6.5 Neurons & Synapses
Structure of the Human Nervous
System
• Composed of cells called neurons that carry
rapid electrical impulses.
Motor Neuron
Structure & Function
• Parts of a motor neuron:
– Cell body = does most of the typical cell functions, contains the nucleus
– Dendrites = branched extensions which connect with other neurons
– Axon = thin long fiber which extends from the cell body carrying
impulses
– Myelin sheath = an
insulating membrane
Nodes of Ranvier
Nucleus
– Nodes of Ranvier =
gaps in
the myelin sheaths
where the
axon is exposed.
– Motor End Plates=
nerve endings that
connect with a new
nerve, muscle, or gland
Motor Neuron
Structure & Function
• How is the overall structure of a motor
neuron related to its function?
Sending a Signal
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Figure 48.5
Synaptic
terminals
The Nerve Impulse
• Nerve impulse is the signal that the nerve sends
through the axon.
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter1
4/animation__the_nerve_impulse.html
• Resting potential: “the state of being where an area
of a neuron is ready to send an action potential (but
is not currently sending).
– During resting potential, the neuron is called polarized.
• The resting potential depends on the ionic
gradients that exist across the plasma
 The concentration
membrane.
+
EXTRACELLULAR
FLUID
CYTOSOL
[Na+]
15 mM
–
[K+] –
150 mM
[A–]
100 mM
Figure 48.10
[Na ]
+ 150 mM
+
[K+]
5 mM
–
+
[Cl–]
120 mM
–
+
–
[Cl–]
10 mM
+
Permanently
Located
in Cytoplasm
Plasma
membrane
of Na+ is higher in
the extracellular
fluid than in the
cytosol

The opposite is
true for K+
Sodium-Potassium Pump
• We are actively pumping Na+
out of the cytoplasm, and K+
into the cytoplasm through
our Sodium-Potassium pump.
– This requires ATP.
– Sends 3 Na+ out, takes 2 K+
in.
• http://highered.mcgrawhill.com/sites/0072495855/st
udent_view0/chapter2/anim
ation__how_the_sodium_pot
assium_pump_works.html
Resting Potential
• Action Potential: The signal conducted by a “self-propagating
wave of ion movement in and out of the neuron membrane”
• When a stimulus depolarizes the membrane…
– Na+ channels open, allowing Na+ to diffuse into the cell
• As the action potential goes away…
– K+ channels open, and K+ flows out of the cell
• This action potential “depolarizes” the cell,
making the cytoplasm & extracellular fluid
equally charged
Axon
Action
potential
–
–
+
+
+
+
+
+
+
+
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
+
+
+
+
+
+
Na+
Action
potential
K+
2
+
+
–
–
+
+
+
+
–
–
+
+
–
–
–
–
–
–
+
+
–
–
–
–
+
–
–
+
+
+
+
+
Na+
1
K+
Action
potential
K+
+
+
+
–
–
–
+
–
–
–
–
–
+
+
+
+
K+
–
+
+
–
Na+
3
–
–
–
+
+
+
+
+
+
–
–
–
An action potential is generated
as Na+ flows inward across the
membrane at one location.
The depolarization of the action
potential spreads to the neighboring
region of the membrane, re-initiating the
action potential there. To the left of this
region, the membrane is repolarizing as K+
flows outward.
The depolarization-repolarization process is
repeated in the next region of the
membrane. In this way, local currents of ions
across the plasma membrane cause the
action potential to be propagated
along the length of the axon.
Review
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter14/animation__the_nerve_impulse.
html
Synaptic Transmission
• Once the impulse reaches the axon terminal, there
must be a chemical communication between the
neurons.
• Synaptic Cleft = area between axon terminal and
postsynaptic neuron or cell
• Neurotransmitter = any chemical
used for synaptic
transmission
– Ex. Acetylcholine
– Neurotransmitters
are held in vesicles
Steps:
• 1. Calcium ions diffuse
into the axon terminal.
• 2. Vesicles with neurotransmitter fuse with cell
membrane, releasing its contents into the
synaptic cleft.
• 3. Neurotransmitters diffuse
across synaptic cleft to the postsynaptic neuron or muscle/gland
cell.
• 4. Neurotransmitters bind with
a receptor protein.
Steps
• 5. When the neurotransmitter
binds with the receptor, it
opens an ion channel and Na+
diffuses
into the cell….
• 6. depolarizing it, and
instigating an action potential.
• 7. Neurotransmitter is broken
down and released from
receptor.
• 8. Na+ ion channels close.
• 9. Neurotransmitters are taken
up again by vesicles into axon
terminal.
Synaptic Transmission
• http://www.youtube.com/watch?v=hqWv0iEF
4ho
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch
apter14/animation__transmission_across_a_s
ynapse.html