Unit 1: Action Potentials, Propagation and synapses

What are the three types of ion channels?

ligand gated→ chemically gated→ neurotransmitters

mechanically gated→respond to physical forces

voltage gated→ respond to changes in voltage

Outline the positive feedback that happens at the action potential threshold.

A stimulus cause an initial depolarization (by opening some sort of ion channel). This initial depolarization stimulates the voltage-gated sodium channels to open, causing a rapid influx of sodium and further depolarizing the membrane. This depolarization causes more channels to open, until all of the available channels are recruited and the action potential moves forward.

Why would a stimulus not initiate this positive feedback cycle?

Because it just wasn’t enough of a depolarization to activate the voltage gated sodium channels.

What is the concentration of sodium inside and outside the cell and RPM

15mM inside the cell and 150mM oitsode the cell

What is the concentration of Potassium at RPM

150mM on the inside and 5mM on the outside

What does ATPase do?

It pumps 3 sodium out of the cell and 2 potassium into the cell, against their concentration gradient because potassium wants to flow out and sodium wants to flow in.

What 2 conditions does electrical activity require

A selectively permeable membrane and a difference in concentration across thy membrane

Is there more sodium or potassium outside of the cell at RPM

More Sodium outside the cell

What are three types of ion channels in our cells?

Ligand gated (chemical), mechanically gated (physical) and voltage gated

What are the sodium potassium channels doing at rest?

The activation gate of the sodium channel is closed and the inactivation gate is open.

What are Schwann cells?

A type of glial cell that forms insulating myelin sheaths around the axons of single neurons in the peripheral nervous system.

How much does myelin increase action potential speed?

50 times

What are oligodendrocytes?

A type of glial cell that forms insulating myelin sheaths around the axons of multiple neurons in the central nervous system.

What is membrane depolarization

Making the membrane more positive. Happens when sodium enters the cell

What is membrane repolarization?

Making the membrane more negative, happens when potassium leaves the cell

What is a graded potential?

Any sort of stimulus that does not initiate an action potential. There will still be a depolarization and repolarization, but because since it does not reach the threshold then there is no action potential that occurs.

What are the voltage gated channels doing at rest?

The sodium channel’s innactivation gate is open at the activation gate is closed. This insures that they are ready as soon as s depolarization happens to allow sodium to enter. The Pottasium gate is closed.

What is happening to the ion channels during the rising phase of an action potential?

The initial depolarization opens the activation gate of the sodium channel and so sodium floods into the cell, depolarizing it to +50.

What is happening to the ion channels at the peak of the action potential and into the falling phase?

The membrane reaches about +50mV and this activates the inactivation gate of the sodium channel, closing it. The activation gate remains open for a time. The potassium gate is now open and potassium rapidly leaves the cell, repolarizing the membrane.

What is happening to the ion channels during the afterhyperpolarization phase of an action potential?

Potassium floods out until it reaches it’s equilibrium potential of -90mV. At this point, the sodium activation gate has closed and the potassium gate slowly beings to close.

What causes the refractory/ recovery period?

The fact that both the inactivation and activation gates on sodium are closed. There cannot be an action potential until the inactivation gate has opened and that will not happen until the cell has returned to the resting membrane potential (because they are voltage-gated). It also takes time for the potassium gate to close.

Compare the relative speed of the ion channel gates and their importance

The sodium activation channel is the quickest, it opens rapidly following a depolarization.

The sodium inactivation gate is medium. It takes its time to open after the action potential has occured.

The potassium channel is the slowest, and this is important because it’s openijg is trigger by the depolarization, but it in order for the action potential to occur there needs to be a delay in when it actually opens.

Explain the Nernst potential equation (what it actually tells us)

It tells us what the equilibrium potential is for our ions. It is the point where the concentration gradient is matched with the electrical gradient. For sodium, this is at +60mV and for potassium this occurs at -90.

why is the resting membrane potential -70?

Because the membrane is much more permeable to potassium than it is to sodium. There are many potassium leak channels which have a slow and constant outflow of potassium. This means that the equilibirum potential of the cell will be more impacted by potassium than it is by sodium and thus sits closer to that side of the equilibrium.

what is electrotonus?

electrical signal propagation, without an action potential. Essentially, how far can one signal passively travel down an axon?

