tendon Skeletal Muscle epimysium endomysium Muscle Fiber perimysium

tendon Skeletal Muscle epimysium endomysium Muscle Fiber perimysium sarcolemma fasicicle mitochondria myofibrils sarcoplasmic reticulum T- tubules muscle fiber endomysium

Z-line sarcomere actin Z-line I-band myosin A-band Z-line I-band

• Resting Membrane Potential • Action Potential • Propagation • Neuromuscular Junction/Synapse • Excitation-Contraction Coupling • Cross-Bridge Movements/Cycling 9 -3

• Na+ and K+ • Na+/K+ pumps • Neurotransmitter • Acetylcholinesterase • Receptors • Binding Sites • ATP ADP • Ca+2 9 -4

• Actin • Troponin • Tropomyosin • Myosin • Sarcolemma • Sarcoplasmic Reticulum • T tubules 9 -5

Resting Membrane Potential • When a neuron is not sending a signal, it is "at rest. " When a neuron is at rest, the inside of the neuron is negative relative to the outside. • Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). • At rest, potassium ions (K+) can leak through the membrane. Sodium can not. • There are negatively charged protein molecules (A-) inside the neuron that cannot cross the membrane. 9 -6

• In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. • When all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. • The resting membrane potential of a neuron is about -70 m. V (m. V=millivolt) - this means that the inside of the neuron is 70 m. V less than the outside. • At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. 9 -7

An action potential occurs when a neuron sends information down an axon, away from the cell body. • The action potential is an explosion of electrical activity that is created by a depolarizing current. ( cell is depolarized) • Some event (a stimulus) causes the resting potential to move toward 0 m. V. When the depolarization reaches about -55 m. V a neuron will fire an action potential. • This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. • When the threshold level is reached, an action potential of a fixed sized will always fire. . . for any given neuron, the size of the action potential is always the same. • The neuron either does not reach the threshold or a full action potential is fired - this is the "ALL OR NONE" principle. 9 -8

Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. 9 -9

This causes the action potential to go back toward -70 m. V (a repolarization). The action potential actually goes past -70 m. V (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 m. V. 9 -10

Action Potential Propagation 1 An action potential in a local area of the plasma membrane is indicated by the orange band. Note the reversal of charge across the membrane. + + – – + +– – – – 1 – – + +– – – – + + + + Stimulus 2 The action potential is a stimulus that causes another action potential to be produced in the adjacent plasma membrane. 3 The action potential propagates along the plasma membrane (orange arrow). Muscle fiber + + – – ++ + + – – + +–– – – 2 – – + +– – – + + – – +++ + + + + – – + +++ – – – + + – – – – + + ++ 3 9 -11

Neuromuscular Junction: • A type of synapse • Also called a myoneural junction • Site where an axon of motor neuron and skeletal muscle fiber interact • Skeletal muscle fibers contract only when stimulated by a motor neuron • Parts of a NMJ: • Motor neuron • Motor end plate • Synaptic cleft • Synaptic vesicles • Neurotransmitters 12

Neuromuscular Junction Structure Presynaptic terminal Axon branch Synaptic vesicles Neuromuscular junction Presynaptic terminal Sarcolemma Capillary Muscle fiber Sarcoplasmic reticulum (a) Myofibrils Axon branches Neuromuscular junctions Skeletal muscle fiber (b) Mitochondrion Postsynaptic Synaptic membrane cleft (sarcolemma) • Synapse: axon terminal resting in an invagination of the sarcolemma • Neuromuscular junction (NMJ): – Presynaptic terminal: axon terminal with synaptic vesicles – Synaptic cleft: space – Postsynaptic membrane or motor end-plate • sarcolemma 9 -13

Function of Neuromuscular Junction • Synaptic vesicles – Neurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane. • Acetylcholine – Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh 9 -14

1. ACh is released at the axon terminal. 2. ACh crosses the synaptic cleft. 3. ACh binds with a receptor on the post-synaptic membrane. 4. ACh. E (E) stops the action of Ach by breaking it down to acetic acid and choline 5. The choline is recycled to make more Ach, acetic acid diffuses 9 -15

