The Nervous System

Nerve Cell Function

History

• Galvani (1759) proposed that nerves work exactly the same way that the wiring in your house. That they carry electrical impulses.

• But in 1900 J. Bernstein demonstrated that nerve impulses were to slow to be electrical singles, so they must be an electrochemical impulse

History and the giant squid

• Bernstein’s theory was supported almost 40 years later by Curtis and Cole. They were able to use a Giant Squid’s nerve and measure the electrical difference across its membrane.

• Why did they use the nerves of the illusive G. Squid?

Experimental Evidence

• The Squids axons are large enough that they were able to place a tiny probes on the inside and the outside of the membrane and measure the potential difference.

• They found that a resting membrane there is about a -70mV difference between out and in.

-70mV

E.E. Cont...

• They then stimulated the nerves and found that the potential difference changed as the nerve impulse traveled down the axon.

• The difference only lasted a few milliseconds.

-70mV+40mV-70mV

Terms

• Resting potential (membrane)– The electrical potential that nerves have a rest -70mV

• Action potential– The reversal of charges as the membrane becomes excited +40mV

• Polarized– The resting membrane is said to be polarized because of the charge

differential

• Depolarized– It then becomes Depolarized as the charge reverses– Through the action of an ion pump the nerve becomes repolarized

Terms Cont...

• Refractory period– The nerve takes a certain period of time to

recharge

• The charge across the nerve membrane is caused by different concentrations of ions on each side (don’t forget ions have charges)

In-depth Look

• When the nerve is at rest there is a high concentration of Na+ on the out side and K+ on the inside. This creates the -70mV difference. As the nerve becomes depolarized Na flows in through specialized doors and K+ runs out.

• They are then re-pumped back into their resting positions.

• Let’s take a look

All or None and Threshold Levels

• Nerves need a minimum stimulus to fire.• Anything below this value and the nerve will do

nothing.• Also the nerve will fire with the same intensity

once that threshold is reached• So it will either fire completely or not at all this is

“All or None”

Things to think about

• If this is the case how do we know How hot something is or if we are being gently poked or stabbed?

The Synapses

• Contrary to popular belief there is not a direct connection between neurons.

• In fact there is a small space called a synapses.• Synapses can also occur between neurons and

effectors.• Small vesicles containing transmitter chemicals are

located at the end of axons terminus.

• When the impulse reaches the end plate calcium rushes in and causes these vesicles to pop and release their neurotransmitters into the synaptic cleft (the space between axon and next nerve.)

• The space between neurons is really small ~20nm, but it still takes time for the transmitter to move across.

• This could be considered the weak link (wrt speed) in nerves.

Terms...

• Presynaptic neuron: PRN– The Neuron which release the transmitter

• Postsynaptic neuron PON– the neuron which dendrites receives the

transmitter.

• NB the greater the number of synapses the longer it takes for a neural transmission.

Neural transmitters

• Acetylcholine– is one of the most common transmitters– when it is released from the PRN it diffuses

across the synaptic cleft where it bonds to specialized receptors in the PON.

– These receptors open the sodium ion channels and cause the wave of transmission to flow down the next neuron.

– What would happen to the PON if Acetylcholine was constantly attached to the PON?

– The PON would be in a constant state of depolarization and never have a chance to recover.

How do we get rid of it then?

• The PRN released an enzyme called cholinesterase soon after the acetylcholine is released

• Cholinesterase destroys acetylcholine and allows the sodium ion channels to close which allows the PRN to recover and get ready for the next wave of excitement.