Message Transmission

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Message Transmission

• How nerve impulses travel

Mrs. S. Taylor
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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)?
• 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 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 Nato 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
  reductions

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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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