Contraction of Muscle. Sliding Filament Mechanism During muscle contraction, myosin cross bridges pull on the thin filaments, causing them to slide inward

Contraction of MuscleContraction of Muscle

Sliding Filament MechanismSliding Filament Mechanism

During muscle contraction, myosin cross During muscle contraction, myosin cross bridges pull on the thin filaments, causing bridges pull on the thin filaments, causing them to slide inward toward the “H” zone. them to slide inward toward the “H” zone.

““Z” discs come towards each other and the Z” discs come towards each other and the sarcomere shortens, but the thick and thin sarcomere shortens, but the thick and thin filaments do not change in length. The sliding filaments do not change in length. The sliding of filaments and shortening of sarcomeres of filaments and shortening of sarcomeres causes the shortening of the whole muscle causes the shortening of the whole muscle fiber and ultimately the entire muscle. fiber and ultimately the entire muscle.


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


The muscle action potential releases calcium The muscle action potential releases calcium ions that combine with troponin, causing it to ions that combine with troponin, causing it to pull on tropomyosin to change its orientation, pull on tropomyosin to change its orientation, thus exposing myosin-binding sites on actin.thus exposing myosin-binding sites on actin.

The immediate, direct source of energy for The immediate, direct source of energy for muscle contraction is ATP. ATPase splits muscle contraction is ATP. ATPase splits ATP into ADP and +P and the released energy ATP into ADP and +P and the released energy activates (energizes) myosin cross bridges.activates (energizes) myosin cross bridges.


Activated cross bridges attach to actin and a Activated cross bridges attach to actin and a change in the orientation of the cross bridges change in the orientation of the cross bridges occurs. This is called a “power stroke.” occurs. This is called a “power stroke.”

This movement results in the sliding of thin This movement results in the sliding of thin

filaments.filaments.

Sliding Filament MechanismSliding Filament Mechanism Once the power stroke is complete, ATP again Once the power stroke is complete, ATP again

combines with the ATP-binding sites on the myosin combines with the ATP-binding sites on the myosin cross bridges; as ATP binds, the myosin head cross bridges; as ATP binds, the myosin head detaches from actin and the cycle may be reinitiated detaches from actin and the cycle may be reinitiated repeatedly.repeatedly.

Relaxation is brought about when Ach is broken Relaxation is brought about when Ach is broken down by the enzyme acetycholinesterase (Ache) and down by the enzyme acetycholinesterase (Ache) and CaCa+2+2 is moved from the sarcoplasm back into the is moved from the sarcoplasm back into the sarcoplasmic reticulumsarcoplasmic reticulum

Calcium removal is accomplished by active transport Calcium removal is accomplished by active transport pumps and a calcium-binding protein called pumps and a calcium-binding protein called calsequestrin. calsequestrin.

Muscle ToneMuscle Tone A sustained partial contraction of portions of a A sustained partial contraction of portions of a

relaxed skeletal muscle results in a firmness known as relaxed skeletal muscle results in a firmness known as “muscle tone.”“muscle tone.”

At any given moment, a few muscle fibers within a At any given moment, a few muscle fibers within a muscle are contracted while most are relaxed. This muscle are contracted while most are relaxed. This small amount of contraction is essential to maintain small amount of contraction is essential to maintain posture.posture.

Hypotonia: decreased or lost muscle tone; such Hypotonia: decreased or lost muscle tone; such muscles are said to be “flaccid.”muscles are said to be “flaccid.”

Hypertonia: increased muscle tone; such muscles are Hypertonia: increased muscle tone; such muscles are said to be either “spastic” or “rigid.”said to be either “spastic” or “rigid.”

Rigor MortisRigor Mortis Rigor mortis is a state of muscular rigidity following death. Rigor mortis is a state of muscular rigidity following death.

It results from a lack of ATP to split the myosin-actin cross 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 bridges. A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This of the body stiffen and become locked in place. This stiffening is called stiffening is called rigor mortisrigor mortis. .

