contents

action potential

  • how do neurons talk?
    • neuron communication protocol
  • neurons communicate in electrical-ese
    • similar to signals in electronics
  • however, neuron and living cell communication electrical-ese is different from how it is in electronics
    • electrical devices use only electrons for transmitting electricity and electronic communication
  • neurons use molecules with charge for communication
    • ions: molecules where number of protons is not the same as number of electrons
  • ions have either lost electrons or gained electrons
    • one electron has one negative charge
    • so ions are either positively charged or negatively charged

cell membrane

  • cell membranes separate the inside of cells from the extracellular space outside the cell
    • all cells have cellular membrane, made mostly of fat
    • a layer of oil surrounded by water on either side
    • continues around the cell
cell and electrical charge
  • the inside of a cell is negatively charged
  • the outside of a cell is zero charge
    • i.e. ground

ions

  • ions are present within the context of cells
    • \(K^+\): positive ion
    • \(Na^+\): positive ion
    • \(Cl^-\): negative ion
  • consider the positive potassium ion \(K^+\)
    • \(K^+\) cannot get through the fat layer of the membrane
    • \(K^+\) can travel through the cell membrane only through an ion channel
chemical gradient
  • \(K^+\) is a chemical
    • there are no \(K^+\) ions around the cell
    • concentration gradient creates a chemical force that pushes the \(K^+\) outside the cell
electrical forces
  • \(K^+\) get an electrical force to stay within the cell
    • as \(K^+\) is positive
    • inside of the cell is negative
cell membrane equilibrium
  • so there exists an equilibrium of chemical and electrical forces across the cell membrane
    • cause by the electrical change forces
    • chemical concentration gradient forces
  • electrically quantifying this equilibrium
    • the cell membrane has a potential difference of \(-70 mV \) to \(-60 mV \)
      • at rest (equilibrium)
    • across the water-fat-water layers

electricity

  • electricity is like water in many ways
  • consider a flat land with some water on it
    • water on the flat land goes nowhere
    • since there is no difference in level water stays where it is
  • for electricity, electrical potential is like ground level
    • if there is no difference in electrical potential, there is no electrical current
  • consider a waterfall
    • there is flow from the top of the bottom
    • because of the difference in ground level
    • the ground level difference drives the water flow of the water fall
  • consider a taller waterfall
    • this will have more level difference
    • so the waterfall will have a larger water flow
  • at the bottom of the waterfall,
    • if there are pipes to capture the water fall along the flat land
    • smaller radii pipes offer more resistance to water flow
    • larger radii pipes offer lesser resistance to water flow
  • electricity is just like that
    • electrical potential
    • electrical current
    • electrical resistance
    • these are the concepts that apply to electrical communication across cells
cell electricity
  • electrical potential across a cell is the difference between the electrical potential inside and outside of the cell
    • around \(-65 mV\)
    • the outside of the cell is considered ground (\(0V\))
  • ion channels provide a path for the ions to go in and out of the cell
    • when ions flow through the cell membrane, it is considered the flow of electric current across the cell membrane
  • the whole cell has an electrical resistance
    • electrical resistance to the flow of current from the inside to the outside of the cell
    • if no ion channels are open, the resistance is infinite
    • if only a few channels are open, then the resistance is high but allows some current
    • if a lot of channels are open, then the cell resistance is low

action potential

  • neurons oscillate around the resting membrane potential in equillibirum
    • small potential differences \( < 1mV \) to \( 5mV \)
    • current induced from these potentials die out at very short distances
    • do not travel all the way along the neuron dendrite
useful potential difference
  • neurons are very long cells
    • compared to other cells in the body
    • the longest neuron goes from toe to the medulla (neck)
      • cell body is located around the hip
      • one process goes from toe
      • another to the medulla
    • the small potential changes between \( < 1mV \) to \( 5mV \) does not drive current along this distance
    • potentials in the range of \( \approx 100mV \) needs to be generated
      • to drive current and consequently, communication within the span of neuron lengths
  • action potential events carry information within the neuron
    • they are critical for carrying signals over long distances
    • it is an event of ion exchange through cell membrane
positive potential
  • consider the following situation
    • \(K^+\) ions are in high concentration within the cell
    • \(Na^+\) ions are in high concentration outside the cell
    • so \(Na^+\) ions are responsible for the large positive change in the membrane potential during an action potential
  • the longer the length of the neuron, more the distance the action potential event has to travel
    • and slower the event is
    • this process is sped up by an insulator called myelin