Nov 30 – Dec 1, 2015
What are the functions of the nervous system?
Receive sensory input, interpret & make a decision, and
effect a response
Individual neurons have two major functional
properties: irritability and conductivity.
 Irritability = ability to respond to a stimulus and
convert it to a nerve impulse
 Conductivity = ability to transmit the impulse to other
neurons, muscles, or glands.
How do irritability and conductivity relate
to the functions of the nervous system?
Irritability = ability to respond to a stimulus and convert
it to a nerve impulse

What type of stimulus?
◦ Receptor neurons respond to specific stimuli (e.g. touch,
temperature, pressure, tension, sight, hearing, chemical)
◦ All other neurons (interneurons, motor neurons) respond to
neurotransmitters released by other neurons

What is a nerve impulse?
◦ A quick switch in voltage potential (charge difference) across
the membrane that travels all the way along the axon of the
neuron
◦ Occurs due to flow of ions across the membrane
◦ All – or - nothing
Now to understand the process of
a nerve impulse / action potential
in much more detail …
Neurons at rest have an electrochemical
gradient across the cell membrane, known as
the resting potential.
The resting potential is approximately
-70mV.
Extracellular fluid
Cell
membrane
cytoplasm
The resting potential is maintained by:

Sodium-potassium pump

Greater membrane permeability of K+

Organic anions (- ions) within cell
(3 Na+ pumped out for every 2 K+ pumped in)
(K+ can diffuse back out to some degree)
Extracellular
fluid
Na+
Cell
membrane
cytoplasm
This
‘rest
state’
takes
energy
to
create
K+
Organic
anion

An action potential involves the rapid
depolarization and repolarization of the
membrane.

When a stimulus is
applied to a nerve, some
Na+ gates open,
allowing Na+ to diffuse
in.
Na+
Extracellular
fluid
Na gate
Cell
membrane
K gate
cytoplasm
Na / K
pump
K+
Organic
anion


Once a threshold is reached,
all Na+ gates open, causing
depolarization of the
membrane.
When the membrane is
depolarized, the inside of the
membrane is more positively
charged than the outside.
Na+
Extracellular
fluid
Na gate
Cell
membrane
K gate
cytoplasm
Na / K
pump
K+
Organic
anion


Membrane repolarization
occurs when Na+ gates close
and K+ gates open, allowing
net diffusion of K+ outside.
Repolarization returns the
membrane to resting potential
(more negatively charged
inside)
Na+
Extracellular
fluid
Na gate
Cell
membrane
K gate
cytoplasm
Na / K
pump
K+
Organic
anion

The K+ gates close and the
resting potential is
maintained by the Na+ / K+
pump
Watch me!
Na+
Extracellular
fluid
Na gate
Cell
membrane
K gate
cytoplasm
Na / K
pump
K+
Organic
anion
Scholar with more siblings …
 At rest, what ions are most abundant outside the
cell, and which are most abundant inside the cell?
 Which side of the membrane is more negative at
rest?
Scholar with less siblings …
 Describe how the movement of ions causes
◦ Depolarization
◦ Repolarization
Scholar with more siblings …
 At rest, what ions are most abundant outside the cell,
and which are most abundant inside the cell?
 Which side of the membrane is more negative at rest?
At rest, Na+ ions are mostly outside the membrane,
while K+ and anions are mostly inside. The inside of
the cell membrane is more negatively charged.
Scholar with less siblings …
 Describe how the movement of ions causes
◦ Depolarization – Na+ ions rush into the cell
◦ Repolarization – K+ ions rush out of the cell
The nerve impulse
moves along the
axon.
The change in voltage of one
area triggers the
depolarization of the
next area.
Repolarization follows
immediately.
In myelinated neurons the impulse “jumps” from node to
node, rather than traveling the whole length of the axon
– makes the impulse transmission much more efficient.
Watch
me!
..and
me!
Generation and propagation of nerve impulse
along one neuron= irritability
Conductivity is the ability of one neuron to
signal another. This occurs in an entirely
different fashion at the synapse, or gap,
between neurons.
• The fluid-filled
space between
neurons is
called a synapse
• Chemicals
called
neurotransmitters
carry the nerve
impulse across
the synapse.
1. The nerve impulse reaches the axon terminal.
2. Ca+ gates open, allowing Ca+ into the axon.
3. The Ca+ causes
vesicles containing
neurotransmitters
to empty into the
synapse
4. The neurotransmitters diffuse across the synaptic
cleft and binds with receptors of the next neuron.
5. Na+ channels open
in the dendrites of
the post-synaptic
neuron
6. Post-synaptic
neuron depolarizes
7. Remaining
neurotransmitter is
broken down.
Pre-synaptic neuron
Action potential  calcium ions release 
neurotransmitter release
Post-synaptic neuron
Neutrotransmitter uptake  sodium gates open 
action potential
Note: information travels as electrical signal within
neurons and as chemical signal between them
Watch me!
Watch me!



Reflexes
are rapid,
predictable,
involuntary
responses
to stimuli.
May be
somatic or
autonomic
Contain 5
elements



What were our objectives today, and what did we
learn about them?
What was our learner profile trait and how did we use
it?
How does what we did today relate to our unit
question?
HOMEWORK: Quiz on Thursday / Friday!
Count off by three.
1) Identify the steps that take place during an
action potential. Make a diagram of the cell
membrane demonstrating the changes that
occur during each step.
2) Draw a diagram of the voltage changes that
occur during an action potential. Label the
diagram with the changes that occur within
the cell during each step.
3) Identify the steps that take place during
synaptic transmission. Make a diagram of the
events that within the pre- and post-synaptic
neurons during each step.