NAS L1&L3 Flashcards
What is an electrical signal?
Change in balance of positive and negative charges
How is an electrical signal generated?
By transfer of ions through ion channels
Conc of K+ in and out of cell?
Inside: 100 high
Outside: 5 low
Conc of Na+ in and out of cell?
Inside: 15 low
Outside: 150 high
Conc of Ca2+ in and out of cell?
Inside: 0.0002 low
Outside: 2 high
Conc of Cl- in and out of cell?
Inside: 13 low
Outside: 150 high
What maintains ionic gradients?
Na+/K+ pump. Exchanges Na+ from inside out and K+ from outside in, against conc gradients. Ca2+ pump transports Ca2+ out of cell. These pumps operate in the background continuously, over long time periods. Constant K+ efflux will lead to significant change in conc. PUMPS MAINTAIN CONCS OVER LONG TERMS NOT FOR CONTROLLING Em.
What is resting membrane potential?
Voltage inside a cell.
What is the RMP for neurons?
-65mV
What determines RMP?
Leak channels, ion movement is passive. Non-gated (leak) - set Em for resting membrane.
There is an excess of negative charge inside.
How does RMP remain stable?
By the movement of ions across the membrane.
At RMP how much Na influx and how much K efflux?
Na influx = K efflux
What factors influence movement of the ions?
Chemical gradient = unequal ion distribution and electrical force = if attracted/repelled by voltage inside cell.
When electrical and chemical influence balance there is no net movement of ions.
What controls ion movement across a resting membrane?
Present of ion channels.
Non-gated: K, Na channels
Therefore membrane has permeability to each ion. Permeability is in part dependent on number of channels for that ion.
Nernst - equilibrium potential of an ion…
the voltage at which equilibrium of ions takes place depends on permeability and conc gradient.
What equil potential for Na and K
ENa = +62mV EK = -80mV
What is ionic driving force?
net force from chemical and electrical influences. Ionic force is present whenever Em is different from equilibrium potential for the ion
Why is Em -65mV in neurons?
Rests between equilibrum potentials for EK and ENa (and other ions; Cl- and Ca2+). But actual value depends on relative permeabilities. PK = 40 x PNa, so Em closer to Ek
How do you achieve positive charge?
Need positive charge influx
So need to open more Na channels i.e. increase in PNa
What causes repolarisation?
Increase in K+ efflux and terminating activity of ‘extra’ Na+ channels
What is conductance (g) of ion channels
= permeability (conductance easily measured). Membrane acts as an electrical resistor (R). conductance (g) = 1/R. Each ion has its own conductance
g = number of open ion channels. Change in g for one ion –> change in Em. This is due to changes in amounts of charge inside the cells.
Which channels open during depolarisation?
Na v.g. and leak Na. V.g. are ‘extra’?
What causes the initial depolarisation?
Synapses and generator potential
Describe the events underlying an action potential.
1) opening of v.g. Na channels (increase in Na conductance). Na entering leads to further depolarisation leading tofurther increase in Na conductance. Em approaches ENa.
2) Na channels inactivate (decrease in Na conductance). Na influx stops despite maintained stimulus. Delayed opening of v.g. K channels leads to increase in K conductance, K efflux, repolarisation (but K conductance increases after Em returns to rest, so Em approaches Ek - hyperpolarisation.
3) V.g. K channels close leading to decrease in conductance
4) Em returns to resting value (leak channels)
During AP why does Em approach ENa?
At rest PNa = PK/40. During AP PNa = 1000 x PK
What are action potentials described as as events?
All or none events. If Na influx by leak and v.g. > K efflux by leak then you get depolarisation. Threshold is the Em value where this condition is satisfied. A stimulus needs to bring cell to threshold. Na influx leads to depo which leads to opening of Na channels which leads to Na influx etc
How does the ap spread in a myelinated axon?
Myelin sheath increases speed of A conduction: ions flow to next node. Depo and A.P. at each node.
How does ap spread in an unmyelinated axon?
Passive spread of depolarisation and v.g. Na channels. Positively charged ions inside repel negative ones out, depo, opening of na channels and this moves along the axon.
