1. Prepared by Jeffrey W. Grimm Western Washington University
PowerPoint Presentation for Biopsychology, 8th Edition by John P.J. Pinel
Prepared by Jeffrey W. Grimm
Western Washington University
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2. How Neurons Send and Receive Signals
Chapter 4
Neural Conduction and
Synaptic Transmission
How Neurons Send and Receive Signals
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3. Resting Membrane Potential
Recording the membrane potential: difference in electrical charge between inside and outside of cell
Inside of the neuron is negative with respect to the outside
Resting membrane potential is about –70mV
Membrane is polarized (carries a charge)
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4. Ionic Basis of the Resting Potential
Factors contributing to even distribution of ions (charged particles)
Random motion – particles tend to move down their concentration gradient
Electrostatic pressure – like repels like, opposites attract
Factors contributing to uneven distribution of ions
Selective permeability to certain ions
Sodium-potassium pumps
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5. Ions Contributing to Resting Potential
Sodium (Na+)
Chloride (Cl-)
Potassium (K+)
Negatively charged proteins (A-)
Synthesized within the neuron
Found primarily within the neuron
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The Neuron at Rest
Ions move in and out through ion-specific channels
K+ and Cl- pass readily
Little movement of Na+
A- don’t move at all, trapped inside
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7. The Neuron at Rest Continued
Equilibrium Potential (Hodgkin-Huxley model)
The potential at which there is no net movement of an ion – the potential it will move to achieve when allowed to move freely
Na+ = 120mV
K+ = 90mV
Cl- = -70mV (same as resting potential)
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8. The Neuron at Rest Continued
Na+ is driven in by both electrostatic forces and its concentration gradient
K+ is driven in by electrostatic forces and out by its concentration gradient
Cl- is at equilibrium
Sodium-potassium pump – active (uses ATP) force that exchanges 3 Na+ inside for 2 K+ outside
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FIGURE 4.2 The passive and active factors that influence the distribution of Na+, K+, and Cl- ions across the neural membrane.
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10. Generation and Conduction of Postsynaptic Potentials (PSPs)
Neurotransmitters bind at postsynaptic receptors
These chemical messengers bind and cause electrical changes
Depolarizations (making the membrane potential less negative)
Hyperpolarizations (making the membrane potential more negative)
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FIGURE 4.3 An EPSP, and IPSP, and an EPSP followed by a typical AP.
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EPSPs and IPSPs
Travel passively from their site of origination
Decremental (graded) – they get smaller as they travel
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13. Integration of PSPs and Generation of Action Potentials (APs)
One EPSP typically will not suffice to cause a neuron to “fire” and release neurotransmitter – summation is needed
In order to generate an AP (or “fire”), the threshold of activation must be reached near the axon hillock
Integration of IPSPs and EPSPs must result in a potential of about -65mV in order to generate an AP
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Integration
Adding or combining a number of individual signals into one overall signal
Spatial summation – integration of events happening at different places
Temporal summation – integration of events happening at different times
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FIGURE 4.4 The three possible combinations of spatial summation.
FIGURE 4.5 The two possible combinations of temporal summation.
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Conduction of APs
All-or-none – when threshold is reached the neuron “fires” and the action potential either occurs or it does not
When threshold is reached, voltage-activated ion channels are opened
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FIGURE 4.6 The opening and closing of voltage-activated sodium and potassium channels during the three phases of the action potential: rising phase, repolarization, and hyperpolarization.
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Refractory Periods
Absolute – impossible to initiate another action potential
Relative – harder to initiate another action potential
Prevent the backwards movement of APs and limit the rate of firing
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19. PSPs vs. Action Potentials (APs)
EPSPs/IPSPs
Decremental
Fast
Passive (energy is not used)
Action Potentials
Nondecremental
Conducted more slowly than PSPs
Passive and active (use ATP)
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20. Axonal Conduction of APs
Passive conduction (instant and decremental) along each myelin segment to next node of Ranvier
New action potential generated at each node
In myelinated axons: instant conduction along myelin segments results in faster conduction than in unmyelinated axons
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21. Velocity of Axonal Conduction
Maximum velocity of conduction in human motor neurons is about 60 meters per second
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22. Conduction in Neurons without Axons
Conduction in interneurons is typically passive and decremental
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23. The Hodgkin-Huxley Model in Perspective
This model was based on squid motor neurons
Cerebral neurons behave in ways that are not always predicted by the model
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24. Snaptic Transmission: Structure of Synapses
Axodendritic are most common; axons synapse onto dendritic spines
Directed synapse: site of release and contact are in close proximity
Nondirected synapse: site of release and contact are separated by some distance
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FIGURE 4.8 The anatomy of the typical synapse.
