BSC2085 Exam #3 Study Guide 1 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 The transmembrane potential of resting cell o 2. Graded potential 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 differenceBSC2085 Exam #3 Study Guide 2 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 PotentialBSC2085 Exam #3 Study Guide 3 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 Have activation gates (open) & inactivation gates (close) Characteristic of excitable membrane Found in – neural axons, skeletal muscle sarcolemma, cardiac muscle o Mechanically-Gated Channels: Respond to membrane distortion Found in sensory receptors – touch, pressure, vibrationBSC2085 Exam #3 Study Guide 4 • 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 cellBSC2085 Exam #3 Study Guide 5 • Transmembrane potential rises • Depolarization occurs o Depolarization: A shift in transmembrane potential toward 0 mV Events include: • Movement of Na+ through channel • Produces local current • Depolarizes nearby plasma membrane (graded potential) • Change in potential = proportional to stimulus Whether potential is depolarizing or hyperpolarizing, they share four (4) basic characteristics:BSC2085 Exam #3 Study Guide 6 • 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 •
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