2. Synapse
•A region where communication occurs between
two neurons, or between an neuron and a
target cell
•A junction between a presynaptic neuron and
postsynaptic neuron
• Presynaptic neuron - conducts impulses toward synapse
• Postsynaptic neuron - transmits impulses away from the synapse
3. Synapse Types
• 2 Main Types:
• Chemical Synapse—such as the Neuromuscular Junction
• Electrical Synapse—faster than chemical synapse
5. Chemical Synapse
• The first neuron secrets a chemical substance (neurotransmitter, or
“NT”) at the synapse
• Transmitter acts on receptor proteins in the membrane of the next
neuron
• To excite, inhibit, or modify it sensitivity
• Signal is transmitted in one direction
6. Chemical Synapse
Events at a chemical synapse
1. Arrival of nerve impulse opens
voltage-gated calcium channels.
2. Ca++ influx into presynaptic term.
3. Ca++ acts as intracellular messenger
stimulating synaptic vesicles to fuse with
membrane and release NT via exocytosis.
4. Ca++ removed from terminal by
mitochondria or calcium-pumps.
5. NT diffuses across synaptic cleft and
binds to receptor on postsynaptic membrane
6. Receptor changes shape of ion channel
opening it and changing membrane potential
7. NT is quickly destroyed by enzymes or
taken back up by astrocytes or presynaptic
membrane.
Note: For each nerve impulse reaching the
presynaptic terminal, about 300 vesicles are
emptied into the cleft.
9. Electrical synapses
• Membranes of presynaptic and postsynaptic cells are fused
• Transmission is faster
• Can be bidirectional
• Generally associated with defensive reflexes
10. Electrical synapses
• Characterized by direct channels that conduct electricity from one
neuron (axon terminal) to the next neuron (dendrites)
• Mostly consist of small protein tubular structure (gap junctions) that
allow free movement of ions from the interior of one neuron to the
next neuron.
11. Electrical Synapses
• Gap junctions that allow local
current to flow between
adjacent cells. Connexons:
protein tubes in cell membrane.
• Found in cardiac muscle and
many types of smooth muscle.
Action potential of one cell
causes action potential in next
cell, almost as if the tissue were
one cell.
• Important where contractile
activity among a group of cells
important.
12. Synapse Types
• Multiple ways of connecting
• Examples
• Axon to Dendrite – excite or inhibit neuron
• Axon to Axon Terminal – moderate NT release
• Axon to Extracellular Space or blood – potential for diffuse effects
17. Receptor Molecules and Neurotransmitters
•Neurotransmitter only "fits" in one receptor.
•Not all cells have receptors.
•Neurotransmitters are excitatory in some cells and
inhibitory in others.
•Some neurotransmitters (norepinephrine) attach to
the presynaptic terminal as well as postsynaptic and
then inhibit the release of more neurotransmitter.
18. Neurotransmitters (Chemical Synapses)
• Chemicals used for neuronal communication with the body
and the brain
• 50 different neurotransmitters have been identified
• Classified chemically and functionally
• Chemically:
• ACh, Biogenic amines, Peptides
• Functionally:
• Excitatory or inhibitory
• Direct/Ionotropic (open ion channels) or Indirect/metabotropic
(activate G-proteins) that create a metabolic change in cell
19.
20.
21. Summary
• Synapse = the region where communication occurs
between two neurons, or between an neuron and a target
cell
• 2 main types of synapses: chemical and electrical
• Processes for both types of synapses
• 3 Types of connections between neurons: axodendritic,
axosomatic, & axoaxonic
• Functions of Synapses
• Neurotransmitters