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26. Synthesis, Packaging, and Transport of Neurotransmitter Molecules
Small
Synthesized in the terminal button and packaged in synaptic vesicles
Large
Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal
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27. Release of Neurotransmitter (NT) Molecules
Exocytosis – the process of NT release
The arrival of an AP at the terminal opens voltage-activated Ca2+ channels
The entry of Ca2+ causes vesicles to fuse with the terminal membrane and release their contents
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FIGURE 4.11 Schematic and photographic illustrations of exocytosis.
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29. Activation of Receptors by NT Molecules
Released NT molecules produce signals in postsynaptic neurons by binding to receptors
Receptors are specific for a given NT
Ligand – a molecule that binds to another
A NT is a ligand of its receptor
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Receptors
There are multiple receptor types for a given NT
Ionotropic receptors – associated with ligand-activated ion channels
Metabotropic receptors – associated with signal proteins and G proteins
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Ionotropic Receptors
NT binds and an associated ion channel opens or closes, causing a PSP
If Na+ channels are opened, for example, an EPSP occurs
If K+ channels are opened, for example, an IPSP occurs
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32. Metabotropic Receptors
Effects are slower, longer-lasting, more diffuse, and more varied
(1) NT 1st messenger binds. (2) G protein subunit breaks away. (3) Ion channel opened/closed OR a 2nd messenger is synthesized. (3) 2nd messengers may have a wide variety of effects.
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FIGURE 4.12 Ionotropic and metabotropic receptors.
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34. Reuptake, Enzymatic Degradation, and Recycling
As long as NT is in the synapse, it is “active” – activity must somehow be turned off
Reuptake – scoop up and recycle NT
Enzymatic degradation – a NT is broken down by enzymes
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FIGURE 4.13 The two mechanisms for terminating neurotransmitter action in the synapse: reuptake and enzymatic degradation.
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36. Glial Function and Synaptic Transmission
Astrocytes appear to communicate and to modulate neuronal activity
Some communication is through gap junctions between cells
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FIGURE 4.14 Gap junctions.
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38. Amino Acid Neurotransmitters
Usually found at fast-acting directed synapses in the CNS
Glutamate – Most prevalent excitatory neurotransmitter in the CNS
GABA
Synthesized from glutamate
Most prevalent inhibitory NT in the CNS
Aspartate and glycine
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Monoamines
Effects tend to be diffused
Catecholamines – synthesized from tyrosine
Dopamine
Norepinephrine
Epinephrine
Indolamines – synthesized from tryptophan
Serotonin
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Acetylcholine
Acetylcholine (Ach)
Acetyl group + choline
First identified at neuromuscular junction
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41. Unconventional Neurotransmitters
Soluble gases – exist only briefly
Nitric oxide and carbon monoxide
Retrograde transmission – backwards communication
Endacannabinoids
anandamide is one of the two known endocannabinoids
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Neuropeptides
Large molecules
Example – endorphins
“Endogenous opioids”
Produce analgesia (pain suppression)
Receptors were identified before the natural ligand was
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FIGURE 4.18 Seven steps in neurotransmitter action.
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44. Pharmacology of Synaptic Transmission
How drugs influence synaptic activity
Agonists – increase or facilitate activity
Antagonists – decrease or inhibit activity
A drug may act to alter neurotransmitter activity at any point in its “life cycle”
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45. Behavioral Pharmacology: Three Influential Lines of Research
Drugs selective to specific receptor subtypes may exert different effects
e.g. nicotinic vs. muscarinic acetylcholine receptors
Discovery of the endogenous opioids provided insight into brain mechanisms of pleasure and pain
Effects of dopamine agonists and antagonists on psychotic symptoms led to new treatments for schizophrenia
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FIGURE 4.19 Some mechanisms of agonistic and antagonistic drug effects.
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