172
Chapter 6
SECTION C SUMMARY
I. An excitatory synapse brings the membrane of the postsynaptic
cell closer to threshold. An inhibitory synapse hyperpolarizes
the postsynaptic cell or stabilizes it at its resting level.
II. Whether a postsynaptic cell fi res action potentials depends on
the number of synapses that are active and whether they are
excitatory or inhibitory.
III. Neurotransmitters are chemical messengers that pass from
one neuron to another and modify the electrical or metabolic
function of the recipient cell.
Functional Anatomy of Synapses
I. A neurotransmitter, which is stored in synaptic vesicles in the
presynaptic axon terminal, carries the signal from a pre- to a
postsynaptic neuron.
Mechanisms of Neurotransmitter Release
I. Depolarization of the axon terminal raises the calcium
concentration within the terminal, which causes the release of
neurotransmitter into the synaptic cleft.
II. The neurotransmitter diffuses across the synaptic cleft and
binds to receptors on the postsynaptic cell; the activated
receptors usually open ion channels.
Activation of the Postsynaptic Cell
I. At an excitatory synapse, the electrical response in the
postsynaptic cell is called an excitatory postsynaptic potential
(EPSP). At an inhibitory synapse, it is an inhibitory
postsynaptic potential (IPSP).
II. Usually at an excitatory synapse, channels in the postsynaptic
cell that are permeable to sodium, potassium, and other small
positive ions open; at inhibitory synapses, channels to chloride
and/or potassium open.
Synaptic Integration
I. The postsynaptic cell’s membrane potential is the result of
temporal and spatial summation of the EPSPs and IPSPs at the
many active excitatory and inhibitory synapses on the cell.
II. Action potentials are generally initiated by the temporal and
spatial summation of many EPSPs.
Synaptic Strength
I. Synaptic effects are infl uenced by pre- and postsynaptic events,
drugs, and diseases (Table 6–5).
Neurotransmitters and Neuromodulators
I. In general, neurotransmitters cause EPSPs and IPSPs, and
neuromodulators cause, via second messengers, more complex
metabolic effects in the postsynaptic cell.
II. The actions of neurotransmitters are usually faster than those
of neuromodulators.
III. A substance can act as a neurotransmitter at one type of
receptor and as a neuromodulator at another.
IV. The major classes of known or suspected neurotransmitters
and neuromodulators are listed in Table 6–6.
Neuroeffector Communication
I. The junction between a neuron and an effector cell is called a
neuroeffector junction.
II. The events at a neuroeffector junction (release of
neurotransmitter into an extracellular space, diffusion of
neurotransmitter to the effector cell, and binding with a
receptor on the effector cell) are similar to those at a synapse.
Additional Clinical Examples
I. Ethanol alters brain function by targeting proteins involved
in synaptic transmission throughout the brain. By inhibiting
glutamate and enhancing GABA signaling, it has a global
depressant effect on the nervous system. Its effects on
dopaminergic and endogenous opioid signaling result in
euphoria, mood elevation, and occasionally addiction. High
doses are fatal due to suppression of cardiovascular and
respiratory centers in the brainstem.
SECTION C KEY TERMS
SECTION C CLINICAL TERMS
acetylcholine (ACh)
166
acetylcholinesterase
167
active zones
161
adenosine
171
adrenergic
168
agonist
166
alpha-adrenergic receptor
168
AMPA receptor
169
antagonist
166
aspartate
169
ATP
171
autoreceptor
165
axo-axonic synapse
164
beta-adrenergic receptor
168
beta-endorphin
170
biogenic amine
167
carbon monoxide
171
catecholamine
167
chemical synapse
159
cholinergic
166
convergence
159
cotransmitter
160
divergence
159
dopamine
167
dynorphin
170
electrical synapse
159
endogenous opioid
170
enkephalin
170
epinephrine
167
excitatory amino acid
169
excitatory postsynaptic potential
(EPSP)
162
excitatory synapse
159
excitotoxicity
170
GABA (gamma-aminobutyric
acid)
170
glutamate
169
glycine
170
inhibitory postsynaptic potential
(IPSP)
162
inhibitory synapse
159
ionotropic receptor
161
L-dopa
167
long-term potentiation
(LTP)
169
metabotropic receptor
161
monoamine oxidase
(MAO)
168
muscarinic receptor
167
neuromodulator
166
neuropeptide
170
nicotinic receptor
167
nitric oxide
171
NMDA receptor
169
noradrenergic
168
norepinephrine (NE)
167
peptidergic
170
postsynaptic density
160
presynaptic facilitation
164
presynaptic inhibition
164
receptor desensitization
165
reuptake
161
serotonin
168
SNARE proteins
161
spatial summation
163
strychnine
170
substance P
171
synaptic cleft
160
synaptic delay
161
synaptic vesicle
160
synaptotagmin
161
temporal summation
163
Alzheimer’s disease
167
analgesics
170
atropine
167
botulism
166
codeine
170
LSD
169
morphine
170
paroxetine (Paxil®)
168
Sarin
167
tetanus toxin
166
Valium
®
170
Xanax
®
170
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