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Bio 100 – Guide 25
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Squid Rig
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Neurons are the functional units of nervous
systems
– Neurons are cells specialized for carrying
signals
• Cell body: contains most organelles
• Dendrites: highly branched extensions that
carry signals from other neurons toward the
cell body
• Axon: long extension that transmits signals
to other cells
– Many axons are enclosed by an
insulating myelin sheath
• Chain of Schwann cells
• Nodes of Ranvier: points where signals
can be transmitted
• Speeds up signal transmission
– Supporting cells (glia) are essential for
structural integrity and normal
functioning of the nervous system
– The axon ends in a cluster of branches
• Each branch ends in a synaptic
terminal
• A synapse is a site of communication
between a synaptic terminal and
another cell
LE 28-2
Signal direction
Dendrites
Cell Body
Cell body
Node of Ranvier
Layers of myelin
in sheath
Axon
Schwann cell
Nucleus
Signal
pathway
Nucleus
Nodes of
Ranvier
Myelin sheath
Synaptic terminals
Schwann cell
NERVE SIGNALS AND THEIR
TRANSMISSION
• A neuron maintains a membrane potential
across its membrane
– A resting neuron has potential energy
• Membrane potential: electrical charge
difference across the neuron's plasma
membrane
• Resting potential: voltage across the
plasma membrane of a resting neuron
– The resting potential depends on differences
in ionic composition inside and outside the
cell
• More K+ than Na+ diffuses inward through
membrane channels
• Sodium-potassium pumps actively transport
Na+ out of cell and K+ in
• The ionic gradient produces a voltage
across the membrane.
http:\\www.geneseo.edu\~simon\bio100\media\Resting_Potential.html
LE 28-3a
Voltmeter
Plasma
membrane
–70 mV
Microelectrode
outside cell
Microelectrode
inside cell
Axon
Neuron
• A nerve signal begins as a change in the
membrane potential
– Electrical changes make up an action
potential, a nerve signal that carries
information along an axon
• Stimulus raises voltage from resting
potential to threshold
• Action potential is triggered; membrane
polarity reverses abruptly
• Membrane repolarizes; voltage drops
• Voltage undershoots and then returns to
resting potential
–Cause of electrical changes of an
action potential
•Movement of K+ and Na+
across the membrane
•Controlled by the opening and
closing of voltage-gated
channels
LE 28-3b
Na+
Outside of cell
Na+
K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
channel
K+
Plasma
membrane
Na+
Protein
K+
Na+
Na+
Na+
Na+ -K+
pump
K+ channel
K+
K+
K+
K+
K+
K+
Inside of cell
Na+
Na+
K+
K+
The Action Potential
http://www.geneseo.edu/~simon/bio100/media/Action_Potential.html
The action potential (Step 5)
Na +
K+
Na +
K+
Additional Na + channels open, K+
channels are closed; interior of cell
becomes more positive.
Na +
Na + channels close and
inactivate. K+ channels open,
and K+ rushes out; interior of
cell more negative than
outside.
Na +
A stimulus opens some Na+ channels; if
threshold is reached, action potential is
triggered.
Membrane potential
(mV)
+ 50
Action
potential
0
– 50
The K+ channels close
relatively slowly, causing a
brief undershoot.
Threshold
Resting potential
– 100
Time (msec)
Neuron
interior
Resting state: voltage-gated
Na + and K+ channels closed; resting
potential is maintained.
Neuron
interior
Return to resting state.
