Grade 9 BIOLOGY
THE NERVOUS SYSTEM
WHAT DOES YOUR NERVOUS
SYSTEM DO?

Your ideas:
Hear what Tim and Moby have to say...
WHAT IS PART OF THE NERVOUS
SYSTEM?
ORGANIZATION OF THE NERVOUS
SYSTEM
How many neuron connections do you have?
 How is the nervous system divided up?


What are major parts of the nervous system…?
WHAT DO NEURONS LOOK LIKE?
Label a neuron!
MAJOR TYPES OF NEURONS
Differences:
Motor
Similarities:
Sensory
HOW DO NEURONS SEND A SIGNAL?

ELECTROCHEMICAL CHANGE that
moves in one direction along
a neuron


involves changes in voltage
(charge) as well as in the
concentrations of ions
What charge does a cell have at rest?

Slightly negative

WHY?

Remember… what does the sodium/potassium
pump do?
Review of the Na+ / K+ pump
Na
+
K+
 The
inside of the cell is negative
because:
More positive ions are pumped out of
the cell than into the cell (3 Na+ out / 2
K+ in)
 Negatively charged proteins already in
the cell
 Some K+ ions leave through potassium
channels



ACTION POTENTIAL
Review:
Change in the relative charges
inside and outside the cell
because of changes in ion
concentration
How do polar molecules get
across membranes?
In the cell membrane there are
two types of gated ion channels
(they open and close):
Na+ channels
 K+ channels


There are also Na+/K+ pumps
HOW DOES IT WORK?

Questions:
Where is there more Na+? Inside or outside the cell?
 Where is there more K+? Inside or outside the cell?
 What do we need to do to make the inside of the cell
change from negative to positive?

Na
+
K+
STEPS IN AN ACTION POTENTIAL
1. Resting potential
 Na+ outside, K+ inside
 Channels closed
2. Stimulus / Threshold
 Some Na+ channels open
 Na+ comes in
 If threshold is passed
First... Watch the tutorial animation!
they all open
3. Depolarization

Na+ channels open, Na+ comes in and the cell becomes
positive
Chain reaction of Na+ channels opening down the axon

4. Repolarization
 Na+ channels close, K+ channels open
 K+ moves out and the cell becomes negative
STEPS IN AN ACTION POTENTIAL
5. Undershoot


First... Watch the tutorial animation!
K+ leaves as K+ channels
slowly close
Cell is MORE negative
than usual
6. Return to Resting
Potential
 Both Na+ and K+ channels

close
Na+ / K+ pump returns
Na+ and K+ to the right
places
Na+
K+
Na+
K+
Na+ channels open,
3 Additional
+
K channels are closed; interior of
cell becomes more positive.
Na+
4 Na+ channels+ close and
inactivate. K channels
open, and K+ rushes
out; interior of cell more
negative than outside.
Action
potential
3
2
Threshold
potential
Na+
2 A stimulus opens some Na+
channels; if threshold is reached,
action potential is triggered.
Neuron
interior
1
Resting potential
4
5
+
5 The K channels close
relatively slowly, causing
a brief undershoot.
1
Neuron
interior
state: voltage gated Na+
1 Resting
+
and K channels closed; resting
potential is maintained.
1 Return to resting state.
Another look at an action potential!
COMPLETE AND KNOW THIS CHART…
Steps in an
Action
Potential
Where is
Na+ ?
Where is
K+ ?
Na+
channels?
K+
channels?
What
charge is
inside?
What
charge is
outside?
Resting
Potential
Depolarization
Repolarization
Undershoot
Return to
Resting
Potential
COMPLETE AND KNOW THIS CHART…
Steps in an
Action
Potential
Resting
Potential
Where is
Na+ ?
Outside
Where is
K+ ?
Inside
Na+
channels?
Closed
K+
channels?
Closed
What
charge is
inside?
Negative
What
charge is
outside?
Positive
Depolarization
Repolarization
Undershoot
For example…
This is Resting
Potential
Return to
Resting
Potential
WHAT DOES THE MYELIN SHEATH DO?


Watch here!
With the myelin sheath you...
Only trade ions in the Nodes of Ranvier instead
of all along the axon
= faster signal along the axon!
Multiple Sclerosis (MS)
happens when you lose your
myelin sheath
 neurons can’t send
messages fast enough!
WHAT IS A REFLEX?

A Reaction Time is...



How fast you can react to something
Uses your brain! (catching a ball, saving a goal)
A Reflex is...
An automatic reaction that does NOT use your brain
 Message goes to the spinal cord and back only
(moving your hand
away from something hot)

THE KNEE-JERK REFLEX...