11.4 Membrane Potentials

11.4 Membrane Potentials • Like all cells, neurons have a resting membrane potential • Unlike most other cells, neurons can rapidly change resting membrane potential • Neurons are highly excitable Basic Principles of Electricity (1 of 8) • Opposite charges are attracted to each other • Energy is required to keep opposite charges separated across a membrane • Energy is liberated when the charges move toward one another • When opposite charges are separated, the system has potential energy Basic Principles of Electricity (2 of 8) • Definitions – Voltage: a measure of potential energy generated by separated charge  Measured between two points in volts (V) or millivolts (mV)  Called potential difference or potential – Charge difference across plasma membrane results in potential  Greater charge difference between points = higher voltage Basic Principles of Electricity (3 of 8) • Definitions (cont.) – Current: flow of electrical charge (ions) between two points  Can be used to do work  Flow is dependent on voltage and resistance – Resistance: hindrance to charge flow  Insulator: substance with high electrical resistance  Conductor: substance with low electrical resistance Basic Principles of Electricity (4 of 8) • Definitions (cont.) – Ohm’s law: gives relationship of voltage, current, resistance Current (I)  voltage (V)/resistance (R) – Current is directly proportional to voltage  Greater the voltage (potential difference), greater the current  No net current flow between points with same potential – Current is inversely proportional to resistance  The greater the resistance, the smaller the current Basic Principles of Electricity (5 of 8) • Role of membrane ion channels – Large proteins serve as selective membrane ion channels  K+ ion channel allows only K+ to pass through – Two main types of ion channels  Leakage (nongated) channels, which are always open  Gated channels, in which part of the protein changes shape to open/close the channel – Three main gated channels: chemically gated, voltage—gated, or mechanically gated Basic Principles of Electricity (6 of 8) • Chemically gated (ligand-gated) channels – Open only with binding of a specific chemical (example: neurotransmitter) • Voltage-gated channels – Open and close in response to changes in membrane potential • Mechanically gated channels – Open and close in response to physical deformation of receptors, as in sensory receptors Operation of Gated Channels Figure 11.7 Operation of gated channels. Basic Principles of Electricity (7 of 8) • When gated channels are open, ions diffuse quickly: – Along chemical concentration gradients from higher concentration to lower concentration – Along electrical gradients toward opposite electrical charge Basic Principles of Electricity (8 of 8) • Electrochemical gradient: electrical and chemical gradients combined • Ion flow creates an electrical current, and voltage changes across membrane – Expressed by rearranged Ohm’s law equation: V = IR Generating the Resting Membrane Potential (1 of 7) • A voltmeter can measure potential (charge) difference across membrane of resting cell • Resting membrane potential of a resting neuron is approximately 70 mV – The cytoplasmic side of membrane is negatively charged relative to the outside – The actual voltage difference varies from 40 mV to 90 mV – The membrane is said to be polarized Generating the Resting Membrane Potential (2 of 7) • Potential generated by: – Differences in ionic composition of ICF and ECF – Differences in plasma membrane permeability Measuring Membrane Potential in Neurons Figure 11.8 Measuring membrane potential in neurons. Generating the Resting Membrane Potential (3 of 7) • Differences in ionic composition – ECF has higher concentration of Na+than ICF   Balanced chiefly by chloride ions (Cl ) – ICF has higher concentration of K+ than ECF  Balanced by negatively charged proteins – K+ plays most important role in membrane potential A&P Flix: Resting Membrane Potential Click here to view ADA compliant Animation: A&P Flix: Resting Membrane Potential Resting Membrane Potential (1 of 2) • Generating a resting membrane potential depends on (1) differences in K+ and Na+concentrations inside and outside cells, and (2) differences in permeability of the plasma membrane to these ions. Focus Figure 11.1-1 Resting Membrane Potential. Generating the Resting Membrane Potential (4 of 7) • Differences in plasma membrane permeability – Impermeable to large anionic proteins – Slightly permeable to Na+(through leakage channels)  Sodium diffuses into cell down concentration gradient – 25 times more permeable to K+ than sodium (more leakage channels)  Potassium diffuses out of cell down concentration gradient – Quite permeable to Cl– Generating the Resting Membrane Potential (5 of 7) • Differences in plasma membrane permeability (cont.) – More potassium diffuses out than sodium diffuses in  As a result, the inside of the cell is more negative  Establishes resting membrane potential – Sodium-potassium pump (Na+K+ ATPase) stabilizes resting membrane potential  Maintains concentration gradients for Na+and K+  Three Na+are pumped out of cell while two K+ are pumped back in Generating a Resting Membrane Potential Depends on (1) Differences in K+ and Na+ Concentrations Inside and Outside Cells, and (2) Differences in Permeability of the Plasma Membrane to these Ions Focus Figure 11.1 Resting Membrane Potential. Changing the Resting Membrane Potential (6 of 7) • Membrane potential changes when: – Concentrations of ions across membrane change – Membrane permeability to ions changes • Changes produce two types of signals – Graded potentials  Incoming signals operating over short distances – Action potentials  Long-distance signals of axons • Changes in membrane potential are used as signals to receive, integrate, and send information Changing the Resting Membrane Potential (7of 7) • Terms describing membrane potential changes relative to resting membrane potential – Depolarization: decrease in membrane potential (moves toward zero and above)  Inside of membrane becomes less negative than resting membrane potential  Probability of producing impulse increases – Hyperpolarization: increase in membrane potential (away from zero)  Inside of membrane becomes more negative than resting membrane potential  Probability of producing impulse decreases Depolarization and Hyperpolarization of the Membrane Figure 11.9 Depolarization and hyperpolarization of the membrane.