Test 3 Lesson 13 14 The Nervous System Synaptic Transmissions and Neurotransmitters Describe the main membrane processes in neurons There are five membrane processes in neurons resting potential graded potential action potential synaptic activity and information processing Resting Potential The transmembrane potential of a resting cell Graded potential Temporary localized change in resting potential Caused by stimulus to the cell Action Potential An electrical impulse produced by graded potential Propagates along surface of axon to the synapse Information Processing Response integration of stimuli of postsynaptic cell Synaptic Activity Releases neurotransmitters at presynaptic membrane Produces graded potentials to postsynaptic membrane Explain the transmembrane potential Three concepts 1 The extracellular and intracellular fluid cytosol differ greatly in ionic composition concentration gradient in ions are Na and K 2 Cells have selectively permeable membranes 3 Membrane permeability varies by ion because of the presence of passive leak channels o Why is it important What are the main ions molecules that generate this potential It composes two passive forces that act across the plasma membrane Chemical Gradients Concentration gradients of ions Main ions molecules Na K Electrical Gradients Result in potential difference Main ions molecules separate charges and ions Electrical current movement of charges to eliminate potential difference in which opposite charges attract Resistance amount of current a membrane restricts Explain the electrochemical gradient For a particular Na and K ion it is the sum of chemical and electrical forces acting on the ion across a plasma membrane o Why is it important It is a form of potential energy Explain the role of sodium and potassium ions on the membrane potential Sodium potassium ATPase are active forces across the membrane that are powered by ATP It carries 3 Na out and 2 K in Na and K balance passive forces of diffusion through passive channels and also maintain resting potential at 70mV This stabilizes the resting potential when the ratio of Sodium entry to Potassium loss through passive channels is 3 2 o Do they enter leave through active or passive channels 3 sodium s leave and 2 potassium s enter both through passive channels Membrane permeability to sodium and potassium determines transmembrane potential Passive channels always open permeability changes with conditions Active channels open and close in response to stimuli usually closed at resting potential Explain how gated channels function Chemically gated channels Opens only to specific chemicals at a binding site Voltage gated channels Responds to changes in transmembrane permeability Acetylcholine pic Located neuron s cell body and dendrites Both open and closed gates and characteristic of excitable membrane 70 60 30mV pic Mechanically gated channels Responds to membrane distortion applied pressure pic Located sensory receptors Located neural axons skeletal muscle sarcolemma and cardiac muscle o What are the different types of gated channels 1 Chemically gated channels 2 Voltage gated channels 3 Mechanically gated channels o What are the different states to a gated channel 1 Closed capable of opening 2 Open activated 3 Inactive closed not capable of opening Describe the difference between action potentials and graded potentials Action Potential Propagated changes in transmembrane potential Affects an entire excitable membrane Links graded potentials at cell body with motor end plate actions Graded Potential Aka Local potentials Changes in transmembrane potential that cannot spread far from site of stimulation Any stimulus that opens a gated channel graded potential o Are they always depolarizing or hyperpolarizing Action potentials and graded potentials are always depolarizing But graded potentials also hyperpolarize Explain how graded potentials can generate action potentials AP s o Which region of the neuron generates the AP The very first action potential gets generated at the initial segment of the axon hillock o What is the all or none principle All or none principle an action potential is either triggered or not Describe the steps to the generation of an action potential There are four steps to the generation of an action potential 1 Depolarization to the threshold a Graded potential reaches the axon hillock to depolarize it to threshold 2 Activation of the Na channels a Voltage gated Na channels cause rapid depolarization Na ions rush into the cytoplasm causing a positive charge in the inner membrane 3 Inactivation of Na channels and activation of K channels a At 30 mV the inactivation Na gates close K channels open and K exits the cell creating repolarization 4 Return to normal permeability a K channels begin to close when membrane reaches normal resting potential of 70 mV however finish completely closing when the membrane reaches hyperpolarization at 90 mV The Na channel gate is closed but inactivation gate is open With the help of sodium potassium pump 3 Na out and 2 K in the normal resting potential is reached Action potential is over o What is the difference between a closed gate and an inactivated gate Closed gate is when is when it is capable of opening whereas inactivated gate is close but not capable of opening Explain the refractory period The refractory period is the time period from beginning of action potential to return to resting state during which membrane will not respond normally to additional stimuli o What is the difference between the absolute refractory period and the relative refractory period Absolute refractory period is when no action potential is possible and the sodium channels are either open or they are inactivated happens at depolarization stage Relative refractory period is when another action potential can be made stimulus has to be VERY large it occurs during repolarization and when membrane potential goes back to normal Describe the role of the sodium potassium pump in action potential generation Like said before during hyperpolarization the sodium potassium pump comes to play 1 ATP 3 Na out 2 K in aka Exchange o Will cessation of this pump cause an immediate halt to action potential generation With a stop to this pump or no ATP Na and K concentration would disappear very slowly This would allow the neurons to stop functioning because an action potential could not be generated Describe the two forms of action potential propagation
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