1 An action potential (orange arrow) arrives at the presynaptic terminal and causes Ca 2+ channels in the presynaptic membrane to open. 2 Calcium ions enter the presynaptic terminal and initiate the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles. 3 ACh is released into the synaptic cleft by exocytosis. 4 ACh diffuses across the synaptic cleft and Ac tio n po te Function of Neuromuscular Junction nt ia l Voltage-gated Ca 2+ channel Synaptic vesicles binds to Na+ channels on the postsynaptic membrane. 5 Na+ channels open and Na+ enters the postsynaptic cell, causing the postsynaptic membrane to depolarize. If depolarization passes threshold, an action potential is generated along the postsynaptic membrane. 1 Ca 2+ Presynaptic terminal Postsynaptic membrane ACh Ca 2+ Acetic acid 9 Synaptic cleft Acetic acid 6 ACh unbinds from the Na+ Choline 8 2 channels, which then close. 7 The enzyme acetylcholinesterase, which is attached to the postsynaptic membrane, removes acetylcholine from the synaptic cleft by breaking it down into acetic acid and choline. 3 Na+ 8 Choline is symported with Na + into the presynaptic terminal, where it can be recycled to make ACh. Acetic acid diffuses away from the synaptic cleft. 9 ACh is reformed within the presynaptic terminal using acetic acid generated from metabolism and from choline recycled from the synaptic cleft. Ach is then taken up by synaptic vesicles. 7 ACh 4 Action potential Ligand-gated Na+ channel (open) Choline ACh receptor site 6 Action potential Acetylcholinesterase 5 Na+ Ligand-gated Na+ channel (closed) 9 -16

Excitation-Contraction Coupling A band • Mechanism where an action potential causes muscle fiber contraction • Involves I band Sarcoplasmic reticulum Sarcolemma Terminal cisterna Transverse tubule Triad (T tubule) Terminal cisterna Capillary Myofibril Mitochondrion – Sarcolemma – Transverse (T) tubules: invaginations of sarcolemma – Terminal cisternae – Sarcoplasmic reticulum: smooth ER – Ca 2+ – Troponin ( on actin ) 9 -17

https: //www. youtube. com/watch? v=Llgazi. PCFU 0

The SARCOLEMMA has a unique feature: it has holes in it. These "holes" lead into tubes called TRANSVERSE TUBULES (or TTUBULES for short). 9 -19

These tubules pass down into the muscle cell and go around the MYOFIBRILS. However, these tubules DO NOT open into the interior of the muscle cell; they pass completely through and open somewhere else on the sarcolemma The function of T-TUBULES is to conduct impulses from the plasma membrane of the cell (SARCOLEMMA) down into the cell and, specifically, to the SARCOPLASMIC RETICULUM. 9 -20

A muscle fiber is excited via a motor nerve that generates an action potential that spreads along the surface membrane (sarcolemma) and the transverse tubular system into the deeper parts of the muscle fiber. A receptor protein senses the membrane depolarization, alters its conformation, and activates the receptor (Ry. R) that releases Ca 2+ from the SR. Ca 2+ then binds to troponin and activates the contraction process. 9 -21

Action Potentials and Muscle Contraction 1 1 An action potential that was produced at the neuromuscular junction is propagated along the sarcolemma of the skeletal muscle. The depolarization also spreads along the membrane of the T tubules. 2 The depolarization of the T tubule causes gated Ca 2+ channels in the sarcoplasmic reticulum to open, resulting in an increase in the permeability of the sarcoplasmic reticulum to Ca 2+, especially in the terminal cisternae. Calcium ions then diffuse from the sarcoplasmic reticulum into the sarcoplasm. Action potential Ca 2+ Sarcolemma Sarcoplasmic reticulum Actin myofilament 2 T tubule Ca 2+ Sarcomere in myofibril Myosin myofilament 3 Ca 2+ 3 Calcium ions released from the sarcoplasmic reticulum bind to troponin molecules. The troponin molecules bound to G actin molecules are released, causing tropomyosin to move, and to expose the active sites on G actin. Tropomyosin Troponin 4 Active sites exposed Actin myofilament G actin molecule Myosin myofilament Active sites not exposed 4 Once active sites on G actin molecules are exposed, the heads of the myosin myofilaments bind to them to form cross-bridges. Ca 2+ bind to troponin. Ca 2+ Cross-bridge 9 -22

