373
CHAPTER TEN
Nervous System I
plasm of the nerve terminals and stored in vesicles. When
an action potential passes along the membrane of a synaptic
knob, it increases the membrane’s permeability to calcium
ions by opening its calcium ion channels. Calcium ions dif-
fuse inward, and in response, some of the synaptic vesicles
fuse with the presynaptic membrane and release their con-
tents by exocytosis into the synaptic cleft. The more calcium
that enters the synaptic knob, the more vesicles release neu-
rotransmitter.
Table 10.4
lists the major neurotransmitters
and their actions.
Tables 10.5
and
10.6
list disorders and
drugs that alter neurotransmitter levels, respectively.
of compounds called
monoamines
(such as epinephrine,
norepinephrine, dopamine, and serotonin), which are modi-
F cations of amino acids; a group of unmodiF ed
amino acids
(such as glycine, glutamic acid, aspartic acid, and gamma-
aminobutyric acid—GABA); and a large group of
peptides
(such as enkephalins and substance P), which are short
chains of amino acids.
The peptide neurotransmitters are synthesized in the
rough endoplasmic reticulum of the neuron cell bodies and
transported in vesicles down the axon to the nerve termi-
nal. Other neurotransmitters are synthesized in the cyto-
TABLE
10.4
|
Some Neurotransmitters and Representative Actions
Neurotransmitter
Location
Major Actions
Acetylcholine
CNS
Controls skeletal muscle actions
PNS
Stimulates skeletal muscle contraction at neuromuscular junctions. May excite or inhibit at autonomic nervous system
synapses
Biogenic amines
Norepinephrine
CNS
Creates a sense of well-being; low levels may lead to depression
PNS
May excite or inhibit autonomic nervous system actions, depending on receptors
Dopamine
CNS
Creates a sense of well-being; deF
ciency in some brain areas associated with Parkinson disease
PNS
Limited actions in autonomic nervous system; may excite or inhibit, depending on receptors
Serotonin
CNS
Primarily inhibitory; leads to sleepiness; action is blocked by LSD, enhanced by selective serotonin reuptake inhibitor
antidepressant drugs
Histamine
CNS
Release in hypothalamus promotes alertness
Amino acids
GABA
CNS
Generally inhibitory
Glutamate
CNS
Generally excitatory
Neuropeptides
Enkephalins, endorphins
CNS
Generally inhibitory; reduce pain by inhibiting substance P release
Substance P
PNS
Excitatory; pain perception
Gases
Nitric oxide
CNS
May play a role in memory
PNS
Vasodilation
TABLE
10.5
|
Disorders Associated with Neurotransmitter Imbalances
Condition
Symptoms
Imbalance of Neurotransmitter in Brain
Alzheimer disease
Memory loss, depression, disorientation, dementia, hallucinations, death
DeF
cient acetylcholine
Clinical depression
Debilitating, inexplicable sadness
DeF
cient norepinephrine and/or serotonin
Epilepsy
Seizures, loss of consciousness
Excess GABA leads to excess norepinephrine and dopamine
Huntington disease
Cognitive and behavioral changes, loss of coordination, uncontrollable
dancelike movements, death
DeF
cient GABA
Hypersomnia
Excessive sleeping
Excess serotonin
Insomnia
Inability to sleep
DeF
cient serotonin
Mania
Elation, irritability, overtalkativeness, increased movements
Excess norepinephrine
Parkinson disease
Tremors of hands, slowed movements, muscle rigidity
DeF
cient dopamine
Schizophrenia
Inappropriate emotional responses, hallucinations
DeF
cient GABA leads to excess dopamine
Tardive dyskinesia
Uncontrollable movements of facial muscles
DeF
cient dopamine
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