NE102 Lecture Notes 3

Resistance if the connection is made of a substance

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RESISTANCE If the connection is made of a substance with low resistance to current flow (resistance  [R], measured in ohms [ Ω ]), current will flow easily, while if the connection has high  resistance, current will flow less easily or not at all. OHM’S LAW I = V/R Current flow between compartments (I) increase with increasing potential difference  between the compartments (V) but decrease with increasing resistance between the  compartments (R). IMPORATNT POINT: It takes energy to generate a potential different (e.g., pumping water uphill/compressing  with a piston).  Given a potential difference, current will flow passively (depending on how much  resistance). (e.g., allowing water to flow downhill or from higher to lower pressure) 3 REMINDERS: 1. Neurons (and most cells) maintain a potential difference between the  intracellular and extracellular space called the  membrane potential  2. The cell membrane is impermeable to charged ions
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Neuronal Biophysics I 18:04 3. But, transmembrane proteins can allow ions to move across the membrane Goal: to understand the molecular events occurring within neurons that produce  membrane potential and changes in it (e.g. APs) GENERATION OF RESTING MEMBRANE POTENTIAL Resting potential is generated by two factors a.1. Separation of ions (principally sodium and potassium) across the  membrane – concentration membrane Ion transporters (pumps) create these ionic concentration  gradients o Ion transporters are transmembrane proteins with  enzymatic properties that  actively  (i.e., using energy from  ATP) transport ions across the membrane. Ex.) Na+/K+ -ATPase is the most important ion  transporter Every cycle separates charges slightly (b/c  one mores positive charge leaves the cell  than comes in), so the pump is electrogenic  but this is NOT the principle cause of the  membrane potential Requires a HUGE amount of energy to  operate (2/3rds of a neuron’s ATP budget!) b. Differential permeability of the membrane to specific ions (potassium) at rest. Ion channels alter the membrane’s permeability to select ions Ion channels are transmembrane proteins that create hydrophilic pores through  which ions can pass through the plasma membrane passively. o Generally selective o Can be gated K+ leak channels Constitutively open (voltage independent) Voltage-gated K+ channels Closed at resting potential
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Neuronal Biophysics I 18:04 Voltage-gated Na+ channels Closed at resting potential Inside of the cell has a high concentration of potassium and a negative charge. Outside  of the cell has a low concentration of potassium and positive charge.
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