What is membrane capacitance?

It is two conductive plates separated by an insulating layer which can store electrical charge.

Basically, the ability of a membrane to store electrical charge→ how much charge must be stored to change voltage by a certain amount.

Having a higher capacitance means that the membrane is storing more charge and so it takes longer to change the voltage.

What is the effect of myelin on capactitance?

myelin decreases membrane capacitance by increasing the thickness of the insulating layer.

what is membrane resistance?

Membrane resistance refers to how much charge is able to leak out of a membrane. More membrane resitance means that more charge can leak out.

what is the effect of myelin on membrane resistance?

myelin decreases membrane resistance, meaning that less charge is able to leak out.

How does reducing capacitance and increasing membrane resistance effect action potential propagation in myelinated areas?

In myelinated areas, the reduced capacitance means that it now takes less time to charge the membrane: as the positive charge is propagated down the axon, the negative areas charge more quickly and the signal travels faster. Because there is increased membrane resistance, there is less charge that is able to leak out and so the signal travels further.

What is saltatory conduction?

At the mylenated sections of an axon the action potential (signal) travels passively and quickly due to the reduced capacitance and high membrane resistance. At the Nodes of Ranvier (without myelin), there are many voltage gated channels which carry the signal through active propagation (many action potentials) as there is increased membrane capacitance and the charge time is much slower. The signal goes super quick along the myelinated regions and then slows right down at the Nodes.

where do synapses occur?

synapses occur from the axon terminal of the presynaptic membrane to the dendrites and cell body of the post synaptic membrane.

Outline what happens on the presynaptic side of the membrane during a synpase

1. The action potential moves down the axon, depolarizing the membrane of the axon terminal.

2. This depolarization cause voltage-gated calcium channels to open, and calcium ions flood into the cell.

3. Calcium binds with presynaptic vesicles which contain neurotransmitters and facilities their transport and binding to the presynaptic membrane (by causing a conformational change which allows the vesicles to bind)

4. The neurotransmitters are now released from the presynaptic membrane and can float across the synaptic cleft where they will eventually bind to post synaptic receptors.

Outline in general terms what happens on the postsynpatic side of the membrane. 

A neurotransmitter binds to a ligand-gated channel, allowing the channel to open. The opening of this channel will cause a change in voltage which is called a post synaptic potential.

What is an excitatory post synaptic potential? (EPSP)

A post synaptic potential which depolarizes the membrane→ more likely for action potential to occur

What is an inhibitory post synaptic potential? (IPSP)

A post synaptic potential which repolarizes (more negative) the membrane→ less likely for action potential to occur

what is dendritic summation of EPSP’s

a single EPSP would never be strong enough to initiate an action potential, so multiple EPSP’s must be summed up to achieve the required depolarization. 

What is temporal summation?

When one neuron (synpase) activated repeatedly and frequently enough can cause an action potential. These are known as compound EPSP’s.

What is Spatial Summation?

the simultaneous activation of multiple synapses from different sources.

What is the most common excitatory neurotransmitter?

Glutamate

What is the difference between AMPA gated channels and NMDA gated channels

AMPA channels are activated by glutamate. NMDA gated channels are activated by a simultaneous depolarization and glutamate binding.

What happens when glutamate binds to an AMPA gated channel?

The channel opens, and both sodium and potassium can flow through. Due to its greater driving force, more sodium will flow out than potassium is flowing in and this will cause a net depolarization, bringing the Vm to about 0mV.

What happens in an NMDA gated channel when glutamate binds and AMPA is present.

When glutamate binds, the NMDA gated channels will open however there are ions (magnesium) that block anything from moving through. Because AMPA is present, a depolarization will clear these ions and the NMDA channel will allow sodium and potassium but most importantly calcium to flow through. The calcium will act as a signal, allowing a vesicle containing more AMPA gated channels to bind with the membrane. 

Explain how AMPA and NMDA are the basis for learning

Because more AMPA gated channels are created after every synapse that happens, the synapse becomes stronger (faster) over time.

what is the most common inhibitory neurotransmitter?

GABA

How do GABA-gated channels work?

They allow the entry of Chloride ions, which decreases the membrane potential, bringing it away from the threshold.