Chapter 3
Synapses
The Concept of the Synapse
• Neurons communicate by transmitting
chemicals at junctions called “synapses”
• In 1906, Charles Scott Sherrington coined the
term synapse to describe the specialized gap
that existed between neurons.
The Concept of the Synapse
• Sherrington observed that repeated stimuli
over a short period of time produced a
stronger response.
• Led to the idea of temporal summation or that
repeated stimuli can have a cumulative effect
and can produce a nerve impulse when a
single stimuli is too weak.
Fig. 3-3, p. 54
The Concept of the Synapse
• Sherrington also noticed that several small
stimuli on a similar location produced a reflex
when a single stimuli did not.
• This led to the idea of spatial summation or
that synaptic input from several locations can
have a cumulative effect and trigger a nerve
impulse.
Fig. 3-4, p. 54
The Concept of the Synapse
• Excitatory postsynaptic potential (EPSP) is a
graded potential that decays over time and
space.
• Graded potentials are different than action
potentials!!
• The cumulative effect of EPSPs are the basis
for temporal and spatial summation.
The Concept of the Synapse
• Sherrington also noticed that during the reflex
that occurred, the foot of a dog that was
pinched retracted while the other three feet
were extended.
• He suggested that an interneuron in the
spinal cord sent an excitatory message to the
flexor muscles of one leg and an inhibitory
message was sent to the other three legs.
Fig. 3-5, p. 55
The Concept of the Synapse
• This led to the idea of inhibitory postsynaptic
potential or the temporary hyperpolarization
of a membrane.
• An IPSP occurs when synaptic input
selectively opens the gates for positively
charged potassium ions to leave the cell or
for negatively charged chloride ions to enter
the cells.
• Serves as an active “brake”, that suppresses
excitation.
The Concept of the Synapse
• Neurons can have thousands of synapses.
• Both temporal and spatial summation can
occur within a neuron.
• The likelihood of an action potential depends
upon the ratio of IPSPs to EPSPs at a given
moment.
Chemical Events at the Synapse
• Transmission of a message across the
synapse occurs by chemical means.
• Neurotransmitters are chemicals that travel
across the synapse and allow communication
between neurons.
Chemical Events at the Synapse
•
The major sequence of events that allow
communication between neurons across the
synapse are as follows:
1. The neuron synthesizes chemicals that
serve as neurotransmitters.
2. Neurons store neurotransmitters in axon
terminals or transport them there.
3. An action potential triggers the release of
neurotransmitters into the synaptic cleft.
Chemical Events at the Synapse (cont.)
4. The neurotransmitters travel across the cleft
and attach to receptors on the postsynaptic
neuron.
5. The neurotransmitters separate from the
receptors.
6. The neurotransmitters are taken back into
the presynaptic neuron (reuptake), diffuse
away, or are inactivated by chemicals.
7. The postsynaptic cell may send negative
feedback to slow the release of further
neurotransmitters.
Fig. 3-8, p. 59
Chemical Events at the Synapse
• Major categories of neurotransmitters include
the following:
– Amino acids — glutamate, GABA,
– Acetylcholine
– Monoamines – serotonin, dopamine,
norephinephrine, epinephrine
– Purines --adenosine
– Gases –nitric oxide (not laughing gas!)
Chemical Events at the Synapse
• Neurons synthesize neurotransmitters and
other chemicals from substances provided by
the diet.
• Smaller neurotransmitters are synthesized in
the presynaptic terminal and held there for
release.
– Example: acetylcholine
• Larger neurotransmitters are synthesized in
the cell body and transported down the axon.
– Example: peptides
Chemical Events at the Synapse
• Vesicles are tiny spherical packets located in
the presynaptic terminal where
neurotransmitters are held for release.
• Exocytosis refers to the excretion of the
neurotransmitter from the presynaptic
terminal into the synaptic cleft.
– Triggered by an action potential arriving fro
the axon.
Fig. 3-10, p. 61
Chemical Events at the Synapse
• Transmission across the synaptic cleft by a
neurotransmitter takes fewer than 10
microseconds.
• Most individual neurons release at least two
or more different kinds of neurotransmitters.
• A neuron may respond to more types of
neurotransmitters than it releases.
Chemical Events at the Synapse
• An ionotropic effect refers to when a
neurotransmitter attaches to receptors and
immediately opens ion channels.
• Ionotropic effects occur very quickly and are
very short lasting.
• Most of the brain’s excitatory ionotropic
synapses use glutamate or acetylcholine as a
neurotransmitter.
Chemical Events at the Synapse
• Metabotropic effects refer to when a
neurotransmitter attaches to a receptor and
initiates a sequence of metabolic reactions
that are slower and longer lasting.
• Metabotropic events include such behaviors
as hunger, fear, thirst, or anger.
Chemical Events at the Synapse
• Metabotropic effects utilize a number of
different neurotransmitters and are often
called neuromodulators because they do not
directly excite or inhibit the postsynaptic cell.
• Instead, neuromodulators:
– increase or decrease the release of other
neurotransmitters
– alter the response of postsynaptic cells to
various inputs.
Chemical Events at the Synapse
• A hormone is a chemical secreted by a gland
or other cells that is transported to other
organs by the blood where it alters activity.
• Endocrine glands are responsible for the
production of hormones.
• Hormones are important for triggering longlasting changes in multiple parts of the body.
