4. Neural Impulse

Neurons are a lot like toilets.

People use them but don’t talk about them. They are usually hidden from view. And you have to wait for them to refill.

After a flush, it can take 3 or 4 minutes until the apparatus is ready for regular use. Neurons are a lot faster but the process is similar. It only takes a neuron 3ms to recover, as it resets its ion channels and finds its resting potential.

Neurons are like living batteries that automatically recharge. It’s a complicated and fascinating process.

Learning Objectives

By the end of this module, you should be able to:

• Describe the properties of a semipermeable membrane.
• Compare and contrast resting- and action-potentials.
• Describe ion channels and depolarization
• How polarity impacts ions

Videos

Slides

Links

• Neuroscience for Kids: Lights, Camera, Action Potential
• For Dummies: Neural Impulse
• Wikipedia: Action Potential

Terms

• – 70 mvolts
• absolute recovery period
• action potential
• active transport
• all-or-none law
• ATP
• cable conduction
• calcium gates
• chloride
• coding
• concentration
• conduction
• depolarization
• diffusion
• electrolyte
• electrostatic pressure
• firing rate
• frequency
• glucose
• hyperpolarization
• inhibition
• ion
• ion channel (gate)
• membrane potential
• myelinated axons
• negative after-potential
• node to node conduction
• passive conduction
• passive transport
• positive ions
• positively charged
• potassium
• potassium gates (channels)
• primary active transport
• refractory period
• relative recovery period
• resting potential
• saltatory conduction
• secondary active transport
• semi-permeable membrane
• sodium channels
• sodium pump (sodium-potassium pump)
• sodium-potassium transporter
• spatial summation
• super-threshold stimulus
• temporal summation
• threshold
• voltage
• Voltage gates open

Notes

• 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 rushes in
    • Impacts voltage for next gate
  • 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

 Quiz

• 1. Neurons have membranes that are:
  • a.           semipermeable
  • b.           transducers
  • c.           permeable
  • d.           unipolar
• 2. Because chloride is stuck inside, a neuron’s resting potential is:
  • a.           reflective
  • b.           centered
  • c.           negative
  • d.           positive
• 3. When the sodium gate open, how much of the available sodium rushes in?
  • a.           1%
  • b.           20%
  • c.           50%
  • d.           99.9%
• 4. Novocain stops pain by blocking:
  • a.           potassium channels
  • b.           sodium channels
  • c.           zinc channels
  • d.           all of the above
• 5. Neural membranes are mostly composed of:
  • a.           chocolate
  • b.           proteins
  • c.           sugar
  • d.           lipids

For the answers: Click Here

Discussion

Any questions about nerve impulses?

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