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Biology 12
Name: ____________________
12.2 Transmission of Nerve Impulses
The nervous system uses the nerve impulse to convey information.
The nerve impulse can be studied using excised axons and a voltmeter called an oscilloscope.
• Voltage: measured in millivolts (mV); a measure of the electrical potential difference
between two points
• In a neuron, the two points are in the inside (axoplasm) and the outside of the axons
• On a voltmeter, voltage is displayed as a trace (pattern) over time
Resting Potential
In an axon that is not conducting an impulse, the voltmeter records a potential difference
across an axon membrane equal to -70mV.
• This reading, known as the resting potential, shows that the inside of the axon is
negative compared to the outside (there is polarity across the axonal membrane)
• The resting potential is the potential difference across the membrane in a resting
neuron
The polarity of the resting axonal membrane is due to a difference in ion distribution on each
side.
• The concentration of Na+ is
greater outside the axon than
inside
• The concentration of K+ is
greater inside the axon than
outside
• This unequal distribution is
maintained by carrier proteins
called sodium-potassium
pumps, which actively
transport Na+ out of the axon
and K+ into the axon
o The pumps are always
working because the membrane is permeable to Na+ and K+
o The membrane is more permeable to K+, therefore there are always more
positive ions outside the membrane than inside
o Negatively charged organic ions on the inside of the axon also contribute to the
polarity across a resting axonal membrane
Biology 12
Name: ____________________
Action Potential
An action potential is a rapid change in polarity across an axonal membrane as the nerve
impulse occurs.
• An all-or-none phenomenon: if a stimulus causes the
membrane to depolarize to a certain level (threshold),
an action potential occurs
• The strength of an action potential does not change,
but an intense stimulus can cause an axon to fire
(start an action potential) more often
• Requires two gated channel proteins in the
membrane:
o One channel protein allows Na+ to pass into
the axon
o One channel protein allows K+ to pass out of
the axon
Action Potential: Sequence of Events
Sodium Gates Open (Depolarization)
• When an action potential begins, sodium channel gates
open, and Na+ flows down its concentration gradient
into the axon
• As Na+ moves inside the axon, the membrane potential
changes from -70 mV to +35 mV
• This is called depolarization because the charge inside
the axon changes from negative to positive
Potassium Gates Open (Repolarization)
• The potassium channel gates open, and K+ flows down
its concentration gradient out of the axon
• As K+ flows out of the axon, the action potential
becomes more negative again (repolarization)
o During this time, it briefly becomes slightly
more negative that its original resting potential
(hyperpolarization)
Conduction of an Action Potential
Action potentials in nonmyelinated axons
• The action potential travels down an axon one small section at a time
• When an action potential has moved on, the previous section undergoes a refractory
period, during which the sodium gates are unable to open
• The action potential cannot move backward; it always moves down an axon
• When the refractory period is over, the sodium-potassium pump has restored the ion
distribution by pumping Na+ out of the axon and K+ into the axon
Biology 12
Name: ____________________
Action potentials in myelinated axons
• The gated ion channels that produce an action potential are concentrated at the nodes
of Ranvier
• Ion exchange only occurs at these nodes, therefore the action potential travels faster
than in nonmyelinated axons
• The action potential “jumps” from node to node (saltatory conduction)
Transmission Across a Synapse
Every axon branches into endings that have a small swelling called an axon terminal
• Each terminal lies close to the dendrite or cell body of another neuron or a muscle cell
• This region of close proximity is called a synapse or chemical synapse
o Membrane of the first neuron: presynaptic membrane
o Membrane of the second neuron: postsynaptic membrane
• Two neurons at a synapse do not physically touch each other; they are separated by a
tiny gap called the synaptic cleft
An action potential cannot cross a synapse.
• Communication between two neurons at a chemical synapse is carried out by
neurotransmitters (chemicals stored in the synaptic vesicles in axon terminals)
• When an action potential arrives at an axon terminal:
• Gated channels for Ca2+ open, and Ca2+ enters the terminal
• Ca2+ interacts with contractile proteins, which contract and pull the synaptic vesicles to
the presynaptic membrane
• Rise in Ca2+ stimulates
synaptic vesicles to merge
with the presynaptic
membrane, resulting in
exocytosis
• Neurotransmitter molecules
are released into the
synaptic cleft and diffuse
across the synapse to the
postsynaptic membrane,
where they bind to specific
receptors
Biology 12
•
Name: ____________________
Depending on the neurotransmitter, the postsynaptic
neuron can either be excited (causing an action
potential) or inhibited (stopping an action potential)
Synaptic Integration
A neuron may receive many excitatory and inhibitory signals
since its dendrites and cell body can have synapses with many
other neurons.
• Excitatory signals: cause a depolarizing effect
• Inhibitory signals: cause a hyperpolarizing effect
Synaptic integration is the summing up of the excitatory and
inhibitory signals in a postsynaptic neuron.
• If the combined signals cause the membrane potential
to rise above threshold, an action potential will occur
Neurotransmitters
Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it
is removed from the cleft.
• This prevents continuous stimulation (or inhibition) of postsynaptic membranes
o In some synapses, the postsynaptic membrane contains enzymes that break
down the neurotransmitter
o In other synapses, the presynaptic membrane reabsorbs the neurotransmitter
for repackaging in synaptic vesicles
Many drugs that affect the nervous system act by interfering or enhancing the action of
neurotransmitters. Drugs can:
• Enhance or block the release of neurotransmitter
• Mimic the neurotransmitter
• Block the receptor for the neurotransmitter
• Interfere with the removal of the neurotransmitter
Example:
• Sarin gas is a chemical weapon that inhibits acetylcholinesterase (AChE), an enzyme that
is responsible for the breakdown of acetylcholine (ACh)
o Leads to prolonged ACh activity (convulsive spasms)