AP Biology Nervous Systems Part 2

Membrane Potential and ion concentrations CYTOSOL EXTRACELLULAR FLUID [Na+] 15 mM [Na+] 150 mM [K+] 150 mM [K+] 5 mM [Cl–] 120 mM [Cl–] 10 mM [A–] 100 mM Plasma membrane

Na+/K+ pumps Cytoplasmic Na+ bonds to the sodium-potassium pump EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high ATP P Na+ P CYTOPLASM ADP Cytoplasmic Na+ bonds to the sodium-potassium pump Na+ binding stimulates phosphorylation by ATP. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. K+ K+ K+ K+ K+ P P K+ Extracellular K+ binds to the protein, triggering release of the phosphate group. Loss of the phosphate restores the protein’s original conformation. K+ is released and Na+ sites are receptive again; the cycle repeats.

Stronger depolarizing stimulus Resting Potential Stimuli Stimuli Stronger depolarizing stimulus +50 +50 +50 Action potential Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) –50 Threshold –50 Threshold –50 Threshold Resting potential Resting potential Resting potential Hyperpolarizations Depolarizations –100 –100 –100 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6 Time (msec) Time (msec) Time (msec) Graded potential hyperpolarizations Graded potential depolarizations Action potential

Impulse generation Na+ Na+ Na+ Na+ K+ Rising phase of the action potential K+ Falling phase of the action potential +50 Action potential Na+ Na+ Membrane potential (mV) –50 Threshold K+ Resting potential –100 Depolarization Time Na+ Na+ Extracellular fluid Potassium channel Activation gates Na+ K+ Plasma membrane Undershoot Cytosol Sodium channel K+ Inactivation gate Resting state

Propagation Axon Action potential An action potential is generated as Na+ flows inward across the membrane at one location. Action potential K+ 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. Action potential K+ 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

Reflex Arc