Neuron Function The Membrane Potential Resting potential Excess

Neuron Function The Membrane Potential – Resting potential • Excess negative charge inside the neuron • Created and maintained by Na-K ion pump Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Function Changes in Membrane Potential – Result from changes in ion movement – Ions move in transmembrane channels – Membrane channels can open or close – If Na+ channels open positive charges enter cell membrane potential moves positive (depolarization) – If K+ channels open positive charges leave cell membrane potential moves negative (hyperpolarization) Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Function Generation of Action Potential – Depolarization of membrane to threshold – Rapid opening of voltage-gated Na+ channels – Na+ entry causes rapid depolarization – Voltage-gated K+ channels open – K+ exit causes rapid repolarization – Refractory period ends as membrane recovers the resting state Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings

Depolarization to threshold Activation of voltageregulated sodium channels and rapid depolarization Sodium ions Local current Potassium ions Inactivation of sodium channels and activation of voltage-regulated potassium channels +30 DEPOLARIZATION 3 REPOLARIZATION Transmembrane potential (m. V) 0 2 _ 60 Threshold _ 70 1 The return to normal permeability and resting state 4 Resting potential REFRACTORY PERIOD 0 1 2 3 Time (msec) Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 8 -8 1 of 5

Control of Muscle Contraction • The Neuromuscular Junction – Synaptic terminal • Acetylcholine release – Synaptic cleft – Motor end plate • Acetylcholine receptors • Acetylcholine binding • Acetylcholinesterase – Acetylcholine removal Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings

Action potential Arrival of an action potential at the synaptic terminal Arriving action potential Synaptic terminal Axon Sarcolemma Vesicles ACh Synaptic cleft Sarcolemma of motor end plate ACh. E molecules ACh receptor site Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Muscle fiber Figure 7 -4(b-c) 2 of 5

Action potential Arrival of an action potential at the synaptic terminal Arriving action potential Synaptic terminal Axon Sarcolemma Vesicles ACh Synaptic cleft Sarcolemma of motor end plate ACh. E molecules ACh receptor site Muscle fiber Release of acetylcholine Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft. Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 7 -4(b-c) 3 of 5

Action potential Arrival of an action potential at the synaptic terminal Arriving action potential Synaptic terminal Axon Sarcolemma Vesicles ACh Synaptic cleft Sarcolemma of motor end plate ACh. E molecules ACh receptor site Muscle fiber ACh binding at the motor and plate Release of acetylcholine Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft. The binding of ACh to the receptors increases the membrane permeability to sodium ions. Sodium ions then rush into the cell. Na+ Na+ Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 7 -4(b-c) 4 of 5

Action potential Arrival of an action potential at the synaptic terminal Arriving action potential Axon Synaptic terminal Sarcolemma Vesicles ACh Synaptic cleft Sarcolemma of motor end plate ACh. E molecules ACh receptor site Muscle fiber ACh binding at the motor and plate Release of acetylcholine Vesicles in the synaptic terminal fuse with the neuronal membrane and dump their contents into the synaptic cleft. The binding of ACh to the receptors increases the membrane permeability to sodium ions. Sodium ions then rush into the cell. Appearance of an action potential in the sarcolemma An action potential spreads across the surface of the sarcolemma. While this occurs, ACh. E removes the ACh. Action potential Na+ Na+ Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 7 -4(b-c) 5 of 5

Anatomy of Skeletal Muscles • The Contraction Process – Actin active sites and myosin cross-bridges interact – Thin filaments slide past thick filaments • Cross-bridges undergo a cycle of movement – Attach, pivot, detach, return – Troponin-tropomyosin control interaction • Prevent interaction at rest Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings

Resting sarcomere ADP + P Tropomyosin Myosin head Troponin Actin ADP P + Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 7 -5 2 of 7

Resting sarcomere ADP + P Myosin head Troponin Active-site exposure ADP + P Sarcoplasm Ca 2+ Tropomyosin Actin ADP P + Active site Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Ca 2+ ADP P + Figure 7 -5 3 of 7

Resting sarcomere ADP + P Myosin head Troponin Active-site exposure ADP + P Sarcoplasm Actin ADP + P Ca 2+ Tropomyosin Cross-bridge formation ADP P + Ca 2+ Active site Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Ca 2+ ADP P + ADP Ca 2+ P + Figure 7 -5 4 of 7

Resting sarcomere ADP + P Myosin head Troponin Active-site exposure ADP + P Sarcoplasm Cross-bridge formation ADP + P Ca 2+ Tropomyosin Actin ADP P + Ca 2+ Active site ADP Ca 2+ P + Ca 2+ ADP P + Pivoting of myosin head ADP + P Ca 2+ ADP + P Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 7 -5 5 of 7

Resting sarcomere ADP + P Myosin head Troponin Active-site exposure ADP + P Sarcoplasm Cross-bridge formation ADP + P Ca 2+ Tropomyosin Actin Ca 2+ Active site ADP P + ADP Ca 2+ P + Ca 2+ ADP P + Cross bridge detachment Pivoting of myosin head ATP ADP + P Ca 2+ ATP Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings Ca 2+ ADP + P Figure 7 -5 6 of 7

Resting sarcomere ADP + P Myosin head Troponin Active-site exposure ADP + P Sarcoplasm Cross-bridge formation ADP + P Ca 2+ Tropomyosin Actin Ca 2+ Active site ADP P + ADP Ca 2+ P + Ca 2+ ADP P + Cross bridge detachment Myosin reactivation Pivoting of myosin head ATP ADP + P Ca 2+ Ca 2+ ADP P + Copyright © 2007 Pearson Education, Inc. , publishing as Benjamin Cummings ATP Ca 2+ ADP + P Figure 7 -5 7 of 7

Control of Muscle Contraction • Summary of Contraction Process Table 7 -1