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Neurons
(Spiral) Neurons
Cornell Notes: How do neurons work?
• Key Points
Summary:
• Notes
What Are Neurons?
• NEURONS = basic units of
the human "biocomputer."
Cells that deal with
information.
• The Nervous System is
made up of long chains of
neurons. No two neurons
are exactly alike in size or
shape. Nerves are large
bundles of neuron fibers.
Nerve cells, arranged in long
chains and dense networks.
Alone, not very smart.
Joined in vast networks,
they produce intelligence
and consciousness.
4 Basic Parts to Neuron
1. Dendrites = They look
like roots and receive
messages from other
neurons
2. Soma = Cell body, it
also accepts incoming
information, which it
collects and combines.
Sometimes messages
cause the soma to send
a nerve impulse.
4 Basic Parts to Neuron
3. Axon = long, thin fiber
down which impulses
are sent. Carries
messages from sensory
organs to the brain.
4. Axon terminals = the
branching at the end of
axons. These branches
link with dendrites and
somas (cell bodies) of
other neurons.
Body Analogy of Neurons
soma
dendrites
axons
What is the Nerve Impulse?
• THE NERVE IMPULSE
(an electrical event):
Each neuron is like a tiny
biological battery ready to be
discharged. It takes about onethousandth of a second for a
neuron to fire an impulse and
return to its resting level. A
maximum of 1,000 nerve
impulses per second is
possible. However, firing rates
of 1 per second to 300-400 per
second are more typical.
A Neuron at Rest
• Cell membrane = skin of cell.
• Resting potential = inactive
state of neurons when they are
negatively charged-- Must
receive electrical message to
activate it. When other neurons
send enough neurotransmitters
to the cell’s dendrites, it
reaches it may reach its
threshold.
• Ions = electrically charged
molecules
 inside and outside each
neuron causing a tiny
difference in electrical NA+ and
charge across the cell K+ ARE
membrane.
MAJOR
PLAYERS!
Lots of NA +
outside the cell
at rest
An Active Neuron: from
Chemical to Electrical
• Threshold = trigger point, for firing.
The threshold for human neurons
averages ca. -50 millivolts (a millivolt
is one-thousandth of a volt.). At this
point a nerve impulse or action
potential sweeps down the axon.
• Action potential = nerve impulse
caused by an exchange of ions (Na+
and K+) across the neuron
membrane. A stimulus first causes
sodium channels to open. The
resulting current sweeps down the
axon after being triggered by
positive sodium ions opening gates
in succession all the way down the
axon.
Parts of Nerve Impulse:
Electricity!
• Ion channels = tiny tunnels
in the axon membrane that
cause action potential.
Normally “closed gates."
The gates pop open during
an action potential, allowing
sodium ions to rush into the
axon--happening near the
soma, first, and then as
action potential moves
along, the gates open in
sequence down the length
of the axon.
• *** NOTE: An impulse
occurs completely or not
at all
NA+ flows into
the axon during
an impulse,
raising its
positive charge
After the Impulse:
Negative After Potential
Negative after-potential = the cell
briefly drops below its resting level
after each nerve impulse--caused by
outward flow of positive potassium
ions that occurs while the membrane
gates are open. The neuron must
recharge after each nerve impulse. It
does this by shifting ions back across
the cell membrane until the resting
potential is restored.
What will happen if he can’t rest
before his next race?
To balance and
be able to rest
again, the cell
moves 3 NA+
ions out for
every 2 K+
ions it brings in
After “After Potential”
The Great Balancing Act
• Positive sodium particles enter to cause
action potential
• Positive potassium particles leave to
rebalance the neuron to -70 millivolts
• The neuron returns to resting potential
+
Leaving the Axon: Neurotransmission between
the Sending and Receiving Neurons
• NEUROTRANSMISSION: When the
action potential impulse reaches the
end of the axon, it stimulates tiny
pouches of chemicals in the axon
terminals. The pouches then release
neurotransmitters such as
dopamine, serotonin, and
epinephrine.
• Communication between neurons,
across the synapses, is
molecular/chemical--in contrast to
electrical event of nerve impulse.
Neurotransmitters
NOTE:
• NA + (sodium) and K+ (potassium)
ARE NOT NEUROTRANSMITTERS!
They are ions that dance in and out
of the axon to stimulate the release
of neurotransmitters at the ends of
the axons.
• Neurotransmitters are chemicals
held in the ends of the axons only
that serve as the “chemical
messengers” which communicate
through the dendrites of other
neurons.
NA+
and
K+
zone
Neurotransmitter
zone
Will the Neurotransmitters Fit the
Receptor Site?
• If yes, and there are no inhibitory
neurotransmitters in the same synapse,
the receiving neuron will fire!
• If no, the sending neuron will reuptake
(vacuum up) the neurotransmitters that it
sent into the synapse!
Terms Regarding
Neurotransmitters
• Neurotransmitters = Potent
chemicals which are released
when a nerve impulse reaches
the tips of the axon terminals.
 At any instant, a neuron
receives messages from
hundreds of thousands of other
neurons.
 If several "exciting" (+)
messages arrive close in time,
and they are not cancelled by
"inhibiting" (-)messages, the
neuron reaches its trigger
point.
 This means that chemical
messages are combined
before a neuron "decides" to
fire its all-or- nothing action
potential.
Terms Regarding
Neurotransmitters
• Synapse = tiny gap
between neurons.
Neurotransmitters cross
these gaps, attach to
sites:
• Receptor sites = on the
soma and dendrites of
the next neuron.
Transmitter molecules
attach to these special
sites. They also activate
receptor sites on muscles
and glands.
FYI: Neuro Regulators
• NEURO REGULATORS:
A new class of brain
transmitters.
Neuropeptides or brain
peptides. They do not
carry messages directly.
Instead, these Chemicals
regulate the activity of
other neurons. Doing so,
they affect memory, pain,
emotion, pleasure, mood,
hunger, sexual behavior,
and other basic
processes