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17 Cards in this Set
- Front
- Back
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2 TYPES OF ELECTRICAL COMMUNICATION SIGNALS |
- Action potentials (APs): Neurons communicate over short and long distances in the body through APs - Grade potentials (GPs): Permit short range communication only |
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DIRECTION OF ACTION POTENTIALS IN NEURONS |
- Action potentials initiated at the axon hillock - Flow in one direction from the axon hillock to the dendrites |
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GENERATION OF APs AND GPs DEPENDS ON 2 CHARACTERISTICS |
- The resting membrane potential - Selectively permeable ion channels |
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MEMBRANE POTENTIAL DIFFERENCES |
- Potential differences occur across plasma cell membranes as cells have selectively permeable membranes - In resting neurons this results in the separation of oppositely charged ions across their plasma membrane - Membrane proteins are responsible for membrane permeability - Resting membrane potential of neurons ranges from -40mV --> 90mV - Most neurons have a resting potential of -70mV |
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2 FACTORS SETTING RESTING MEMBRANE POTENTIAL |
- The relative distribution of ions across the membrane - The relative permeability of the membrane to ions that are present at the highest concentration across the membrane
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ION CONCENTRATIONS AT REST |
- In the extracellular fluid (ECF) the neurilemma of Schwann cells is rich in Na+ and Cl- ions - The cytoplasm is high in K+ and and the major anions are organic phosphates and negatively charged proteins |
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TWO PASSIVE FORCES ACTING ON IONS ACROSS THE CELL MEMBRANE ESTABLISHING THE RESTING MEMBRANE POTENTIAL |
- Electrical gradients - Concentration gradients
The resting membrane potential is established when these two opposing forces acting on the K+ are balanced
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MEMBRANE PERMEABILITY |
- Plasma membranes are impermeable to negatively charged proteins as they are too big - However some ions can cross the plasma membrane through leakage channels - The neuronal plasma membrane is 50-100 times more permeable to K+ than Na+ as there are more K+ leakage channels - Therefore ion currents that flow through K+ leakage channels largely determine the value of the resting membrane potential - -40mV is Na+ channel average |
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K+ MOVEMENTS |
- K+ ions can flow down concentration gradient out of neuron more readily than Na+ can flow down concentration gradient into cell - Flow of K+ out of cell results in a build up of negative charge in the cell interior generating an electrical gradient - This electrical gradient will tend to pull K+ back into cell |
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ELECTROCHEMICAL GRADIENT |
The sum of concentration gradient and electrical gradient acting on a particular ion is termed the electrochemical gradient of the ion |
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EQUILIBRIUM POTENTIAL |
- The membrane potential at which there is no net flow of a specific ion across the plasma membrane is the ions equilibrium potential - The Nernst equation can be used to calculate the equilibrium potential for a specific ion
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AT RESTING POTENTIAL |
- No net movement of K+ - Equilibrium potential of K+ is -90mV - Typical neuronal potential is -70mV - Difference is due to slight permeability of Na+ - Combined effects of the electrochemical gradients of K+ ions and Na+ ions determine the resting membrane potential of a neuron |
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MAINTING THE RESTING MEMBRANE POTENTIALS |
- Preventing the dissipation of the Na+ and K+ concentration gradients is prevented by Na+K+ATPase pumps - ATP required as pumping both against their concentration gradients - 3Na+:2K+ - This pump is electrogenic (production of electrical activity in living tissues) because it contributes to the negativity of the membrane potential
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SELECTIVELY PERMEABLE ION CHANNELS |
When open channels allow specific ions to flow through down their electrochemical gradient |
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GATED ION CHANNELS |
- Controlled by gates - Gates are the part of the ion channel that seals the ion pore - In response to an appropriate stimulus the gate undergoes a conformational change that leads to opening the channel |
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ION CHANNELS INVOLVED IN AP GENERATION |
- Leakage ion channels: gates alternate randomly between an open and closed stated, plasma membranes of neurons have significantly more leakage of K+ than Na+ - Voltage-gated ion channels: channels open in response to a change in membrane potential, key role in AP generation and conduction reflected by predominance at nodes of Ranvier
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IMPORTANCE OF ION CHANNELS |
- Essential for the normal functioning of the nervous system - However targeted by potent neurotoxins: tetrodotoxin from the puffer fish blocks voltage gated Na+ channels |
