1 Membrane Potentials (Polarity) Information found in 2 places: –Chapter 3 - pp –Chapter 9 - pp /22/12 MDufilho

Generation of a Resting Membrane Potential Resting membrane potential (RMP) –Produced by separation of oppositely charged particles (voltage) across membrane in all cells Cells described as polarized –Voltage (electrical potential energy) only at membrane Ranges from –50 to –100 mV in different cells –"–" indicates inside negative relative to outside 6/22/122

3 Membrane Potentials - Introduction What you must know 1.Locations of ECF vs. ICF 2.Normal distribution of Na+, K+, Cl-, Ca++ and proteins between ECF vs. ICF 3.Cell membranes are much more permeable to K+ than Na+ 4.Composition of ECF is different from that of ICF, but both are electrically neutral and isotonic 6/22/12 MDufilho

Figure 3.15 The key role of K + in generating the resting membrane potential. 1 K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. 2 K + also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 3 A negative membrane potential (–90 mV) is established when the movement of K + out of the cell equals K + movement into the cell. At this point, the concentration gradient promoting K + exit exactly opposes the electrical gradient for K + entry. Extracellular fluid Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm – – – – – – – – Slide 1 6/22/124

Normal Distribution of Electrolytes IonsICF ( mM/Liter ) ECF ( mM/Liter ) mV Na K Cl Ca cytosol Higher in SR /22/12 5MDufilho

Selective Diffusion Establishes RMP In many cells Na + affects RMP –Attracted into cell due to negative charge  RMP to –70 mV –Membrane more permeable to K + than Na +, so K + primary influence on RMP Cl – does not influence RMP— concentration and electrical gradients exactly balanced 6/22/126

Figure 11.8 Finally, let’s add a pump to compensate for leaking ions. Na + -K + ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential. Suppose a cell has only K + channels... K + loss through abundant leakage channels establishes a negative membrane potential. Now, let’s add some Na + channels to our cell... Na + entry through leakage channels reduces the negative membrane potential slightly. The permeabilities of Na + and K + across the membrane are different. The concentrations of Na + and K + on each side of the membrane are different. Na + (140 mM ) K + (5 mM ) K + leakage channels Cell interior –90 mV Cell interior –70 mV Cell interior –70 mV K+K+ Na + Na+-K+ pump K+K+ K+K+ K+K+ K+K+ Na + K+K+ K+K+ K K+K+ K+K+ K+K+ K+K+ Outside cell Inside cell Na + -K + ATPases (pumps) maintain the concentration gradients of Na + and K + across the membrane. The Na + concentration is higher outside the cell. The K + concentration is higher inside the cell. K + (140 mM ) Na + (15 mM ) 6/22/12 7MDufilho

8 What happens to establish resting membrane potential K+ diffuses out of cell – Why? Protein anions do not diffuse – Why? Therefore membrane interior becomes more negative Negativity becomes great enough to attract K+ back into the cell When K+ concentration gradient is balanced by membrane potential (-70mV) one K+ enters cell as one leaves The tendency for K+ to diffuse out of the cell is the most significant factor in establishing RMP 6/22/12 MDufilho

9 How to Alter Resting Membrane Potential Alter the permeability to one or more ions This typically happens at a chemical synapse 6/22/12 MDufilho

Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released. Slide 1 Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Synaptic vesicle containing ACh Synaptic cleft Junctional folds of sarcolemma Sarcoplasm of muscle fiber Postsynaptic membrane ion channel opens; ions pass. Ion channel closes; ions cannot pass. Action potential arrives at axon terminal of motor neuron. Voltage-gated Ca 2+ channels open. Ca 2+ enters the axon terminal moving down its electochemical gradient. Ca 2+ entry causes ACh (a neurotransmitter) to be released by exocytosis. ACh diffuses across the synaptic cleft and binds to its receptors on the sarcolemma. ACh binding opens ion channels in the receptors that allow simultaneous passage of Na + into the muscle fiber and K + out of the muscle fiber. More Na + ions enter than K + ions exit, which produces a local change in the membrane potential called the end plate potential. ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction. Axon terminal of motor neuron Fusing synaptic vesicles Degraded ACh ACh Acetylcho- linesterase ACh /22/1210

11 Action Potential A transient depolarization event that includes polarity reversal of a membrane and the propagation of an action potential along the membrane 6/22/12 MDufilho

Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. Slide 1 Open Na + channel Na + Closed K + channel K+K+ Action potential Axon terminal of neuromuscular junction ACh-containing synaptic vesicle Ca 2+ Synaptic cleft Wave of depolarization An end plate potential is generated at the neuromuscular junction (see Figure 9.8). Depolarization: Generating and propagating an action potential (AP). The local depolarization current spreads to adjacent areas of the sarcolemma. This opens voltage-gated sodium channels there, so Na + enters following its electrochemical gradient and initiates the AP. The AP is propagated as its local depolarization wave spreads to adjacent areas of the sarcolemma, opening voltage-gated channels there. Again Na+ diffuses into the cell following its electrochemical gradient. Repolarization: Restoring the sarcolemma to its initial polarized state (negative inside, positive outside). Repolarization occurs as Na + channels close (inactivate) and voltage-gated K + channels open. Because K + concentration is substantially higher inside the cell than in the extracellular fluid, K + diffuses rapidly out of the muscle fiber Closed Na + channel Open K + channel Na + K+K+ −  −  −  −  −  −  −  − −  −  −  −  − −  −  −  −  −  −  −  −  −  −     −  −  −  −  −  −  −  − −  −  −  −−  −  −  −  −  −  −  − 6/22/1212

13 Time from the opening of the Na + activation gates until the closing of inactivation gates The absolute refractory period: –Prevents the neuron from generating an action potential –Ensures that each action potential is separate –Enforces one-way transmission of nerve impulses Absolute Refractory Period 6/22/12 MDufilho

14 The interval following the absolute refractory period when: –Sodium gates are closed –Potassium gates are open –Repolarization is occurring The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events Relative Refractory Period 6/22/12 MDufilho

Figure 9.10 Action potential tracing indicates changes in Na + and K + ion channels. Membrane potential (mV) – Depolarization due to Na + entry Na + channels close, K + channels open Repolarization due to K + exit K + channels closed Na + channels open Time (ms) 6/22/1215

16 What if ……..? 1.The amount of extracellular K+ were below normal? 2.The release of the NT from the synaptic knob were prevented? 3.Receptor sites for the NT were blocked? 4.The amount of EC Ca++ were below normal 5.The number of NT receptor sites were reduced? 6.Opening of the voltage-gated sodium channels were prevented? 7.You grab a live 120V electric wire? 6/22/12 MDufilho