Neurotransmitters

Introduction

  • Neurotransmitters   * More than 100 different ones in animals   * Categorized by size or structure
  • Excitatory and inhibitory neurotransmitters   * Like brakes and accelerators on a car
  • All nervous systems operate with combined excitatory and inhibitory neurotransmitters

Five Classes of Neurotransmitters

  1. Acetylcholine    * One of most widespread neurotransmitters    * Released at neuromuscular junctions    * Excitatory in brain and skeletal muscles but inhibitory in cardiac muscles
  2. Biogenic amines    * Widespread physiological effects and psychoactive    * Abnormally high or low levels associated with a variety of mental illnesses (schizophrenia, depression)
  3. Amino Acids    * Glutamate most widespread excitatory neurotransmitter    * GABA (gamma aminobutyric acid) most common inhibitory neurotransmitter
  4. Neuropeptides    * Often called neuromodulators – can alter response of postsynaptic neuron to other neurotransmitters    * Example: Opiate peptides
  5. Gaseous Neurotransmitters    * Nitric oxide, NO    * Carbon monoxide, CO    * Not sequestered into vesicles    * Produced locally as required    * Short-acting – influence other cells by diffusion    * Drugs for male sexual dysfunction enhance erections by increasing or mimicking action of NO on smooth muscle    * Function of CO uncertain

Otto Loewi Discovered Acetylcholine

  • He was interested in how nerves communicate with muscles
  • A certain nerve attached to the heart increased contraction rate while another nerved decreased it
  • Placed two frog hearts in separate, connected chambers
  • Stimulated vagus nerve on first heart and rate slowed
  • Transferred sample of saline solution from first to second heart - rate also slowed
  • Acetylcholine was the first neurotransmitter discovered

Postsynaptic Receptors

  • In some cases, same neurotransmitter can have excitatory or inhibitory effects
  • Response of postsynaptic cell depends on receptor type
  • Ionotropic receptors: ligand-gated ion channels open in response to neurotransmitter
  • Metabotropic receptors: G-protein coupled receptors (GPCRs) initiate changes in postsynaptic cell

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