Download SBI4U - 9.2

Chapter 9.2: Electrochemical
Impulse
Pages 418-426
Recap
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The Importance of the Nervous System
CNS (Brain and Spinal Cord)
PNS (Somatic and Autonomic Nerves)
Anatomy of a Nerve Cells (Dendrites, Axon, Cell
Body, Nodes of Ranvier etc)
• Types of Neurons (SIM)
• Reflex Arc (5 steps)
The Impulse
18th century:
• Galvani realized that the
leg muscle of a dead frog
could be made to twitch
 electrical stimulation
• Lead to scientists
questioning how electric
current is generated in the
body
• Electrical vs. Neural
transmission
Current in Wire, Current in Nerves
There are differences between current in wire and
those that travel through nerves:
1.Current in a wire travels much faster
1.The cytoplasmic core of a nerve cell offers great
resistance to the movement of electric current
1.Electric currents diminish as they move through
a wire
Current in Wire, Current in Nerves
1. Nerve impulses remain as strong at the end as
they were at the beginning of the impulse
1. Nerves  cellular energy to generate current
Electric conductor  An external force to push
the current through the conductor
Figure 2. page 418
Placed an electrode inside a large nerve cell
- 70 mV
+ 40 mV
• The resting membrane normally had a
potential near – 70 mV and registered + 40
mV when the nerve became excited
• The voltage difference across a nerve cell
membrane during the resting stage is called
the resting potential
• The reversal of potential is described as an
action potential – the voltage difference
across a nerve cell membrane when the nerve
is excited
How do nerve cells become charged?
• Molecular level  nerve cells have a rich supple of + and
– ions both inside and outside of the cell
• negative ions stay inside the cell
• positive ions move and cause an unequal concentration
50x more
permeable to
potassium
• The membranes become charged due
to the movement of positive ions
• Potassium is high inside, and sodium
is high outside
• As potassium diffuses out, sodium
diffuses in
The Resting Membrane
Rapid diffusion of K+ ions  nerve
cells loses a greater number of
positive ions than it gains
Exterior membrane is + compared
to interior
The resting membrane:
• Charged
• Called a polarized membrane 
unequal distribution of + ions
• Difference between + ions inside
nerve membrane relative to
outside = -70 mV
When the nerve gets excited…
• Nerve cell membrane  more permeable to Na+
than K+
• Na+ ions rush into nerve cell through diffusion
and charge attraction
• Sodium inflow causes charge reversal 
depolarization
• A sodium-potassium pump restores the original
polarity of the nerve membrane  repolarization
When the nerve gets excited…
Resting membrane
Depolarization
Repolarization
Sodium-Potassium Pump
(Repolarization)
Active Transport
• Nerve cells conducting an impulse cannot be
activated until resting membrane condition is
restored
• Once depolarization is complete, nerves must
repolarize before next action potential takes
place
• Time required to become repolarized 
refractory period (1-10 ms)
Movement of the Action Potential
• For the impulse to be
conducted on the
axon, the impulse
moves from zone of
depolarization to
adjacent areas
Action Potential
• The flow of positively charged ions from the area of
the action potential toward the adjacent regions of
the resting membrane causes a depolarization in the
adjoining area
• This creates an electric disturbance, which causes
adjacent sodium channels to open
• The result is a wave of action potential that moves
along the cell membrane
• The wave of action potential and depolarization is
followed by a wave of repolarization
Action Potential
Threshold Levels: All or None
• Minimum level of a stimulus required to produce a response
Figure 8. page 422
Threshold Levels: All or None
• Increasing the intensity of the stimulus above
critical threshold  does NOT produce an
increased response
• Neurons fire maximally or none at all  all-ornone response
Detecting Intensity
1. The more intense the
stimulus  the greater
the frequency of
impulses
Warm rod: lower
frequency of impulse
sent to brain
Hot rod: higher
frequency
2. Different threshold
levels of neurons
Synaptic Transmisson
• Synapse: regions between neurons, or between
neurons and effectors
• A single neuron may branch off and join with
many different neurons
• Involves neurotransmitters: chemicals release
from vesicles to synapses
• Presynaptic neuron: neuron that carries
impulses to the synapse
• Postsynaptic neuron: neuron that carries
impulses away from the synapse
Synaptic Transmisson
• Neurotransmitters released from presynaptic neuron,
diffuse across the synaptic cleft, create a depolarization of
dendrites of the postsynaptic neuron
Acetylcholine
• An example of an excitatory neurotransmitter
• Acts on many postsynaptic neurons by opening the sodium
ion channels
Causes depolarization
Cholinesterase
• An enzyme that breaks down acetylcholine to
reverse depolarization
• Once acetylcholine is destroyed, Na+ close 
neuron begins recovery phase
Any Questions?