Exam 1 - Lecture 05 Human Physiology:Skeletal Muscle Contraction

Describe the steps that occur from Action potential in the motor neuron to the final sliding filament action of actin and myosin

1. Arrival of action potential at terminal end of nerve fiber. 2. Opening of voltage-gated calcium channels on nerve fiber ending. Calcium enters into axon ICF which cause movement of Ach vesicles 3. Release of neurotransmitter (Ach) from synaptic vesicles into synaptic cleft 4.Once two Ach molecules are bound to ligand-gated sodium channels of in the sub neural cleft on the sarcolemma. This gate allows sodium in. 5. This generates an action potential (end plate potential) on sarcolemma. Which causes voltage gated sodium channels to depolarize and generate an action potential on the sarcolemma 6. This action potential travels to the T tubules where voltage-gated channels on T tubules (DHP ─ dihydropyridine ─ channels) interact with ryanodine receptors on SR membrane 7. Opening of ryanodine-sensitive calcium ion release channels 8. Increase in calcium ion concentration in cytosol. (.1 ECF to 10 ECF) 9. Activation of sliding filament mechanism

Describe the steps that occur from Action potential in the motor neuron to the final sliding filament action of actin and myosin(2)

10. Released calcium ions bind to troponin C. which causes a conformation change in troponin. 11. As the result of this tropomyosin is pulled away from myosin binding sites on actin. 12. M-ADP-Pi binds to actin. Pi releases. Then ADP. 13. Once ADP is released then the power stroke occurs. Stored energy in myosin head causes deformation such that thick and thin filaments slide past one another. 14. A second ATP binds to myosin and causes it to release actin. 15. ATP is hydrolyzed in myosin to create ADP and Pi. Then Process is repeated over and over. 16. Contraction stops when ATP-dependent calcium pump sequesters calcium ions back into SR. (SERCA)

Explain where ATP is required in the contraction of a sarcomere.

ATP is required to release the myosin head from actin and be hydrolyzed to get recocked to be ready to bind to myosin again

Describe the role of the T-tubules and sarcoplasmic reticulum in muscle contraction.

After the synaptic motor junction and end plate potential is created. Action potential travels down the sarcolemma into the T tubules, which has a DHP receptor on it which induces a change in ryanodine, which then releases calcium. It transmits the AP created on the sarcoma (from the motor neuron) to the

Explain the function of the DHP and ryanodine channels in muscle contraction and describe their location in relation to the sarcomere.

Dihydropyridine (DHP) receptors: • Voltage-sensitive L-type calcium channels arranged in quadruplets • Located on the sarcolemma T-tubules • Cause a conformational change in the ryanodine receptors • A minute amount of calcium flows into the cytosol via these channels. Ryanodine receptors (RyRs or Ca2+ - release channels): • Located on the cisternae of the sarcoplasmic reticulum • Open in response to conformational change in DHP receptors • Allow calcium into the cytosol from the SR

Compare characteristics of fast fibers and slow fibers.

Fast twitch fibers contract rapidly but have less endurance. • Characteristics include: • Fewer mitochondria • Primarily use anaerobic respiration resulting in a buildup of pyruvic and lactic acids • Little myoglobin • Larger concentration of ATPase (think chicken breast) Slow twitch fibers contract more slowly but have more endurance. • Characteristics include: • More mitochondria • Primarily use aerobic respiration • More myoglobin • Smaller concentration of ATPase (think duck Breast)

Describe and recognize examples of each of the three types of lever systems as they apply to skeletal muscles

First-class; fulcrum is in the middle: • Example = raising chin using sternocleidomastoids or similar muscles (fulcrum = atlas/axis complex) • In-force and out-force move in opposite directions. Second-class; Resistance (out-force) is in the middle: • Example: Raising the body on the ball of the foot. • Fulcrum = ball of foot. • Both in and out forces are on the same side of the fulcrum. Third-class; effort (in-force) is in the middle: • Example: Lifting a weight in the palm of your hand • Both in and out forces are on the same side of the fulcrum. • Both forces move in same direction.

Describe the components, including connective tissue, of the hierarchical organization of a skeletal muscle.

Hierarchical organization (Refer to Figure 6-1): • Epimysium: Connective tissue surrounding entire muscle • Muscle: Made up of multiple fascicles • Perimysium: Connective tissue surrounding individual fascicle • Fascicle: A bundle of myofibers • Endomysium: Delicate connective tissue around each myofiber • Sarcolemma (= plasmalemma): Cell membrane of muscle fiber • Myofiber (= muscle cell): Individual multinucleated muscle cell • Myofibril: A chain of sarcomeres within a myofiber • Myofilament: Actin and myosin filaments that make up a sarcomere

Compare isotonic and isometric contractions and recognize examples of each.

Isometric: • An isometric contraction occurs when there is an increase in tension but not in length. Isotonic: • Muscle length changes in an isotonic contraction. Eccentric: • An eccentric contraction occurs when the muscle lengthens. Concentric: • A concentric contraction occurs when the muscle shortens.

Isotonic versus Isometric

Isotonic is when the muscle generates more force than the force opposing it. Isometric is when the force doesn't change length of muscle.

List the sources for rephosphorylation during muscle contraction and the relative significance of each of these sources.

Phosphocreatine: • Releases energy rapidly • Reconstitutes ATP • ATP + phosphocreatine provides enough energy for 5-8 seconds of contraction. Glycolysis: • Lactic acid build-up • Can sustain contraction for 1 minute. Oxidative metabolism: • Provides more than 95% of all energy needed for long-term contraction.

Describe the function of SERCA and calsequestrin in controlling calcium concentration.

SERCA* uses ATP to pump calcium back into the SR • *(Sarcoplasmic Reticulum Calcium ATPase) Calsequestrin in the SR maintains an optimum calcium concentration gradient to facilitate return of calcium to SR.

Recognize the sarcomere as the functional component of the myofibril and describe the structural and molecular components of the sarcomere.

Sarcolemma = Plasmalemma • T-tubules • Invaginations of sarcolemma • Lie close to cisternae of sarcoplasmic reticulum • Form triads with cisternae • Two per sarcomere • Sarcoplasmic reticulum • Sarcomere

Define summation and tetany and explain the mechanism behind each.

Summation (Refer to Figure 6-14): • Electrical events occur faster than mechanical events: • An additional spike can occur before the previous calcium ions have been returned to the SR. • This increases the total amount of calcium ion in the cytosol and increases the rate of cycling between the myosin and actin cross-bridges. • This increases muscle tension. • Each additional spike adds to the effects of the previous spikes. Tetany (Refer to Figure 6-14): • If the frequency of spikes is fast enough, there is no time for relaxation between spikes. • The muscle remains at maximal contraction.

Describe the role of calcium in muscle contraction.

binds to troponin C to move tropomyosin which allows the mechanical part of this whole electromechanical coupling to occur.

Describe the muscle length-tension relation relative to changes in sarcomere length and explain on the basis of actin and myosin relationship.

when sarcomere length is between 2.2 and 1.65 then we have maximum tension

Differentiate between active and passive tension.

• Passive: produced by the preload • Active: produced by cross-bridge cycling • Total: sum of active and passive tension