1. The Synapse and Synaptic Transmission

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

4. Neuron - 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.

7. Neuron – Excitation & Inhibition

8. Electrical synapses
Membranes of presynaptic and postsynaptic cells are fused
Transmission is faster
Can be bidirectional
Generally associated with defensive reflexes
B5revisedintronervous.ppt

9. 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.
B5revisedintronervous.ppt

10. 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.

11. 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

12. Synapses Axodendritic synapse Axosomatic synapse Axoaxonic synapse
Figure 11.17

13. Synapse Function Neurotransmitter cycle in Axon Terminals
Synthesis
Storage
Release
Inactivation
Reuptake
Degradation
Neural transmission problems if cycle disrupted (e.g., drugs) at any step

14. Synapse Function

15. Synapse Function

16. 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.

17. 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

18. Chemical Neurotransmitters
Acetylcholine (ACh)
Biogenic amines
Amino acids Endocrine system
Peptides
Novel messengers: ATP and dissolved gases NO and CO

19. 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