Download 13.2 part 2

Electrochemical Impulse #2
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A nerve impulse is carried along the full length of
the axon by generating a series of action potentials
one after another along the axon, causing a wave
of depolarization.
The electrical charge keeps shifting because of the
movement of ions.
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When axons are myelinated, nerve impulses travel by a
process called saltatory conduction.
In myelinated axons, the gated ion channels of K+ and
Na+ are concentrated at the nodes of Ranvier between
the Schwann cells.
The flow of ions across the cell membrane can only
happen at the nodes and so therefore action potentials
can only happen at the nodes as well so the action
potentials have to “jump” from node to node.
This causes a nerve signal to be transmitted down an
axon much faster.
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In a classic experiment, a single neuron leading
to a muscle is isolated and a mild electrical
shock is applied to the neuron.
A special recorder measures the strength of the
muscle contraction. The following table shows
data from a sample experiment.
Strength
of stimuli
Force
of contraction
1 mV
no contraction
2 mV
3N
3 mV
3N
4 mV
3N
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1.
In this example, stimuli of less than 2 mV does
not produce any muscle contraction, whereas
anything 2 mv and over produces the same force
of muscle contraction.
This experiment shows us two important things:
All neurons have a threshold level or a
minimum level that must be reached in order for
an action potential to be generated.
 In our example 2 mV is the minimum voltage required to
produce a response from the muscle.
 Stimuli below threshold levels do not initiate a response.
 Threshold levels are different for each neuron.
2.
Increasing the intensity of the stimuli above the
threshold level value will not produce an
increased response. This is known as the all-or
none response
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It doesn’t matter if the stimuli is 2 mV or 10 mV the
muscle produced exactly the same response.
So just to clarify:
Threshold level – the minimum level of a
stimulus required to produce a result.
All-or-None Response – a nerve or muscle fibre
responds to a stimulus completely or not at
all.
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If the all or none response is true and our
neurons either fire action potentials or don’t,
how can we tell the difference between the
intensity of stimuli?
The result of the stimulus remains the same,
however the frequency of the nerve impulses
changes.
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The more intense a stimulus (hand on stove), the
greater the frequency of impulses. There are more
impulses travelling at a faster rate.
The less intense a stimulus (hand under warm
water) the lower the frequency of impulses. The
nerve impulses will be fewer and travelling
slower.
Our brain interprets the number of impulses and
rate at which they enter the brain in order to relay
to us how much pain or sensation to feel.
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Remember that our nervous system is made up of
billions of nerve cells (glial cells and neurons).
Action potentials are used to carry the message
along one neuron, however action potentials end
when the message reaches the end of the neuron.
The end of the neuron is made up of terminal
branches with end plates.
In order to carry the message across the gap to the
next neuron, a chemical called a neurotransmitter
is used.
Terminal
Branches
End Plates
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The gap between two neurons or a neuron and
an effector is known as a synapse.
The neuron carrying the impulse into the
synapse is called the presynaptic neuron.
The neuron leaving the synapse is called the
postsynaptic neuron.
The neurotransmitters that carry the impulse
across the synapse are contained in small
vesicles located in the end plates of axons.
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The transmitter diffuses across the synaptic gap
and binds to a receptor on the postsynaptic
neuron.
Here another action potential is produced and the
signal continues on through the next neuron.
Once the signal has been delivered the transmitter
must be removed so that new signals may be
received.
In some cases the transmitter is broken down by an
enzyme in the synapse.
In other cases the transmitter is recycled- it is
transported back into the presynaptic neuron to be
used again.
Presynaptic
Neuron
Post
Synaptic
Neuron
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2 main classes of neurotransmitters:
1. Excitatory – speed nerve transmission on post
synaptic neuron.
Ex. Acetlycholine – opens Na+ channels on
post synaptic neuron so the neuron becomes
depolarized and an action potential occurs
immediately.
Cholinesterase – enzyme that destroys
acetylcholine, so the Na+ channels close.
2. Inhibitory – slow down nerve transmission on
post synaptic neuron.
Inhibitory neurotransmitters also make post
synaptic neurons permeable to K+ ions so
resting potential is achieved and the nerve
impulse is not sent immediately.
Ex. GABA, Serotonin
Neurotransmitter
Action
Location of Action
Major Effects
Acetylcholine
Excitatory to
skeletal muscles,
excitatory or
inhibitory at other
locations.
CNS, PNS
Skeletal muscle
contraction
Norepinephrine
Excitatory in PNS.
Excitatory or
inbihitory in CNS.
CNS, PNS
Wakefulness
Dopamine
Excititory
CNS, PNS
Voluntary
movement and
emotions
Serotonin
Inhibitory
CNS
Sleep
GABA
Inhibitory
CNS
Motor behaviour
How are the lessons?
1.
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Too fast? long? slow? Boring?
2. How are you finding the pace of the course?
Difficult? Easy? Challenging?
3. What do you wish we could do more of in
class? Less of in class?
4. Did you go on the website last night?