The Excitable Cell Flashcards
What is the nervous system and what does it allow?
System of communication that allows an organism to react rapidly and modifiable to changes in its environment
Neurones need to:
- Collect
- Integrate
- Output
Why might chemical activity be used instead of electrical activity?
Provides more flexibility between and within cells i.e. inhibition
How can electrical signals be divided?
Action potentials and graded potentials
Action potentials
- Fixed size (all-or-nothing)
- Propagate along axon
- Usually one way
Graded potentials
- Variable size
- Not propagated over long distances - usually generated in the dendrites
- Can pass both ways
Information Coding
Action potentials are coded by frequency as they are of unit size
Graded potentials are coded by size and vary according to the strength of the stimulus
How do action potentials differ with a stronger stimulus?
Increased frequency
How do graded potential differ with stronger stimuli?
Bigger size
What is the resting potential?
-70mV
Why is there a resting potential?
- Selectively permeable membrane
- Unequal distribution of charged molecules/ions
- Physical forces e.g. diffusion, electrostatic
Why is the membrane selective and unequal?
- Channels are highly selective for specific ions
- Pumps assist unequal distribution
Equilibrium potential (E ion)
The membrane potential that would be achieved if the neurones membrane were selectively permeable to that ion
What equation can be used to calculate the equilibrium potential for an ion?
Nernst equation
What is the resting membrane potential for K+?
No voltage
How would you estimate the real membrane potential for K+?
Use the Goldman equation as it takes other ions into account, not just K+
How would you derive the Nernst equation from the Goldman equation?
Make permeability of Na = 0 in Goldman equation
What is the ionic driving force?
The ion movement rate
What is the ionic driving force proportional to?
Membrane potential to equilibrium potential to an ion
Action potential steps
DEPOLARISATION - Stimulus causes Na+ channels to open. Once threshold is released, voltage-gated Na+ channels open. Influx of Na+ down electrochemical gradient
Na+ channels close at +40mV and K+ channels open
REPOLARISATION - K+ diffuses out of cell down the concentration gradient
HYPERPOLARISATION - K+ channels reamin open and Na+ channels inactivated
Some K+ channels open so diffuses out and Na+/K+ pump restores resting potential
How is the action potential propagated?
The Na+ diffuses out although in both direction Na+ channels behind are inactivated as they are in the refractory period so only Na+ channels ahead can open
What are some factors affecting conduction velocity?
- Axon diameter
- Temperature
- Permeability of membrane
- Myelination
How does myelination alter the conduction velocity?
Myelin is an electrical insulator so no depolarisation of the membrane can occur where it is wrapped around the axon. There are regular breaks in the myelin called nodes of Ranvier in which depolarisation can occur. So this means longer local circuits and the impulse jumps from node to node which is called saltatory conduction and increases conduction velocity of an action potenetial
2 reasons for why are there so many unmyelinated, small axons?
- The benefit of a high membrane resistance is reduced by the high internal resistance
- Metabolic and volume costs of myelination
Which channels are blocked by tetraethylammonium?
K+ channels
K+ channels can be blocked by which poison?
Tetraethylammonium
Which poisons can block Na+ channels?
Tetrodotoxin and saxitoxin
Where are saxitoxins from?
Shellfish
Which poison is present in puffer fish?
Tetrodotoxin
Why do axons produce action potentials whereas dendrites don’t?
Axons contain a lot more voltage-gated Na+ channels
What kind of channels double as a GABA receptor?
Cl- channels
The nerve action potential involves a very large amount of Na+ ions moving into the axon: true or false?
False, although ion flow is rapid. The actual amount is very little
Are synaptic potentials graded potentials?
Yes
Where are action potentials generated in neurones?
Axon hillock
