4 Notes: Neural Impulse – biologicalpsych.com

Neurons are electrical
- Communication within a neuron
- Membrane potential
  - inside vs. outside
Two Basic Principles
- 1. Resting Potential
- 2. Action Potential
1. Resting Potential
- Semi-Permeable Membrane
  - Positive ions outside
  - Negative inside
  - – 70 mvolts
  - Chloride stuck inside (negative)
- Ion gates are specific
  - Sodium gates
  - Potassium gates
  - Calcium gates
- Flow for 2 reasons
  - 1. Passive Transport
    - From more to less concentrated
    - Passive transport (no energy)
  - 2. Active Transport
    - Primary active transport
    - Use chemical like ATP
    - Use fuel = glucose
    - Sodium Pump
      - Secondary active transport
      - Use electrochemical gradient
- How does it work?
  - NT activates dendrite receptors
  - NT opens lipid gate
  - Intracellular charge more positive
  - More NT
  - Higher internal positive charge
  - No major change until -55
- Hit Threshold of -55
  - Voltage gates open
  - 2 kinds: Sodium & potassium
2. Action Potential
- Sodium voltage gates open
- Sodium rushes in
- Only about 1% of available
- Impacts voltage for next gate
- If sodium gates don’t open… neuron doesn’t fire!
  - Local anesthetics block gates
  - Novocain, Xylocaine, etc.
Down the axon
- Sodium gates open
- Sodium gates wide open
  - Cell is already positive
  - Potassium rushes OUT
1-WAY ONLY
- Sodium gates close quickly
- Can’t reopen (too positive in cell)
How Membranes Work
- Primarily composed of lipids
  - Other molecules move from high concentration to low across it
  - Diffusion
- Selectively Permeable
  - Some go through easily
    - Water, oxygen, carbon dioxide
  - Some go through slowly
    - Potassium
    - Its gates are slightly open
  - Some don’t go through at all
    - Sodium channels are closed
    - Chloride can’t get out
Electrolyte = Water solution
- adds or removes electrons
- ionizes a soluble acid, base or salt
Ion = charged molecule
- positively charged
- negatively charged
Electrostatic pressure
- charged atomic particles
- opposite signs attract
- same sign repulsive
Ion channels
- Specialized protein molecule
- Lets specific ions enter or leave
Why open?
- Chemical
- Voltage
Sodium-potassium transporter
- protein in all cell membranes
- extrudes sodium ions (pushes)
- sodium pump
  - 2 in , 3 out
Conduction
- 1. Cable conduction
  - Passive conduction
  - Decreases as goes down length of axon
  - Less as you go
- 2. Saltatory conduction
  - Node to node
    - in myelinated axons
    - increases the speed of impulse; 10x faster
  - No sodium channels under sheath
    - breaks in sheath every 1mm
    - have sodium channels
    - un-mylinated sections
  - nodes of Ranvier
BOTH Cables & Node work together
- Cable conduction
- Node to node
Action Potential
- the nerve impulse
- +50 millivolts
- 200 mph
Negative After-Potential
- More negative than resting potential
- Reaches resting potential by diffusion
- Sodium-potassium pump
Principles
- All-or-None Law
  - Either neuron fires or it does not
  - It fires if it passes a certain threshold
  - If it fires, it does so at full strength
  - Can’t change intensity
  - Can’t go fast or slow
- Firing rate = frequency
- Refractory Period
  - Can’t fire until it recovers
  - 3 millisecond to totally “reload”
    - Refill (toilet tank level)
  - 2 part recovery process
  - Absolute Recovery Period
    - Not recovered
    - Can’t fire for 1 ms
  - Relative Recovery Period
    - Fire if lots of $
    - Resting potential 2 ms later
Thresholds
- Super-threshold stimulus
- 1 neuron releases enough NT
- activates (depolarize) the post-synaptic neuron in 1 shot
Summation
- Not enough NT to trigger firing
- 1. Temporal Summation
  - 1 neuron, several times
  - So NT doesn’t dissipate
- 2. Spatial Summation
  - Many neurons, 1 time
1 cell can code 2+ percept. experiences
- 1 Cell: 2 codings
- excitation signals one quality
- inhibition signals another
  - Fire = yes Not fire = no
  - Fire = left Not fire = right
  - Fire = blue Not fire = yellow

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