Transcript Biology 30 Unit 1

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 -70mV
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.
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.
+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.