Neurobiology
Principles of Neural Science
- Kandel, Schwartz and Jessell:
Read pages 175-227 for this lecture
end
Neurobiology |
text:
Principles of Neural Science - Kandel, Schwartz and Jessell: Read pages 175-227 for this lecture end |
XIII. Synapses
are the connection between neurons and other cells back to X. Synthesis
(between neurons or with non-neural cells)
A. Communication occurs via synapses
1. basis for much of neural function
a. perception, motor action, learning
b. avg. neuron makes 1000 synaptic connections
i. receives more
c. 1014 (= 100 trillion) synaptic connections in the brain
2. Synaptic Transmission can be Electrical or Chemical
a. direct signal transduction
b. neurotransmitter as messenger
3. neurons can communicate rapidly over great distances
B. Electrical Synapses
1. ion channels connect cytoplasm of pre- and postsynaptic cells
a. cytoplasmic continuity via gap junctions
i. 1.5 nm communication channel across 3.5 nm synapse
ii. no delay in ionic transmission
2. Not unique to nerve cells - heart, smooth muscle, epithelial muscle
a. rare in the brain
3. Unidirectional or Bi-directional
a. rectifying synapses allow for only one direction of transmission by
rotation of connexins in response to voltage changes, gating the channel
b. most are bi-directional (nonrectifying)
i. not usually sensitive to voltage, but may be to pH or Ca++
4. Rapid, stereotypic responses
a. Mauthner neuron escape response in fish
5. Synchronization
a. several electrically coupled neurons decreases resistance
i. high threshold ® results in all or none action potentials behaviors
C. Chemical Synapses and the Synaptic Cleft
1. not connected structurally; 20-40 nm extracellular space: synaptic cleft
2. pre- and postsynaptic membranes have morphologically specialized regions
a. active zones for focused secretory release
b. postsynaptic density - proteins that anchor and
focus receptor function
3. presynaptic terminals contain localized collections of vesicles
a. synaptic vesicles are filled with chemical neurotransmitter
4. presynaptic action potential stimulates exocytosis of transmitter into cleft
a. transmitter diffuses and binds to a receptor on postsynaptic membrane
5. receptor binding causes ion channels to open or close
a. alteration of membrane conductance and potential of postsynaptic cell
b. several steps to postsynaptic depolarization result in delay of conduction
i. 0.3 to several ms
6. amplification
a. large quantities of transmitter can be released ® may depolarize
a large postsynaptic cell
7. similar to endocrine gland secretion
a. except faster and more directed
i. active zones - presynaptic secretory machinery focused for release
(1) presynaptic autonomic cells do not
ii. unfocused secretion may lead to multiple functions
- neurotransmitter, neuromodulator, endocrine
8. Neurotransmitters can be small molecules, such as: ACh, GABA,
glycine, glutamate, aspartate, 5-HT, DA, NE, Epi...
a. or peptides such as: Substance P, NT, insulin, somatostatin,
endorphins, VIP, CCK, PRL, AVP, OXY, CRH, GnRH, TRH, NMn...
b. or gas such as: nitric oxide (NO) or CO
c. action of neurochemical messengers depends upon the properties
of the receptors and not the transmitter (or modulator)
i. eg. ACh can be both excitatory and inhibitory,
or both simultaneously
9. All receptors are membrane-spanning proteins
a. extracellular region recognizes and binds transmitter (or modulator)
b. act directly by gating an ion channel
or
c. indirectly by initiating a 2nd messenger cascade
i. also may open/close ion channel
ii. and/or activate enzymes
iii. and/or bind to DNA ® transcription
10. Transmitters or Modulators in the cleft may be catabolized by specific enzymes
or taken back up into the presynaptic cell and used again (re-uptake)
a. even metabolites are taken back up by endocytosis and re-synthesized
into transmitter
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