neural conduction and synaptic transmission (i.e., electricity and chemistry)
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Neural Conduction and Synaptic Transmission(i.e., Electricity and Chemistry)
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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
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Neural Conduction
An Electrical Process
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Resting Membrane Potential
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Figure 4.2 Recording the resting membrane potential of a neuronKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Ions and the resting membrane potential
• K+
– Potassium ions, positive charge
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Ions and the resting membrane potential
• Na+
– Sodium ions, positive charge
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Ions and the resting membrane potential
• Cl-
– Chloride ions, negative charge
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Ions and the resting membrane potential
• Inside the neuron– K+
– Protein-
• Outside the neuron– Na+
– Cl-
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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 neuronKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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
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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 pumpKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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
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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
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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
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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
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• Like relay team passing a baton• Causes something called
“postsynaptic potentials” to happen
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Postsynaptic potentials can do one of 2 things…
• Depolarize neuron
• Hyperpolarize neuron
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Postsynaptic potentials
• Depolarize
• Decrease resting potential
• Become less negative
• E.g., from -70 mV to – 67 mv
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Postsynaptic potentials
• Increase likelihood that neuron will fire
• Excitatory postsynaptic potentials: EPSPs
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Postsynaptic potentials
• Hyperpolarize
• Increase the resting potential
• Become more negative
• E.g., from -70 mV to -72 mV
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Postsynaptic potentials
• Decrease likelihood that neuron will fire
• Inhibitory postsynaptic potentials: IPSPs
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Characteristics of EPSPs and IPSPs
• Notes
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There are a bunch of EPSPs and IPSPs There are a bunch of EPSPs and IPSPs happening in the same neuron at oncehappening in the same neuron at once
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How EPSPs or IPSPs add up
• Spatial summation – A bunch of EPSPs/IPSPs combine together
Figure 4.14 Spatial summation and temporal summationKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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 summationKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Getting a neuron to fire
• EPSPs and IPSPs travel until they reach near the axon hillock
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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
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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)
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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
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Figure 4.7 Changes in the membrane potential during the action (spike) potentialKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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)
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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
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Figure 4.7 Changes in the membrane potential during the action (spike) potentialKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Getting a neuron to fire
• The end result that matters is how the EPSPs and IPSPs cancel each other out near the axon hillock
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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
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Figure 4.7 Changes in the membrane potential during the action (spike) potentialKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Action potential
• Membrane becomes depolarized to about 40 mV
• All-or-nothing
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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
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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
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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) potentialKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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) potentialKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Action potential travels down axon
Figure 4.9 Propagation of the action potential along an unmyelinated axonKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Action potential travels down axon
• Action potentials are nondecremental – do not become weaker as they travel
• Travel very slowly
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• 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
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Conduction of Action Potential in Myelinated Axons
Figure 4.10 Propagation of the action potential along an myelinated axonKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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
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Chemical signals
• In your nervous system there are chemicals called “neurotransmitters.”
• Neurons produce neurotransmitters.
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Neurotransmitters
• Neurotranmsitters are packed into synaptic vesicles.
• Synaptic vesicles are found at the terminal buttons.
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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 transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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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 transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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What happens next?
It’s like playing pinball
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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
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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
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What happens to the leftover neurotransmitters?
• Reuptake– Neurotransmitters return to presynaptic
buttons
Figure 4.18 Termination of neural transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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What happens to the leftover neurotransmitters?
• Degradation– Neurotransmitters broken apart in the
synapse by enzymes
Figure 4.18 Termination of neural transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Neurotransmitters
Chemicals in the Nervous System
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Neurotransmitters
• Acetylcholine (Ach)– Muscles– Memory: Alzheimer’s
• Gamma-aminobutyric acid (GABA)– Seizures– Huntington’s disease
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Neurotransmitters
• Epinephrine (aka adrenaline)
• Norepinephrine
– Activation of cardiovascular system
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Neurotransmitters
• Dopamine– Schizophrenia– Parkinson’s
• Serotonin– Depression– Aggression
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Agonists and Antagonists
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Agonists
• Synthesis of neurotransmitter
• Helps with release
• Obstructs autoreceptor
• Pretends to be neurotransmitter
• Prevents reuptake
Figure 4.16 AutoreceptorsKlein/Thorne: Biological Psychology© 2007 by Worth Publishers
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Antagonists
• Obstacle to synthesis
• Obstacle to neurotransmitter release
• Fools autoreceptor
• Blocks receptor
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How some drugs work
• Cocaine– Agonist of norepinephrine and dopamine– Prevents reuptake of leftover norepinephrine
and dopamine– Therefore, effects of these neurotransmitters
are increased
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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