TR

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