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4. Membrane Potentials.pdf

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Anatomy & Cell Biology
ANAT 262
John Presley

Membrane Potentials Neurons are a special type of cell because they can use their electrical properties to produce signals, called action potentials, to transfer information to one another Neurons communicate through joinings called synapses, through which the APs are sent from one neuron to another Ion channels play a large role in the production and propagation of action potentials o Remember, they only mediate passive transport A neuron has 3 parts: a soma, the cell body, an axon and dendrites; the somatodendritic part is used to receive information while the axon is used to send information Neurons process information in the form of electrical signals (nerve impulses or action potentials) that travel along their axons, usually to the somatodendritic portion of another neuron Electrical charges move across the membrane in the form of charged ions. Their movement is selectively controlled by ion channels and ion pumps The ion channels and pumps are responsible for establishing and maintaining a voltage difference across the plasma membrane at rest, i.e. resting membrane potential Ion channels cycle between open and closed conformations (gated); when open, a channel provides a continuous pathway through the bilayer, allowing flux of many ions down their electrochemical gradient Gating an ion channel is a form of allosteric regulation Channels transiently increase the membranes permeability of an ion such that the transmembrane potential will move in the direction of that ions equilibrium potential Cellular ion channels usually consist of large, 3-5 subunit protein complexes with multiple transmembrane -helices that form a narrow highly selective (ion specific) pore (with saturable kinetics) o Ions move through the hydrophilic pore in a single file, losing any water molecules they associate with along the way o Because of the pore, they never come into contact with the hydrophobic core of the lipid bilayer Channels can flux up to 100 million ions per second (x10 transporters)-FAST Control of channel gating (opening & closing) is a form of allosteric regulation. Conformational changes associated with channel opening may be regulated by one of the following: Voltage (voltage gated) Binding of a ligand (a regulatory molecule-neurotransmitter, ion, nucleotide) (ligand gated) o Similar to a receptor Membrane stretch (e.g., via a link to the cytoskeleton) (mechanically gated) Neurons express multiple ion channels types, located in different domains, which are responsible for mediating most forms of neuronal electrical signalling o The localization of these receptors is important for their function Most channels exist at any given time in one of three states (open, closed and inactivated or desensitized), only one of which allows the flow of ions: The open state is the only one that permits a flow, while closed acts like a dormant state waiting to be activated o The inactive state is a short period between the two where the channel cannot be opened Desensitization/inactivation is crucial for allowing membranes to re-establish ion flow prior to re-opening of these channels The study of ion channels has focused on three major aspects: Gating, mechanism of channel opening Selectivity: nature of ion transported Inactivation: mechanism of channel inactivation A lot more is known about the voltage gates channels than the ligand gated ones What we know mostly comes from studies on: Neuronal voltage-gated Na+ and K+ channels, a tetramer in which each subunit has 6 TM domains and one re-entrant loop (not a full TM domain, not hydrophobic enough) Bacterial/ Drosophila K+ Channels To look at the structure function relationship in gating, we will look at the voltage gates potassium channel, the tetramer mentioned above As with all others of its type, this channels open/close cycle depends on a change in voltage The S4-S5 helix responds to changes in the membrane potential by twistingfrom a horizontal position to a slightly more vertical position This causes the S5 and S6 transmembrane helices to also adopt a more vertical orientation, which opens up the pore This change happens on each of the subunits, which each have a voltage sensing S4-S5 portion The switch between the open and closed conformations is extremely rapid and it is very sensitive to changes in the membrane potential Unlike carriers, the conformational change is very small, which allows for the high speeds In terms of specificity, ions can get through channels on the basis of size and charge In the potassium channel, a TXGVG signature peptide within a P-loop confers specificity by mimicking the hydration shell of a potassium ion o The P-loop is the region of the protein which faces the pore o This allows the potassium ion to lose the water it associates with and travel through the pore Any other cation will have a different hydration shell, and wont fit properly into the pore; in this case, Na wont be able to lose all of its water molecules because its too small Upon long period of depolarization, a channel will inactivate by block
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