【正文】 charge. ? In order for an ion to cross the membrane, it must use a channel that facilitates its movement. Extracellular Intracellular Neuron Cell Membrane Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ A A A A A A A A A A A A A A A A A K+ Na+ 14 The Charge Across Cell Membranes ? Passive ion channels allow diffusion of K+ and Na+ across the membrane. ? There are MANY passive K+ channels and very few Na+ channels making the membrane 100x more permeable to K+ than Na+. Extracellular Intracellular Neuron Cell Membrane Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ A A A A A A A A A A A A A A A A A K+ K+ K+ K+ Na+ 15 CQ4: The movement of charged ions based on concentration gradients when the nerve is at rest…. Examine the figure to the right and predict the direction Na+ and K+ will move via diffusion based on concentration if all ion channels are open. A. Both Na+ and K+ will move OUT of the cell. B. Both Na+ and K+ will move INTO the cell. C. Na+ will move OUT and K+ will move INTO the cell. D. Na+ will move IN and K+ will move OUT of the cell. E. Neither Na+ nor K+ will move across the cell membrane via diffusion. Extracellular Intracellular Neuron Cell Membrane Na+ 150mM Na+ 15mM K+ 150mM K+ 5mM A 110mM 16 Resting Potential in Neurons Diffusion of K+ (and less Na+) leads to a separation of charges across the membrane, and the resting potential. – Remember: There are MANY K+ and very few Na+ channels, thus membrane permeability is 100x for K+ than Na+. – Movement of K+ increases the positive charge outside the membrane relative to the inside. Thus an ELECTRICAL gradient is formed that can also influence ion flow. 17 Maintaining the Resting Potential: the SodiumPotassium Pump ? The pump moves three sodium (Na+) ions out of the cell for every two potassium (K+) ions that it brings in. ? This helps the cell maintain a separation of charges. 18 Resting Potential TIME Membrane Voltage (mV) 70 55 0 30 At Resting Potential, the inside of the neuron is negative relative to the outside and is thus polarized. (70Mv). 19 Action Potentials: How Nerves Communicate ? Neurons are specialized to use changes in membrane potential for fast munication called an ACTION POTENTIAL. ? Neurons have special gated ion channels, that open or close in response to stimuli. ? Membrane potential may change in response to stimuli that open or close those channels. 20 CQ5: When a neuron is stimulated, the area of the membrane at the point of stimulation bees more permeable to Na+. If a cell starts at resting potential (70mv), and then is stimulated: A. The membrane voltage will bee 70mV, because Na+ will move OUT of the cell B. The membrane voltage will bee 70mV, because Na+ will move OUT of the cell. C. The membrane voltage will bee 70mV because