Question

READ THIS WELL: If you answer with something stupid/inccorect (e.g. telling me what a depolarization is or telling me that a small postsynaptic cell helps a signal carry, I'll give you no credit)

One property of electrical synapses is that the postsynaptic neuron tends to be smaller than the presynaptic neuron. Based on what you know about the relationship between neuron size and input resistance, what is the advantage of this arrangement? Please answer this in terms of Ohm's law considering input resistance and delta voltage. I'm not sure if I should say in my answer that a greater delta-v (which corresponds to a greater input resistance seen in smaller cells) causes the action potential to actually OCCUR in the postsynaptic cell, or if it just makes the action potential happen QUICKER? is the rate of propagation changed or is there just more likely that an actual action potential will happen (with a bigger postsynaptic cell than presynaptic cell)?

Answer 1

Internal resistance is determined by characteristics inside the neuron that impede current flow (i.e., length/size of the neuron, length/diameter of the axon; internal neuronal constituents).

Membrane resistance is a term that refers to the general state of the neuronal membrane. In this case, as more channels are open, the membrane resistance is lower (i.e., the easier passage of ions and thus, less "resistance" of current flow).

Input resistance and membrane resistance are closely related but not exactly the same thing. Input resistance is essentially a measure of how much current is needed to change the voltage of the membrane wherein higher input resistance means that less synaptic input (or current injection) can lead to larger changes in the voltage.

In Ohm's law, it is "input resistance" (and not membrane resistance) that is measured in

"V=IR" (where "R" = input resistance, "V" = membrane Voltage potential, and "I" = current). It has been said that, mathematically,

"input resistance" is the sum of "membrane resistance" and "electrode resistance" (Barbour, 2014).

Eg: The more channels open in a membrane = lower membrane/input resistance.

Action potentials (those electrical impulses that send signals around your body) are nothing more than a temporary shift (from negative to positive) in the neuron’s membrane potential caused by ions suddenly flowing in and out of the neuron. During the resting state (before an action potential occurs) all of the gated sodium and potassium channels are closed. These gated channels are different from the leakage channels, and only open once an action potential has been triggered. We say these channels are “voltage-gated” because they are open and closed depends on the voltage difference across the cell membrane. Voltage-gated sodium channels have two gates (gate m and gate h), while the potassium channel only has one (gate n).

• Gate m (the activation gate) is normally closed and opens when the cell starts to get more positive.
• Gate h (the deactivation gate) is normally open, and swings shut when the cells get too positive.
• Gate n is normally closed but slowly opens when the cell is depolarized (very positive).

Once your action potential reaches the terminal bouton (or synaptic bulb or whatever), it triggers the opening of Ca2+ channels, and because a high extracellular concentration of Ca2+ was maintained, it will rush into the terminal region. synaptic vesicles are then prompted to fuse with the presynaptic membrane so it can expel neurotransmitters via exocytosis to the synapse.

The action potential travels down the axon, opening/closing voltage-gated proteins (etc.) toward the terminal where voltage-gated Ca2+ channels will open and let Ca2+ inside where the synaptic vesicles will fuse with the presynaptic membrane and let out their contents in the synapse (typically neurotransmitters).