- Slides: 8
AP Biology Nervous Systems Part 2
Membrane Potential and ion concentrations CYTOSOL EXTRACELLULAR FLUID [Na+] 15 m. M [Na+] 150 m. M [K+] 5 m. M [Cl–] 10 m. M [Cl–] 120 m. M [A–] 100 m. M Plasma membrane
Na+/K+ pumps EXTRACELLULAR [Na+] high FLUID [K+] low Na+ Na+ CYTOPLASM [Na+] low [K+] high Na+ Cytoplasmic Na + bonds to the sodium-potassium pump P ATP P ADP Na+ binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. K+ K+ P Extracellular K+ binds to the protein, triggering release of the phosphate group. K+ Loss of the phosphate restores the protein’s original conformation. K+ is released and Na + sites are receptive again; the cycle repeats.
Resting Potential Stimuli +50 – 50 Threshold Resting potential Hyperpolarizations 0 1 2 3 4 5 Time (msec) Graded potential hyperpolarizations +50 Membrane potential (m. V) +50 – 100 Stronger depolarizing stimulus 0 – 50 Threshold Resting potential – 100 0 – 50 Threshold Resting potential Depolarizations 0 1 2 3 4 5 Time (msec) Graded potential depolarizations Action potential – 100 0 1 2 3 4 5 6 Time (msec) Action potential
Impulse generation Na+ Na+ K+ K+ Rising phase of the action potential Falling phase of the action potential Na+ Membrane potential (m. V) +50 Action potential 0 – 50 Threshold K+ – 100 Depolarization Resting potential Time Na+ Extracellular fluid Potassium channel Na+ Activation gates K+ Plasma membrane Cytosol Resting state Undershoot Sodium channel K+ Inactivation gate
Propagation Axon Action potential Na+ An action potential is generated as Na + flows inward across the membrane at one location. K+ Action potential Na+ K+ The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K + flows outward. K+ Action potential Na+ K+ The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.
Saltatory Conduction Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon