The Electrochemical Impulse
Section 9.2
Page 418
Background
 Messages are sent throughout the nervous system by
electrochemical impulses
 Electrical impulses transmitted by movement of ions
 Hence the “electro” and the “chemical”
Recall: a long, long time ago...
 Passive transport: Movement of a substance along its
concentration gradient
 Charged entities cannot pass freely through the cell
membrane
 Can move by passive transport through ion channels
Recall: a long, long time ago...
 Ion channels are not constantly open:
 Voltage-gated – Open when a change in voltage across
the membrane is detected
 Ligand-gated – Open when something binds to the
channel
Nerve membrane potential
 Electrical potential – A difference in charge
Resting potential
 The normal “resting” potential of a cell membrane is -70 mV
 The membrane has more
negative charges inside
than it does outside
 The charge difference
between the outer and
inner surfaces is 70 mV
Nerve membrane potential
 When a nerve is excited, the potential becomes +40 mV
 Reversal of potential (+110 mV)
 Called an action potential
Outline
1.
The resting potential
2.
The action potential




3.
Generation
The all-or-nothing response
Propagation along an axon
Synaptic transmission
Diseases and disorders (if time)
Establishing the resting potential
At rest,
 The concentration of sodium ions (Na+) outside the
membrane is higher than the concentration of
potassium ions (K+) inside.
 Uneven
concentrations
cause the inside to
be more negative
 Resting potential
is actually an
accumulation of
a lesser amount
of positive
charges
Maintaining the resting potential
 The cell membrane is “leaky”: Some ions diffuse across
 Resting potential is maintained by the sodium-
potassium pump (active transport – antiport)
 3 Na+ out, 2 K+ in
The Action Potential
Generating the action potential
 Na+ ions can also diffuse in through ion channels
 Recall that channels can be voltage-gated
 they can be stimulated to open by disrupting membrane
potential
Generating the action potential
During excitation,
 A stimulus disrupts the resting potential of the cell
 The Na + channels are opened
 Na + rushes in through voltage-gated ion channels
 The inner surface of the membrane becomes positive
relative to the outer
 The process of Na + rushing in is called depolarization.
Repolarization
 Once the membrane is depolarized, Na + gates close
 K+ channels open, allowing K+ to exit the cell
 The original polarity of the membrane is
restored to -70 mV
 The membrane is said to be repolarized
Then...
 The Na +/ K+ pump restores the balance of ions
Refractory period
 The membrane must be repolarized before it can
generate another nerve impulse
 Refractory period: The time required for repolarization
 1 to 10 ms
The all-or-nothing response
 What can stimulate initial depolarization of the
membrane?
 Pressure
 Changes in pH
 Electrical shock
 The intensity of the stimulus must reach a
threshold level in order to stimulate an action
potential.
The all-or-nothing response
What is the stimulus threshold?
What happens when the stimulus is beyond the
threshold?
The all-or-nothing response
 Any stimulus below the threshold will not elicit a
response.
 Additionally, increasing the stimulus intensity beyond
the threshold will not change the degree of response.
The all-or-nothing response:
Neurons either fire maximally, or they do not fire at all.
 All action potentials travel with the same speed and
intensity.
 Differences in stimulus intensity are detected by changes
in the number and frequency of action potentials.
More intense stimulus = More + More frequent action potentials
Propagation of the action potential
 Nerve impulses are transmitted:
a. along the length of neurons
b. from neuron to neuron (synaptic transmission)
 The action potential
travels down axons,
like a wave
Propagation along a neuron
Propagation along a neuron
Animation
http://highered.mcgrawhill.com/olc/dl/120107/bio_d.swf
Propagation along a neuron
How?
 The positive charges inside the cytoplasm are attracted to the
negatively-charged areas adjacent to them
 Movement of the positive charges depolarizes the area
"downstream"
 Upstream area is still experiencing a refractory period, so will be
unaffected
 Na + channels in this area now open, causing sodium to rush in
 The cycle repeats itself in order to propagate the action
potential
Result
 Each successive region becomes depolarized, passing the
impulse along the length of the axon
 One direction only
 The wave of depolarization is followed by a wave of
repolarization
Synaptic transmission
 Neurons are separated by a gap (the synapse)
 Nerve impulses must somehow get across the gap
from the axon of one neuron to the dendrites of
another
Synaptic transmission
 Synaptic transmission occurs due to neurotransmitters
 Chemicals contained within vesicles, at the ends of axons
 The action
potential reaches
the end of the
axon, which
stimulates the
release of
neurotransmitters
Synaptic transmission
 Neurotransmitters diffuse across the synapse and bind to
the dendrites post-synaptic neuron
 Ligand-gated Na+ channels
 Binding can either:
 Stimulate another action potential (excitatory effect); or
 Make another action potential less likely (inhibitory effect)
Animation
Featuring the excitatory effect of neurotransmitters:
http://highered.mcgraw-hill.com/olc/dl/120107/anim0015.swf
Excitatory effect
 Binding depolarizes the membrane of the dendrites
 Na+ channels open, and the action potential is reintiated
Inhibitory effect
 Opens K + channels in the membrane, so K + diffuses out
 The inner surface becomes even more negative in
relation to the outer
 the membrane is hyperpolarized
 makes it harder to depolarize the membrane in order to initiate
an action potential
Acetylcholine and cholinesterase
Two neurotransmitters:
 Acetylcholine (ACh) acts has an excitatory effect on most
neurons
 Causes depolarization
 Cholinesterase is then released by the postsynaptic
neuron
 It destroys Ach so that the sodium channels close, and the
cell can be repolarized
Summation
 Often the axons of many neurons will synapse
with the dendrites of another.
 Some of the presynaptic neurons may release excitatory
neurotransmitters, while some may release inhibitory
ones.
Summation
 The effect on the postsynaptic neuron will be determined
by the sum of all the effects of the neurotransmitters
released.
 This principle is called summation.
Diseases and disorders
Parkinson’s disease
 Linked to decreased production of the
neurotransmitter dopamine.
 Dopamine is the messenger between
parts of the brain that control smooth
muscle movement.
Diseases and disorders
Depression
 Linked to imbalances in neurotransmitters serotonin,
norepinephrine, and dopamine.
 The nature of the relationship is currently unclear.
 Anti-depressant drugs act on pre- and post-synaptic
neurons to alter rate of neurotransmitter breakdown.
Diseases and disorders
Addiction
 Dopamine is associated with
feelings of pleasure.
 Many drugs mimic, or stimulate
the release of, dopamine.
 Massive stimulation of
dopamine receptors gives
the “high”
 Overstimulation of dopamine receptors on postsynaptic
neurons causes the neuron to decrease the number of
dopamine receptors.
 This is responsible for drug tolerance.
 Progressively increasing amounts
of drug are required to achieve
the same effect.
 Body also decreases the amount
of dopamine it makes.
 This is responsible for withdrawal once
drug use is discontinued.
Homework
 Pg. 426 #3-10