the axons of the nerve meet with the muscle cell.

Transcription

1 Steps to Contraction 1. A nerve impulse travels to the neuromuscular junction on a muscle cell. The neuromuscular junction is the point where the axons of the nerve meet with the muscle cell. 2. Ach is released from the axon to s located on the sarcolemma 3. The binding Ach causes depolarization of the sarcolemma by opening ion channels and allowing Na ions into the muscle cell. 4. Na ions diffuse into the muscle and depolarization continues. 5. Depolarization creates a wave of action potential (electrical current) across the sarcolemma 6. travels across the sarcolemma and down the T-tubules which triggers the sarcoplasmic reticulum SR to release Ca 7. As Ca levels rise, Ca ions bind with which removes the blocking action of from the binding s. 8. Myosin is now ready to bind with the actin and form cross bridges which begins the contraction process 9. In order to contract, AT binds to the Myosin 10. AT is then hydrolyzed (broken down) to and i, which gives the myosin the energy to cock its head to the highenergy position. 11. and myosin bind together to form a cross bridge 12. The myosin heads then pull the actin filaments inward and release the and i and return to a low energy position. The myosin is now ready for more AT to bind and repeat the cycle. This process will continue for as long as there are Ca ions and AT available. RED = the E-C coupling stage

2 Overview of Events at the neuromuscular junction An action potential (A), an electrical impulse, travels down the axon of the motor neuron to the end bulbs (synaptic terminals) The A causes the synaptic vesicles to fuse with the end bulb membrane, resulting in the release of Acetylcholine (Ach) into the synaptic Ach diffuses across the synaptic & binds to Ach s on the The binding of Ach to its s causes a new A to be generated along the muscle cell membrane Immediately after it binds to its s, Ach will be broken down by Acetylcholinesterase (AchE) an enzyme present in the synaptic of E molecules An action potential (A), an electrical impulse, travels down the axon of the motor neuron to the end bulbs (synaptic terminals) Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 2 of 5 of E molecules in the synaptic terminal fuse their contents into the synaptic. The A causes the synaptic vesicles to fuse with the end bulb membrane, resulting in the release of Acetylcholine (Ach) into the synaptic Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 3 of 5

3 E molecules of in the synaptic terminal fuse their contents into the synaptic. The A causes the synaptic vesicles to fuse with the end bulb membrane, resulting in the release of Acetylcholine (Ach) into the synaptic 3 of 5 Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings E molecules of binding at the motor and plate in the synaptic terminal fuse their contents into the synaptic. Ach diffuses across the synaptic & binds to Ach s on the The binding of to the s increases the membrane permeability to sodium ions. Sodium ions then rush into the cell. Na Na Na 4 of 5 Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings The binding of Ach to its s causes a new A to be generated along the muscle cell membrane Immediately after it binds to its s, Ach will be broken down by Acetylcholinesterase (AchE) an enzyme present in the synaptic of E molecules binding at the motor and plate in the synaptic terminal fuse their contents into the synaptic. The binding of to the s 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, E removes the. Action potential Na Na Na lay I neuromuscular junction p. 14

4 hysiology of Skeletal Contraction Table 7-1 Once an action potential (A) is generated at the it will spread like an electrical current along the sarcolemma of the muscle The A will also spread into the T-tubules, exciting the terminal cisternae of the sarcoplasmic reticula This will cause Calcium (Ca 2 ) gates in the SR to open, allowing Ca 2 to diffuse into the sarcoplasm Calcium will bind to troponin (on the thin myofilament), causing it to change its shape. This then pulls tropomyosin away from the active s of actin molecules. The exposure of the active s allow the sliding of the filaments hysiology of skeletal muscle contraction events at the myofilaments Myosin reactivation Cross bridge detachment ivoting of myosin head AT AT Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 1 of 7 Calcium (Ca 2 ) gates in the SR open, allowing Ca 2 to diffuse into the sarcoplasm Calcium will bind to troponin (on the thin myofilament), causing it to change its shape. This then pulls tropomyosin away from the active s of actin molecules. Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 3 of 7

5 s are energized by the presence of O 4 3- at the AT binding (energy is released as phosphate bond of AT breaks) Once the active s are exposed, the energized myosin heads hook into actin molecules forming cross-bridges Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 4 of 7 Using the stored energy, the attached myosin heads pivot toward the center of the sarcomere The & phosphate group are released from the myosin head ivoting of myosin head Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 5 of 7 A new molecule of AT binds to the myosin head, causing the cross bridge to detach from the actin strand The myosin head will get re-energized as the AT à Cross bridge detachment AT AT ivoting of myosin head As long as the active s are still exposed, the myosin head can bind again to the next active

6 Myosin reactivation Cross bridge detachment ivoting of myosin head AT AT LAY I Sliding filament theory p single cross bridge; p. 27 multiple cross bridges Copyright 2007 earson Education, Inc., publishing as Benjamin Cummings 7 of 7