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
      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