Download The Nervous System

The Nervous System
A great example of cell to cell
communication and multicellular
coordination…
Anatomy of the Nervous System
• There are 2 main branches of
the nervous system
• Central Nervous System
– Brain
– Spinal Cord
• Peripheral Nervous System
– All nerves leading to rest of
body
Anatomy of a Neuron
• The main cells in the nervous system are called
Neurons. They consist of three main parts
– Cell body = contains the organelles, nucleus
– Dendrites = short branches off the cell body where a
signal is received
– Axon = long branch off the cell body that leads to the
next neuron
Continued Anatomy of a Neuron
• Myelin = insulating layer that surrounds the
axon to speed up the signal transfer
• Synaptic Terminal = end of the neuron
• Synapse = gap/space between neurons
where neurotransmitters transfer
How do neurons communicate?
• Signals transfer through neurons using
synaptic signaling. It follows this general
pathway
– Dendrites  cell body  Axon  Synaptic
Terminals (then into the synapse to get to the
next neuron or other cell)
Talking Cells
• The “signal” sent through a neuron is
actually an electrical signal
• Electricity is generated anytime an ion
moves. By using the dam metaphor, the
cells create ion movement cascades called
action potentials
– The movement of the ions cause changes in the
+ and – charges inside the cell
A Neuron at Rest
• A neuron at rest (unstimulated) is slightly
negatively charged thanks to the careful
arrangement of ions by the cell. This is
called the resting potential
– This means that it is more negative inside than
outside
• The cell controls this potential by moving
ions in or out as needed to adjust the
charges
A Neuron at Rest
• Resting potential and proper location of
ions is maintained mostly by the SodiumPotassium Pump
K+
K+
– Pumps Na+ out of cell
– Pumps K+ into the cell
– Active transport
– More sodium outside than
potassium inside = negative
cell
Na+
Na+
K+
Na+
K+
K+
Na+
Na+
Na+
K+
K+
K+
Na+
Na+
Na+
K+
K+
Ion Concentrations at Rest
Ion
Inside neuron
Outside neuron
Na+
Lower
Higher
Cl-
Lower
Higher
K+
Higher
Lower
• Leak channels allow some K+ to escape out of the
cell in case it gets too positively charged on the
inside
• Leaves negatively charged molecules behind (Clions, etc.)  more negative on the inside than on
the outside.
Sending a Signal: Action Potential
• GATED Na+ channels are embedded in the
membrane of the neuron, but they are
normally CLOSED. These channels can be
triggered to open by a stimulus such as a
touch or smell or ligand
– When the gate opens, the sodium come rushing
into the cell (dam metaphor) and cause the
voltage to change.
Sending a Signal: Action Potential
• More sodium channels that are voltage gated exist in
the nearby membrane. These channels open in
response to a change in voltage or charge near them.
When that first gate opens by the stimulus and lets
in the sodium, the other gates are triggered to open
in a chain, adding more sodium and triggering the
next in line.
The Cascade of Opening Sodium
Gates is called an Action Potential
• STEP 1: To start an
action potential, some
kind of stimulus (light,
pressure, chemical, etc.)
causes Na+ channels in
the dendrite to open.
• This causes Na+ to flood
into the neuron from
outside 
DEPOLARIZATION
Questions…
• Why does Na+ diffuse in from the outside?
– Higher concentration on the outside
• When depolarization occurs, how is the
charge inside the neuron affected?
– Becomes more positively charged inside
Action Potential
• STEP 2: The change in
voltage triggers the next Na+
channel (voltage gated
channel) to open.
• STEP 3: As Na+ diffuses
down the neuron, it continues
to trigger voltage gated Na+
channels to open.
– This is what sends a signal
through the individual neurons
towards the axon terminal.
Action Potential
• STEP 4: Na+ voltage gated channels only
open temporarily. After a short period of
time, they close and an inactivation gate
opens to prevent them from opening again
for a little while  REFRACTORY
PERIOD
Action Potential
• STEP 5: The neuron must be
“reset” (REPOLARIZED) by
the opening of voltage gated K+
channels.
• K+ flows out of the neuron,
making the inside more
negative again.
– Why does K+ flow out?
– Higher K+ concentrations inside
neuron
• The Na+/K+ pump helps
reestablish resting potential.
Saltatory Conduction
• Depolarization &
Repolarization happens over
and over down the axon, so
the nerve impulse travels.
• Myelin sheaths insulate the
axon, keeping ions from
getting too far away from
the cell.
How does the signal jump the gap?
• Remember that a gap exists
between neurons that the
action potential cannot
“jump”. They are just too far
apart. When the signal
reaches the end of the axon
and wants to go to the next
cell in line, it must change to
a chemical messenger instead
of an electrical impulse.
These chemical messengers
are called neurotransmitters
How does the signal jump the gap?
• The last voltage gated
channel triggers the release of
Calcium ions.
• This triggers exocytosis and
vesicles that contain
neurotransmitter molecules
fuse with the plasma
membrane and expel the
neurotransmitters into the
synaptic cleft (space between
neurons)
Communication between Neurons
• The neurotransmitters diffuse across the cleft
and bind to receptors on the next neuron
• This triggers a Na+ chemical gated channel to
open on that neuron, creating a new action
potential
Communication between Neurons
• After the signal has been sent,
neurotransmitters must be eliminated. This
is done by
– Diffusion = diffuse away
– Reuptake (proteins that reabsorb the
neurotransmitters for recycling)
– Enzyme degradation (proteins that break down
the neurotransmitters completely)
Question…
• Why is it important that our bodies /
medications control nerve communication?
– So that signals are only sent to neurons when
needed.