The Action PotentialAnd the synaptic junction

http://groups.google.com/group/APBIO2010

Joy KilloughRound Rock ISD

The Big Picture

• Neuron function: send a signal

• How does it work? Changes in the permeability of the neuron membrane to ions.

The Resting Potential

• + and - ions are not equal across a membrane.

• In a nerve or muscle cell this membrane potential has a special name- “Resting Potential.”

• Although there is a range, we will use an average of -70mV.

The Sodium Potassium Pump

• Helps maintains the resting potential. The resting potential is negative inside a cell and positive out.

Take out the two pieces of the pump,

the circles (Na+ ), and squares

(K+)

The inside of the cell is negative largely due to phosphates, proteins and amino

acids .

The Sodium Potassium Pump

• Helps maintains the resting potential. To Model:

The Sodium Potassium Pump

• Helps maintains the resting potential. To Model:

K+K+

The Sodium Potassium Pump

• Helps maintains the resting potential. To Model:

K+

K+

The Sodium Potassium Pump

• Helps maintains the resting potential. To Model:

K+K+ Na +

Na +Na +

The Sodium Potassium Pump

• Helps maintains the resting potential. To Model:

K+K+

Na +

Na +

Na +

The Sodium Potassium Pump• Helps maintains the resting potential. Place in the

cell membrane of your model:

Operate your pumps. Start with sodium (round) in the cell and potassium (square) out.

The uneven transfer is creating a charge difference. The membrane is positive outside and negative in. The concentration gradient the

sodium potassium pump is creating is very important.

Looking at the Axon

Interstitial Fluid

Inside the axon

[Na+]15 mM

[K+]

150 mM

[Cl-]10 mM

[A-]100 mM

[Na+]150 mM

[K+]5 mM

[Cl-]120mM

Which way will Sodium want to move?

DOWN ITS CONCENTRATION GRADIENT.

Which way is that? Which way will potassium want to move?

DOWN ITS CONCENTRATION GRADIENT.

Which way is that?

Also maintaining the resting potential:

Ion leakage channels

Ungated Ion Channels(Ion leak channels)

Potassium Ion K+

Sodium Ion Na+

There are more K+ leak channels than Na+ leak

channels. The positive charge increases outside the

membrane.

Add 2 K and 1 Na leak channels

Place two K+ leak channels and one Na+ leak channel in your membrane.

Allow a few K+ to move out of the cell and a Na+ to move back in

through the channels.

Let’s look at two kinds of gated channels

• Ligand gated-open or close when a little molecule (ligand) binds to the channel.

• Voltage gated ion channels- In axons- open or close when the membrane potential changes.

Ion Channels

• Voltage gated ion channels- these open when the voltage reaches a certain level.

• Place the sodium and potassium ion channels in the neuron cell membrane.

Voltage gated ion channel

Take out your green and pink voltage

gated ion channels.Place in membrane.

Voltage Gated Ion ChannelsPotassium Ion K+

Sodium Ion Na+Activation Gates

Inactivation Gate

Neuron with pumps and ion channels.

• How do we trigger the voltage gated ion channel?

50 40 30 20 10 0-10-20-30-40-50-60-70-80

We are a go!

A hyperpolarization

A depolarization

Ligand Gated Channels

• Neurotransmitters are ligand• They fit in a receptor• Which changes the permeability of the channel.• Opens a Na channel? Depolarizes..more

positive.• Opens a K+ channel? Hyperpolarizes… K+

moves out leaving the cell interior more negative.

When the membrane potential reaches -55…

• From the axon down • Na + channels open sequentially. Na moves

in, down its concentration gradient.• K+ channels open right behind them, moving

K+ out.• This restores the Membrane potential.

At the end of the neuron

• Depolarization opens voltage gated Ca++ channels.

• Ca++ moves in• This moves transmitter vesicles to the

membrane. Transmitter is released.

Line students up down both sides of the neuron model.

Students

Habituation

• If a synapse has high levels of neurotransmitter for a long time…

• The receiving cell (postsynaptic) may decrease the number of receptors.

• Normal feedback mechanism

Drugs

• Many add neurotransmitter• Because of habituation more drugs are

needed for the same effect.

Curare

• Paralyzes skeletal Muscles • Nerves can still carry impulse• Muscle can still contract if it received direct

stimulation.• Works at the synapse on the post synaptic

cell.• It is a competitive antagonist of acetylcholine.

– (binding sites are filled)

In Taiwanese snake venom…

• Is a compound that binds to the nAChR on postsynaptic skeletal muscles.

• Causes paralysis• ACh (acetlycholine) can’t bind.

Karp, Gerald, Cell and Molecular Biology, 5th edition, 2008.

Nicotine

• The receptor on the muscle cells at the neuromuscular junction is called the nAChR or nicotinic acetlycholine receptor.

• The nAChR can be activated by nicotine or acetlycholine.

• Quit smoking? Withdrawal results from reduced stimulation of the nAChR.

Cocaine• Works in the pleasure center of the brain (limbic

system) with the neurotransmitter dopamine.• On the presynaptic membrane are proteins that take

up dopamine from the synapse.• Cocaine fills these transporters. Dopamine stays in

the synapse.• Eventually the number of receptors is reduced

resulting in addiction. The user needs drugs for a normal feeling.

Nerve Gas

• Inhibits acetylcholinesterase (AChE) which breaks down acetylcholine (ACh).

• ACh builds up in the synapse. The muscles are unable to respond. Paralysis results.

Prozac

• In depression, serotonin is in short supply.• Prozac inhibits reuptake of serotonin.• More serotonin is left in the synapse.