Local Potentials

Terminology

We have seen these terms and this graph before when we talked about how a motor neuron excites a skeletal muscle fiber.  Now, we will speak about these terms in application to neuronsThe functional cells of the nervous system that transmit signals..  A Skeletal muscle is like a dead end.  It can receive the action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. but it can’t give that action potential to another cell.  Neurons can.

Let’s review.  A neuron sits or chills out at the resting potential, which we will set as -70mV.  It’s negative because of all those proteinsLarge molecules made of amino acids with various functions in the body. in a cell.  This side of the first hill on this graph represents depolarizationThe loss of electrical charge across a membrane, triggering an action potential..  In this process, the cell is becoming more positive.  This results from the influx of sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function..  The cell gains these cations and they make it more positive. 

This side of the hill is repolarizationThe return of membrane potential to a negative value after depolarization..  We Depolarized the cell, so now we have to Repolarize it back to the resting potential.  In repolarization, potassium(K⁺): Major ICF cation; essential for muscle and nerve function. cations move out of the cell.  The cell is losing positives and thus becomes more negative again.  Hyperpolarization is also defined as the cell becoming more positive.  The difference is that hyperpolarizationAn increase in membrane potential, making the inside of the neuron more negative. is defined as occurring when the cell is becoming more positives BELOW the resting potential.  Therefore, this is also attributed to the opening of potassium channelsProtein passages in the cell membrane that allow specific molecules to pass through..

Any and all channels we are referring to here are channels that are voltage gated.  Those are the channels that open in action potentials.  I know, you’re asking, “Amy, but how does the depolarization start if these are all voltage gated?”  Yes.  Depolarization associated with the local potential opens ligand gated channels.  If we can open enough of them, these voltage gated channels will start to open.

Local Potentials

Local or graded potentials are only able to create very localized potentials or changes in voltage.  Like a match, they fizzle with distance from the spark.  These local potentials only open ligand-gated channels and therefore can’t be propagated down an axon.  They get stuck at the axon hillockCone-shaped region of the neuron where the axon originates and where action potentials begin..  These ligand gated channels are only on the membrane where other neurons would be sending a signal to this one.  This would be only in the dendritesBranch-like extensions from a neuron that receive signals from other neurons. of this neuron.  Dendrites are full of ligand-gated channels.  If we open enough of them with a neurotransmitterChemicals that transmit signals across synapses., we will open voltage gated channels. This will occur on the axon hillock and axon.  This is somewhat where the axon hillock gets its name as a hill, or an area of resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter..

Post Synaptic Potentials

Post-synaptic potential is a fancier word for local or graded potentials.  We use this term post-synaptic potential to indicate the receptiveness of the receiving or the post-synaptic neuron.  A post-synaptic potential can make a receiving neuron more likely or less likely to have an action potential. A post synaptic potential that moves the resting potential more toward thresholdThe minimum voltage needed to trigger an action potential. would be called an excitatory post synaptic potential. We’re moving closer to threshold. Local or graded potentials in a post synaptic neuron increase the likelihood of an action potential. Notice that an EPSP is just a small little bump of depolarization.  The cell is becoming more positively charged from sodium cations moving into the neuron.  An EPSP does not guarantee that an action potential will occur. However, it makes the probability of an action potential more likely.

Inhibitory post synaptic potentials work in exactly the opposite way. With an inhibitory post synaptic potential, the resting potential is moved away from threshold, making an action potential less likely.  In the same way as EPSPs, IPSPs don’t guarantee that an action potential won’t happen. They make it harder to bring that neuron to threshold.  A neuron undergoing an IPSP will need to open additional ligand-gated channels. These channels are located in the dendrites. More channels are needed to create enough depolarization to carry the signal over the axon hillock.

Most general anesthetics cause IPSPs, deadening the activity of your nervous systemThe organ system that controls body functions using electrical and chemical signals..  They achieve this by opening potassium channels. This action allows potassium to flow out of the cell. It drives the resting potential to a more negative value.  This is not depolarization and it is not repolarization, this is hyperpolarization.

Summation

The previous slide talk about ONE action potential and its likelihood of happening.  Neurons are being bombarded with neurotransmittersChemicals released by neurons to transmit signals across a synapse.. These neurotransmitters constantly open up ligand gated channels on the dendrites. If the presynaptic neuronThe neuron that sends the signal across a synapse sends one action potential across the synapseThe junction between two neurons where communication occurs., it may send another shortly after. These two depolarizing events could sum. This could create an action potential in the post-synaptic neuron.  One of the sending Aps wouldn’t be strong enough to cause an AP in the post-synaptic neuron. However, both of them together are strong enough.  This is called temporal summationThe addition of multiple stimuli to increase contraction strength. because the post synaptic neuron is receiving a signal at two points in time.  These two points in time are close enough to build upon each other.

A post synaptic neuron can receive two Aps at the same time. These can occur at different places on the neuron.  For example, the post synaptic neuron in pink is receiving signals. These signals come from the axon terminals of the purple neuron. But if we were to add a third presynaptic neuron, I will do this in blue here. The post synaptic neuron might simultaneously receive two action potentials. These would come from two separate pre synaptic neurons. The sum of the two action potentials received at different locations could be enough to cause enough depolarization and cause an action potential

In both of these situations, the post synaptic neuron needs more than one sending action potential. This is required for it to have its own action potential.