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Chapter 5

Chapter 5 Notes

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Department
Neuroscience
Course Code
NROB60H3
Professor
Janelle Leboutillier

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Chapter 5 Notes
Synaptic Transmission: /the process of information transfer a synapse
Electrical Synapse: One hypothesis of synaptic transmission whereby electrical current flowed from one
neuron to the next
Chemical Synapses: Another hypothesis of synaptic transmission whereby chemical neurotransmitters
transfer information from on neuron to another at the synapse
-Supported by Otto Loewi
oElectrically stimulated axons which innervated the frogs heart caused the release of
a chemical and this chemical could mimic the effects of neuron stimulation of the
heart beat
oProved that synaptic transmission between nerve and heart is chemically mediated
oHeart is supplied with two types of innervation; one type speeds the beating of the
heart, the other slows (supplied by the vagus nerve)
oIsolated a frog heart with the vagal innervation left intact, stimulated the nerve
electrically and observed the expected effect, the slowing of the heartbeat
The critical demonstration that this effect was chemically mediated came
when he took the solution that bathe this heart, applied it to a second
isolated frog heart applied it to a second isolated frog heart, and found that
the beat of this one also slowed
Neurotransmitter here termed vagusstoff turned out to be acetylecholine
oChemical synapses comprise the majority of synapses in the brain
Synapse: Specialized junction where one part of a neuron contacts and communicates with another neuron
or cell type (such as muscle or glandular cell)
-Information flows in one direction (from neuron to target cell)
Electrical Synapses
-Simple in structure and function and allow direct transfer of ionic current from one cell to
the next
-Occur at gap junctions – membranes of two cells are separated by only 3mm
oGap is spanned by special proteins called connexins
oSix connexins combine to form a channel (connexon) and two connexons (on from
each cell) combine to form a gap junction channel
Channel allows ions to pass directly from the cytoplasm of one cell to the
cytoplasm of the other
Pore of most gap junction channels is large: 1-2nm which is big enough for
all major cellular ions and many small organic molecules to pass through
oElectrical synapses are bidirectional, transmission is very fast, fail-safe
oAn AP in the presynaptic neuron can produce an AP in the postsynaptic neuron
almost instantaneously
-Electrical synapses are common in every part of the mammalian CNS
-When two neurons are electrically coupled, an AP in the presynaptic neuron causes a small
amount of ionic current to flow across the gap junction channels into the other neuron
oThis current causes a postsynaptic potential (PSP) in the second neuron (and
therefore induces a PSP in the first neuron)
www.notesolution.com
oPSP generated by a single electrical synapse in the mammalian brain is usually about
1mV, and may not, by itself be large enough to trigger an AP in the postsynaptic cell
oElectrical synapses vary and are often found where normal function requires that the
activity of neighboring neurons be highly synchronized
oCritical during early embryonic stages
Chemical Synapses
- Most synaptic transmission in the mature human NS is chemical
- Universal characteristics of chemical synapses:
- Presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft (20-50
nm) wide
- Ten times the width of the separation at gap junctions
- Cleft is filled with a matrix of fibrous extracellular proteins which adheres each synaptic end to each other
- Presynaptic end is usually an axon terminal, which contains dozens of small membrane-enclosed spheres
each about 50nm in diameter (synaptic vesicles)
- Vesicles store neurotransmitter, chemical used to communicate with the postsynaptic neuron
- Many axon terminals also contain larger vesicles (100 nm in diameter) called secretory granules
- Contain soluble protein that appears dark in the electron microscope (dense-core
vesicles)
- Membrane Differentiations: Dense accumulations of protein adjacent to and within
the membranes on either side of the synaptic cleft
- Action Zones: Pyramids and membrane associated with them are the actual sites of
neurotransmitter release
- Protein Density: The protein thickly accumulated in and just under the posysynaptic
membrane
- Contains neurotransmitter receptors which convert the intercellular chemical
signal (NT) into an intracellular signal (e.g. change in membrane potential) in the
postsynaptic cell
CNS Synapses
-Different types of synapse may be distinguished by which part of the neuron is postsynaptic
to the axon terminal
oIf the postsynaptic membrane is on a dendrite synapse = axodendritic
oIf the postsynaptic membrane is on a cell body = axosomatic
oIf postsynaptic membrane is on another axon = axoaxonic
oIf dendrites form synapses with one another = dendrodendritic
-CNS synapses may be classified into two categories:
oGrays Type I Synapses: Synapses in which the membrane differentiation on the
postsynaptic side is thicker than that on the presynaptic side (asymmetrical)
oGrays Type II Synapses: Synapses in which the membrane differentiations are of
similar thickness (symmetrical)
www.notesolution.com
These structural differences predict functional differences
Grays Type I synapses are usually excitatory
Gray’s Type II synapses are usually inhibitory
The Neuromuscular Junction
- Synaptic junctions also exist outside the CNS (axons of the ANS innervate glands, smooth muscle, and
the heart)
Neuromuscular Junction: Chemical synapses that occur between the axons of motor neurons of the spinal
cord and skeletal muscle
-These synapses are always reliable
-An AP in the motor axon always causes an AP in the muscle it innervates
-Reliability is accounted for by its structural specialization
oIt is one of the largest synapses in the body
oThe post-synaptic membrane is called the motor end plate containing a series of
shallow folds
oPresynaptic action zones are precisely aligned with these folds, and folds are packed
with NT receptors
Chemical Synaptic Transmission
Neurotransmitters
Three Categories:
-Amino Acids
-Amines
-Peptides
Amino AcidsAmines Peptides
GABA Acetylcholine (ACh) Cholecystokinin (CCK)
GlutamateDopamine (DA)Dynorphin
Gycline EpinephrineEnkephalins (Enk)
Histamine NAAG
Norepinephrine (NE)Neuropeptide Y
Serotonin (5-HT) Somatostatin
Substance P
Thyrotropin-releasing hormone
Vasoactive Intestinal Polypeptide
(VIP)
-Amino and amine NTs are small organic molecules containing at least one nitrogen atom
-They are stored and released from synaptic vesicles
-Peptide NT are large molecules stored in and released from secretory granules
-Fast synaptic transmission at most CNS synapses is mediated by the amino acids glutamate,
GABA, and glycerine
-ACh mediates fast synaptic transmission at the neuromuscular junction
www.notesolution.com

