Excitable Cells & Synapses Flashcards
Which cell membrane leak channel contributes most to the resting potential, K+ or Na+?
K+ leak channel
At rest, the membrane is 10x more permeable to K+ than to Na+ (i.e. there are 10x more leak channels for K+ than for Na+)
What ions (and in what direction) are moved by the Na+/K+ ATPase pump?
For one ATP, 3 Na+ are moved out of the cell and 2 K+ are moved in.
How does the inactivation gate of a sodium channel work?
The gate closer (ball) is positively charged. When the gate opens and sodium (Na+) rushes in, the local area becomes positively charged and the positively-charged ball is repulsed and wants to get out of the area - tries to go out the gate, which blocks it. This causes the refractory period (the gate can’t be opened, and sodium can’t enter). When enough sodium is pumped back out, or diffuses away from that area, the charge becomes less positive and the ball isn’t repulsed anymore and it falls out of the hole and gate closes. Now the gate is reactivated.
Which voltage-sensitive channel opens first in response to a depolarization? (Na+ or K+)
Na+ channel opens first. K+ channels open more slowly.
Do all of the voltage gated sodium channels open at the exact same time? Why or why not?
No, they do not all open at exactly the same time. Some of the channels have lower opening thresholds and open first (say at -50 mV) and some have higher thresholds and open after (say at 0 mV).
This establishes a positive feedback loop that accelerates the opening of all Na+ channels at peak of action potential
What is the difference between the absolute refractory period and the relative refractory period?
Absolute refractory period:
~No Na+ channels can open (all are inactivated) and so NO action potential can be generated
Relative refractory period:
~Na+ channels are resetting…some are inactivated still, some are available = the membrane is not yet back to resting potential but a HIGHER stimulus could generate an action potential (and this action potential would be SMALLER)
Why is the movement of an action potential unidirectional?
Because behind it everything is in refractory period (just opened)….the only available active gates are the ones ahead of the action potential
Where along the axon are voltage-gated Na+ channels located, in relation to myelin?
What happens in a demyelinating disease?
Voltage-gated Na+ channels are located at the spaces between clumps of myelin (these spaces are called Nodes of Ranvier).
In a demyelinating disease, the channels spread back out all along the axon, and the axon starts to act like an unmyelinated axon.
What cells produce myelin?
In CNS: oligodendrocytes
Outside CNS: Schwann cells
What ions are involved in the release of neurotransmitter into the synaptic cleft?
An action potential (propagated by voltage-gated SODIUM channels) arrives at the axon terminal.
The action potential causes voltage-gated CALCIUM channels in the axon terminal to open. Calcium flows into the axon terminal, and via second messenger system of some kind, causes neurotransmitter vesicles to release into the cleft.
What would happen to synaptic transmission in a calcium-free medium?
No release of neurotransmitter.
What kind of ions would produce an excitatory postsynaptic potential?
Excitatory = depolarizing = less negative
Na+ rushing in, Ca2+ rushing in
What kind of ions would produce an inhibitory postsynaptic potential?
Inhibitory = hyperpolarizing = more negative
K+ rushing out, Cl- rushing in
What are the 2 types of cholinergic receptors? Where are they found?
nicotinic (neuromuscular junctions of skeletal muscles)
muscarinic (smooth muscle. cardiac muscle. glands.)
The entire acetylcholine molecule is taken back up into the synaptic knob.
TRUE or FALSE?
FALSE: acetylcholinesterase breaks the molecule up into acetate and choline. CHOLINE is taken up into the synaptic knob and used to regenerate acetylcholine.
