Excitatory vs. Inhibitory Synapses ( 1i )

When talking about Polarization of graded potentials

Depending on what ion channels open

I could have a

• Depolarizing Graded Potential

OR

• Hyper-polarizing Graded Potential

Therefore this can establish

• Excitatory Synapse

OR

• Inhibitory Synapse

Difference is

What Ion channel’s opening

when the neurotransmitter binds

Excitatory Synapse

can vary on

speed

Some are gonna be

FAST !!

because

the receptor’s an ion channel

( ion can immediately start crossing )

Some are gonna be

SLOW !!

because

the receptors a G-Protein Coupled

• sometimes with enzymes too

( extra steps needed before ion channel can open )

FAST Excitatory Synapse

Preview:

• Opens a monovalent cation channel

  ( new channel! )

• Get depolarizing graded potential called an “Excitatory Post-Synaptic Potential”

  ( EPSP )

“Opens a Monovalent Cation Channel”

Breakdown of phrase in next few cards

"Cation”

positively charged ion that’s going through

“Mono”

one

“Valence”

ion’s charge

All together this means

One positive charged ion!

What specific ions can go through this

Monovalent Cation Channel ?

• Sodium ( Na⁺ )

AND

• Potassium ( K⁺ )

= Both fit the criteria!

Review:

In the past,

We’ve looked at voltage-gated channels that let ONLY ONE type of ion through

example

voltage-gated Na⁺ channels ONLY let Na⁺ through

= VERY SELECTIVE

While

The Monovalent Cation Channels are

more UNSELECTIVE !!

In fact

Monovalent Cation Channels lets

BOTH Na⁺ AND K⁺

go though it

AT THE SAME TIME !!

Na⁺ goes down it’s electrochemical gradient

at the same time

K+ goes down it’s electrochemical gradient

= This pic shows that

they can both move through at the same time!

If a monovalent cation channel opens

Which ion will move faster through it?

( Hint: This neuron’s resting at -70 mV )

Sodium ( Na⁺ ) ions!

Why?

Two reasons

1. It’s very far from it’s equilibrium of ( +60 mV )

so there’s a

→ STEEP concentration gradient

2. So there’s a bigger electrochemical gradient for Na⁺ moving IN

Both

Concentration/Chemical Gradient

and

Electrical Gradient

pushes Na⁺ the same way!

Notice:

The Na⁺ arrow was drawn LONGER than the K⁺ arrow

because

Na⁺ is moving more than K⁺

( because it’s faster - because of the bigger electrochemical gradient for Na⁺ )

If sodium is moving in more than the potassium’s moving out

What’s gonna happen to the membrane potential?

( more positive or negative? )

It’ll become

More Positive

“Get’s Depolarized”

neuron’s more positive than how it was before

→ because positive Na⁺ ions coming in

Review

A membrane potential change happening right where the receptor is as an ion goes in/out the membrane

This is called a

Graded Potential

→ which dissipates as it spreads

( when you measure it further away from the start it looks smaller )

Each graded potential has it’s own name!

Depolarizing Graded Potential is called

Excitatory Post-Synaptic Potential ( EPSP )

Called “Post-Synaptic” because

The postsynaptic neuron is where the graded potential’s happening!

( as it happened right where ion channel opened )

Called “Excitatory” because

it’s depolarizing

• that’s putting the membrane CLOSER TO threshold

  • closer to firing action potential

So what’s produced by an Excitatory Synapse?

An EPSP

→ Gets us CLOSER to threshold!

Inhibitory Synapse

there’s many mechanisms!

For example:

It can open a

Ligand-Gated K⁺ Channel

What’s the ligand here?

Neurotransmitter

( from the pervious neuron )

If this K⁺ Channel opens

K⁺ will moved in what direction?

OUT the cell

( because of the’s electrochemical gradient )

Positive K⁺ moving out will leave

Negative ions behind inside

This makes the membrane even more

negative than it was before

The term for what’s happening is

( talking about graded potential )

Hyper-polarizing Graded Potential

Hyper-polarizing Graded Potential is called

Inhibitory Postsynaptic Potential ( IPSP )

Called “Inhibitory” because

it’s hyper-polarizing

• that’s pushing the membrane FURTHER AWAY from threshold

  • less likely to fire action potential

Showing

EPSP ( excitatory )

and

IPSP ( inhibitory )

Excitatory because

depolarization has moved membrane closer to threshold

Inhibitory because

hyper-polarization has moved membrane further from threshold

= now harder for postsynaptic neuron to fire an action potential

Whether a graded-potential at a synapse is EPSP or IPSP depends on

Which ion channel opens

Assume there’s a large number of neurotransmitter leading to a large EPSP

Big enough to get postsynaptic membrane to -50mV ( threshold )

Will the postsynaptic cell DEFINITELY fire an action potential?

No

Why

Where do we have our first action potential?

Axon Hillock

Where does the graded potential start?

At the synapse

→ On the dendrites + cell body

_{( dendrites “ antennas receiving info” )}

So we can notice that

they are at DIFFERENT SPOTS!

→ Graded potential at the cell body has to

move to the axon hillock

What happens as the graded potential moves?

It loses charge!

gets less and less and less

By the time it gets to the axon hillock

It’s NOT gonna be above threshold anymore

_{( is now less than -50 mV )}

So even if there was a single HUGE graded potential

It still WON’T fire an action potential

Why?

• Graded potential’s current decrease as they travel

→ it’s not gonna be above threshold at the axon hillock

= Will NOT fire an action potential

So how do we ever get the Postsynaptic Neuron to fire an action potential?

