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Message
Transmission
How nerve impulses travel
Mrs. S. Taylor
Electric Cells
• Nerve cells have an electric charge on their cell
membranes (we call this polarized)
– Yes, even when they are not stimulated (resting)
they have an uneven concentration of positive and
negative ions on opposite sides of their
membranes
• Due to active transport there are more sodium
ions (Na+) outside
• Inside we have potassium ions (K+) as well as
many large negatively charged particles
– Phosphate and sulfate
– proteins
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Nerves have potential
• Two types actually: resting potential and
active potential
• Is maintained by diffusion. The positive ions
defuse in to balance out the charges, and
the cell pumps the positive charges out (as
well as diffusion due to concentration) to
maintain it.
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Resting potential
• This is a nerve cell that is ready to work.
• The cell membrane is more permeable to
K+ than to Na+, so more positives are
leaving than entering. Add the Na+ K+ pump
( a mechanism in the cell membrane that
shunts the Na+ back out) and the inside is
decidedly negative and the outside is
definitely positive. This is potential...
something that can change.
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Action potential
• Permeability changes at the trigger zone
and this opens channels specifically
designed to let Na+ in.
– The membrane depolarizes
• Almost immediately, K+ channels open and
the K+ rushes out
– The membrane repolarizes with K+ on the
outside and Na+ on the inside
• This flipping between states is Action
potential
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What causes the potentials to change?
• It is because nerves are excitable.
– Special nerve cells respond to special things
• Pain, temperature, pressure, light
– Some respond to other neurons
• One signal cause a slight depolarization and
more and you can have summation occur.
(remember muscle cramps)
– Get enough and you have reached the threshold
potential. Once you've reached here, the nerve
acts. (Action potential starts but is localized)
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Cool, now how does the signal travel?
• Well, the first flip at the trigger zone stimulates the
next part to flip, and so on.
– It is a wave.
• No myelin sheath – impulse is conducted over the
entire surface of the axon
– takes a while longer
• Myelin sheath – the impulse jumps from one node
of Ravnier to the next (called saltatory
conduction)
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– if there were no nodes there would be no impulse
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Speed? What else affects it?
• The speed is proportional to the diameter of
the axon
– The thinner the axon the slower the signal
• A relatively thick, myelinated axon might
travel at 120 m/s
• A thin, unmyelinated axon might travel at
0.5 m/s
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All the way or nothing!
• Nerve conduction is follow the all-or-none
response rule.
– Once you have action, it is going to send it to
the end, the axon terminals
• None? How does that happen
– The signal doesn't cross the threshold point
– Refractory period – a short rest period after the
nerve has passed a message.
Animation
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So, what happens at the end of the
axon?
• You run into a synapse.
– This is the junction between any two
communicating neurons
– It really is a gap (the synaptic cleft), the cells
don't actually touch each other.
• The sender neuron is the presynaptic neuron
• The receiving one is the postsynaptic neuron
• Crossing the cleft is called synaptic transmission
– One-way process handled by neurotransmitters released
from synaptic vesicles located in the synaptic knobs and
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react with receptors on the other side.
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Neurotransmitters? How do they
work?
• Well, remember that nerves work by
moving Na+ and K+ across the cell
membrane
– The more Na+ that crosses the closer you
are to the threshold potential
– The less Na+ the further the neuron is from
reacting.
• So,the neurotransmitter either helps this
movement or prevents it.
• Animation
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Two type of transmitters
• Excitatory - allows more Na+to cross the
membrane (continues the message)
• Inhibitory – lesses the likelihood that the
other nerve will make it to threshold.
• Both are present, and can both can be
released at the same time. So, it is all up to
the amount of each that are released to
determine if the next nerve will continue the
message.
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Excitatory Types
• PNS
• Skeletal – acetylcholine
• General - Substance P– neuropeptides (pain)
• CNS
• Norepinephrine - creates sense of feeling good
• Dopamine – feeling good, low levels assoc with
Parkinson disease
• Histamine – promotes alertness
• Glutamic acid -general
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Inhibitory types
• PNS
– Norepinephrine – can excite or inhibit
dependent on receptors
– Dopamine – can excite or inhibit dependent on
receptors
• CNS
– Serotonin – leads to sleepiness
– GABA – general
– Endorphines and enkephalins – general – pain
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reductions
What happens to them?
• The neurotransmitters have a couple of
options for removal
1 decomposed by enzymes
2. transported back into the synaptic knob that
released them
3. transported into a nearby neuron
4. taken up by a neuroglial cell
• Removal prevents constant stimulation of
the postsynaptic neuron
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Impulse processing
• The organization of the nerves reflects in
part on how they process info.
– Neuronal Pools – CNS, makes hundreds of
connections, reacts together
– Facilitation -If a neuron is not excited,but is
more excitable to incoming stimulation
– Convergence – a single neuron may receive
impulses from two or more incoming axons,
has a additive effect on the neuron
– Divergence – a single neuron sends messages
to many neurons amplifying the signal.
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