Cross-Bridge Cycle 1. Cross bridge formation: Phosphorylated (from ATP) myosin head attaches to an actin myofilament 2. The power stroke: 1) ADP and Pi are released from the myosin head 2) Myosin head changes to bend, low-energy state 3) Shape change pulls the actin towards the M line 3. Cross bridge detachment: ATP attaches to myosin, breaking the cross bridge 4. Cocking of the myosin head: attached ADP is hydrolyzed by myosin ATPase into ADP + Pi, bringing it back to a high-energy state 9 -23

https: //www. youtube. com/watch? v=WA 5 mwv k. BMy. A

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Actin and Myosin Myofilaments Sarcomere Cross-bridge M line Actin myofilament. Myosin myofilament Z disk Titin Z disk (a) Myosin molecule F actin molecules Tropomyosin Troponin Actin (thin) myofilament Active sites (b) Myosin (thick) myofilament Myosin Rod portion Troponin Tropomyosin Coiled portion of the two α helices Myosin light chains G actin molecules Active sites (c) Binds to G actin Binds to Two myosin heavy chains Binds tropomyosin to Ca 2+ Hinge region of myosin

Adenosine Triphosphate 9 -27

ATP is needed for to form the cross-bridge and then to break it. ATP prepares myosin for binding with actin by moving it to a higher-energy state and a "cocked" position. Once the myosin forms a cross-bridge with actin, the Pi disassociates and the myosin uses the energy to make the power stroke, reaching a lower energy state when the sarcomere shortens. ATP must bind to myosin to break the cross-bridge and enable the myosin to rebind to actin at the next muscle contraction. 9 -28

Calcium ions are required for each cycle of myosin-actin interaction. Calcium is released into the sarcomere when a muscle is stimulated to contract. This calcium binds to troponin, causing the tropomyosin to move and uncover the active binding sites on actin for the myosin head. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage. 9 -29

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Summary of Steps from Stimulation of the Nerve to the Production of a contraction 1. A stimulus is propagated down the alpha motor neuron. 2. Acetylcholine (Ach) is released from the endplate and crosses the synapse. 3. Ach causes Na+ and K+ channels to open up on the sarcolemma. 4. Na+ flows into the cell and K+ flow out of the cell, generating a muscle fiber action potential. 9 -33

5. Na+ spreads downward into the T-tubule system causing Ca++ to be released from the sarcoplasmic reticulum. 6. Ca++ binds with troponin, a change in configuration of actin that exposes the actin binding site. 9 -34

7. A cross-bridge is formed between actin and myosin. 8. The ATP in the myosin head is downgraded to ADP + Pi 9. Once the Pi is released the myosin head is tightly bound to actin. 10. The myosin arm does work on actin and a contraction is generated. 9 -35

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During contraction of a muscle, calcium ions bind to: A. the actin myofilament. B. the troponin molecule. C. the tropomyosin molecule. D. the sarcoplasmic reticulum. E. the sarcolemma. 9 -37

During contraction of a muscle, calcium ions bind to: A. the actin myofilament. B. the troponin molecule. C. the tropomyosin molecule. D. the sarcoplasmic reticulum. E. the sarcolemma. 9 -38

True or False: The sequence of cross bridge formation and myofilament movement will be repeated as long as calcium ions are present. 9 -39

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True or False: When cross bridges form and the muscle fibers contract, the actin myofilament slides past the myosin myofilament. 9 -41

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Muscle Relaxation • Ca 2+ moves back into sarcoplasmic reticulum by active transport. Requires energy • Ca 2+ moves away from troponintropomyosin complex • Complex re-establishes its position and blocks binding sites. 9 -43