Membrane Potential

Introduction

• Membrane potential: difference in charge inside and outside the cell
  • Plasma membrane barrier separating charges
  • Ion concentration differ between the inside and outside or outside cell
  • Polarized
• Resting membrane potential: when neurons are not sending signals
• Plasma membrane is not very permeable to cations and anions
  • Separates charge by keeping different ions largely inside or outside the cell
• -70 mV resting potential inside cell
  • Interior more negative than exterior
• Negative ions within the cell are drawn to the positive ions arrayed on the outer surface

Factors Contributing to Resting Potential

1. NA⁺/K⁺ -ATPase (sodium-potassium pump): transports 3 Na+ out for every 2 K+ moved in
2. Ion specific channels: allow passive movement of ions
   • More ungated K+ channels than ungated Na+ channels 
   • Membrane more permeable to K+ at rest
3. Negatively charged molecules such as proteins more abundant inside cell

Electrochemical Gradient

• Electrochemical gradient: combined effect of electrical and chemical gradient
• Equilibrium potential: no net movement due to opposing forces of chemical and electrical gradients

Communication Between Neurons

• Changes in membrane potential are changes in the degree of polarization
• Depolarization: cell membrane less polarized and less negative relative to surrounding solution
  • Gated channels open allowing Na+ to flow in and membrane potential becomes more positive (less negative)
• Hyperpolarization: cell membrane more polarized and more negative 
  • K+ moves out of the cell making the cell membrane less positive (more negative)
• All cells have a membrane potential
• Only neurons and muscle cells are excitable
  • Excitable: capacity to generate electrical signals
• Use gated ion channels
  • Voltage-gated ion channel: open and close in response to voltage changes
  • Ligand-gated ion channel: open and close in response to ligands or chemicals

Two Types of Changes

1. Graded potentials
   • Depolarization or hyperpolarization
   • Varies depending on strength of stimulus
   • Occur locally on dendrites or cell body 
   • Spreads a short distance and dies out
   • Act as triggers for action potential
2. Action potentials
   • Carry electrical signal along an axon
   • Always the large same amplitude depolarization
   • All-or-none - cannot be graded
   • Actively propagated - regenerates itself as it travels

Generation and Transmission of Electrical Signals Along Neurons

• Action potential begins when graded potential depolarizes to threshold potential (-50mV)
• Voltage-gated Na+ channels, triggering action potential
• Na+ rapidly diffuses into cell causing spike
• Inactivation gate in Na+ channel shuts when membrane sufficiently positively polarized 
  • Cannot reopen until resting potential is restored
• Voltage-gated K+ channels also open at threshold potential, but 1 msec later than Na+ channels
• K+ leave cell and membrane becomes negative again
• So many K+ leave that membrane hyperpolarizes
• Voltage-gated K+ channels close and resting membrane potential is restored
• Evolution of K+ channels with a slightly slower opening time than Na+ channels was a key event that led to the formation of nervous systems
• If both opened at the same time, they would negate each other’s effects
• Absolute refractory period
  • While inactivation gates of Na⁺ channels are closed, cell is unresponsive to another stimulus
  • Places limits on the frequency of action potentials
  • Also ensures action potential does not move backward toward cell body

Speed Variation

• Speed varies depending on
  • Axon diameter
  • Broad axons provide less resistance and action potential moves faster
  • Myelination 
  • Myelinated axons are faster then unmyelinated
  • Oligodendrocytes and Schwann cells make myelin sheath
  • Not continuous: gaps at nodes of Ranvier
  • Saltatory conduction: action potential seems to “jump” from node to node

Synapses

• Junction where nerve terminal meets a neuron, muscle cell, or gland
• Presynaptic cell: sends signal
• Synaptic cleft and postsynaptic cell: receives signal
• Two types
  • Electrical synapses: electric charge freely flows through gap junctions from cell to cell
  • Chemical synapses: neurotransmitter acts as signal from presynaptic to postsynaptic cell
  • Presynaptic nerve cell contains vesicles of neurotransmitter
  • Exocytosis releases neurotransmitter into \n synaptic cleft
  • Diffuses across cleft
  • Binds to channels or receptors in postsynaptic cell membrane
• Binding of neurotransmitter changes membrane potential of postsynaptic cell
• Excitatory postsynaptic potential (EPSP): brings membrane closer to threshold potential
• Inhibitory postsynaptic potential (IPSP): takes membrane further from threshold potential (hyperpolarization)
• Synaptic signal ends when neurotransmitter is broken down by enzymes or taken back into presynaptic cell for reuse

Neuron Response

• Synaptic integration: integrates multiple inputs to single neuron
• Spatial summation: when two or more EPSPs or IPSPs are generated at one time along different regions of the dendrites and cell body, their effects sum together
• Temporal summation: two or more EPSPs arrive at same location is quick succession