Physiology · MU Pleven

Synaptic Transmitters

Covers the classes of small-molecule, rapidly acting transmitters (acetylcholine, amines, amino acids, nitric oxide) and the slowly acting neuropeptides, including hypothalamic-releasing hormones and pituitary peptides. It then explains how postsynaptic receptor proteins work through ligand-gated ion channels or G-protein second-messenger cascades, contrasts cation and anion channels, and sets out the excitatory and inhibitory mechanisms. The note also outlines the four effects a second messenger system can have, from ion channel opening to gene transcription.

Inside: 5 sections · 2 labelled pictures · 15 flashcards · 7 min

Topic 7 of 17 in the MU Pleven syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 5 sections, about 1,055 words (7 min of reading), with 15 flashcards.

In this note

  1. TOPIC 7. SYNAPTIC TRANSMITTERS. TRANSMITTER RECEPTORS IN THE POSTYNAPTIC MEMBRANE. NEUROPEPTIDES.
  2. SYNAPTIC TRANSMITTERS
  3. TRANSMITTER RECEPTORS IN THE POSTYNAPTIC MEMBRANE
  4. SECOND MESSENGER
  5. EXCITATORY AND INHIBITORY RECEPTORS IN THE POSTSYNAPTIC MEMBRANE

SYNAPTIC TRANSMITTERS

  • SMALL-MOLECULE, RAPIDLY ACTING TRANSMITTERS
  • Class I
  • Acetylcholine
  • Class II → Amines
  • Norepinephrine
  • Epinephrine
  • Dopamine
  • Serotonin
  • Histamine
  • Class III → Amino Acids
  • Gamma-aminobutyric acid (GABA)
  • Glycine
  • Glutamate
  • Aspartate
  • Class IV
  • Nitric oxide (NO)
  • NEUROPEPTIDES – SLOWLY ACTING TRANSMITTERS OR GROWTH FACTORS
  • Hypothalamic-releasing hormones
  • Thyrotropin-releasing hormone
  • Luteinizing hormone–releasing hormone
  • Somatostatin (growth hormone inhibitory factor)
  • Pituitary peptides
  • Adrenocorticotropic hormone (ACTH)
  • Beta-Endorphin
  • Alpha-Melanocyte-stimulating hormone
  • Prolactin
  • Luteinizing hormone
  • Thyrotropin
  • Growth hormone
  • Vasopressin
  • Oxytocin

TRANSMITTER RECEPTORS IN THE POSTYNAPTIC MEMBRANE

ACTION OF THE TRANSMITTER SUBSTANCE ON THE POSTSYNAPTIC NEURON – FUNCTION OF “RECEPTOR PROTEINS”

The membrane of the postsynaptic neuron contains large numbers of receptor proteins.

The molecules of these receptors have two important components:

  1. 1. A binding component that protrudes outward from the membrane into the synaptic cleft -here it binds the neurotransmitter coming from the presynaptic terminal,
  2. 2. An ionophore component that passes all the way through the postsynaptic membrane to the interior of the postsynaptic neuron.
  • The ionophore in turn is one of two types:
  • An ion channel that allows passage of specified types of ions through the membrane
  • A “second messenger” activator that is not an ion channel but instead is a molecule that protrudes into the cell cytoplasm and activates one or more substances inside the postsynaptic neuron. These substances in turn serve as “second messengers” to increase or decrease specific cellular functions.

Post synaptic receptors create an EPSP or IPSP by:

  • LIGAND-GATED / DIRECT GATING → transmitter reconfigures the channel when it attaches, allowing for the movement of ions either in or out of the cell.
  • INDIRECT GATING / SECONDARY MESSENGERS → transmitter binds to the receptor a GTP-binding protein activates a second-messenger cascade that modulates channel activity.

ION CHANNELS

The ion channels in the postsynaptic neuronal membrane are usually of two types:

  1. 1. CATION CHANNELS → allow mainly sodium ions to pass when opened, but sometimes allow potassium and/or calcium ions as well
  • The cation channels that conduct sodium ions are lined with negative charges. These charges attract the positively charged sodium ions into the channel when the channel diameter increases to a size larger than that of the hydrated sodium ion. But those same negative charges repel chloride ions and other anions and prevent their passage.
  1. 2. ANION CHANNELS → allow mainly chloride ions to pass but also minute quantities of other anions.

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