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Summary Problem 3
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Summary Problem 3 - Inside the Neurons
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Problem 3 – Inside the neuronResting Membrane Potential
- membrane potential: difference in electrical charge/ voltage between the in- and outside of a cell
→when at rest the membrane maintains an electrical guarding – called polarization:
- to record a neuron’s potential the tip of an electrode is positioned inside the neuron and another
electrode outside the neuron in the extracellular fluid
- resting potential: the membrane potential of -70mV (inside)
→ the potential inside a resting neuron is about 70 mV less than outside the neuron
- neurons are said to be polarized in the resting state
Iconic Base of the Resting Potential
- is a salt solution
- there are many different kind of ions in neurons
- most important ions:
• sodium ions – Natrium (Na+)
• potassium inos – Kalium (K+)
In resting Neurons
- concentration gradient:
• there are more Na+ions outside the cell than inside the cell
• there are more K+ions inside the cell than outside
→ there are ion channels in neural membranes through which ions can pass
→ each ion channel is specialized for the passage of particular ions
→ the unequal distribution remains are maintained even though there are ion channels
Two types of pressure on Na+ ions to enter the resting neurons:
1. electrostatic pressure
→ from the resting membrane potential
→ -70mV attracts positive Na+ ions
2. pressure from random motion
→ for Na+ions to move down their concentration gradient
→ area of high concentration to area of low concentration
(more likely to move down than up → are more likely to move from areas of high
concentration to low concentration than vice versa)
Why Na+ ions don’t rush in:
- the sodium ion channels in the resting neurons are closed reducing their flow into the neuron
→ there is selective permeability which only lets certain things through
- potassium channels are open but only a few K+ ions exit – due to the negative resting membrane
potential
→ they are largely held inside by the negative resting membrane potential
Sodium-Potassium Pumps:
- ion transport in the cell membrane of neurons that exchange three Na+ ions inside the neuron with
two K+ ions outside
→ the ions are continually exchanged
→at the same rate that Na+ions leak into the resting neurons other Na+ions are actively transported
out (vice versa for K+ ions – same out and back in)
- the resting membrane potential stays fixed despite some Na+ions leaking inside and some K+ions
leaking outside
, - potassium is positively charged and the inside if the cell is negatively charged – so the electrical
gradient tend to pull potassium in
- potassium is more concentrated inside the cell than outside – so the concentration gradient tends to
drive potassium out
Three factors influencing ion distribution during resting stage:
1. Sodium-Potassium pump For every 2 Potassium going inside, 3 sodium going out →
more positive ions outside → higher potential (electrochemical gradient)
2. Electrostatic pressure → more positive ions inside due to electrical charge of ion
(positive ions attract negative ions), reason for sodium entering
3. Pressure from random motion due to gradient → influences diffusion (random
movement of ions from high to low concentrated density) and osmosis (liquid changing
from high to low concentration → movement of molecules)
Generation and conduction of postsynaptic potentials:
- neurotransmitters: a chemical that is released from the terminal buttons of neurons when they fire
→ it diffuses across the synaptic clefts
→ interacts with specialized receptor molecules on the receptive membrane of the next neurons in
the circuit
- when neurotransmitter molecules bind to the postsynaptic receptors
→ they may either depolarize the receptive membrane or hyperpolarize it
- depolarize: decrease in the resting membrane potential
- hyperpolarize: increase in the resting membrane potential
- Postsynaptic depolarization = excitatory postsynaptic potentials (EPSPs)
→ they increase the likelihood that the neuron will fire
→ Sodium gates open
- Postsynaptic hyperpolarizations = inhibitory postsynaptic potentials (IPSPs)
→ decrease the likelihood that the neuron will fire
→ cannot flow along axon – decay over time and distance
→ potassium or chloride gates open
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