374
UNIT THREE
TABLE
10.6
|
Drugs That Alter Neurotransmitter Levels
Drug
Neurotransmitter Af
ected*
Mechanism oF Action
Ef
ect
Tryptophan
Serotonin
Stimulates neurotransmitter synthesis
Sleepiness
Reserpine
Norepinephrine
Decreases packaging of neurotransmitter into vesicles
Decreases blood pressure
Curare
Acetylcholine
Blocks receptor binding
Muscle paralysis
Valium
GABA
Enhances receptor binding
Decreases anxiety
Nicotine
Acetylcholine
Activates receptors
Increases alertness
Dopamine
Elevates levels
Sense of pleasure
Cocaine
Dopamine
Blocks reuptake
Euphoria
Tricyclic antidepressants
Norepinephrine
Blocks reuptake
Antidepressant
Serotonin
Blocks reuptake
Antidepressant
Monoamine
oxidase inhibitors
Norepinephrine
Blocks enzymatic degradation of neurotransmitter in presynaptic cell
Antidepressant
Selective serotonin
reuptake inhibitors
Serotonin
Blocks reuptake
Antidepressant,
Anti-anxiety agent
Dual reuptake inhibitors
Serotonin, norepinephrine
Blocks reuptake
Mood elevation
*Others may be aF
ected as well.
TABLE
10.7
|
Events Leading to
Neurotransmitter Release
1. Action potential passes along an axon and over the surface of its
synaptic knob.
2. Synaptic knob membrane becomes more permeable to calcium
ions, and they diF
use inward.
3. In the presence of calcium ions, synaptic vesicles fuse to synaptic
knob membrane.
4. Synaptic vesicles release their neurotransmitter by exocytosis into
the synaptic cleft.
5. Synaptic vesicles become part of the membrane.
6. The added membrane provides material for endocytotic
vesicles.
RECONNECT
To Chapter 3, Exocytosis, page 97.
After a vesicle releases its neurotransmitter, it becomes
part of the cell membrane. Endocytosis eventually returns
it to the cytoplasm, where it can provide material to form
new secretory vesicles.
Table 10.7
summarizes this process,
called vesicle trafF
cking.
To keep signal duration short, enzymes in synaptic
clefts and on postsynaptic membranes rapidly decompose
some neurotransmitters. The enzyme
acetylcholinesterase,
for example, decomposes acetylcholine on postsynaptic
membranes. Other neurotransmitters are transported back
into the synaptic knob of the presynaptic neuron or into
nearby neurons or neuroglia, a process called
reuptake.
The
enzyme
monoamine oxidase
inactivates the mono amine
neurotransmitters epinephrine and norepinephrine after
reuptake. This enzyme is found in mitochondria in the syn-
aptic knob. Destruction or removal of neurotransmitter pre-
vents continuous stimulation of the postsynaptic neuron.
Neuropeptides
Neurons in the brain or spinal cord synthesize
neuropep-
tides.
These peptides act as neurotransmitters or as
neuro-
modulators
—substances that alter a neuron’s response to a
neurotransmitter or block the release of a neurotransmitter.
Among the neuropeptides are the
enkephalins,
present
throughout the brain and spinal cord. Each enkephalin mol-
ecule is a chain of F ve amino acids. Synthesis of enkephalins
increases during periods of painful stress, and they bind
to the same receptors in the brain (opiate receptors) as the
narcotic morphine. Enkephalins relieve pain sensations and
probably have other functions. Another morphinelike pep-
tide,
beta endorphin,
is found in the brain and cerebrospinal
uid. It acts longer than enkephalins and is a much more
potent pain reliever (Clinical Application 10.4).
Substance P
is a neuropeptide that consists of eleven
amino acids and is widely distributed. It functions as a neu-
rotransmitter (or perhaps as a neuromodulator) in the neu-
rons that transmit pain impulses into the spinal cord and on
to the brain. Enkephalins and endorphins may relieve pain
by inhibiting the release of substance P from pain-transmit-
ting neurons.
10.8
IMPULSE PROCESSING
The way the nervous system processes and affects nerve
impulses refl ects, in part, the organization of neurons and
axons in the brain and spinal cord.
Neuronal Pools
Interneurons, the neurons completely in the CNS, are orga-
nized into
neuronal pools.
These are groups of neurons that
previous page 404 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 406 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off