Nervous System 1

• Principles of Electrical Signalling

  • All animals but sponges have neurons and muscle cells
  • Two basic types of nervous system
  • Diffuse arrangements of cells called a nerve net, found in cnidarians and ctenophores
  • A central nervous system (CNS) - includes large numbers of neurons aggregated into clusters called ganglia (most animals with CNS have large cerebral ganglia or brain)
• Introduction to Animal Nervous System

  • Neurons conduct information in the form of electrical signals from point to point
  • Neurons can integrate incoming signals
• Types of Neurons

  • In vertebrates, CNS made up of brain and spinal cord - integrate information from many sensory neurons
  • Interneurons pass signals from one neuron to another
  • Motor neurons - send signals to effector cells in glands and muscles
  • Motor neurons and sensory neurons are bundled together into long strands called nerves
• Anatomy of a Neuron

  • Most neurons have cell body (or soma), dendrites, axon
  • Dendrites receive signals from the axons of other neurons, and a neuron’s axon sends signals to the dendrites and cell bodies of other neurons
• Membrane Potentials

  • Ions carry electrical charge - cytoplasm and extracellular fluids adjacent to the plasma membrane contain electrically unequal distributions of ions
  • Difference in charge creates electrical potential across the plasma membrane (membrane potential (Vm))
• Membrane Potentials

  • Membrane potentials refer only to a separation of charge immediately adjacent to the plasma membrane
  • Sodium pump contributes to membrane potential
  • Resting membrane potential is negative
  • “Resting” means membrane potential when neuron isn’t sending/receiving signals, or the normal membrane potential of non-excitable cells
• Measuring Membrane Potential

  • Membrane potentials (Vm) are measured in millivolts (mV) - expressed in terms of inside relative to outside
  • Outside value defined as 0 mV
  • Generally, more negative ions are on the inside surface of the plasma membrane than the outside - membrane potentials usually negative
• Resting Potential

  • When a neuron isn’t communicating with other cells, the difference in charge across its membrane is called the resting potential
  • Potential exists because inside cell: low Na+ and Cl- and relatively high K+ and some organic anions and outside cell: Na+ and C- predominate
  • If each ion diffuse according to its concentration gradient, anions and K+ would leave the cell and Na+ and Cl- would enter
• K+ Leak Channel (Part 1)

  • At rest, plasma membrane of neuron is relatively impermeable to most cations
  • Neurons have K+ leak channels allowing K+ to leak across the membrane - concentration gradient favours net diffusion of K+ out of cell
• K+ Leak Channels (Part 2)

  • K+ moves out of cell, inside becomes more negatively charged relative to outside - buildup of negative charge begins to attract K+ and counteract concentration gradient
  • Membrane eventually reaches voltage with equilibrium between concentration gradient that moves K+ out and the electrical gradient that moves K+ in - can be calculated using the Nernst equation, assuming electrochemical equilibrium
• K+ Leak Channels (Part 3)

  • Cl- and Na+ cross plasma membrane much less readily than K+, but each type of ion still has own equilibrium potential - membrane potential is combined effect of all these individual equilibrium potentials
• The Action Potential 

  • Neurons have excitable membranes, as they’re capable of generating action potentials the propagate quickly along length of axons
  • Mechanism for electrical signalling
  • In nervous system, information is coded in the form of action potentials that travel along axons
• Action potential has three phases:

  • Depolarization - phase where membrane becomes less negative and moves toward a positive charge
  • Rapid repolarization - changes membrane back to negative charge
  • Hyperpolarization - when membrane becomes more negative then it was during resting potential
• Threshold potential (-65 mV to -55mV) - membrane must depolarize from former resting potential to latter resting potential for action potential to begin

  • If reached, certain channels in axon open
  • Allowing ions to rush into axon along electrochemical gradient
  • Causing inside of membrane to become less negative and then positive with respect to the outside
• When membrane potential reaches about +40 mV, repolarization phase begins; triggered by closing of certain ion channels and opening of other ion channels in membrane.
• At the end of the actin potential there’s a hyperpolarization phase where the membrane becomes more negative than the resting membrapotential.

  • During this phase it’s more difficult for the neuron (requires greater depolarization to reach threshold) to initiate another action potential - relative refractory period
• An action potential occurs because specific ion channels in the plasma membrane open or close in response to changes in voltage

  • Always has the same three-phase form
  • Even though size of resting potential, threshold potential and peak depolarization may vary among species
  • All action potentials for a given neuron are identical in magnitude and direction
  • Action potentials are propagated down the length on the axon
• Action potential begins when Na+ flows into neuron - peak of action potential parallels Na+ concentration.
• Voltage-Gated Channels

  • Action potential depends on voltage-gated channels - ion channels that open and close in response to changes in membrane voltage - shape of voltage-gated channel changes in response to charges present at membrane surface
• Voltage clamping - technique which allows researchers to hold an axon at any voltage and record the electrical currents that occur.
• • Patch clamping - a form of voltage clamp that allows isolation and measurement of electric activity of a single ion channel - determined that:
  • Voltage-gated channels are either open or closed
  • Sodium channels open quickly after depolarization
  • Potassium channels open with a delay during depolarization and continue to flip open and closed until membrane depolarizes
• Initial depolarization leads to opening of more Na+ channels, which depolarizes membrane further, leads to opening of more Na+ channels, etc. - exemplifies positive feedback (occurrence of an event making same event more likely to occur).
• Tetrodoxin blocks the voltage-gated Na+ channel by binding to a specific site on the channel protein.