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The Nervous System &
Neurons
Chapter 7
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Overview of the Nervous
System
• STRUCTURES: brain, spinal
cord, & peripheral nerves
• FUNCTION: Recognizes and
coordinates the body’s
response to changes in its
internal and external
environments
Overview of the
Nervous System
• The ultimate
control of all the
organ systems
is done by the
nervous system.
A. Neurons
• Neurons are nerve
cells that carry
electrical impulses
through the body
• Impulse = message
• Neurons are
classified according
to the direction an
impulse travels.
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• 3 Types of Neurons
– 1. Sensory: carry impulses from sense
organs to the brain
– 2. Motor: carry impulses from the brain to
muscles & glands
– 3. Interneurons: connect sensory and
motor neurons and process impulses
Interneuron
Sensory Neuron
Motor Neuron
Reflex Arc
Sensory Neuron
Interneuron
Motor Neuron
• Structure of a Neuron
Nucleus
Dendrites
Axon
terminals
Cell body
Myelin sheath
Nodes
Axon
– 1. Dendrites: receive impulses
– 2. Cell body: contains nucleus &
cytoplasm, largest part of cell
– 3. Axon: transmit impulses away
from cell body
– 4. Myelin Sheath: covering that
insulates the axon, sending the
impulse faster and gives axon a
whitish appearance
– Neurons with axons that have myelin make up
“white matter” in the brain, while neurons without
myelin are called “gray matter”
– 5. Nodes: gaps in the myelin
sheath where membrane is
exposed
– Impulses jump from one node to
the next
– 6. Axon terminals: transmits
impulse (message) to next cell
II. Transmission of an Impulse
• A nerve impulse is an electrical
message.
• At rest, no impulse is being sent.
• The neuron must be stimulated
(receive input) to send a impulse.
+
A. Resting Neuron
+ ---- +
+
• STEP 1: At rest, the neuron is not transmitting
an impulse
– inside of cell has a net negative charge –
– outside of cell has a net positive charge +
• The cell membrane has Membrane Potential,
the potential to carry an electrical current
because there is a difference in charge
– The ions sodium (Na+) and potassium (K +) cause
the potential
(K+)
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• Sodium-potassium pumps in the nerve cell
membrane pumps sodium (Na+) ions out of the
cell and potassium (K+) ions into the cell by
means of active transport.
• As a result, the inside of the cell contains
more K+ ions and fewer Na+ ions than the
outside.
B. Stimulated Neuron
• STEP 2: An impulse begins when a
neuron is stimulated by another
neuron or the environment
– The stimulated impulse must reach
threshold, the minimum impulse strength
needed to create a new impulse (all or
none)
• STEP 3: At the leading edge of the
impulse, gates in the sodium
channels open allowing positively
charged sodium (Na+) ions to flow
into the cell membane.
• This reversal of charges is called is
called the nerve impulse or an
ACTION POTENTIAL (AP)
• As the action potential passes, gates in
the potassium channels open, allowing
potassium (K+) ions to flow OUT of the
cell
– This restores the negative potential inside
the axon.
• STEP 4: The AP jumps from node to
node along the axon
STEP 5: The AP continues along the axon
in one direction to the axon terminals.
1.
At Rest
------------2.
Action Potential
++---------3.
----++----4.
---------++-
Action Potential
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C. Synapse
• Synapse: the space
where the axon
terminal of one
neuron can transfer
an impulse to another
neuron (or cell)
– The small space
between cells is the
synaptic cleft
– Axon terminals
contain vesicles filled
with neurotransmitter
• Neurotransmitters
(NTs) are chemical
signals of neurons
that transmit an
impulse across the
synapse to another
cell
– NTs fit like a lock and
key with receptors
Steps of Transmission
•
•
•
•
STEP 6: Action Potential
reaches axon terminal
STEP 7: Neurotransmitter is
released from vesicles into
synaptic cleft
STEP 8: Neurotransmitter
binds to receptors on next
neuron.
STEP 9: Channels open,
which creates a new action
potential in next neuron
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Re-uptake of NTs
• Re-uptake: After the NT has done its
job, it is taken back into the axon
terminal so it can be used again or
broken down.
Types & Effects of
Neurotransmitters
►There are dozens of different NTs,
but a few do most of the work.
►Each NT fits with its own specific
receptor.
►It won’t cause a response without the
right receptor
Acetylcholine
Serotonin
Endorphins
GABA
• NTs can be excitatory or inhibitory
– Excitatory NTs cause impulses in the
next neurons
– Inhibitory NTs stop an impulse from
being sent in the next neuron
Click here: Chemistry
of Addition Video
Major NTs
• 1) GABA (Gammaaminobutyric acid) –
Inhibitory
– Effects: Relaxation,
calming, anti-anxiety, and
sleep
– Depressant drugs like
heroin, marijuana, and
alcohol intensify effects
of GABA
• 2) Glutamate – Excitatory
– Effects: Stimulates
learning and memory
formation
– Some drugs can interfere
with glutamate, causing
short-term memories not
to be formed
• 3) Dopamine – Excitatory
– Effects: Pleasure,
Euphoria, Reward-centers,
addiction, positive
reinforcement, movement
and posture
– Most addictive drugs
(cocaine, PCP, heroin,
opiates, marijuana, and
amphetamines) work by
flooding the synapse with
dopamine
• 4) Serotonin – Excitatory
– Effects: Sleep, mood, appetite, pain,
body temperature
– Low serotonin linked to depression,
suicide, impulsive behavior, and
aggression
– Alcohol, stimulants, marijuana and
hallucinogens alter serotonin levels
• 5) Endorphins – Excitatory &
Inhibitory
– Effects: alertness, blocks pain, happy
(runner’s high), sexual arousal
– Often released after physical activity
• 6) Norepinephrine – Excitatory
– Effects: alertness, energy, stress,
sexual arousal
• 7) Acetylcholine – Excitatory
– Effects: alertness, memory, muscle
contraction, appetite, sexual arousal
Click here: Crash Course-The Chemical Mind
Effects of Drugs
• Many prescription and illegal drugs
can alter the brain’s chemistry.
– Many drugs are agonists (enhancers) of
certain NTs that cause many of the
pleasurable sensations of drugs and
lead to addiction.
– Addiction: chemical dependency on a
substance as a result of it altering the
brain’s chemistry after repeated use.
Ways Drugs interfere with
Neurotransmission
– 1) Increase number of impulses
– 2) Release NT from vesicles with or
without an impulse
– 3) Block reuptake or block receptors
– 4) Produce more or less NT
– 5) Prevent vesicles from releasing NT
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