The Action Potential And the synaptic junction Joy Killough Round 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+

The Sodium Potassium Pump Helps maintains the resting potential. To Model: K+

The Sodium Potassium Pump Helps maintains the resting potential. To Model: K+ Na +

The Sodium Potassium Pump Helps maintains the resting potential. To Model: K+ 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 Channels Potassium Ion K + Sodium Ion Na + Activation Gates Inactivation Gate

Neuron with pumps and ion channels.

How do we trigger the voltage gated ion channel?

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, 5 th 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.