Neural Conduction and
Synaptic Transmission
(i.e., Electricity and Chemistry)
Neurons
Figure 2.5 A typical neuron and synapse
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Figure 2.6 The four major types of synapses
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Neural Conduction
An Electrical Process
Resting Membrane Potential
Figure 4.2 Recording the resting membrane potential of a neuron
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Ions and the
resting membrane potential
• K+
– Potassium ions, positive charge
Ions and the
resting membrane potential
• Na+
– Sodium ions, positive charge
Ions and the
resting membrane potential
• Cl– Chloride ions, negative charge
Ions and the
resting membrane potential
• Inside the neuron
– K+
– Protein-
• Outside the neuron
– Na+
– Cl-
What the ions naturally want to do
• Force of diffusion
– It’s getting crowded in here
• Electrostatic pressure
– Opposites attract
– Similarities repel
Figure 4.4 The influence of diffusion and electrostatic pressure on the
movement of ions into and out of the neuron
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
What the neural membrane is
making the ions do
• Differential permeability
– Playing favorites
• K+ and Cl- pass through easily
• Na+ -- not so easy to pass through the membrane
• Proteins: not a chance!
– Ion channels: like doors
What the neural membrane is
making the ions do
• Sodium-potassium pump
• Three Na+ out for every two K+ cells in
Figure 4.5 The sodium-potassium pump
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Putting it all together…
• Na+ ions
– Want to go inside neuron because
• There are fewer of them inside (force of diffusion)
• There is a negative charge inside (opposite to their positive
charge)
– But
• Neuron’s membrane not very permeable to Na+ ions
• Sodium-potassium pump keeps kicking them out
– Therefore, most Na+ ions stay outside neuron
Putting it all together…
• K+ ions
– Want to go outside neuron because
• There are fewer of them outside (force of diffusion)
• Neuron’s membrane very permeable to K+ ions
– But
• There is a positive charge outside (similar to their positive
charge), so they are repelled by the outside
• Sodium-potassium pump keeps kicking them back into
neuron
– Therefore, most K+ ions stay inside neuron
Putting it all together…
• Cl- ions: can’t make up their minds
– Want to go inside neuron because
• There are fewer of them inside
• Neuron’s membrane very permeable to Cl- ions
– Also want to stay outside of neuron because
• The charge outside is positive (and their own charge is
negative
– Therefore, Cl- ions keep going back and forth,
distribution of Cl- ions is held at equilibrium.
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_
Postsynaptic Potentials
Getting the membrane potential to
change from -70 mv
Figure 2.6 The four major types of synapses
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Figure 4.11 Overview of synaptic transmission
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
• Like relay team passing a baton
• Causes something called
“postsynaptic potentials” to happen
Postsynaptic potentials can do one
of 2 things…
• Depolarize neuron
• Hyperpolarize neuron
Postsynaptic potentials
•
•
•
•
Depolarize
Decrease resting potential
Become less negative
E.g., from -70 mV to – 67 mv
Postsynaptic potentials
• Increase likelihood that neuron will fire
• Excitatory postsynaptic potentials: EPSPs
Postsynaptic potentials
•
•
•
•
Hyperpolarize
Increase the resting potential
Become more negative
E.g., from -70 mV to -72 mV
Postsynaptic potentials
• Decrease likelihood that neuron will fire
• Inhibitory postsynaptic potentials: IPSPs
Characteristics of EPSPs and
IPSPs
• Notes
There are a bunch of EPSPs and IPSPs
happening in the same neuron at once
How EPSPs or IPSPs add up
• Spatial summation
– A bunch of EPSPs/IPSPs combine together
Figure 4.14 Spatial summation and temporal summation
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
How EPSPs or IPSPs add up
• Temporal summation
– When EPSPs/IPSPs are coming in real fast,
the next one happens before the previous one
fades away
– They add together over time
Figure 4.14 Spatial summation and temporal summation
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Getting a neuron to fire
• EPSPs and IPSPs travel until they reach
near the axon hillock
Getting a neuron to fire
• Remember…
– EPSP make membrane’s resting potential
less negative (e.g., from -70 mv to -68 mv)
– IPSPs make membrane’s resting potential
more negative (e.g., from -70 mv to -75 mv)
• When combined they cancel each other
out, and whichever is stronger wins
Getting a neuron to fire: Example 1
• EPSPs add up to change resting potential
from -70 mv to -60 mv (change of +10 mv)
• IPSPs add up to change resting potential
from -70 mv to -75 mv (change of -5 mv)
Getting a neuron to fire: Example 1
• Net difference of +5 mv, from -70 mv to 65 mv
• The resting membrane potential to
become less negative
• The end result is the membrane is
depolarized
Figure 4.7 Changes in the membrane potential during the action (spike) potential
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Getting a neuron to fire: Example 2
• EPSPs add up to change resting potential
from -70 mv to -68 mv (change of +2 mv)
• IPSPs add up to change resting potential
from -70 mv to -75 mv (change of -5 mv)
Getting a neuron to fire: Example 2
• Net difference of -3 mv, from -70 mv to -73
mv
• The resting membrane potential to
become more negative
• The end result is the membrane is
hyperpolarized
