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 Stimulus generated capabilities: ◦ Irritability (also called responsiveness)—ability to receive and respond to a stimulus ◦ Contractility—ability to shorten when an adequate stimulus is received  Movement capabilities: ◦ Extensibility—ability of muscle cells to be stretched ◦ Elasticity—ability to recoil and resume resting length after stretching © 2015 Pearson Education, Inc.

 Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract  Motor unit: consists of one motor neuron and all the skeletal muscle cells stimulated by that neuron ◦ (page 232 provides more elaboration about the neurological make up of the motor unit) © 2015 Pearson Education, Inc.

Figure 6.4a Motor units. Spinal cord Motor neuron cell bodies Muscle Muscle fibers (a) Axon terminals at neuromuscular junctions Motor unit 1 Motor unit 2 Nerve Axon of motor neuron

Figure 6.4b Motor units. Branching axon to motor unit (b) Axon terminals at neuromuscular junctions Muscle fibers

 Neuromuscular junction ◦ Where the axon terminal (end) of the motor neuron “meets up with” the sarcolemma (plasma membrane) of a muscle  These two components NEVER touch © 2015 Pearson Education, Inc.

 Synaptic cleft ◦ Gap/space between axon terminal and muscle ◦ This gap is filled with interstitial (tissue) fluid  Neurotransmitter ◦ A chemical messenger released by the nerve when the nerve impulse reaches the end of the axon terminal ◦ Acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle © 2015 Pearson Education, Inc.

Action potential reaches axon terminal of motor neuron. 1 Slide 2 Nerve impulse Nucleus Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Synaptic vesicle containing ACh Axon terminal of motor neuron Mitochondrion Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Folds of sarcolemma ACh receptor Ca 2+ Synaptic cleft ACh

1. Calcium channels open ◦ calcium ions enter the axon terminal 2. The presence of Calcium causes the release of acetylcholine (ACh) by way of vesicles ◦ ACh diffuses across the synaptic cleft (the gap) and attaches to receptors on the sarcolemma (membrane) of the muscle cell © 2015 Pearson Education, Inc.

Slide 3 Action potential reaches axon terminal of motor neuron. 2 Calcium (Ca 2+ ) channels open, and Ca 2+ enters the axon terminal. 1 Synaptic vesicle containing ACh Axon terminal of motor neuron Mitochondrion Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Folds of sarcolemma ACh receptor Ca 2+ Synaptic cleft ACh

Slide 4 Action potential reaches axon terminal of motor neuron. 2 Calcium (Ca 2+ ) channels open, and Ca 2+ enters the axon terminal. 3 Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis. 1 Synaptic vesicle containing ACh Axon terminal of motor neuron Mitochondrion Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Folds of sarcolemma ACh receptor Ca 2+ Synaptic cleft ACh

4. If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium (Na  ) and potassium (K  ) ions ◦ Sodium rushes into the cell ◦ Potassium leaves the cell ◦ This causes an imbalance of charge: the sarcolemma becomes depolarized © 2015 Pearson Education, Inc.

Slide 5 Action potential reaches axon terminal of motor neuron. 2 Calcium (Ca 2+ ) channels open, and Ca 2+ enters the axon terminal. 4 3 Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis. Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 1 Synaptic vesicle containing ACh Axon terminal of motor neuron Mitochondrion Sarcolemma Fusing synaptic vesicle Sarcoplasm of muscle fiber Folds of sarcolemma ACh receptor Ca 2+ Synaptic cleft ACh

Slide 6 Ion channel in sarcolemma opens; ions pass. Na + K+K+ 5 ACh binds and channels open that allow simultaneous passage of Na + into the muscle fiber and K + out of the muscle fiber. More Na + ions enter than K + ions leave, producing a local change in the electrical conditions of the membrane (depolarization). This eventually leads to an action potential.

5. Depolarization opens more sodium channels that allow sodium ions to enter the cell  Once started, the action potential cannot be stopped  The action potential travels throughout the surface of the sarcolemma via t-tubules of the sarcolemma, causing the muscle to contract 6. The enzyme Acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid and choline to end muscle contraction

Slide 7 Ion channel closed; ions cannot pass. Degraded ACh K+K+ Na + ACh Acetylcholine- sterase The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft, ending the process. 6

 Cell returns to its resting state when: 1.Potassium ions diffuse back out of the cell  Sodium-potassium pump moves sodium and potassium ions back to their original positions  The muscle is ready to receive another stimulus © 2015 Pearson Education, Inc.

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 Calcium binds to regulatory proteins called troponin and tropomyosin ◦ troponin stimulates tropomyosin to uncover the actin binding sites ◦ This exposes myosin-binding sites ◦ myosin heads on the thick filaments attach © 2015 Pearson Education, Inc.

 The attached heads pivot, sliding the thin filaments toward the center of the sarcomere, and contraction occurs (muscle shortens)  ATP provides the energy ◦ This continues as long as ionic calcium is present

Figure 6.7 Diagrammatic views of a sarcomere. Myosin Actin ZIZI ZIZI HAHA (a) Relaxed sarcomere A ZIZI ZIZI (b) Fully contracted sarcomere

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