Exam 3 Review Topics Listed below are some of the key topics discussed in class This is not meant to be a comprehensive review of lessons 13 15 for that you should go over your notes However it does cover some of the key topics and concepts discussed in lecture Lesson 13 14 1 What is the difference between a resting potential a graded potential and an action potential What are passive channels chemically gated channels and voltage gated channels Graded Potential The transmembrane potential of resting cell A typical neuron has a resting Resting Potential potential of approximately 70 mV All neural activities begin with a change in the resting potential of a neuron The sodium potassium exchange pump ejects 3 Na ions for every 2 K ions that it brings into the cell which stabilizes the resting potential when the ratio of Na entry to K loss through passive channels is 3 2 If the graded potential is large enough it triggers an action potential in the Temporary localized change in resting potential Caused by stimulus to the cell Are changes in transmembrane potential that cannot spread far from site of stimulation Includes any stimulus that opens a gated channel which produces a graded potential Involves Voltage gated Channels which respond to changes in transmembrane potential Action Potential membrane of the axon An action potential is an electrical impulse that is propagated spread along the surface of an axon and does not diminish as it moves away from its source This impulse travels along the axon to one or more synapses Channels i Passive channels are always open however their permeability can vary from moment to moment as the proteins that make up the channel change shape in response to local conditions Leak channels are important in establishing the normal resting potential of the cell ii Chemically gated channels open or close when they bind specific chemicals The receptors that bind acetylcholine ACh at the neuromuscular junction are chemically gated channels Chemically gated channels are most abundant on the dendrites and cell body of a neuron the areas where most synaptic communication occurs iii Voltage gated Channels open or close in response to changes in the transmembrane potential They are characteristic of areas of excitable membrane a membrane capableof generating and conducting an action potential The most important voltage gated channels are voltage gated sodium channels potassium channels and calcium channels These sodium channels have two gates that function independently an activation gate that opens on stimulation letting sodium ions into the cell and an inactivation gate that closes to stop the entry of sodium ions 2 How is the resting membrane potential generated and maintained Why is the sodium potassium exchange pump important Because the plasma membrane is highly permeable to potassium ions the electrochemical gradient for sodium ions is very large but the membrane s permeability to these ions is very low The sodium potassium exchange pump ejects 3 Na ions for every 2 K ions that it brings into the cell It serves to stabilize the resting potential when the ratio of Na entry to K loss through passive channels is 3 2 At the normal resting potential these passive and active mechanisms are in balance 3 What is the difference between chemical and electrical gradients and why are they important in the transmembrane potential Concentration gradients chemical gradient of ions Na K Because the Chemical Gradient intracellular concentration of potassium ions K is relatively high these ions tend to move out of the cell through open potassium channels The movement is driven by a concentration gradient or chemical gradient Similarly a chemical gradient for sodium ions Na tends to drive those ions into the cell Electrical Gradient The Chemical Gradient causes the extracellular fluid ECF and intracellular fluid cytosol to differ greatly in ionic composition which affects the transmembrane potential Potential difference due to an Electrical Gradient as well as Chemical Gradients play an important role in the membrane permeability also creating transmembrane potential Separate charges of positive and negative ions Result in potential difference Membrane permeability to Na and K determines transmembrane potential 4 What are the three states of gated channels Why is that important in action potential generation Open activated Closed not capable of opening inactivated Closed but capable of opening at rest gated channels are usually in this state At the resting potential most gated channels are closed When gated channels open the rate of ion movement across the plasma membrane increases changing the transmembrane potential 5 What does depolarization repolarization and hyperpolarization mean in terms of the transmembrane potential What are the four basic characteristics of graded potentials Depolarization A shift in transmembrane potential toward 0 mV remember the resting membrane potential of a cell is negative and approximately 70 mV Repolarization to normal inside becomes negative again caused by increasing the negativity of the resting potential occurs when the stimulus is removed the transmembrane potential returns Hyperpolarization Result of opening a potassium channel Opposite effect of opening a sodium channel Positive Ions move OUT of the cell Note in hyperpolarization the membrane potential becomes more polarized so more negative than the resting membrane potential Four Basic characteristics of graded potentials The transmembrane potential is most changed at the site of stimulation and the effect decreases with distance ex Ripples in a pool The effect spreads passively due to local currents The spread in all directions not just in one direction ex Creating ripples when dropping a pebble into a pool of water The graded change in transmembrane potential may involve either depolarization or hyperpolarization The properties and distribution of the membrane channels involved determine the nature of the change i e in a resting membrane the opening of sodium channels causes depolarization whereas the opening of potassium channels causes hyperpolarization The stronger the stimulus the greater the change in the transmembrane potential and the larger the area affected 6 What are the four steps in action potential generation Are you able to draw the action potential curve and label the movement of ions and the state of the gated channels at specific regions of the curve Four
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