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Transcript of Neurobiology
ions eg Na+
ligand -G-Na+ channel
Nerves and muscles are "excitable" - they propagate electrical signals. The basis of this excitability is ion movement
The opening and closing of ion channels underlies even the most complicated of activities
Wow! Ion channels are responsible for that awesome performance?
Tell me more!
Action potentials (AP's)
The action potential is an "all-or-none" sequence of opening and closing of voltage-gated sodium and voltage-gated potassium channels, that cause sequential depolarization and repolarization of the axonal membrane, until the depolarization reaches the axon terminal. A threshold level of depolarization is needed to open the first sodium channel
For any given excitable cell, every action potential is exactly the same. They are not summable and do not decay over distance
Nice video showing an overview of the action potential
Gating of sodium channels and refractory period
Graded potentials are small, variable changes in membrane potential that occur in the dendritic region when ligand or mechanically gated ion channels open. The ligand is very often a neurotransmitter
Graded potentials are summable and can be excitatory (EPSP's) (eg. sodium entry) or inhibitory (IPSP's) (eg. chloride entry)
If the graded depolarization is high enough at the trigger zone after the positive ion (eg. sodium) has diffused through the cell, then the first V-G Na+ channel can open and start an action potential
The equilibrium potential (E) of an ion is the voltage that exactly opposes the chemical concentration gradient of that ion
E Na+ = +60 mV
E K+ = -90 mV
E Ca++ = + 122 mV
E Cl- = -63 mV
For example, if you open sodium channels, sodium will diffuse into the cell, and the membrane potential will get more positive, but will not exceed +60 mV. 60 mV is so positive that is opposed any further sodium entry, regardless of the sodium concentration gradient
Membrane potential is the difference in charge between the inside and outside of the cell, which is established by difference in ion and protein concentration gradients across the membrane.
A typical cell has a resting membrane potential of -70mV, meaning that the inside of the cell is -70mV more negative compared to the outside
Ion channels are often "gated". The gate can be opened or closed in response to changes in voltage, mechanical stretch or binding with ligands eg. a neurotransmitter
Equilibrium potentials and diffusion of ions are the key to understanding graded and action potentials!
This video about EPSP's and IPSP's (graded potentials) is 12 min long but TOTALLY worth watching even though it adds a little bit more complexity than Silverthorn! He has an awesome accent too :)