CJH332H1 Lecture Notes - Lecture 4: Resting Potential, Nernst Equation, Membrane Potential
Lecture 4: Retracing Steps for Resting Membrane Potential
Squid Giant axon - squid not giant but the squid axons big enough to be seen with naked eye
Membrane potentials in squid axons
• The first experiments examining how changes in ion concentrations affect the
membrane potential were done on squid
• Why squid axons?
- Large diameter (mm – very few nerves that are quite large; bundles seen easily)
- Internal and external ion compositions can be controlled
• Nerve is so big that we can squeeze out the intracellular ions (axoplasm)
• With intact nerve, they were able to create AP
- Squeezing out nerve and fusing with new intracellular concentrations, the AP
were identical – they had right concentrations of different ions (figured out
internal and external concentrations)
The effect of Na+ permeability
• If membrane were only permeable to K+, Vm = Ek = -93mV according to equation
• But they measured Vm = -65 to -75mV
• Why the difference?
- Membrane is also slightly permeable at rest, to Na+
- Leak channels that cause Na+ enter and K+ leave – open all the time but still selective
- Resting membrane potential is dominated by leak channels while AP dominated by voltage gated channels
- Cells are less than 10% as permeable to Na+ as they are to K+
Taking stock
• Resting membrane potential compared to the outside is negative (Vm is -65mV)
• There are many different ion channels in neurons
• Some channels are open at rest (leak channels)
• Some channels which are gated by the membrane potential (voltage) or by neurotransmitter binding (ligand)
• How is the membrane potential thus calculated since it is dependent on all the ionic equilibrium potentials and
the ionic permeabilities?
What accounted for this missing gap?
• Current for any ion is determined by the difference between the membrane potential and the equilibrium
potential for that ion
• K+ Driving force: Ik = Vm - Ek
- Difference between membrane potential and equilibrium potential for particular ion
- This gives how many K ions can flow in at a given time = driving force
How to examine all possible currents
• Nernst equation considers only 1 ion species at a time
• To find out more about the actual membrane potential, we have to consider all the ions present
- i = current and g = conductance (average number of channels open at rest)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture 4: retracing steps for resting membrane potential. Squid giant axon - squid not giant but the squid axons big enough to be seen with naked eye. Large diameter (mm very few nerves that are quite large; bundles seen easily) Internal and external ion compositions can be controlled: nerve is so big that we can squeeze out the intracellular ions (axoplasm, with intact nerve, they were able to create ap. Squeezing out nerve and fusing with new intracellular concentrations, the ap were identical they had right concentrations of different ions (figured out internal and external concentrations) Membrane is also slightly permeable at rest, to na+ Leak channels that cause na+ enter and k+ leave open all the time but still selective. Resting membrane potential is dominated by leak channels while ap dominated by voltage gated channels. Cells are less than 10% as permeable to na+ as they are to k+