Figure 4.7 Changes in the membrane potential during the action (spike) potential
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Getting a neuron to fire
• The end result that matters is how the EPSPs
and IPSPs cancel each other out near the axon
hillock
Getting a neuron to fire
• If it so happens that, near the axon hillock
– The net combination of EPSPs/IPSPs
– Depolarizes the membrane (makes it less negative)
– To a point called threshold potential
Figure 4.7 Changes in the membrane potential during the action (spike) potential
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Action potential
• Membrane becomes depolarized to about
40 mV
• All-or-nothing
Voltage-activated ion channels
• When a neuron’s membrane reaches the
threshold of excitation, ion channels open
• Na+ ions (previously could not permeate
the membrane) can now rush into the
neuron
• As a result, membrane potential goes to
about 40mv
Voltage-activated ion channels
• K+ ions (they start out being inside the
neuron) now rush out of the neuron
– Force of diffusion
– When membrane potential is now positive,
also driven out by electrostatic pressure
Refractory period
• Absolute refractory period
– Lasts 1 to 2 milliseconds
– Impossible for another action potential to
happen
Figure 4.7 Changes in the membrane potential during the
action (spike) potential
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Refractory period
• Relative refractory period
– Possible for another action potential to
happen
– But need extra-strength stimulation
Figure 4.7 Changes in the membrane potential during the
action (spike) potential
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Action potential travels down axon
Figure 4.9 Propagation of the action potential along an
unmyelinated axon
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Action potential travels down axon
• Action potentials are nondecremental – do
not become weaker as they travel
• Travel very slowly
• Action potential only causes those ion
channels in one small spot of the
membrane to open
• To travel down the axon, needs to nudge
the adjacent ion channels
Conduction of Action Potential in Myelinated Axons
Figure 4.10 Propagation of the action potential along an myelinated axon
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Conduction of Action Potential in Myelinated Axons
• This time,
– Action potential travels rapidly
– Action potential simply hops from one node of
Ranvier to another
• Saltatory conduction (saltare = dance)
– Action potential grows weaker as it travels
• But, still strong enough to initiate another action
potential at the next node of Ranvier
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Synaptic Transmission
of Signals
A Chemical Process
Chemical signals
• In your nervous system there are
chemicals called “neurotransmitters.”
• Neurons produce neurotransmitters.
Neurotransmitters
• Neurotranmsitters are packed into synaptic
vesicles.
• Synaptic vesicles are found at the terminal
buttons.
Release of neurotransmitters
• Action potential travels down axon and reaches synapse
• This causes Ca2+ (calcium) ion channels to open
Figure 4.11 Overview of synaptic transmission
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Release of neurotransmitters
• Ca2+ ions cause synaptic vesicles to join to presynaptic
membrane
• Vesicles release neurotransmitters into synaptic cleft
• Neurotransmitters get passed on to the next neuron
Figure 4.11 Overview of synaptic transmission
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
What happens next?
It’s like playing pinball
Back to where we started…
• Neurotransmitter binds with receptor,
causes ion channels to open
• If Na+ channels open, then Na+ ions enter
neuron, depolarizes membrane  EPSP
Back to where we started…
• If chloride channels open, the Cl- ions
enter neuron, hyperpolarizes membrane
 IPSP
• If potassium channels open, the K+ ions
leave the neuron, hyperpolarizes
membrane  IPSP
What happens to the leftover
neurotransmitters?
• Reuptake
– Neurotransmitters return to presynaptic
buttons
Figure 4.18 Termination of neural transmission
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
What happens to the leftover
neurotransmitters?
• Degradation
– Neurotransmitters broken apart in the
synapse by enzymes
Figure 4.18 Termination of neural transmission
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Neurotransmitters
Chemicals in the Nervous System
Neurotransmitters
• Acetylcholine (Ach)
– Muscles
– Memory: Alzheimer’s
• Gamma-aminobutyric acid (GABA)
– Seizures
– Huntington’s disease
Neurotransmitters
• Epinephrine (aka adrenaline)
• Norepinephrine
– Activation of cardiovascular system
Neurotransmitters
• Dopamine
– Schizophrenia
– Parkinson’s
• Serotonin
– Depression
– Aggression
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Agonists and Antagonists
Agonists
•
•
•
•
•
Synthesis of neurotransmitter
Helps with release
Obstructs autoreceptor
Pretends to be neurotransmitter
Prevents reuptake
Figure 4.16 Autoreceptors
Klein/Thorne: Biological Psychology
© 2007 by Worth Publishers
Antagonists
•
•
•
•
Obstacle to synthesis
Obstacle to neurotransmitter release
Fools autoreceptor
Blocks receptor
How some drugs work
• Cocaine
– Agonist of norepinephrine and dopamine
– Prevents reuptake of leftover norepinephrine
and dopamine
– Therefore, effects of these neurotransmitters
are increased
How some drugs work
• Botulinium toxin
– Antagonist of acetylcholine
– Prevents acetylcholine from being released
– Therefore, effects of these neurotransmitters
are decreased
– Small amounts used to paralyze certain
muscles