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Summary Excitability and ionic transport/ Action potential
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Lecture 2 : Excitability and ionic transportSelective membrane permeability : the lipid barrier of the cell membrane & cell membrane transport proteins
1) Diffusion
- Simple diffusion
● lipid-soluble subst (O2, CO2, Alcohols) → through intermolecular spaces of lipid
barrier
● water, lipid-insoluble (water solubles and small enough), through a membrane
opening- protein channels
○ selective permeable channels - PORE, always open (diameter/ shape/
internal electrical chage, chemical bonds)
○ non-gated/ gated (open/ close by a gate)
■ voltage gated (changes in membrane potential)
■ ligand-gated (chemical-gated)
- Facilitated diffusion = carrier mediated diffusion (AA. & GLUCOSE)
2) Active transport
- Primary Active (pumps) - CARRIER
- Secondary Active (co- and counter-transport)- CARRIER
Membrane ionic transport system (MITS)
1) Ion channels
- Gated (active) Ion channels
● Voltage gated
○ Na+ channel
■ rapid electrical signaling
■ activation :
● resting membrane : (-90-70mV) closed
● opens when depolarize, -50mV
■ Components :
● Ion selective filter for Na+
● Activation gate, open and close as controlled by voltage
sensors; respond to level of the membrane potential
● inactivation gate : limits the period of time the channel
remains open, despite steady stimulation
● ɑ subunit : poly chain embedded in cell memb.
○ non polar side chain face : outward, interact w
lipids of the membrane.
○ polar side chain face inward, separated by the
lipide membrane
● β subunit : anchor the channel to the plasma membrane
● Ligand gated
a) Ionotropic receptor - directly gated ion channels
● Receptors for
○ Ach/ nicotine
○ serotonin
○ GABA
○ glycine
● IP3 receptors & the Ca2+ - release channel (Ryanodine receptor),
both are Ca2+ channels located in the membrane of intracellular
organelles
b) Metabotropic, G-protein coupled receptor (GPCRs) - indirectly gated ion
channels, act via ‘’ metabolic ‘’ path / 2nd messengers
● largest family of receptor on the cell surface
● Glutamate receptors !!!!!!!
○ NMDA & AMPA ionotropic receptors
○ metabotropic receptors, grp I & II (G-protein coupled rec)
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