XI. Skeletal Muscle Physiology Chapter 10.3-10.5A. Electrically Excitable Cells have:1. Resting membrane potentiala. Voltage difference across a membraneb. Maintained by Na+/K+ pumpsc. -90mV2. Action Potentiala. Quick, reversible change in polarity3. Gated channelsa. Voltage gated channels i. Open to large enough charge change ii. AKA Thresholdb. Ligand gated channels i. Open to chemical signalii. Neurotransmitters or hormonesB. The Neuromuscular Junction1. Synapse between axon terminal and muscle fiber2. Skeletal Muscle fibers are controlled by motor neurons3. motor unitsa. 1 neuron and the fibers it innervatesb. All contract togetherC. Triggering Muscle Contraction1. At the Neuromuscular Junctiona. Action potential travels down motor neuronb. Neuron releases Acetylcholine (Ach) into synapsec. ACh crosses synaptic cleft by diffusiond. ACh Binds to ligand gated channels’ receptors on sarcolemmae. Receptors’ ion channels openi. Lots of Na+ in ii. A little K+ out iii. Change in RMP = end plate potential2. On the Sarcolemmaa. Depolarizationi. End plate potential reaches threshold ii. Voltage-gated Na+ channels open in nearby membraneiii. Lots of Na+ enters through channels into cell b. Action Potentiali. Charge reversal spreads from one voltage-gated Na+ channel to next along membrane ii. Voltage-gated K+ channels in membrane are triggered but are slower to openD. Excitation - Contraction Coupling 1. AP travels down the T tubules of sarcolemma2. Triggers Ca2+ released from the sarcoplasmic reticulum into the cytosol3. Ca2+ binds to regulatory proteins in myofibrils 4. Regulatory proteins change shape and reveal Myosin binding sites on Actin5. Phosphorylated Myosin can now bind to Actin E. Steps in cross bridge cycling1. Myosin phosphorylationa. ATP donated phosphate to myosin2. Cross Bridge formationa. Phosphorylated Myosin binds to exposed site on Actin3. Power Strokea. After binding, myosin heads “flex”, pulling actin along myosin and releasing phosphate 4. Cross bridge detachmenta. New ATP binds to myosinb. Releases myosin from actinF. Steps in Relaxation1. Ach signal endsa. ACh broken down by acetylcholinesterase enzymeb. Ligand Gated channels close, Ion signal ends2. Membrane Repolarizationa. Voltage gated Na+ channels close on a chemical timer 1ms after triggeringb. Voltage gated K+ channels open slowly, 1ms after triggeringi. K+ exits cell down concentration gradientii. RMP is restoredc. Na+/K+ Pump re-sets initial Ion positions3. Calcium actively transported into sarcoplasmic reticulum4. Regulatory proteins return to blocking myosin from rebinding to actin5. Cell lengthened by gravity or antagonistG. Graphing Muscle cell APsH. Refractory period1. Membrane cannot depolarize again until ions are reset2. Potassium channels must be closing3. Na/K pump returns ions to starting