• The action potential propagates itself along
the neuron
– An action potential transmits a signal in a
domino effect
1. Na+ channels open, Na+ rushes inward
2. K+ channels open, K+ diffuses
outward; Na+ channels are closed and
inactivated
3. Membrane returns to resting potential
LE 28-5
Axon
Action potential
Axon
segment
Na+
K+
Action potential
Na+
K+
K+
Action potential
Na+
K+
–Action potentials are propagated
only from cell body to synaptic
cleft
•Cannot be generated where K+
is leaving axon and Na+
channels are inactivated
–Action potentials are all-or-none
events
•Same events occur no matter
how strong or weak the
stimulus
•Intensity of stimulus determines
frequency of action potentials
• Neurons communicate at synapses
– The transmission of signals occurs at synapses
• Junction between synaptic terminal and
another cell
– Electrical synapse
• Electrical current passes directly from one
neuron to the next
• Receiving neuron stimulated quickly and at
same frequency as sending neuron
• Found in human heart and digestive tract
– Chemical synapse
1. Action potential arrives in sending
neuron
2. Vesicle containing neurotransmitter fuses
with plasma membrane
3. Neurotransmitter is released into synaptic
cleft
4. Neurotransmitter binds to receptor on
receiving neuron
– Following events vary with different
types of chemical synapses
The Synapse
http://www.geneseo.edu/~simon/bio100/
media/Synapse.html
Neuron communication (Step 1)
Sending neuron
Action
potential
arrives
Vesicles
Axon of
sending
neuron
Synaptic
terminal
Synapse
Vesicle fuses
with plasma
membrane
Neurotransmitter
is released into
synaptic cleft
Synaptic
cleft
Receiving
neuron
Receiving
neuron
Ion channels
Neurotransmitter
molecules
Neurotransmitter binds
to receptor
Neuron communication (Step 2)
Sending neuron
Action
potential
arrives
Vesicles
Axon of
sending
neuron
Synaptic
terminal
Synapse
Vesicle fuses
with plasma
membrane
Neurotransmitter
is released into
synaptic cleft
Synaptic
cleft
Receiving
neuron
Receiving
neuron
Ion channels
Neurotransmitter
molecules
Neurotransmitter
Receptor
Ions
Ion channel opens
Neurotransmitter binds
to receptor
Neuron communication (Step 3)
Sending neuron
Action
potential
arrives
Vesicles
Axon of
sending
neuron
Synaptic
terminal
Synapse
Vesicle fuses
with plasma
membrane
Neurotransmitter
is released into
synaptic cleft
Synaptic
cleft
Receiving
neuron
Receiving
neuron
Ion channels
Neurotransmitter
molecules
Neurotransmitter
Receptor
Neurotransmitter binds
to receptor
Neurotransmitter broken
down and released
Ions
Ion channel opens
Ion channel closes
• Chemical synapses make complex information
processing possible
– A neuron may receive information from
hundreds of other neurons via thousands of
synaptic terminals
– Some neurotransmitters excite the receiving cell
– Other neurotransmitters inhibit the receiving
cell's activity by decreasing its ability to
develop action potentials
– If excitatory signals are strong enough to
initiate an action potential, a neuron will
transmit a signal
Neuron communication (Step 1)
Sending neuron
Action
potential
arrives
Vesicles
Axon of
sending
neuron
Synaptic
terminal
Synapse
Vesicle fuses
with plasma
membrane
Neurotransmitter
is released into
synaptic cleft
Synaptic
cleft
Receiving
neuron
Receiving
neuron
Ion channels
Neurotransmitter
molecules
Neurotransmitter binds
to receptor
LE 28-7
Synaptic terminals
Dendrites
Inhibitory Excitatory
Myelin
sheath
Receiving
cell body
Axon
SEM 5,500×
Synaptic
terminals
LE 28-6
Sending neuron
Action
potential
arrives
Vesicles
Axon of
sending
neuron
Synaptic
terminal
Synapse
Vesicle fuses
with plasma
membrane
Neurotransmitter
is released into
synaptic cleft
Synaptic
cleft
Receiving
neuron
Receiving
neuron
Ion channels
Neurotransmitter
molecules
Neurotransmitter
Receptor
Neurotrans mitter binds
to receptor
Neurotransmitter broken
down and releases
Ions
Ion channel opens
Ion channel closes
• A variety of small molecules function as
neurotransmitters
– Many small, nitrogen-containing
molecules serve as neurotransmitters
• Acetylcholine
– Important in brain and at synapses
between motor neurons and muscles
• Biogenic amines
–Important in central nervous system
–Seratonin, dopamine
– Amino acids
• Important in central nervous system
– Peptides
• Substance P, endorphins influence
perception of pain
– Dissolved gases
• NO functions during sexual arousal
• Many drugs act at chemical synapses
– Many psychoactive drugs act at synapses and
affect neurotransmitter action
• Caffeine
• Nicotine
• Alcohol
• Psychoactive prescription drugs
• Stimulants
• THC (marijuana)
• Opiates
The End