Trans membrane potential and Nerve Impulse Transmission

• Concept 1: A neuron at rest has a membrane potential of -70 millivolts.


  Membrane potential

  ◦ Charge difference/voltage across plasma membrane

  ◦ Inside of cell is (-) charged relative to outside of the cell
• Concept 1: A neuron at rest has a membrane potential of -70 millivolts.



  Resting neuron

  ◦ Not sending a signal (Electrical Impulse)

  ◦ Resting potential

  ◦ Membrane potential of resting neuron

  ◦ Typically between -60 and -80 mV

  Stimulus --> membrane potential changes (action potentials) --> actions/responses
• • Sodium and Potassium influences membrane potential.

  
• Sodium-potassium pump

  • Maintain Na+ and K+ concentration gradients
  • Uses ATP

  ◦ Transport 3 Na+ out of cell for every 2 K+ transported into cell

  ◦ Net export of (+) charge but pump acts slowly -->small change in membrane potential (few mV)
• Formation of Resting Potential

  Ion Channels

  • Pores formed by clusters of specialized proteins spanning membrane
  • Allow ions to diffuse back and forth across membrane

  ◦ Resulting current: net movement of +/- charge  generates membrane

  potential/voltage across membrane

  • Have selective permeability

  ◦ Allow only certain ions to pass

  ◦ Ex. Potassium channel only for K+

  • In a Resting neuron

  ◦ Many open potassium channels (leak channels)

  ◦ Very few open sodium channels

  ◦ Na+ can’t readily cross membrane

  ◦ K+ outflow --> (-) charge inside cell --> major source of membrane

  potential
• Concept 2: A stimulus may bring a change in the resting potential of a neuron through the gated ion channels.


  Stimulus --> change in membrane potential

  • Gated ion channels in a neuron

  ◦ open/close in response to stimuli

  ◦ Opening/closing --> change in membrane permeability of particular ions --> rapid flow of ions across membrane --> change in membrane potential

  ◦ Voltage-gated ion channel - Opens/closes due to shift in voltage across plasma membrane of neuron
• Stimulus --> voltage-gated ion channels open

  ◦ Opening of gated potassium channels in resting neuron --> K+ membrane permeability --> net diffusion of K+ out of neuron --> shift in membrane potential toward (-90 mV).
• Hyperpolarization


  ◦ Increase in magnitude of membrane potential

  ◦ Makes inside of membrane more (-)

  ◦ Results from any stimulus that increases outflow of + ions or inflow of (–) ions
• Depolarization


  ◦ Reduction in magnitude of membrane potential

  ◦ Inside of membrane less (-)

  ◦ Often involves gated sodium channels

  ◦ If open, Na+ permeability increases --> Na+ diffuses into cell along gradient --> depolarization (+62 mV)
• Gradient Potential


  ◦ Shift in membrane potential in response to hyperpolarization/depolarization

  ◦ Magnitude varies with strength of stimulus

  ◦ Larger stimulus --> greater change in membrane potential

  ◦ Induce small electrical current; dissipates

  ◦ Decay with time & distance from source
• Action potential

  ◦ Massive change in membrane voltage

  ◦ Depolarization shifts membrane potential sufficiently

  ◦ Constant magnitude as long as it reaches threshold

  ◦ Can regenerate in adjacent regions of membrane --> can spread along axons

  ◦ For transmitting signal over long distances
• Action Potential


  Action potential

  ◦ Depolarization increases membrane potential up to threshold --> voltage-gated sodium channels open --> flow of Na+ into neuron --> further depolarization --> more Na+ channels open --> greater flow of current

  ◦ Positive-feedback loop: channel opening --> depolarization --> action potential

  ◦ Threshold: about -55 mV

  ◦ Magnitude is independent of strength of triggering

  stimulus

  ◦ All-or-none response to stimuli

  ◦ Occur fully or not at all
• How Action Potential is generated step-by-step

  1. Resting state - The voltage gated ion channels are closed
  2. Depolarization - The channels open (sequential and independent) --> sodium ions enter --> neurons become positively charged
  3. Rising phase of the action potential - Sodium channels open, potassium does not.
  4. Falling phase - Potassium channels open, sodium channels starts closing --> cell becomes more negative
  5. Undershoot - Potassium channels remains open and will eventually close --> goes back to resting phase
• Concept 2: A stimulus may bring a change in the resting potential of a neuron through the gated ion channels.


  Inactivation of channels during action

  potential

  • Sodium channels open upon threshold

  ◦ Open throughout action potential

  • To restore resting potential

  ◦ Na+ inflow should stop --> inactivation

  ◦ End of Na+ inflow allows K+ outflow --> repolarization
• Refractory period

  ◦ “downtime” when a second action potential cannot be initiated

  ◦ sodium channels remain inactivated during falling phase and the early part of undershoot

  ◦ membrane’s permeability to K+ is higher than at rest.
• Concept 3:

  The myelin sheath across the length of the axon speeds up the transmission / conduction of impulse.


  Conduction - how action potential is transmitted along the axon.
• Conduction of action potential

  • Axon hillock – where action potential is initiated
  • Na+ inflow depolarizes neighboring region of axon membrane --> large enough to reach threshold --> action potential in neighboring region

  ◦ Repeated many times along axon

  ◦ Magnitude and duration of action potential are similar at each position along axon

  - Action potential is all-or-none event

  ◦ Net result: movement of nerve impulse from cell body to synaptic terminals
• Conduction of action potential

  • Action potential moves along axon only toward synaptic terminals (unidirectional)

  ◦ Refractory period

  • Frequency of action potentials conveys information

  ◦ Rate of action potential production = input signal strength

  ◦ Louder sounds --> more frequent action potentials (same magnitude)
• Myelin sheath


  ◦ Electrical insulation surrounding vertebrate axons

  ◦ Mostly lipid – poor conductor of electrical current --> good insulator

  ◦ Produced by glia

  ◦ Oligodendrocytes in CNS

  ◦ Schwann cells in PNS
• Nodes of Ranvier


  ◦ Gaps in myelin sheath

  ◦ Where voltage-gated sodium channels can only be found
• Saltatory conduction

  (L., saltare, to leap)

  ◦ action potential appears to jump from node to node along axon

  ◦ more rapid propagation of action potentials in myelinated axons
• Concept 4:

  Impulses are transmitted from one neuron to another at synapses.


  Synapses

  ◦ Transmission from neurons to other cells

  ◦ Either:

  ◦ Electrical synapses

  ◦ Chemical synapses
• Electrical Synapses

  • electrical current generated by action potential flows from one neuron to another
  • Often play role in synchronizing activity of neurons that direct rapid, unvarying behaviors
  • Ex. Giant axons of squids & lobsters; also found in vertebrate heart & brain
• Chemical Synapse


  the depolarization of an axon triggers the release of chemical neurotransmitters --> will diffuse into the next neuron --> action potential