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Description
Chapter 5 Notes Synaptic Transmission: the process of information transfer a synapse Electrical Synapse: One hypothesis of synaptic transmission whereby electrical current flowed from one neuron to the next Chemical Synapses: Another hypothesis of synaptic transmission whereby chemical neurotransmitters transfer information from on neuron to another at the synapse - Supported by Otto Loewi o Electrically stimulated axons which innervated the frogs heart caused the release of a chemical and this chemical could mimic the effects of neuron stimulation of the heart beat o Proved that synaptic transmission between nerve and heart is chemically mediated o Heart is supplied with two types of innervation; one type speeds the beating of the heart, the other slows (supplied by the vagus nerve) o Isolated a frog heart with the vagal innervation left intact, stimulated the nerve electrically and observed the expected effect, the slowing of the heartbeat The critical demonstration that this effect was chemically mediated came when he took the solution that bathe this heart, applied it to a second isolated frog heart applied it to a second isolated frog heart, and found that the beat of this one also slowed Neurotransmitter here termed vagusstoff turned out to be acetylecholine o Chemical synapses comprise the majority of synapses in the brain Synapse: Specialized junction where one part of a neuron contacts and communicates with another neuron or cell type (such as muscle or glandular cell) - Information flows in one direction (from neuron to target cell) Electrical Synapses - Simple in structure and function and allow direct transfer of ionic current from one cell to the next - Occur at gap junctions membranes of two cells are separated by only 3mm o Gap is spanned by special proteins called connexins o Six connexins combine to form a channel (connexon) and two connexons (on from each cell) combine to form a gap junction channel Channel allows ions to pass directly from the cytoplasm of one cell to the cytoplasm of the other Pore of most gap junction channels is large: 1-2nm which is big enough for all major cellular ions and many small organic molecules to pass through o Electrical synapses are bidirectional, transmission is very fast, fail-safe o An AP in the presynaptic neuron can produce an AP in the postsynaptic neuron almost instantaneously - Electrical synapses are common in every part of the mammalian CNS - When two neurons are electrically coupled, an AP in the presynaptic neuron causes a small amount of ionic current to flow across the gap junction channels into the other neuron o This current causes a postsynaptic potential (PSP) in the second neuron (and therefore induces a PSP in the first neuron) www.notesolution.com
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