within a muscle are contracted while most are relaxed. This small amount of contraction is essential to maintain posture. At any given moment, a few muscle fibers within a muscle are contracted while most are relaxed. This small amount of contraction is essential to maintain posture. Hypotonia: decreased or lost muscle tone; such muscles are said to be “flaccid.” Hypotonia: decreased or lost muscle tone; such muscles are said to be “flaccid.” Hypertonia: increased muscle tone; such muscles are said to be either “spastic” or “rigid.” Hypertonia: increased muscle tone; such muscles are said to be either “spastic” or “rigid.”

8. Rigor Mortis Rigor mortis is a state of muscular rigidity following death. It results from a lack of ATP to split the myosin- actin cross bridges. A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. Rigor mortis is a state of muscular rigidity following death. It results from a lack of ATP to split the myosin- actin cross bridges. A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. Depending on temperature and other conditions, rigor mortis lasts approximately 72 hours. The muscles are unable to relax, so the joints become fixed in place. The muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as acetylcholine and ATP are present. Depending on temperature and other conditions, rigor mortis lasts approximately 72 hours. The muscles are unable to relax, so the joints become fixed in place. The muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as acetylcholine and ATP are present.

9. Rigor Mortis However, muscles need ATP in order to release from a contracted state (it is used to pump the calcium out of the cells so the fibers can unlatch from each other). However, muscles need ATP in order to release from a contracted state (it is used to pump the calcium out of the cells so the fibers can unlatch from each other). ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. This means that the actin and myosin fibers will remain linked until the muscles themselves start to decompose. ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. This means that the actin and myosin fibers will remain linked until the muscles themselves start to decompose.

10. Rigor Mortis Rigor mortis can be used to help estimate time of death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it reoccurs upon thawing). Maximum stiffness is reached around 12-24 hours post mortem. Rigor mortis can be used to help estimate time of death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it reoccurs upon thawing). Maximum stiffness is reached around 12-24 hours post mortem. Facial muscles are affected first, with the rigor then spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax permanently. Facial muscles are affected first, with the rigor then spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax permanently.

11. Muscle Metabolism On demand, skeletal muscle fibers can step up ATP production. On demand, skeletal muscle fibers can step up ATP production. Creatine phosphate (phosphocreatine) and ATP constitute the phosphagen system. This energy system can power maximal muscle contraction for about 15 seconds and is used for maximal short bursts of energy, such as those used by athletes in the 100 m dash. Creatine phosphate (phosphocreatine) and ATP constitute the phosphagen system. This energy system can power maximal muscle contraction for about 15 seconds and is used for maximal short bursts of energy, such as those used by athletes in the 100 m dash.

12. Muscle Metabolism The partial catabolism of glucose to generate ATP occurs in the glycogen-lactic acid system. The partial catabolism of glucose to generate ATP occurs in the glycogen-lactic acid system. This is an anerobic system. This is an anerobic system. This system can provide energy for approximately 30 to 40 seconds of maximal muscle activity. This would be useful in the 300 meter dash. This system can provide energy for approximately 30 to 40 seconds of maximal muscle activity. This would be useful in the 300 meter dash.

13. Muscle Metabolism Muscular activity requiring more than 30 seconds of maximal energy requires oxygen and thus utilizes the aerobic system. Muscular activity requiring more than 30 seconds of maximal energy requires oxygen and thus utilizes the aerobic system. This system of ATP production involves the complete oxidation of glucose via cellular respiration or biological oxidation. This system of ATP production involves the complete oxidation of glucose via cellular respiration or biological oxidation. Muscle tissue has two sources of oxygen: oxygen can diffuse into muscle fibers from the blood and oxygen that is released by myoglobin from inside the muscle tissues. Muscle tissue has two sources of oxygen: oxygen can diffuse into muscle fibers from the blood and oxygen that is released by myoglobin from inside the muscle tissues. The aerobic system will provide enough ATP for prolonged activity so long as sufficient oxygen and nutrients are available to the muscles. The aerobic system will provide enough ATP for prolonged activity so long as sufficient oxygen and nutrients are available to the muscles.

14. Muscle Fatigue Muscle fatigue: the inability of muscle to maintain its strength of contraction or tension. Muscle fatigue: the inability of muscle to maintain its strength of contraction or tension. It occurs when a muscle cannot produce enough ATP to meet its needs. It occurs when a muscle cannot produce enough ATP to meet its needs.