1. 11
Fundamentals of the Nervous System and Nervous Tissue: Part C

2. The Synapse
How does the nervous system flow information from neuron to neuron/affector/effector organ?
Action potentials
Synapses
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3. Synapse Classification
What is the difference?
Axodendritic
Axosomatic
Other types (less common):
Axoaxonal
Dendrodendritic
Somatodendritic
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4. Synapse Classification
What is the difference?
Axodendritic axon terminals to dendrites
Axosomatic axon terminals to soma
Other types (less common):
Axoaxonal
Dendrodendritic
Somatodendritic
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5. Synapse Classification
What is the difference?
Axodendritic axon terminals to dendrites
Axosomatic axon terminals to soma
Other types (less common):
Axoaxonal axon to axon
Dendrodendritic dendrite to dendrite
Somatodendritic dendrite to soma
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6. Important Terminology
What is the presynaptic neuron?
Neuron conducting impulses toward synapse
Sends the information
What is the postsynaptic neuron?
May be a (PNS) neuron, muscle, or gland
Neuron transmitting electrical signal away from synapse
Receives the information
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7. Figure 11.16 Synapses. Axodendritic synapses Dendrites Axosomatic
Cell body
Axoaxonal
synapses
Axon
Axon
Axosomatic
synapses
Cell body (soma)
of postsynaptic
neuron
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8. Varieties of Synapses Electrical Chemical Joined by gap junctions
Communication: RAPID
May be unidirectional or bidirectional
Synchronize activity
Found in embryonic nervous tissue
Chemical
Most common
Uses neurotransmitters and synaptic vesicles
Fluid filled space
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9. Prevents nerve impulses from directly passing from one neuron to next
Synaptic Cleft
Super small
30 – 50 nm wide
Prevents nerve impulses from directly passing from one neuron to next
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10. Transmission across synaptic cleft
Chemical event (as opposed to an electrical one)
Depends on release, diffusion, and receptor binding of neurotransmitters
Ensures unidirectional communication between neurons
PLAY
Animation: Neurotransmitters
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11. Information Transfer Across Chemical Synapses
AP arrives at axon terminal of presynaptic neuron
Causes voltage-gated Ca2+ channels to open
Ca2+ floods into cell
Where have we seen this before?
Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane
Exocytosis of neurotransmitter into synaptic cleft occurs
Higher impulse frequency  more released
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12. Termination of Neurotransmitter Effects
Within a few milliseconds neurotransmitter effect terminated in one of three ways
Reuptake
By astrocytes or axon terminal
Degradation
By enzymes
Diffusion
Away from synaptic cleft
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13. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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14. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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15. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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16. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane. 4
Postsynaptic
neuron
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17. ion channels, resulting in graded potentials.
Figure Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Ion movement
Graded potential 5
Binding of neurotransmitter opens
ion channels, resulting in graded
potentials.
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18. terminated by reuptake through transport proteins, enzymatic
Figure Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Enzymatic
degradation
Reuptake
Diffusion away
from synapse 6
Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
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19. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane. 4
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
Binding of neurotransmitter opens
ion channels, resulting in graded
potentials. 5
Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse. 6
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20. The synapse needs time. Why?
Synaptic Delay
The synapse needs time. Why?
Neurotransmitters need to
Be released
Diffuse across synapse
Bind to receptors
How long does this take? 0.3–5.0 ms
Synaptic delay is rate-limiting step of neural transmission
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21. Table 11.2 Comparison of Graded Potentials and Action Potentials (1 of 4)
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22. Table 11.2 Comparison of Graded Potentials and Action Potentials (2 of 4)
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23. Table 11.2 Comparison of Graded Potentials and Action Potentials (3 of 4)
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24. Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4)
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25. Postsynaptic Potentials
What is the difference between these types of postsynaptic potentials?
