A bear walks into the room. In response, you run away. Trace the events that occur from the initial release of epinephrine to the release of glucose into the blood to the generation of ATP (which ultimately leads to muscle contraction). In your answer explain the processes of a. epinephrine signal transduction leading to glucose release b. cellular respiration using glucose as the substrate and generating ATP c. acetyl choline signal transduction at the neuromuscular junction d. generation of the action potential e. muscle contraction that results

a. Epinephrine >> G protein-coupled receptor >> cAMP >> phosphorylation of glycogen phosphorylase and glycogen synthase >> glucose

b. Cellular respiration >> glycolysis >> pyruvate oxidation >> Krebs cycle >> acetyl CoA>> oxidative phosphorylation

c and e. Acetylcholine >> nicotinic receptors >>  sodium ions (enter to the cells) >> muscular action potential >> contraction

d. Action potential >> resting potential >> potassium channels open >> sodium channels open >> threshold potential >> voltage-gated sodium channels and potassium channels open >> membrane  repolarization >> resting membrane potential (steady state of the cell)

​Explanation:

Epinephrine binds to G protein-coupled receptors, triggering the production of cyclic AMP (cAMP). cAMP is a second messenger associated with the phosphorylation of 1-glycogen phosphorylase (GP) that breaks down glycogen (the storage form of glucose) into glucose, and 2-glycogen synthase (GS), involved in the production of glycogen (i.e., phosphorylation inhibits GS activity). On the other hand, during cellular respiration, glucose is used to synthesize ATP via three sequential steps: glycolysis, Krebs cycle and oxidative phosphorylation. During glycolysis, glucose is converted into pyruvate that is subsequently oxidated into Coenzyme A (acetyl CoA), generating NADH and ATP. In the Krebs cycle, acetyl CoA is combined with the oxaloacetic acid to form citric acid, generating NADH, FADH2 and ATP. During oxidative phosphorylation, electrons from NADH and FADH2 are used to pump protons against an electrochemical concentration gradient, which is finally used to synthesize more ATP. On the other hand, during muscle contraction, acetylcholine binds to nicotinic receptors and sodium ions enter the muscle fiber, thereby generating a muscular action potential that travels across muscle cells and triggers muscle contraction when calcium ions (Ca2+) bind to the protein complex troponin by sarcomere shortening (sarcomeres are the functional units of muscle fibers). This contraction ends when Ca2+ ions are pumped back into the sarcoplasmic reticulum (a unique organelle of endoplasmic reticulum in the sarcoplasm). On the other hand, an action potential is defined as a fast and propagating change of the resting membrane potential of neuron cells. In the resting potential, potassium ion (K+) channels open, thereby K+ ions can enter/exit inside the cell. A stimulus causes the depolarization of the cell by opening Na+ channels that enter into the neuron. At the threshold potential, more sodium channels open, thereby voltage across the membrane reaches its most positive value. Subsequently, channels begin to close and more potassium channels open. Finally, the membrane repolarizes (K+ ions leave the cell) and cells return to the resting membrane potential, i.e., the steady-state of the cell.