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13 Cards in this Set
- Front
- Back
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Neurons are electrically excitable cells. Explain how electrical signalling in neurons takes place. |
They use to types of electrical signals to communicate: 1. Action Potentials (APs) - Neurons communicate over short and long distances in the body through APs, which are initiated at the axon hillock. 2. Graded Potentials (GPs) - permit short-range communication only. |
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What does the generation of APs and GPs depend on? |
They depend on two important characteristics of the neuronal membrane: 1. The resting membrane potential 2. Selectively permeable ion channels (when open, the channels allow specific ions e.g. K+ ions to flow through don their electrochemical gradient. |
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State the Na+, K+, A- and Cl- conc inside and outside the membrane and state the overall resting membrane potential in mV. |
Inside: -Na+ = 15mM -K+ = 150 mM -Cl- = 10mM -A- = 100mM Outside: -Na+ = 150mM -K+ = 5mM -Cl- = 120mM -A- = 0.2mM
Overall resting membrane potential mV = -70 across plasma membrane (Ranges from -40 to -90) |
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Why do electrical potential differences occur across cell membranes and what is the result of this? |
Because neurons have selectively permeable membranes and in resting neurons, this results in the separation of oppositely charged ions across their plasma membranes. |
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What 2 factors is the resting membrane potential set by? |
1. The relative distribution of ions across the membrane. 2. The relative permeability of the membrane to the cations and anions that are present at the highest concentrations across the membrane. |
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-What is the ECF outside the neurilemma of Schwann cells like? -State the major intracellular cation and anions present. |
-Rich in Na+ and Cl- ions > cytsolic concentrations. -Cation = K+ and the main anions are organic phosphates and negatively charged proteins. |
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How is the resting membrane potential established? |
-The plasma membrane is impermeable to the large negatively charged cytosolic proteins, they cannot cross the phospholipid bilayer. - However, some ions can cross the plasma membrane through leakage channels. - The neuronal plasma membrane is approx 50-100 times more permeable to K+ than Na+ ions because the conc of K+ ions inside is greater. - Thus K+ ion currents that flow through K leakage channels largely determine the value of the resting membrane potential. |
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If the plasma membrane were only permeable to K+ ions at rest, what would happen to K+ ions inside the cell? |
- They would flow out of the cell through and down the K+ ion conc gradient more readily than Na+ ions can flow down the Na+ ion conc gradient into the neuron. - The flow of K+ ions out of the cell results in a build up of -ve charge (-vely charged proteins) in the cell interior, which generated an electrical gradient. - This electrical gradient will tend to pull some K+ ions back into the cell interior until equilibrium is established. |
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When is the resting membrane potential established? |
When the two opposing forces due to the conc gradient and electrical gradient, acting on the K+ ions are balanced. The sum of the conc gradient and electrical gradient acting on a particular ion is termed the electrochemical gradient of the ion. There is no net flow of K+ ions across the plasma membrane at the resting membrane potential, it has reached it's equilibrium potential; Eq (can be worked out using the Nernst equation), which in this case is approx -90mV, a little lower than the typical neuronal membrane potential of -70mV due to the slight permeability of the plasma membrane to Na+ ions (the combined effect of the EG's of K+ and Na+ ions determine the resting membrane potential of a neuron). |
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How is the resting membrane potential maintained? |
- The dissipation of the conc gradients of K+ and Na+ ions due to leakage across the plasma membrane is prevented by the Na+K+ ATPase pumps. -Each ejects 3Na+ ions from the cytoplasm and simultaneously transports 2K+ ions back into the cell interior. - This pump is electrogenic (producing an electrical potential of a cell) because it contributes to the negativity of the resting membrane potential. |
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Explain gated ion channels. |
-Ion channels are controlled by 'gates' -Gates are the part of the ion channels which seal the channel pore. -In response to the appropriate stimulus the gate undergoes a conformational change that leads to channel opening. |
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State the ion channels required for APs and explain how they work. |
-Leakage Ion Channels - the gates alternate randomly between a closed and open state, the plasma membrane of neurons have significantly more leakage K+ ion channels than Na+ ion leakage channels. -Voltage-Gated Ion Channels - these 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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Ion channels are essential for the normal functioning of the NS and are targeted by potent neurotoxins. Give an example and state how it works. |
Tetrodotoxin from the Blowfish (puffer fish) blocks the voltage gated sodium channels. |
