Download 9.2 Electrochemical Impulses

6.5 Nerves, Hormones, and Homeostasis
How do neurons
communicate and relay
information?
http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000089&ptid=17
Action Potential
 ~1900 Julius Berstein suggested that nerve impulses
were an electrochemical message created by the
movement of ions through the nerve cell membrane.
 Through experimentation it was found that every time
a nerve is excited, there was a rapid change in
electrical potential difference (electric potential)
Some important terms….
 Resting potential: the voltage difference (mV) across a
nerve cell membrane during the resting stage (when it
is not relaying information)
 -70 mV (average)
 (The area outside the neuron is more positive than the
inside.)
 Action potential: the voltage difference (mV) across a
nerve cell membrane when the nerve is excited.
How do nerve cells become
charged?
 Nerve cells have a rich supply of positive and negative
ions inside and outside the cell.
 However, the negative ions are not really involved in
creating the charged membrane – they are large and
cannot easily cross the membrane.
 The membrane becomes charged because of an
unequal concentration of positive ions across the
membrane (possibly through protein channel/ion
gates)
Pre-Excitation
 Before an action potential is initiated, there is a high
concentration of Na+ outside the neuron and a high
concentration of K+ inside the neuron
 Negative ions (Cl-) will want to move out of the axon
(where it is more positive) but they cannot get through
the membrane, so they will accumulate along the
inside the membrane.
At Resting Potential
Pre-Excitation / Resting Potential
 The membrane is now a polarized membrane -
charged by the unequal distribution of ions.
 the membrane has the potential to do work –
expressed in mV.
Excitation
1. When the nerve cell becomes excited (due to a
stimulus), the membrane becomes more permeable to
sodium than potassium.
i.e. Na+ gates open and K + gates are closed
2. Na+ moves into cell following a concentration gradient
(diffusion) and also an electrical potential gradient.
The positive charge moving into the neuron reduces the
potential difference of the membrane . This is
depolarization.
 When the potential difference is above zero, Na+
movement will now only be driven by diffusion.
3. When the membrane potential is +40mV, the sodium
gates will close. And the K+ gates will open.
4. K+ will now exit the neuron following the rules of
diffusion and following the electric potential gradient.
 As the positive K+ leave the neuron, the potential
difference across the membrane will start to decrease:
Repolarization
 When the potential difference falls below zero again,
the movement of K+ is due to diffusion alone.
 5. The K+ pores will close when the resting potential
(of -70mV) is reached again.
 However, the Na+ and K+ ions are on the “wrong” side
of the cell membrane.
6. Refractory Period - a resting period; during this time,
the neuron cannot start another action potential.
-The ions will move back to their original positions
through sodium/potassium pumps (active transport
requiring ATP)
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__voltage-gated_channels_and_the_action_potential__quiz_2_.html
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__sodium-potassium_exchange_pump__quiz_1_.html
“Undershoot” =
Hyperpolarization:
excess K+ diffuse out of
cell (more K+ leave
than Na+ enters).
The excess K will
diffuse away.
Movement of Action Potential
 Movement of Na+ ions into the neuron causes
depolarization and action potential.
 For the impulse to be conducted along the axon, the
impulse must move from the zone of depolarization to
an adjacent region.
 When the Na+ ions move in, they will be attracted to
the adjacent negative charges and cause depolarization
in the adjacent area.
 This previously resting membrane would have had
negative charges lining the inside membrane.
 With depolarization, it becomes positive.
 This electrical disturbance causes the Na+ channels in
the adjoining area to open resulting in another action
potential.
Unmylenated vs Mylenated?
 http://highered.mcgraw-
hill.com/sites/0072507470/student_view0/chapter11/a
nimation__action_potential_propagation_in_an_unm
yelinated_axon__quiz_1_.html
Synaptic Transmission
 A single neuron many branch many times and at its
end join with many different neurons.
 (The end of a neuron may be referred to as an “axon
end plate” or a synaptic knob” or a terminal button”)
 Small spaces between neurons or between neurons
and effectors are known as synapses.
 Small vesicles containing neurotransmitters are located in
the end plates of axons.
 Ex of a neurotransmitter: acetylcholine
 As the impulses moves along the axon, the axon releases
neurotransmitters from the end plate which will diffuse
into the dendrites of an adjacent neuron and create a
depolarization of the dendrites.
 The neuron that releases the neurotransmitters is the
presynaptic neuron
 The neuron that receives them is the postsynaptic neuron.
Synaptic Transmission
1.
When an action potential reaches a synaptic knob, it
causes the membrane to be more permeable to
calcium ions Ca2+
2. As Ca2+ move into the neuron, this causes the axon to
release neurotransmitters into synaptic cleft.
 The synapses are very small (~20nm), however nerve
transmission slows across a synapse.
 The greater the number of synapses, the slower the
transmission.
3.
The neurotransmitters will bind to receptors on the cell
membranes of adjacent neuron (post-synaptic neuron)
4. This will cause Na+ channels to open up and cause an
action potential to start in that neuron.

But with the Na+ channels open, the neuron would
remain in a constant state of depolarization. The
neurotransmitter needs to be released from the receptor
sites.

Enzyme s such as cholinesterase are released by the
postsynaptic neuron to destroy acetylcholine , thus
closing the Na+ channels.
 (Acetylcholine is changed into choline and ethanoic acid,
and will diffuse back to the presynaptic neuron to be
reused).
 Inhibitory neurotransmitters make postsynaptic
membranes more permeable to K+ and Cl K+ will diffuse out the neuron (and Cl- in) creating an even
more negative resting membrane which is said to be
hyperpolarized.
 This increases the “distance” to the threshold value
 To achieve depolarization, even more Na+ channels must
be opened.

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__transmission_across_a_synapse.html
Threshold Levels and the All or
None Response
 Experiments with neurons have shows that there is a
threshold level
 Threshold level – is the minimum level of stimulus
required to produce a response.
 Experiments also show that increasing the intensity of
the stimuli above the threshold value does not change
the intensity of speed of transmission – this is known
as the all-or-non-response.
So, how to we detect intensity of
stimuli if nerve fibres fire
completely or not at all?
 Frequency!
 The more intense the stimulus, the greater the
frequency of the impulse.
 When you put your hand on a warm surface – a fewer
impulses are sent to your brain than if you were to put
your hand on a hot surface.
 Also, different threshold levels of neurons provide a
way for intensity to be detected.
 Each nerve is compose of many individual neurons
each with different threshold levels.
 Ex: touching a surface at a temp of 40°C may cause 1
neuron to reach threshold and fire a signal, but
touching a surface at a temp of 50 °C may cause 2
neurons to reach threshold temps and fire a signal.
Side note…
 The interaction of excitatory and inhibitory
neurotransmitters is what allows you to do everything
you do: throwing a ball, talking….
 Inhibitory impulses in your CNS allows for
prioritization of information received by your brain.
 Ie: when you are listening to a biology lecture, your
sensory info should be directed to Ms. De Souza.
 Information from other sensory nerves (ie, temp in the
room, the pressure receptors confirming you are
wearing clothes) should be suppressed.