Question

Neuroscience

1. Many sensory and synaptic ion channels are non-selective cation channels that pass Na+ and K+ equally well. Explain why these sensory ion channels are excitatory in neurons. Base your answer on what you know about typical ion equilibrium potentials, control of resting membrane potential in neurons and control of action potential threshold in neurons.

Answer 1

The binding of the chemical neurotransmitters on the receptors present on the dendrites causes opening of the ion channels to flow ions in and out of the neuron resulting the chemical signal converted into an electrical signal.Thus the inner resting potential which is negative(-70 mV); becomes less negative and the action potential is generated throughout the axons.There are more Na+,Cl- and Ca2+ outside the cell and more K+ and negatively charged proteins inside the cell.This gives an overall negative potential inside the cell.When a neurotransmitter binds to the receptors present on the dendrites, the ligand gated ion channels opens to flow the ions in.E.g.The ligand gated Na+ channels open to flow Na+ ions into the cell.The extra positive charge makes the inside environment less negative.This gain i positive charge is called depolarization.Neurotransmitters typically open various ligand gated ion channels.So Na+ and Ca2+ ions flow in.Meanwhile K+ ions go out which allows some positive charge to leave the cell.The net influx of positive charge is called Excitatory Post Synaptic Potential (EPSP).In contrast, the opening of the ligand gated Cl- channel causes influx of negatively charged ions causes net negative charge creating a =n inhibitory post synaptic potential(IPSP) making the cell potential more negative or repolarizing. A single EPSP or IPSP causes a small change in resting membrane potential.But, if there are multiple EPSPs throughout the dendrites, it collectively they can push the membrane potential to a specific threashold value typically about -55mV. When it hits this threashold value, it opens up the voltage gated Na+ channels at the start of the axon known as 'Axon hillock'.Na+ rushes into the cell.This will change the membrane potential to open up more voltage gated Na+ channels.This spread as a chain reaction extending throughout the axon.Thus finally cell become positively charged when compared to the external environment(+40mV).This depolarization process ends as the Na+ closes to cause stop the Na+ influx.This is called inactivation.For this,Na+ gated Na+ channels have inactivation gates which blocks the Na+ influx shortly after depolarization and stays in this state until the cell repolarizes and the channel enters the closed state again.So no Na+ further enters the cell.

Like Na+ voltage gated  channels, there are K+ voltage gated channels too, which are slow to respond and don't open along with Na+ channels until the already opened Na+ voltage gated channels get inactivated. . After the initial rush of Na+,K+ flows out of the cell down its electrochemical gradient removing some positive charge and affecting the Na+ depolarization.These K+ channels don't have a separate inactivation gates.And therefore they stay open for slightly longer time; which means there is a period when the net positive ions flow out of the cell.This results in membrane potential to become more negative or Repolarization.During this repolarization phase, the cell relies on the Na+-K+ pump which actively transport 3 Na= out of the cell and 2 K+ into the cell.The cell is in absolute refractory period during this repolarization period since Na+ channels are inactivated and won't respond to any type of stimuli.This absolute refractory period keeps the action potential to happen too closely and action potential moving in one direction only.The combined effect of this pump and extended opening of the K+ channels results in a small period of overproduction with n=the neuron becoming hyperpolarized in relation to resting membrane potential.During this time Na+ goes back to initial closed state and the K+ channels still in an open state.This period is known as relative refractory period.Finally the K+ channels also closes to form the resting membrane potential again.