Chemical Events at the Synapse
• Neurotransmitters released into the synapse
do not remain and are subject to either
inactivation or reuptake.
• Reuptake refers to when the presynaptic
neuron takes up most of the neurotransmitter
molecules intact and reuses it.
• Transporters are special membrane proteins
that facilitate reuptake.
– Example: Serotonin is taken back up into
the presynaptic terminal.
Chemical Events at the Synapse
• Examples of inactivation and reuptake include:
– Acetylcholine is broken down by
acetylcholinesterase into acetate and choline.
• Some serotonin and catecholamine molecules are
converted into inactive chemicals:
– COMT (catechol-o-methyltranferase)and MAO (monoamine
oxidase) are enzymes that convert catecholamine
transmitters into inactive chemicals.
Chemical Events at the Synapse
• Research has begun to investigate the role of
events at the synapse and their effects on
personality.
• Research suggests that some dopamine
receptors may be related to “pleasureseeking” and “thrill-seeking” behaviors.
Drugs and the Synapse
• The study of the influence of various kinds of drugs
has provided us with knowledge about many aspects
of neural communication at the synaptic level.
• Drugs either facilitate or inhibit activity at the
synapse.
– Antagonistic drugs block the effects of
neurotransmitters (e.g., novacaine, caffeine).
– Agonist drugs mimic or increase the effects of
neurotransmitters (e.g., receptors in the brain
respond to heroin, LSD and cocaine)
• Drugs alter various stages of synaptic processing.
Drugs and the Synapse
•
Drugs work by doing one or more of the
following to neurotransmitters:
1. Increasing the synthesis.
2. Causing vesicles to leak.
3. Increasing release.
4. Decreasing reuptake.
5. Blocking the breakdown into inactive
chemical.
6. Directly stimulating or blocking
postsynaptic receptors.
Drugs and the Synapse
• A drug has an affinity for a particular type of
receptor if it binds to that receptor.
– Can vary from strong to weak.
• The efficacy of the drug is its tendency to
activate the receptor .
• Drugs can have a high affinity but low
efficacy.
Drugs and the Synapse
• Almost all abused drugs stimulate dopamine
release in the nucleus accumbens
– small subcortical area rich in dopamine
receptors
– an area responsible for feelings of pleasure
• Sustained bursts of dopamine in the nucleus
accumbens inhibit cells that release the
inhibitory neurotransmitter GABA
Fig. 3-18, p. 72
Drugs and the Synapse
• Drugs are categorized according to their
predominant action or effect upon behavior
• Stimulant drugs increase excitement,
alertness, motor activity and elevate mood.
• Examples: amphetamines, cocaine,
methylphenidate (Ritalin), MDMA (Ecstasy),
nicotine
• Stimulant drugs directly stimulate dopamine
receptor types D2, D3, and D4.
Fig. 3-19, p. 72
Drugs and the Synapse
• Amphetamine stimulate dopamine synapses
by increasing the release of dopamine from
the presynaptic terminal.
• Cocaine blocks the reuptake of dopamine,
norepinephrine, and serotonin.
• Methylphenidate (Ritalin) also blocks the
reuptake of dopamine but in a more gradual
and more controlled rate.
Drugs and the Synapse
• Ecstasy increases the release of dopamine at
low doses that account for its stimulant
properties.
• Ecstasy increases the release of serotonin at
higher doses accounting for its hallucinogenic
properties.
• Research indicates ecstasy use may
contribute to higher incidences of anxiety and
depression as well as memory loss and other
cognitive deficits.
Fig. 3-20, p. 73
Drugs and the Synapse
• Nicotine stimulates one type of acetylcholine
receptor known as the nicotinic receptor.
• Nicotinic receptors are found in the central
nervous system, the nerve-muscle junction of
skeletal muscles and in the nucleus
accumbens (facilitate dopamine release).
Drugs and the Synapse
• Opiate drugs are those that are derived from
(or similar to those derived from) the opium
poppy.
• Opiates decrease sensitivity to pain and
increase relaxation.
• Examples: morphine, heroin, methadone.
Drugs and the Synapse
• The brain produces peptides called
endorphins.
• Endorphin synapses may contribute to certain
kinds of reinforcement by inhibiting the
release of GABA indirectly.
• Inhibiting GABA indirectly releases dopamine.
• Endorphins attach to the same receptors to
which opiates attach.
Drugs and the Synapse
• Opiates also block the locus coeruleus.
– involved in our response to arousing
stimuli by release of norepinephrine
– also involved in memory storage.
Drugs and the Synapse
• Tetrahydocannabinol (THC) is the active
ingredient in marijuana.
• THC attaches to cannabinoid receptors
throughout the brain but especially the
cerebral cortex, cerebellum, basal ganglia,
and hippocampus.
• Anandamide and 2-AG are the endogenous
chemicals that attach to these receptors.
Drugs and the Synapse
• The location of the receptors in the brain may
account for the subjective effects of loss of
time, an intensification of sensory experience,
and also memory impairment.
• The cannabinoid receptors are located on the
presynaptic neuron and inhibit the release of
glutamate and GABA.
Drugs and the Synapse
• Hallucinogenic drugs cause distorted
perception.
• Many hallucinogenic drugs resemble
serotonin in their molecular shape.
• Hallucinogenic drugs stimulate serotonin
type 2A receptors (5-HT2A) at inappropriate
times or for longer duration than usual thus
causing their subjective effect.