You Should Be Able To Exam 3 1 Describe the main membrane processes in neurons a Five main membrane processes in neural activities i Resting potential ii Graded potential 1 The membrane potential of resting cell 1 Caused by stimulus 2 Temporary localized change in resting potential iii Action potential an electrical impulse 1 Produced by graded potential 2 Propagates along surface of axon to the synapse iv Synaptic activity 1 Releases neurotransmitters at presynaptic membrane 2 Produces graded potentials in postsynaptic membrane v Information processing 1 Response integration of stimuli of postsynaptic cell 2 Explain the transmembrane potential a Transmembrane potential refers to the difference in electrical potential voltage across b Why is it important What are the main ions molecules that generate this potential the membrane lining of a cell i 3 important concepts 1 The extracellular fluid and intracellular fluid differ greatly in ionic composition a More Na outside more K inside 2 Cells have selectively permeable membranes a Water soluble substances including ions cannot pass through hydrophobic region of lipid bilayer they need a channel or carrier protein 3 Membrane permeability varies by ion a Due to the presence of specific passive leak channels 3 Explain the electrochemical gradient a Gradient of electrochemical potential usually for an ion that can move across a membrane b The difference in charge and chemical concentration across a membrane c Gradient consists of 2 parts the electrical potential and a difference in the chemical concentration across a membrane d Chemical gradient i Concentration gradients of ions Na and K e Electrical gradients i Separate charges of positive and negative ions ii Results in potential differences f The electrical and concentration gradients of a membrane tend to drive sodium into and potassium out of the cell and active transport works against these gradients g To move substances against a concentration or electrochemical gradient the cell must utilize energy in the form of ATP during active transport h Why is it important 4 Explain the role of sodium and potassium ions on the membrane potential a Sodium potassium ATPase exchange pump i Powered by ATP ii 3 NA OUT and 2 K IN iii Balances passive forces of diffusion iv Maintains resting potential 70 mV b Plasma membrane is highly permeable to potassium ions c Electrochemical gradient for sodium ions is very large but the membrane s permeability to these ions is very low d Membrane permeability to Na or K determines the membrane potential e They are either passive or active i Passive Channels Leak Channels 1 Always open 2 Permeability changes with conditions ii Active Channels Gated Channels 1 Open and close in response to stimuli 2 At resting potential most gated channels are closed 5 Explain how gated channels function a What are the different types of gates channels i Chemically gated channels 1 Open in the presence of specific chemicals at the binding site 2 Found on neuron cell body and dendrites ii Voltage gated channels 1 Respond to changes in membrane potential 2 A change in membrane potential causes the channels to open or close 3 Have activation gates open and inactivation gates close 4 Na has 2 gates while K has only 1 gate 5 Found in neural axons skeletal muscle sarcolemma cardiac muscle iii Mechanically gated channels 1 Respond to membrane distortion 2 Found in sensory receptors touch pressure vibration b What are the different states to a gated channel i Closed but capable of opening ii Open activated iii Closed not capable of opening inactivated 6 Describe the difference between action potentials and graded potentials a Both result from a depolarization in the resting potential of a plasma membrane b Difference is found in strength c Graded potentials i Weaker but have the ability to generate action potentials ii Local do not spread very far iii Magnitude of potential is determined by magnitude of stimulus 1 Like throwing a pebble into a pond the ripples that form travel a short distance before they disappear If the stimulus is not constant the graded potential will die out iv v Graded potentials are normal in cells with no action potentials d Action Potentials Na rapidly explode potential i Start off slow depolarization and due to positive feedback loop from incoming ii The magnitude of the depolarization is uniform all or none to trigger the action iii Travels long distances iv Is self perpetuating in one direction down the axon 7 Explain how graded potentials can generate action potentials a The resting state i Opening sodium channel produces graded potential 1 Resting membrane is exposed to chemical 2 Sodium channel opens 3 Sodium ions enter the cell 4 Membrane potential rises 5 Depolarization occurs because the ion is positively charged ii Depolarization 1 A shift in membrane potential toward 0 mV a Movement of Na through the channel b Produces local current c Depolarizes nearby plasma membrane d Change in potential is proportional to stimulus i Small pebble small ripples Rock bigger ripples iii Whether it is depolarizing or hyperpolarizing there are 4 basic characteristics 1 Membrane potential is most changed at the site of stimulation and the effect decreases with distance 2 The effect spreads passively due to local currents a All directions 3 Graded change in membrane potential may involve either depolarization or hyperpolarization a Example In a resting membrane opening of sodium channels depolarization opening of potassium channels hyperpolarization 4 Stronger the stimulus the greater the change in the membrane potential and the larger the area affected 1 When stimulus is removed membrane potential returns to normal iv Repolarization v Hyperpolarization Increasing the negativity of the resting potential 1 2 Result of opening potassium channel 3 Opposite effect of opening a sodium channel 4 Net movement of positive ions out vi Effects of Graded Potentials 1 At cell dendrites or cell bodies a Trigger specific cell functions 2 At motor end plate a Release Ach into synaptic cleft b Which region of the neuron generates the AP i Trigger zone 8 Describe the steps to the generation of an action potential a Initiating Action Potential i Initial Stimulus 1 A graded depolarization of axon hillock large enough to change resting potential to threshold level of voltage gated sodium channels 2 The graded depolarization can be from one source or multiple as long as it depolarizes
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