BIOB30 Ch8 (273-end)

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8 Apr 2012
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BIOB30 Chapter 8
Neurons: Cellular and Network Properties
1
Cell-to-Cell Communication in the Nervous System
The specificity of neural communication depends on several factors:
o The signal molecules secreted by the neurons
o The target cell receptors for these chemicals
o The anatomical connections between neurons and their targets (occur
at synapses)
Neurons Communicate at Synapses
Each synapse has two parts:
o The presynaptic cell’s axon terminal
o The postsynaptic cell’s membrane
In a neural reflex, information moves from the presynaptic to postsynaptic
cell
o The postsynaptic cell isn’t always a neuron
o In most neuron-to-neuron synapses, the axon terminal is next to the
dendrites or cell body of the postsynaptic neuron
Sometimes on axon or axon terminal
Cells average 10,000 synapses (can be more than 150000)
Electrical Synapses
Electrical synapses pass an electrical signal or current directly from the
cytoplasm of one cell to another through gap junctions
o Rectifying synapse gap junctions in which current can only flow in
one direction
Occur mainly in CNS neurons
o Also found in glial cells, cardiac and smooth muscle, and in
nonexcitable cells that use electrical signals
The advantage is a rapid conduction of signals that can synchronize activity
within a network of cells
Gap junctions also allow diffusion of chemicals
Chemical Synapses
Chemical synapses the electrical signal of the presynaptic cell is converted
into a chemical signal (neurotransmitters) that crosses the synaptic cleft to
the bind with receptors on the postsynaptic cell initiating an electrical
response
o Most neurons in the nervous system use this
o An electrical response is a rapid response
A second messenger pathway is a slower response
Neurotransmitter synthesis can occur at the nerve cell body or the axon
terminal
o But, since axon terminals don’t have organelles for protein synthesis,
the proteins must be made in the cell body
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BIOB30 Chapter 8
Neurons: Cellular and Network Properties
2
o Enzymes are brought to the axon using slow axonal transport
(consumed slowly)
o Neurotransmitters are moved using fast axonal transport (consumed
fast)
Axon terminal contents
o Small synaptic vesicles
Some are docked at active zones along the membrane waiting
for signal to release contents
Some act as a reserve pool for neurotransmitters (cluster close
to docking sites)
o Large mitochondria in the cytoplasm
Calcium Is the Signal for Neurotransmitter Release at the Synapse
Release of neurotransmitters into the synaptic cleft takes place by exocytosis
Process:
o Depolarization in the axon terminal sets off a sequence of events
o The axon terminal membrane has voltage-gated Ca2+ channels that
open in response
o Ca2+ is more concentrated outside the cell so it moves in
o Ca2+ binds to regulatory proteins and initiates exocytosis
o Membrane proteins help the vesicle fuse with the cell membrane
The fused area opens and neurotransmitter is released into the
cleft
o The neurotransmitter diffuses across the gap to bind with
postsynaptic membrane receptors
Classic model of exocytosis
o The membrane of the vesicle becomes part of the axon terminal
membrane
o Endocytotic recycling of the vesicles occurs away from the active site
New model is the kiss-and-run pathway
o Synaptic vesicles fuse to the presynaptic membrane at a complex
called the fusion pore
o The fusion opens a small channel large enough for neurotransmitter
to pass through
o Then the vesicle pulls back from the fusion pore and returns to the
pool of vesicles in the cytoplasm
Neurons Secrete Chemical Signals
Neurocrine signal molecules may function as neurotransmitters,
neuromodulators, or neurohormones
Neurotransmitters and neuromodulators act as paracrine signals (target cells
located close to the neuron that secretes them)
o Some are autocrine signals too
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BIOB30 Chapter 8
Neurons: Cellular and Network Properties
3
Neurohormones are secreted into the blood and distributed throughout the
body
Neurotransmitters elicit a rapid response
o Usually act at a synapse
Neuromodulators are slower acting
o Are at synaptic and non-synaptic sites
The Nervous System Secretes Neurocrines
Neurocrines can be informally grouped into seven classes by structure
1. Acetylcholine
2. Amines
3. Amino acids
4. Purines
5. Gases
6. Peptides
7. Lipids
CNS releases many different neurocrines
PNS secretes only 3 major ones
o Neurotransmitters, acetylcholine and norepinephrine, and the
neurohormone epinephrine
Acetylcholine
Acetylcholine (Ach) a molecule synthesized by an enzyme from choline and
acetyl coenzyme A (acetyl CoA)
o Synthesis happens in the axon terminal
Choline a small molecule also found in membrane phospholipids
Acetyl CoA the metabolic intermidiate that links glycolysis to the citric acid
cycle
Cholinergic neurons that secrete ACh and receptors that bind to ACh
Amines
Amine neurotransmitter and hormones are derived from single amino acids
o All are active in the CNS
Tyrosine is converted to dopamine, norepinephrine, and epinephrine
o These neurocrines function as neurohormones when secreted by the
adrenal medulla
Adrenergic neurons (or noradrenergic neurons) neurons that secrete
norepinephrine
Serotonin (or 5-hydroxytryptamine or 5-HT) made from tryptophan
(amino acid) and histamine (derivative of histadine)
Norepinephrine is the major neurotransmitter of the peripheral autonomic
sympathetic division
Amino Acids
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