1. Muscle Physiology Human Anatomy and Physiology

2.  Beneath the sarcolemma of a muscle fiber lies the sarcoplasmic reticulum (endoplasmic reticulum), which is associated with transverse (T) tubules (invaginations of the sarcolemma).  Each T tubule lies between two cisternae of the sarcoplasmic reticulum and is open to the outside of the muscle fiber.  The sarcoplasmic reticulum and transverse tubules activate the muscle contraction mechanism when the fiber is stimulated. 2 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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4. Neuromuscular Junction  The site where the motor neuron and muscle fiber meet is the neuromuscular junction.  The muscle fiber membrane forms a motor end plate in which the sarcolemma is tightly folded and where nuclei and mitochondria are abundant.  The cytoplasm of the motor neuron contains numerous mitochondria and synaptic vesicles storing neurotransmitters. 4 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. Motor Units  A motor neuron and the muscle fibers it controls make up a motor unit; when stimulated to do so, the muscle fibers of the motor unit contract all at once. 5

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7. Skeletal Muscle Contraction  Muscle contraction involves several components that result in the shortening of sarcomeres, and the pulling of the muscle against its attachments. 7 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

8. Stimulus for Contraction  The motor neuron must release the neurotransmitter acetylcholine from its synaptic vesicles into the synaptic cleft in order to initiate a muscle contraction.  Protein receptors in the motor end plate detect the neurotransmitters, the permeability of the sarcolemma changes, allowing sodium ions to enter the muscle cell.  This produces an electrical current, the action potential, which flows across the entire sarcolemma and into the T tubules, finally resulting in release of calcium ions from the sarcoplasmic reticulum. 8 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

10.  Upon receipt of the action potential, the sarcoplasmic reticulum releases its stored calcium to the sarcoplasm of the muscle fiber.  The high concentration of calcium in the sarcoplasm interacts with the troponin and tropomyosin molecules, which move aside, exposing the myosin binding sites on the actin filaments.  Myosin cross-bridges now bind and pull on the actin filaments, causing the sarcomeres to shorten.  After the nervous impulse has been received, acetylcholinesterase rapidly decomposes the acetylcholine.  Then, calcium is returned to the sarcoplasmic reticulum, and the linkages between myosin and actin are broken. 10 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

12. Role of Myosin and Actin  Myosin consists of two twisted strands with globular cross-bridges projected outward along the strands.  Actin is a globular protein with myosin binding sites; tropomysosin and troponin are two proteins associated with the surface of the actin filaments. 12 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

13. Sliding Filament Theory  According to the sliding filament theory of muscle contraction, the myosin crossbridge attaches to the binding site on the actin filament and bends, pulling on the actin filament; it then releases and attaches to the next binding site on the actin, pulling again.  Energy from the conversion of ATP to ADP is provided to cross-bridges from the enzyme ATPase, causing them to be in a “cocked” position. 13 Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

14. Sliding Filament Theory  Video clips of sliding filament theory  http://www.sci.sdsu.edu/movies/actin_myosin.html http://www.sci.sdsu.edu/movies/actin_myosin.html  http://media.pearsoncmg.com/bc/bc_campbell_bi ology_6/cipl/ins/49/HTML/source/71.html http://media.pearsoncmg.com/bc/bc_campbell_bi ology_6/cipl/ins/49/HTML/source/71.html  http://www.wisc- online.com/objects/index_tj.asp?objID=AP2904 http://www.wisc- online.com/objects/index_tj.asp?objID=AP2904

16. Sliding Filament Theory  In the contracted sarcomere,  the light H zone in the center of the A band has disappeared,  the Z lines are closer to the thick filaments,  and I bands have nearly disappeared.  The A bands move closer together but do not change in length

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18. Sliding Filament Theory  Sarcomere Shortening Animation Sarcomere Shortening Animation