Chapter 48
Neurons, Synapses, and
Signaling
Information Processing
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Sensory neurons –
Interneurons –
Motor neurons –
Central vs. Peripheral NS --
Neuron Structure and Function
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Cell Body –
Dendrites –
Axon –
Axon hillock –
Synapse –
Synaptic terminal –
Neurotransmitters –
Presynaptic cell –
Postsynaptic cell --
I. Nerve Signals and Their Transmission
A. Resting Potential – maintained by the Sodium Potassium
Pump
1. Cells pump 3 Sodium ions (3Na+) out
2 Potassium ions (2K+) in
a. Cell is more negative inside than out
b. This makes the cell like a battery
B. How do the ions get “pumped” in/out?
1. An integral protein (enzyme) uses ATP
a. its name is ATPase
b. it “countertransports” – pumping two things in
opposite directions
c. it is also referred to as an electrogenic pump because
it generates an electrical charge
C. Now that the electrical gradient is established, how is it
used?
1. Cell’s “resting membrane potential” is -70mV
2. Depolarizing a cell:
1. enables us to use the charge in our cell membrane
2. There are sodium channels in our membrane. They
open when we need to depolarize the membrane.
a. this allows Na+ to rush in which makes the inside
of the cells less negative.
b. cells depolarize to +30mV (when Na+ is at
equilibrium with itself, there is still an excess of
K+)
D. How do we “repolarize” our cell membranes?
1. NOT through the Sodium Potassium Pump…would
take too long
a. it keeps membrane charged continually
2. Potassium channels open briefly
Resting
“polarized”
Na+ channel open
“depolarized”
Na+ closed, K+ open
“repolarization”
Resting
“polarized”
E. This “depolarization” (opening Sodium gates) and
“repolarization (opening Potassium gates) is called an
action potential.
1. What signals a cell to depolarize?
a. a receptor (lock) on a cell membrane binds to its
ligand (key) causing the receptor to change shape
-- change involves the opening of the sodium
channel
b. the receptor is called a neurotransmitter receptor
and the ligand is called the neurotransmitter
-- Acetylcholine is the neurotransmitter in
skeletal muscle cells.
2. How does an Action Potential work?
a. Begins in a location where chemically gated sodium
channels are present. These locations are called
“postsynaptic membranes” in neurons and “motor
endplates” in muscle cells.
b. Neurotransmitter (ligand) binds to a neurotransmitter
receptor. The receptor changes shape.
c. A chemically gated sodium channel opens. Na+ flow in
d. Once one Na+ channel opens, the others follow
due to the change in electrical conditions. This
point is called “threshold”
e. K+ channels open slowly so that when the Na+
channels close, the K+ channels are fully open
(repolarization). System resets when K+ channels
close.
f. this causes a moving wave of depolarization and
repolarization within the cell
Post-synaptic membranes or Motor Endplates 
Neurotransmitter binds to Neurotransmitter receptor 
Neurotransmitter receptor changes shape 
Sodium Channel opens 
Sodium flows in 
Other Sodium Channels open = “Threshold” 
Potassium Channels open slowly 
Sodium Channels close 
Potassium Channels close.
Resting
“polarized”
Na+ channel open
“depolarized”
Na+ closed, K+ open
“repolarization”
Resting
“polarized”
F. Action potential is how information is carried along a
neuron, and how an entire muscle contraction is
stimulated through one muscle cell.
1. Neuron to neuron communication occurs
between structures called synapses.
2. Neurons communicate with other cells (effector
cells) and muscle cells in structures called
neuroeffector junctions
a. the term synapse is used for both even though
it is incorrect
G. How this communication takes place:
1. Action potential moves down the neuron through the axon,
then through the axon terminals (“telodendria”), and finally
through the synaptic end bulbs.
a. Synaptic end bulbs contain vesicles of neurotransmitter
2. Voltage-gated calcium channels in the membrane of the
end bulb open.
3. Calcium diffuses in.
4. Calcium’s presence causes the synaptic vesicles to fuse
with the cell membrane to release neurotransmitter
(exocytosis) into the synaptic cleft.
5. Neurotransmitters diffuse across the synaptic cleft and
bind to neurotransmitter receptors on the membrane on
the opposite side of the cleft.
6. Neurotransmitters are broken down by enzymes in the
synaptic cleft.
Action Potential 
axon 
axon terminals 
Synaptic end bulbs 
Calcium Channels open 
Calcium diffuses in 
Synaptic vesicles leave via exocytosis 
Neurotransmitters diffuse across synaptic cleft 
Neurotransmitters bind to neurotransmitter receptors 
Action potential triggered in next cell.
H. The All-or-None Principle
1. If stimulus is too small….no action potential
2. If stimulus is “threshold”…action potential
3. If stimulus is larger than threshold…same sized action
potential as above
I. The Absolute Refractory Period
1. A brief period of time after a stimulus in which an additional
stimulus will not produce an additional action potential
J. The Relative Refractory Period
1. A certain time period after a stimulus during which a
second stimulus will produce a second action potential, but
only if it is a larger stimulus than usual.
Neurotransmitters
• Acetylcholine
– transmit signal to skeletal muscle
• Epinephrine (adrenaline) & norepinephrine
– fight-or-flight response
• Dopamine
– widespread in brain
– affects sleep, mood, attention & learning
– lack of dopamine in brain associated with
Parkinson’s disease
– excessive dopamine linked to schizophrenia
• Serotonin
– widespread in brain
– affects sleep, mood, attention & learning
Neurotransmitters
• Weak point of nervous system
– any substance that affects neurotransmitters
or mimics them affects nerve function
• gases: nitrous oxide, carbon monoxide
• mood altering drugs:
– stimulants
» amphetamines, caffeine, nicotine
– depressants
» quaaludes, barbiturates
• hallucinogenic drugs: LSD, peyote
• SSRIs: Prozac, Zoloft, Paxil
• poisons