A Realistic Postsynaptic Neuron

has A LOT of synapses!!

each carrying information

How much synapses?

hundreds to thousands!

Which are ALL coming to

this one same postsynaptic neuron

What this postsynaptic neuron is going to do is

Summate all of that information

( add together )

★ So the axon hillock is actually receiving the

SUM of all these graded potentials! ★

1 synapse is

NOT enough to get this postsynaptic neuron to fire an action potential

How many synapses do we need?

50-100 synapses

( aka 50-100 graded potentials )

that all have to get ADDED TOGETHER to get the neuron to threshold at the axon hillock

The Summation of Graded Potentials

→ each graded potential that happens on the same synapses added together

Here we have 3 Synapses

( not real just simplified! )

Ex1 and Ex2 are

excitatory ( EPSP )

In1 is

inhibitory

Frequency measures

how often a repeating event occurs

in a given amount of time

1. Ex1 causes a depolarizing graded potential

2. BUT the graded potential dissipates ( goes away )

3. Later on → we get another Ex1 graded potential

   (presynaptic sent another AP)

Will we get an action potential? Why?

No

→ graded potential went away

• The frequency of action potential was too slow

What is this Summation Type?

No Summation

= Slow Frequency of Action Potential

What happens if the frequency of action potentials went up?

1. I have an action potential in a presynaptic neuron

   → gets a graded potential

2. BUT BEFORE that graded potential disappears -

ANOTHER action potential happens!

Which

releases more neurotransmitters

→ opens more channels on postsynaptic neuron

This allows us to get

A graded potential that

ADDS to the one that’s ALREADY THERE FROM BEFORE

Will it fire an action potential in the postsynaptic neuron?

Yes!

What is this Summation Type?

Temporal Summation

“Temporal” referes to

Timing ( think: tempo )

of the action potentials

Action potential goes down axon

releases neurotransmitters

gets graded potential

BUT THEN

★ right behind it ANOTHER action potential happens BEFORE first graded potential goes away!! ★

This is caused by

Increasing the Frequency of Action Potential

( increasing the amount of action potentials in a certain amount of time )

( going by a spot on the membrane )

Why would there be a different frequency of Action Potentials?

Note: think about the

size of the stimulus

In action potential it’s “All or Nothing”

= big stimulus gives us the SAME action potential level ( +30 mV )

So how does the body know if it was a

• small stimulus ( that reached threshold )

or

• REALLY BIG stimulus ??

The really big one is going to increase the frequency of action potential

How?

Although there’s hyper polarization

If the stimulus is strong enough, the neuron can fire again, though it requires a stronger-than-usual stimulus ( a big one! )

= more can get fired again more quickly!

So the action potential strength is still exactly the same ( +30 mV , + 30 mV , + 30 mV )

BUT what changes is

how often the Action Potential is going by

( the frequency! )

The really big stimulus opens voltage-gated Na⁺ channels

→ fires an action potential

BUT

Stimulus is still there!

What will it do now?

Fires the next batch of action potential!

= Higher Frequency of Action Potential

That’s how our body distinguishes whether something we touch is cold or hot

When we touch something hot →

there’s a higher frequency of action potential

( heat is a stimulus that can send signals )

= increases likelihood of postsynaptic cell that receives it to release it’s own action potential

Would there be a maximum limit to the frequency of action potential?

Yes

There’s a limit of how frequent action potentials can be

What limits it?

The Absolute Refractory Period!

Remember: action potentials CANNOT happen on top of each other

Analogy:

Blue Angels

Blue angels fly with computers ( not with their own hands )

→ Because they can’t fire fast enough frequency of action potential to have the reaction time needed to move at the plane’s insanely fast speeds

What happens if Ex1 and Ex2 neurons have action potentials at the SAME TIME?

we get a

excitatory graded potential for Ex1

AND

excitatory graded potential for Ex2

= both currents get ADDED TOGETHER

This is called a

Spatial Summation

“ have on graded potential here and another one over there "

→ many different presynaptic neurons at the same postsynaptic neuron

Spatial Summation means

having currents form

graded potentials from DIFFERENT locations/spaces

→ ADDS UP to a bigger current

In reality what summation type(s) do we have?

BOTH

• Spacial summation

  AND

• Temporal summation

What’s important from either one is that we end up needing 50-100 of these graded potentials added together

in order to

make it big enough to reach threshold at an axon hillock

Complication:

NOT all of the graded potentials are exitatory!

There can be inhibitory synapses having

action potentials that lead to

inhibitory graded potential!

( IPSP )

There can be inhibitory synapses happening at the SAME TIME as excitatory

Which will cause them to

CANCEL each other OUT

What type of Summation is this?

EPSP - IPSP cancellation

EPSP-IPSP cancellation

means that

Depolarization and Hyper-polarization cancel each other out

→ we’re back at -70 mV

Review

What are the 4 Summation Types?

1. No Summation

2. Temporal Summation

3. Spatial Summation

4. EPSP - IPSP cancelation

So in Reality

The postsynaptic neuron has A LOT of input coming in

→ some say go! ( yes AP )

→ some say stop! ( no AP )

What does this neuron do with these graded signals that the action potential brought in??

Summate them all together!

If we sum all the graded potentials together and it’s above threshold

We get an action potential to start at the axon hillock!!

Revisiting Compare and Contrast Chart between Graded and Action Potential

a new explanation can be added with this info

In relation to summation:

Graded Potential:

Can be summed

• can pile on top of each other

In relation to summation:

Action Potential:

Cannot be summed

→ because of

• threshold

• absolute refractory period