Electrical Activity of a Membrane

Resting Potential

  • Difference of charge on the inside and outside of the neuron’s membrane produces an electrical potential

    • Electrical potential: the ability to use its stored power, like a charged battery
    • Membrane’s Resting Potential: the charge storing potential energy
    • Store of energy that can be used later
    • Rest potentials can vary from -40 mV to -90 mV

  • Channels, gates and pumps main the potential because

  1. the membrane is relatively impermeable to large molecules, the negatively charged proteins remain inside the cell
  2. Ungated potassium and chloride channels allow potassium and chloride ions to pass more freely, but gates on sodium channels keep out positively charged sodium ions
  3. Sodium (+) -potassium (+) pumps extrude sodium from the intracellular fluid and inject potassium

  • Inside the Cell

    • Large, negatively charged proteins inside that cannot move across the membrane
    • Making most cells have a charge across the membrane
    • Not all potassium that could enter do enter because the internal concentration of potassium is much higher than external (gradient)
    • A few potassium ions on the outside of the membrane are enough to contribute to the charge across the membrane
    • All the positive potassium ions inside the cell can’t make it positive because they do not overpower the protein’s negative charge
    • Potassium inside the cell is limited because when there is too much inside, it flows with its gradient and some go outside the cell where the concentration is lower
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  • Outside the Cell

    • Positively charged sodium cannot move freely across the membrane into the cell or it would eliminate the charge produced by the unequal distribution of potassium ions inside and outside the cell
    • Sodium channel has a gate that blocks most Na(+) from coming in
    • Sodium Potassium pumps 
    • Brings the Na(+) back out of the cell again if it happens to make its way in
      • 3 Na(+) for 2 K (+)
    • Chloride 
    • Contributes little to the resting potential 
    • Chloride is at equilibrium at resting potential
      • Concentration gradient = voltage gradient
      • Graded Potentials

  • Graded potentials: small voltage fluctuations across the cell membrane 

    • Due to change in ion concentration

  • Hyperpolarization: charge of the membrane increases

    • When a negative voltage is applied

  • Depolarization: membrane charge decreases

    • When a positive voltage is applied 

  • Usually lasting milliseconds

  • Hyperpolarization and Depolarization usually take place in the soma and dendrites

  • Potassium, Chloride, Sodium Channels

    • Potassium Channels
    • Efflux of potassium makes outside more positive
    • Even though potassium channels always open, there is resistance for outward flow of potassium
      • Reducing the resistance enables hyperpolarization
    • Chloride Channels
    • Influx of Chloride can cause hyperpolarization
    • Even though channel is open, more Chloride remain on outside
      • Decreasing resistance will increase chloride inside the cell
    • Sodium Channels
    • Opening of normally closed sodium channels allow influx of sodium ions
      • Depolarization

  • Action potential: a brief reversal in an axon membrane’s polarity

    • Making inside more positive in relation to outside
    • Then abruptly reversed again back to resting potential 

  • Occurs when sodium and then potassium cross the membrane rapidly

    • Depolarizing phase = sodium influx
    • Hyperpolarizing phase = potassium efflux
    • Triggered when a cell membrane is depolarized to about -50 mV

  • Threshold Potential: membrane charge undergoes further change with no stimulus necessary

    • Relative voltage drops to 0 and continues to depolarize until reaching +30 mV
    • Reversing again to become hyperpolarizing -- returning to -70 mV
    • \n
    • Role of Voltage-Sensitive Ion Channels

  • Voltage sensitive channels: closed when an axon’s membrane is at resting potential

    • When reaching threshold voltage…

    • 1. Both sodium and potassium voltage sensitive channels are adjusted to threshold voltage of about -50 mV

    • 2. Voltage-sensitive sodium channels are more sensitive than potassium channels and open first

      • Voltage change due to sodium influx happens before change due to potassium efflux begins

    •  3. Sodium channels have 2 gates

      • Once membrane depolarizes to about +30 mV -- one opens

    •  4. Potassium channels remain open longer than sodium because they open slower

      • Efflux reverses depolarization and hyperpolarizes

    • Action Potentials Refractory Periods

    • Absolutely refractory

      • Stimulation of axon membrane during depolarizing will not produce another action potential
      • Axon cannot produce another action potential when it is repolarizing
      • During Resting Potential
      • Gate 1 of sodium channel is closed; gate 2 is open
        • Gate 1 also opens at threshold
        • Gate 2 closes very quickly after gate 1 opens
      • In this state when both sodium gates are open or when gate 2 is closed

    • Relatively Refractory

      • If axon membrane is stimulated during hyperpolarization, an action potential can be created if the second stimulation is greater than the first
      • Refractory Periods: result from the way gates of voltage-sensitive sodium and potassium channels open and close
      • Sodium channels have 2 gates; potassium has 1 gate
      • Opening of potassium channels repolarizes and eventually hyperpolarizes the membrane
      • Potassium channels open and close more slowly than sodium 
      • It is in this state of refractory when the membrane is hyperpolarizing

  • Nerve impulse: the spreading of an action potential along an axon

    • Total voltage change during an action potential is 100 mV
    • When the voltage change in part of the membrane where an action potential first occurs, it is large enough to make adjacent parts reach the threshold of -50 mV 
    • When reaching threshold at each adjacent part, the voltage sensitive channels open to produce action potential there as well (domino effect)
    • Action potentials do not dissipate; either generated completely or not at all
    • Is a constant size and arrives unchanged to every nerve terminal
    • Refractory Periods and Nerve Action

  • Action Potential’s refractory phase has 2 uses for nerves

    •  Max rate at which action potential can occur is about 200 per second
    • Sensitivity of voltage channels affects firing frequency
    • Prevents the action potential from reversing directions and returning to its point of origin
    • \n

  • Saltatory Conduction and the Myelin Sheath

  • Humans have very thin axons because we require so many for complex function

  • Glial Cells

    • Speed up nerve impulses in vertebrae nervous system
    • Schwann Cells = PNS --- Oligodendroglia = CNS
    • Both wrap around the axon and form the insulating myelin sheath
      • Action potentials cannot occur when the myelin is wrap
      • Barrier to ionic current flow
      • Areas under myelin sheath have few channels for ions to flow
    • Nodes of Ranvier
    • Areas between sheath
    • Action potential at one node can open voltage gated channel at adjacent node even though sheath is in between
    • Saltatory Conduction: this type of energy flow
    • Consequences of Myelin
    • Less energy used because action potentials regenerates only at the nodes
    • Improves action potential’s conduction speed
    • Multiple Sclerosis (MS)
    • Myelin formed by oligodendroglia is damaged
      • Disrupts the function of the neurons