Depending on temperature and other conditions, rigor mortis Depending on temperature and other conditions, rigor mortis lasts approximately 72 hours. The muscles are unable to lasts approximately 72 hours. The muscles are unable to relax, so the joints become fixed in place. The muscle cells relax, so the joints become fixed in place. The muscle cells become more permeable to calcium ions. Living muscle become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and promote the cross-bridge attachment between actin and myosin. The muscle fibers ratchet shorter and shorter until myosin. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as acetylcholine and they are fully contracted or as long as acetylcholine and ATP are present. ATP are present.

Rigor MortisRigor Mortis

However, muscles need ATP in order to However, muscles need ATP in order to release from a contracted state (it is used to release from a contracted state (it is used to pump the calcium out of the cells so the fibers pump the calcium out of the cells so the fibers can unlatch from each other). can unlatch from each other).

ATP reserves are quickly exhausted from the ATP reserves are quickly exhausted from the muscle contraction and other cellular muscle contraction and other cellular processes. This means that the actin and processes. This means that the actin and myosin fibers will remain linked until the myosin fibers will remain linked until the muscles themselves start to decompose. muscles themselves start to decompose.

Rigor MortisRigor Mortis

Rigor mortis can be used to help estimate time of death. Rigor mortis can be used to help estimate time of death. The onset of rigor mortis may range from 10 minutes to The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it (rapid cooling of a body can inhibit rigor mortis, but it reoccurs upon thawing). Maximum stiffness is reached reoccurs upon thawing). Maximum stiffness is reached around 12-24 hours post mortem.around 12-24 hours post mortem.

Facial muscles are affected first, with the rigor then Facial muscles are affected first, with the rigor then

spreading to other parts of the body. The joints are stiff spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax permanently. cause the muscles to relax permanently.

Muscle MetabolismMuscle Metabolism

On demand, skeletal muscle fibers can step up On demand, skeletal muscle fibers can step up ATP production.ATP production.

Creatine phosphate (phosphocreatine) and Creatine phosphate (phosphocreatine) and ATP constitute the phosphagen system. This ATP constitute the phosphagen system. This energy system can power maximal muscle energy system can power maximal muscle contraction for about 15 seconds and is used contraction for about 15 seconds and is used for maximal short bursts of energy, such as for maximal short bursts of energy, such as those used by athletes in the 100 m dash.those used by athletes in the 100 m dash.


The partial catabolism of glucose to generate The partial catabolism of glucose to generate ATP occurs in the glycogen-lactic acid system. ATP occurs in the glycogen-lactic acid system.

This is an anerobic system. This is an anerobic system. This system can provide energy for This system can provide energy for

approximately 30 to 40 seconds of maximal approximately 30 to 40 seconds of maximal muscle activity. This would be useful in the muscle activity. This would be useful in the 300 meter dash.300 meter dash.


Muscular activity requiring more than 30 seconds of Muscular activity requiring more than 30 seconds of maximal energy requires oxygen and thus utilizes the maximal energy requires oxygen and thus utilizes the aerobic system. aerobic system.

This system of ATP production involves the complete This system of ATP production involves the complete oxidation of glucose via cellular respiration or biological oxidation of glucose via cellular respiration or biological oxidation.oxidation.

Muscle tissue has two sources of oxygen: oxygen can Muscle tissue has two sources of oxygen: oxygen can diffuse into muscle fibers from the blood and oxygen that diffuse into muscle fibers from the blood and oxygen that is released by myoglobin from inside the muscle tissues. is released by myoglobin from inside the muscle tissues.

The aerobic system will provide enough ATP for The aerobic system will provide enough ATP for prolonged activity so long as sufficient oxygen and prolonged activity so long as sufficient oxygen and nutrients are available to the muscles. nutrients are available to the muscles.

Muscle FatigueMuscle Fatigue

Muscle fatigue: the inability of muscle to Muscle fatigue: the inability of muscle to maintain its strength of contraction or tension. maintain its strength of contraction or tension.

It occurs when a muscle cannot produce It occurs when a muscle cannot produce enough ATP to meet its needs. enough ATP to meet its needs.

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Contraction of Muscle. Sliding Filament Mechanism During muscle contraction, myosin cross bridges pull on the thin filaments, causing them to slide inward