Q. 18

[li5504552]

-is extracellular fluid the inside of a cell? What exactly does it refer to?
Thanks.

[admin]

intra = in (ie, intramural means within the walls of your school, intravascular means within blood vessels, intracellular means within cells)

extra = outside (ie, extracurricular means outside of your curriculum/studies, extracellular means outside of our cells)

There are 3 places where fluid can be in your body: (1) intracellular, (2) intravascular or (3) litterally called in medicine "the 3rd space" or interstitial space which is extracellular.

[tjk447963]

I thought membrane hyperpolarization results from extra potassium leaking out of the cell, which removes positive charge from the cell making the potential more negative. Therefore, I don't understand why the correct choice involves Cl- and not K+?

[emilie.maz5603]

Remember: when the postsynaptic membrane potential is more negative than the threshold, firing of another action potential is inhibited.

As an example of inhibitory postsynaptic action, consider a neuronal synapse that uses GABA as its transmitter. At such synapses, the GABA receptors typically open channels that are selectively permeable to Cl-. When these channels open, negatively charged chloride ions can flow across the membrane.

Assume that the postsynaptic neuron has a resting potential of -60 mV and an action potential threshold of -40 mV. If ECl is -70 mV, transmitter release at this synapse will inhibit the postsynaptic cell (because ECl is more negative than the action potential threshold). In this case, the electrochemical driving force (Vm - Erev) causes Cl- to flow into the cell, generating an outward PSC (because Cl- is negatively charged) and consequently a hyperpolarizing IPSP. Because ECl is more negative than the action potential threshold, the postsynaptic membrane potential is more negative than threshold, inhibiting an action potential.
Here is the same thing, explained another way:
Despite their name, some neurotransmitters inhibit the transmission of nerve impulses. They do this by opening

•chloride channels and/or
•potassium channels in the plasma membrane.
In each case, opening of the channels increases the membrane potential by
•letting negatively-charged chloride ions (Cl-) IN and
•positively-charged potassium ions (K+) OUT

This hyperpolarization is called an inhibitory postsynaptic potential (IPSP).

Although the threshold voltage of the cell is unchanged, it now requires a stronger excitatory stimulus to reach threshold.

Example: Gamma amino butyric acid (GABA). This neurotransmitter is found in the brain and inhibits nerve transmission by both mechanisms:

•binding to GABAA receptors opens chloride channels in the neuron.
•binding to GABAB receptors opens potassium channels.