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Synapses
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.17
Synaptic Cleft

Fluid-filled space separating the presynaptic and
postsynaptic neurons

Prevents nerve impulses from directly passing
from one neuron to the next

Transmission across the synaptic cleft:

Is a chemical event (as opposed to an electrical
one)

Ensures unidirectional communication between
neurons
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Axon terminal of
presynaptic neuron
Axon of
presynaptic
neuron
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Axon terminal of
presynaptic neuron
Mitochondrion
Axon of
presynaptic
neuron
Synaptic vesicles
containing
neurotransmitter
molecules
2
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Axon terminal of
presynaptic neuron
Mitochondrion
Postsynaptic
membrane
Axon of
presynaptic
neuron
Synaptic vesicles
containing
neurotransmitter
molecules
2
Synaptic
cleft
Ion channel
(closed)
3
Ion channel (open)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel (open)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Cleft: Information Transfer
Ca2+
1
Neurotransmitter
Axon terminal of
presynaptic neuron
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Na+
Receptor
Postsynaptic
membrane
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules
5
Degraded
neurotransmitter
2
Synaptic
cleft
Ion channel
(closed)
3
4
Ion channel closed
Ion channel (open)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.18
Synaptic Delay



Neurotransmitter must be released, diffuse across
the synapse, and bind to receptors
Synaptic delay – time needed to do this (0.3-5.0
ms)
Synaptic delay is the rate-limiting step of neural
transmission
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Postsynaptic Potentials

The two types of postsynaptic potentials are:

EPSP – excitatory postsynaptic potentials

IPSP – inhibitory postsynaptic potentials
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Excitatory Postsynaptic Potential (EPSP)
Open Na+ channels
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.19a
Inhibitory Postsynaptic (IPSP)
Open K+ and Cl- channels
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.19b
Summation
A single EPSP cannot induce an action potential
Temporal summation
Spatial summation
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 11.20
Chemical Neurotransmitters
50 different neurotransmitters

Acetylcholine (ACh)

Biogenic amines

Amino acids

Peptides

Novel messengers: ATP and dissolved gases NO
and CO
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Acetylcholine


Degraded by the enzyme
acetylcholinesterase
(AChE)
Released by:

All neurons that
stimulate skeletal muscle

Some neurons in the
autonomic nervous
system
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Tetanus – bacterial toxin
Neurotransmitters: Biogenic Amines

Catecholamines – dopamine, norepinephrine (NE),
and epinephrine


Indolamines – serotonin and histamine
emotional behaviors
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Amino Acids


Include:

GABA – Gamma ()-aminobutyric acid

Glycine

Aspartate

Glutamate
Found only in the CNS
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Peptides

Include:

Substance P – mediator of pain signals

Beta endorphin, dynorphin, and enkephalins

Act as natural opiates; reduce pain perception

Bind to the same receptors as opiates and
morphine

Gut-brain peptides – somatostatin, and
cholecystokinin
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Novel Messengers

ATP

Is found in both the CNS and PNS

Produces excitatory or inhibitory responses
depending on receptor type

Induces Ca2+ wave propagation in astrocytes

Provokes pain sensation
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitters: Novel Messengers


Nitric oxide (NO)

Activates the intracellular receptor guanylyl
cyclase

Is involved in learning and memory
Carbon monoxide (CO) is a main regulator of
cGMP in the brain
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of Neurotransmitters

Two classifications: excitatory and inhibitory

Excitatory neurotransmitters cause depolarizations
(e.g., glutamate)

Inhibitory neurotransmitters cause
hyperpolarizations (e.g., GABA and glycine)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of Neurotransmitters

Some neurotransmitters have both excitatory and
inhibitory effects

Determined by the receptor type of the
postsynaptic neuron

Example: acetylcholine

Excitatory at neuromuscular junctions with
skeletal muscle

Inhibitory in cardiac muscle
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neurotransmitter Receptor Mechanisms

Direct: neurotransmitters that open ion channels

Promote rapid responses

Examples: ACh
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings