BSC2085 Exam 3 Study Guide Lesson 13 14 Membrane Potential and Synaptic Transmission Ion Movements and Electrical Signals All plasma cell membranes produce electrical signals by ion movements Transmembrane potential important to neurons Five 5 Main Membrane Processes in Neural Activities o 1 Resting potential o 2 Graded potential The transmembrane potential of resting cell Temporary localized change in resting potential Caused by stimulus to the cell o 3 Action potential An electrical impulse produced by graded potential Propagates along surface of axon to the synapse o 4 Synaptic activity Releases neurotransmitter at presynaptic membrane Produces graded potentials in postsynaptic membrane o 5 Information processing Response integration of stimuli of postsynaptic cell Transmembrane Potential Three 3 important concepts o 1 The extracellular fluid ECF intracellular fluid cytosol differ greatly in ionic composition Concentration gradient of ions Na K o 2 Cells have selectively permeable membranes o 3 Membrane permeability Due to the presence of passive leak channels Passive Forces Acting across the Membrane o Chemical Gradients Concentration gradients chemical gradient of ions Na K o Electrical Gradients Separate charges of positive and negative ions Result in potential difference 1 BSC2085 Exam 3 Study Guide o Electrical currents resistance Electrical Current Movement of charges to eliminate potential differences Opposite charges attract Resistance The amount of a current a membrane restricts o Electrochemical Gradient The sum of the chemical and electrical forces acting on the ion across a plasma membrane Form of potential energy Active Forces across the Membrane o Sodium Potassium ATPase Exchange Pump Powered by ATP Carries 3 Na out 2 K in Balances passive forces of diffusion through passive channels Maintains resisting potential 70 mV Resting Potential o Plasma membrane is highly permeable to potassium ions o Electrochemical gradient for sodium ions is very large but the membrane s permeability to these ions is very low o Sodium potassium exchange pump ejects 3 Na ions for every 2 K ions that it brings into the cell Serves to stabilize the resting potential when the ratio of Na entry to K loss through passive channels is 3 2 o At the normal resting potential these passive active mechanisms are in balance The resting potential varies widely with the type of cell Typical neuron resting potential 70 mV Changes in the Transmembrane Potential 2 BSC2085 Exam 3 Study Guide o Transmembrane potential rises or falls Occurs in response to temporary changes in membrane permeability from opening or closing specific membrane channels Sodium and Potassium Channels o Membrane permeability to Na and K determines transmembrane potential o These channels are either passive or active Passive Channels Are always open permeability changes with conditions Flow is from greater lesser Active Channels Open close in response to stimuli At resting potential most gated channels are closed Three 3 States of Gated Channels o Closed but capable of opening o Activated open o Inactivated closed not capable of opening Three 3 Classes of Gated Channels o Chemically Gated Channels Open in presence of specific chemicals at a binding site Ex Ach neurotransmitter acetylcholine Found on neuron cell body dendrites o Voltage Gated Channels Respond to changes in transmembrane potential A change in membrane potential causes channels to open close Have activation gates open inactivation gates Characteristic of excitable membrane Found in neural axons skeletal muscle sarcolemma cardiac muscle o Mechanically Gated Channels Respond to membrane distortion vibration Found in sensory receptors touch pressure 3 BSC2085 Exam 3 Study Guide Transmembrane potential exists across plasma membrane because o Cytosol extracellular fluid have different chemical ionic balance Inside of cell has negatively charged proteins not usually found outside of cell o The plasma membrane is selectively permeable Transmembrane potential changes with plasma membrane permeability in response to chemical or physical stimuli Graded Potentials Local Potentials Changes in transmembrane potential that cannot spread far from the site of stimulation o Includes any stimulus that opens a gated channel which produces a graded potential o Resting State Opening sodium channels produces a graded potential Events include Resting membrane exposed to chemicals Sodium channel opens Sodium ions enter the cell 4 BSC2085 Exam 3 Study Guide Transmembrane potential rises Depolarization occurs 0 mV Events include o Depolarization A shift in transmembrane potential toward Movement of Na through channel Produces local current Depolarizes nearby plasma membrane graded potential Change in potential proportional to stimulus 5 Whether potential is depolarizing or hyperpolarizing they share four 4 basic characteristics BSC2085 Exam 3 Study Guide 1 The transmembrane potential is most changed at the site of stimulation and the effect decreases with distance o Ex Ripples in a pool 2 The effect spreads passively due to local currents o The spread is in all directions not just in one direction 3 Graded change in transmembrane potential may involve depolarization or hyperpolarization o The properties and distribution of the membrane channels involved determine the nature of the change o In a resting membrane Opening of sodium channels depolarization Opening of potassium channels hyperpolarization o The change in transmembrane potential reflects wehtehr positive charges enter or leave the cell 4 The stronger the stimulus the greater the change in the transmembrane potential the larger the area affected Repolarization Occurs when the stimulus is removed transmembrane potential returns to normal Hyperpolarization Caused by increasing the negativity of the resting potential Result a potassium channel opens Opposite effect of opening a sodium channel Net positive ions move out of cell o Effects of Graded Potentials At dendrites or cell bodies Trigger specific cell functions o Ex Exocytosis of glandular secretions At motor end plate Release of Ach into synaptic cleft Action Potential Action Potentials Propagated changes in transmembrane potential o Affect an entire excitable membrane o Link graded potentials at cell body with motor end plate actions 6 BSC2085 Exam 3 Study Guide Initial stimulus a graded depolarization of the axon hillock large enough 10 to 15 mV to change resting potential 70
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