Assignment 7 o The production of graded potentials in the plasma membrane of the postsynaptic cell is the function associated with the indicated neural activity o Synaptic activity results in the production of graded potentials in the plasma membrane of the postsynaptic cell o o Ions are unequally distributed across the plasma membrane of all cells This ion distribution creates an electrical potential difference across the membrane What is the name given to this potential difference o Resting membrane potential RMP The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation 1 o Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of leak channels passive o Leak channels for Na and K are ubiquitous and they allow for the diffusion of these ions across plasma membranes o On average the resting membrane potential is 70 mV What does the sign and magnitude of this value tell you o The inside surface of the plasma membrane is much more negatively charged than the outside surface The inside surface of the plasma membrane accumulates more negative charge because of the presence of Na and K gradients and the selective permeability of the membrane to Na and K o The plasma membrane is much more permeable to K than to Na because there are many more K leak channels than Na leak channels in the plasma membrane o More leak channels translates into more leakiness Thus the outward flux of K is greater than the inward flux of Na o The resting membrane potential depends on two factors that influence the magnitude and direction of Na and K diffusion across the plasma membrane Identify these two factors o The presence of concentration gradients and leak channels The concentration gradient and the large number of K leak channels allow for rather robust K diffusion out of a cell In contrast the concentration gradient and the relatively few Na leak channels allow for much less Na diffusion into a cell o Na K ATPase prevents the Na and K gradients from dissipating o Yes Also known as the Na K pump or simply the pump this transporter moves three Na out of the cell and two K into the cell for every ATP it hydrolyzes This pumping action prevents the Na and K gradients from running down as these ions passively move through leak channels o The membranes of neurons at rest are very permeable to K but only slightly permeable to Na Na into the cell o During depolarization Both the electrical and chemical gradients move s o A positive ion is driven into the cell because the inside of the cell is negative compared to the outside of the cell and Na is driven into the cell because the concentration of Na is greater outside the cell o What is the value for the resting membrane potential for most neurons o 70 mV 2 o The Na K pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage In which direction is each ion pumped o Na is pumped out of the cell and K is pumped into the cell Na is pumped out of the cell against its electrochemical gradient and K is pumped into the cell against its concentration gradient o The concentrations of which two ions are highest outside the cell o Na and Cl o In a neuron sodium and potassium concentrations are maintained by the sodium potassium exchange pump such that the sodium concentration is higher outside the cell than inside the cell and the potassium concentration is higher inside the cell than outside the cell o Because the sodium potassium exchange pump moves sodium and potassium ions in opposite directions the pump generates concentration gradients for these ions that are opposite in direction The opposite direction of these concentration gradients explains why the equilibrium potentials for those ions are opposite in sign that is 90 mV for potassium and 66 mV for sodium o The sodium potassium exchange pump transports potassium and sodium ions in o Sodium ions are transported out of the cell Potassium ions are transported which direction s into the cell The energy of ATP is used to actively transport potassium and sodium ions against their electrochemical gradients Potassium and sodium ions diffuse in the opposite direction through channels o Leak channels allow the movement of potassium and sodium ions by channel mediated diffusion gradients o Ions move through leak channels because of chemical and electrical o The electrochemical gradient for potassium ions when the transmembrane potential is at the resting potential 70 mV is caused by what o A chemical gradient going out of the cell and an electrical gradient going into the cell The higher concentration of potassium inside the cell than outside the cell results in an outward chemical gradient However the electrical gradient is in the opposite direction inward because at the resting potential the inside of the cell is more negative which is attractive to the positively charged potassium ions 3 o What is the electrochemical gradient of an ion o The sum of the electrical and chemical gradients for that ion Together these two gradients determine the net movement of a particular ion across the plasma membrane o The events that occur at a functioning cholinergic synapse cause synaptic delay o Due to the time involved in calcium influx and neurotransmitter release the transmission of an action potential is delayed at the synaptic cleft o Passive ion channel is always open o The sodium potassium exchange pump stabilizes resting potential at about 70mV o o Where do most action potentials originate o The initial segment The first part of the axon is known as the initial segment The initial segment is adjacent to the tapered end of the cell body known as the axon hillock o What opens first in response to a threshold stimulus o Voltage gated Na channels The activation gates of voltage gated Na channels open and Na diffuses into the cytoplasm o What characterizes depolarization the first phase of the action potential o The membrane potential changes from a negative value to a positive value The plasma membrane which was polarized to a negative value at the RMP depolarizes to a positive value 4 o What characterizes repolarization the second phase of the action potential o Once the membrane depolarizes to a peak value of 30 mV it repolarizes to its negative resting value of 70 mV The plasma membrane was depolarized to a positive value at the peak
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