The resting neuron

• Neurons

  • Transmit information to other neurons muscles of gland cells
  • 80% of neurons are in the brain
• Neuron structure

  • Dendrites
  • Cell body/soma
  • Axon
  • Axon Terminal/ Terminal buttons
• Sensory neurons

  • Part of PNS
  • Contain sensory receptors for detecting sensory changes
  • sends information about these changes to CNS
  • Cell body in PNS, axon enters CNS (axon terminals located in CNS )
• Dendrites

  • Receive neural message transmitted across the synapse
• Axon

  • Outer surface of the axon carries information from the cell body to the terminal buttons
  • Carries action potential
• Anterograde

  direction along an axon = cell body toward the terminal buttons
• Retrograde

  direction along an axon = terminal buttons towards the cell body
• myelin sheath 


  surrounds axons + insulates them, preventing messages from spreading between adjacent axons
• Motor neurons

  • part of PNS
  • Synapses to skeletal muscles to command movement
  • Synapses to glands to release hormones
  • relays signal from CNS to PNS
  • Dendrites & cell body in CNS , axon enters PNS
• Interneurons

  • In CNS
  • Receives info from sensory neurons
  • Sends info to motor neurons
  • Integrate / change signal
  • Integrate: inputs from multiple afferent neurons - average signal
  • Change: interneurons can provide excitatory or inhibitory signals
• Neuronal membrane

  • Made of two layers of lipid molecules
  • Lipid molecules
  • Hydrophilic (water loving) head
  • Hydrophobic (water hating) tails
  • Barrier: water soluble molecules cannot pass through
  • Particularly impermeable to ions
• Fluid environment

  • Fluid environment containing ions
  • Intracellular fluid: fluid contained within cells
  • Extracellular fluid: fluids contained outside of cells
• Cations

  • Positive
  • Sodium (Na+)
  • Predominantly extracellular
  • potassium (K+)
  • predominanlty intracellular
• Anions

  • Negative
  • Organic ions (A-)
  • Only intracellular
  • Chloride (Cl-)
  • Predominantly extracellular
• Movement of ions

  • Ions move because of:
  • Concentration gradients (via diffusion)
  • Electrical force (via electrostatic pressure)
• Diffusion

  • Ions are subject to the force of diffusion:
  • even distribution within a given medium
  • Ions move from an area of high concentration to an area of low concentration, moving down the concentration gradient
• Electrostatic pressure

  • electrical charge produces an action:
  • charges of opposite sign attract
  • charges of same sign repel
  • electrostatic pressure
• Electrical polarity of neuron

  • Neuron is polarised
  • At rest, neurons are negatively charged compared to extracellular fluid
  • negative charge occurs if there are less positive ions and/or more negative ions inside cell
  • Whilst there is a difference in charge, an electrical force tends to move ions across the membrane
• Border guards

  Ion channels (leak channels):

  • passive ion specific conduits
  • selected ion rush down gradients concentration & electric potential
  • controlled by gates
• Border guards

  Ion pumps

  • Energy consuming
  • Active transport - against gradient
  • Maintains and builds gradients
  • Slower
• Potassium ions (K+)

  • Diffusion
  • K+ highly concentrated in cell
  • K+ wants to move out of cell down concentration gradient
  • at rest, K+ leak channels allow K+ to leave neuron down concentration gradient
  • Inside cell becomes more negative
• Potassium ions (K+)

  • Electrostatic pressure
  • Not a lot of K+ moves out
  • Ions will stop moving when opposing forces are equal (at equilibrium)
  • NB: this happens in a resting cell
• Chloride ion (Cl-)

  • Diffusion
  • Cl - highly concentrated outside cell
  • Cl - wants to move into cells down concentration gradient
• Chloride ion (Cl-)

  • Electrostatic pressure
  • Inside of cell is negatively charged
  • Cl- also wants to move out of cell due to repel of electric charge
  • Net force for Cl- = stay where it is
• Sodium ions (Na+)

  • Diffusion
  • Na+ is highly concentrated outside cell
  • Na+ wants to move into cell down concentration gradient
• Sodium ions (Na+)

  • Electrostatic pressure
  • inside of cell is negatively charged
  • Na+ also wants to move into cell due to electric charge attraction
  • Net force for Na+ = move into cell
  • 📢 There are few sodium channels, so ion movement is slight
• Resting potential

  • Resting membrane potential
  • Two forces act on ions
  • membrane is a barrier to ion movement
  • at rest membrane is permeable to K+ so mainly K+ ions move
  • K+ ion movement stops once opposing forces reach equilibrium
  • Result: unequal distribution of positive & negative ions on the inside & outside of membrane
  • Resting membrane potential
  • difference in charge across membrane at rest = -70 mV
• Sodium-potassium pump

  • a transport protein that uses energy (given by ATP) to constantly pump three sodium ions out of the cell while at the same time pumping two potassium ions into the cell.
  • more positive ions being pumped out than negative helps to keep the membrane potential negative.