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The Nervous System
Chapter 14
Plan – Field Trip
• Wednesday 5/23 – to Case Western Neurology
Laboratory:
• Dr. Hillel Chiel:
http://www.case.edu/artsci/biol/people/chiel.
html
• Time TBD.
Outline
• Different types of neurons
• Structure of a neuron
• How do neurons send messages?
a) Electrical
b) Chemical
Purpose
• The purpose of the Nervous System is to
respond to change in the environment of an
organism.
• Example: An approaching predator, sunrise, a
moving baseball.
Definition
• The Nervous System is a network of
specialized cells (called “neurons”) that carry
messages throughout the body.
• [The study of these neurons is called
Neurology
______________
]
Nervous System
• The Nervous System is made up of three
different types of neurons:
1. Sensory Neurons
2. Interneurons
3. Motor neurons
1- Sensory Neurons
• Sensory neurons are neurons that detect a
stimulus.
• Stimuli can be sounds, heat, light, chemicals,
pressure, etc.
• Example: Sensory neurons in your ear detect
sound, nerves on your neck can detect touch
(pressure), in your nose can detect smell, etc.
• These neurons then pass the message to…
2- Interneurons
• Interneurons relay messages from one neuron to
another. These can be divided into two different
types:
• A) Messengers- Neurons that are messengers
send receive a message from the previous neuron
and pass it along to the next neuron in line.
• B) Interpreters- Neurons that receive a message
and interpret the message and come up with a
proper response. (Example: neurons in the brain).
• Interneurons continue passing the message along
until it reaches a…
3- Motor Neuron
• Motor neurons receive a message from
interneurons and cause a reaction in the body
to the original stimulus. Motor neurons
usually control a muscle and cause it to
contract.
Label
Sensory Neuron
Stimulus
Interneuron
Motor Neuron
Effector- organ that actually reacts
(usually a muscle)
Majority of nerves in the body
• The overwhelming majority of neurons in the
body are…. Interneurons
• This is because the chain to relax information
is fairly long and involves many different
neurons and only the very first one is a
sensory neuron and the very last one is a
motor neuron.
How does a neuron differ from an
“average animal cell”?
Average animal cell (epithelial
cell, white blood cell, gland
cell, etc).
Neuron
Structure of the Neuron
• Now we are zooming in on a single neuron.
The neuron has 3 sections:
1. Dendrites
2. Cell body
3. Axon
Structure of the Neuron –
Similarities to other cells (Cell Body)
The cell body of a neuron is similar to any other type of cell. It
has mitochondria, ribosomes, a cell membrane, a nucleus,
etc.
This is because the cell needs to survive just like any other
Similar
cell.
Different
Dendrites
• Dendrites are short,
highly branched fibers
that receive an
impulse (message)
from another neuron.
• It transmits the
message toward the
cell body.
Axon
• The axon is a single long
fiber that carries an impulse
away from the cell body
toward a different neuron.
• This is the longest part of a
neuron.
• It generally does not branch
until the very end, where it
branches into many
terminals.
• Axons can be as long as a
meter in certain areas.
Axon
Terminal
Dendrite
Cell
body
Nucleus
Axon
Axon (contd.)
• The length of the axon is
mostly covered by a white,
fatty material known as
myelin.
• There are also gaps in the
myelin known as the nodes of
Ranvier.
• We will get to the function of
these different areas soon.
The Nerve Impulse
1. Resting potential
2. Nerve Impulse
3. Refractory Period
Resting Potential
• When a neuron is at rest, the
sodium-potassium pump moves
sodium (Na+) out of the cell and
moves potassium (K+) into the
cell. This is done by active
transport.
• After this, some of the potassium
leaks out of the cell, making the
outside more positive than the
inside.
• This causes the inside of the cell
has a net negative charge and
the outside of the cell has a net
positive charge.
Nerve Impulse
• When a nerve impulse reaches
the dendrite of a neuron, the
sodium channels open. This
causes sodium to rush in (diffuse)
to the cell and make the inside of
the cell more positive.
• This, in turn, causes “voltagegated sodium channels” to open
in response to the change in
charge.
• This causes many more sodium
to rush in.
The transmission of the nerve impulse
1. The sodium rushes in to the
cell.
2. This changes that area of the
cell to become more
positive.
3. This affects the voltage-gated
sodium channels nearby
causing them to open.
4. This allows sodium to rush
in, etc.
Speed of
Transmission
• This impulse moves rapidly
down the dendrite membrane,
down the cell body and down
the axon to the axon terminal.
• The impulse is able to move so
rapidly down the long axon
because it skips all of the
regions that are covered by
myelin. The impulse only affects
the Nodes of Ranvier, which are
not covered by myelin.
• In neurons without myelin, the
impulse moves at ~2 m/s. In
myelinated cells, it moves at
~100 m/s.
Refractory Period
• After the sodium rush has caused a
certain segment of the cell to
become positive, the cell reverses
this process by opening potassium
channels and allowing the
potassium (K+) to rush out,
restoring the negative charge inside
the cell.
• After a while, the sodiumpotassium pump will exchange Na+
for K+ and the normal balance will
be restored in the cell.
• During this time, the nerve cannot
respond to a new impulse. (This
takes a few thousandths of a
second).
Review
At a given spot in a neuron, the following events will happen:
1. Resting potential: _______ inside, _______outside. More
_______ inside and more ________ outside.
2. Action potential: _______ flows inside the cell. This
causes the inside to become more ________. This causes
_____________ to open, allowing more ________ to flow
inside. Now the inside is more ________ and the outside
is more _________.
3. Refractory Period: The cell opens channels to let
________ flow out of the cell. This causes the inside to be
more __________ and the outside to be more
__________. Eventually, the sodium-potassium pump
restores the balance for the cell, with more ________
outside and more ________ inside. We are now back to
resting potential.
Summary Movie
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter14/animation__the_nerve_impulse.html
Synapse
• The gap between different
neurons is called a
synapse.
• We have established how
an electrical impulse can
travel down a neuron. How
does the impulse travel
between different
neurons? How does an
impulse travel across a
synapse?