Outlines
• Basic Functions of the Synapses
• Types of Synapses
• The Physiologic Anatomy of the Synapses
• Mechanism of Neurotransmission
• Synaptic Transmitters
How is Information Transmitted in the Nervous System?
• Nerve action potentials, simply called “nerveimpulses”
• Each impulse may be blocked, changed or integrated
• Collectively, these functions can be classified assynaptic functions of neurons
National Institutes of Health (US); Biological Sciences Curriculum Study.Bethesda (MD): National Institutes of Health (US); 2007-
Overview
• Almost all the synapses used for signaltransmission are chemical synapses
• Neurotransmitter acts on the membrane of thenext neuron to excite the neuron, inhibit it, ormodify its sensitivity
Neuroscience. 2nd edition.Purves D, Augustine GJ, Fitzpatrick D, et al., editors.Sunderland (MA): Sinauer Associates; 2001.
• Electrical synapses, in contrast, are characterized bydirect open fluid channels that conduct electricity fromone cell to the next
From: Electrical SynapsesCopyright © 2001, Sinauer Associates, Inc.
“One-way conduction”
• Chemical synapses always transmit signals in onedirection (exceedingly important characteristic)
Function of Receptor Proteins on Postsynaptic Neuron
1. Binding component: bindsthe transmitter coming fromthe presynaptic terminal
2. Ionophore component: canbe either an ion channel or a“second messenger”activator
Neuroscience. 2nd edition.Purves D, Augustine GJ, Fitzpatrick D, et al., editors.Sunderland(MA): Sinauer Associates; 2001
Excitatory or Inhibitory Receptors in the Postsynaptic Membrane
Excitation
• Opening of sodium (Na+) channels > flow of positivecharges interiorly > raised membrane potential tothreshold for excitation
• Decreased conduction through chloride (Cl-) channels >decreased diffusion of negative charges to the inside >more positive membrane potential
Inhibition
• Opening of chloride (Cl-) channels > influx of negativecharges to the inside > more negative membranepotential, which is inhibitory
• Increased conductance of potassium (K+) out of theneuron > less positive charges inside > increasednegativity
Overview
• More than 50 chemical substance have been proved
• Two groups: small-molecule (rapidly acting) andneuropeptides (slowly acting)
Small-Molecule Transmitters
• Synthesized in the cytosol of the presynapticterminal
• Absorbed by means of active transport intotransmitter vesicles
• Continuous recycling of vesicles
1. Acetylcholine
• Typical small-molecule transmitter
Acetyle CoA Choline
Acetylcholine + CoA
Synthes
is
• Acetylcholine is secreted by neurons in many areas ofthe nervous system:
1. Large pyramidal cells from the motor cortex
2. Basal Ganglia
3. Somatic neurons
4. All Preganglionic autonomic neurons
5. Parasympathetic postganglionic neurons
• Excitatory centrally, mixed action peripherallydepending on type of receptor
3. Amino Acids
• Glycine: Secreted mainly at synapses in the spinalcord. It is always inhibitory
• GABA: secreted in the spinal cord, cerebellum, basalganglia and cortex. It is always inhibitory
• Glutamate: secreted by sensory pathways enteringthe CNS, as well as cerebral cortex. It is alwaysexcitatory
4. Nitric Oxide (NO)
• Secreted in areas responsible for long-term behaviorand for memory
• NO has different chemical properties. It is a diffusiblegas
• It regulates cerebral and peripheral blood flow
Neuropeptides
• Slow acting > much more prolonged actions
• They are synthesized as integral parts of large-protein molecules by ribosomes in the cell body
Small Molecule Neurotransmitter SubstancesAcetylcholine (ACh) Dopamine (DA) Norepinephrine (NE)
Serotonin (5-HT) Histamine Epinephrine
Amino AcidsGamma-aminobutyric
acid (GABA)Glycine Glutamate
Aspartate
Neuroactive Peptides - partial list
bradykinin beta-endorphin bombesin calcitonin
cholecystokinin enkephalin dynorphin insulin
gastrin substance P neurotensin glucagon
secretin somatostatin motilin vasopressin
oxytocin prolactin thyrotropin angiotensin II
sleep peptides galanin neuropeptide Ythyrotropin-
releasing hormone
gonadotropnin-
releasing hormone
growth hormone-
releasing hormone
luteinizing
hormone
vasoactive
intestinal peptide
Soluble GasesNitric Oxide (NO) Carbon Monoxide
Summary of Neurotransmitters