EPSP
Excitatory PostSynaptic Potentials
IPSP
Inhibitory PostSynaptic Potentials
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26. Excitatory Synapses and EPSPs
NT-binding opens chemically gated channels
Allows flow of Na+ and K+ in opposite directions
EPSP: Na+ influx is greater than K+ efflux
EPSP help trigger AP if EPSP is of threshold strength
Can spread to axon hillock to cause AP
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27. Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
+30
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
Na+ and K+ to pass
through simultaneously.
Membrane potential (mV)
Threshold
–55
–70
Stimulus 10 20 30
Time (ms)
Excitatory postsynaptic potential (EPSP)
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28. Inhibitory Synapses and IPSPs
Makes membrane more permeable to K+ or Cl–
If K+ channels open, it moves out of cell.
If Cl- channels open, it moves into cell.
Why?
Therefore NT hyperpolarizes cell
Inner surface of membrane becomes more neg.
AP less likely to be generated
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29. Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
+30
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
that drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
Membrane potential (mV)
Threshold
–55
–70
Stimulus 10 20 30
Time (ms)
Inhibitory postsynaptic potential (IPSP)
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30. Synaptic Integration: Summation
A single EPSP cannot induce an AP
Both can summate to influence postsynaptic neuron
Most neurons receive both inputs from thousands of other neurons
If EPSP's reach threshold then AP will occur
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31. Two Types of Summation Temporal summation Spatial summation
One or more presynaptic neurons transmit impulses in rapid-fire order
Spatial summation
Postsynaptic neuron stimulated simultaneously by large number of terminals at same time
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32. Figure 11.19a Neural integration of EPSPs and IPSPs.
Threshold of axon of
postsynaptic neuron
Membrane potential (mV)
Resting potential
–55
–70 E1 E1
Time
No summation:
2 stimuli separated in time
cause EPSPs that do not
add together.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
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33. Figure 11.19b Neural integration of EPSPs and IPSPs.
Threshold of
axon of
postsynaptic
neuron
Resting
potential
Membrane potential (mV)
–55
–70 E1 E1
Time
Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
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34. Figure 11.19c Neural integration of EPSPs and IPSPs.
Threshold
of axon of
postsynaptic
neuron
Resting
potential
Membrane potential (mV)
–55
–70
E1 + E2
Time
Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
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35. Figure 11.19d Neural integration of EPSPs and IPSPs.
Threshold of axon of
postsynaptic neuron
Membrane potential (mV)
Resting potential
–55
–70 l1
E1 + l1
Time
Spatial summation of
EPSPs and IPSPs:
Changes in membane potential
can cancel each other out.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
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36. Classification of Neurotransmitters: Chemical Structure
Acetylcholine (ACh)
First identified; best understood
Released at neuromuscular junctions ,by some ANS neurons, by some CNS neurons
Synthesized from acetic acid and choline by enzyme choline acetyltransferase
Degraded by enzyme acetylcholinesterase (AChE)
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37. Classification of Neurotransmitters: Chemical Structure
Biogenic amines
Catecholamines
Dopamine, norepinephrine (NE), and epinephrine
Synthesized from amino acid tyrosine
Indolamines
Serotonin and histamine
Serotonin synthesized from amino acid tryptophan; histamine synthesized from amino acid histidine
Broadly distributed in brain
Play roles in emotional behaviors and biological clock
Some ANS motor neurons (especially NE)
Imbalances associated with mental illness
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38. Classification of Neurotransmitters: Chemical Structure
Amino acids
Glutamate
Aspartate
Glycine
GABA—gamma ()-aminobutyric acid
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39. Classification of Neurotransmitters: Chemical Structure
Peptides (neuropeptides)
Substance P
Mediator of pain signals
Endorphins
Beta endorphin, dynorphin and enkephalins
Act as natural opiates; reduce pain perception
Gut-brain peptides
Somatostatin and cholecystokinin
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40. Classification of Neurotransmitters: Chemical Structure
Purines
ATP!
Adenosine
Potent inhibitor in brain
Caffeine blocks adenosine receptors
Act in both CNS and PNS
Produce fast or slow responses
Induce Ca2+ influx in astrocytes
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41. Classification of Neurotransmitters: Chemical Structure
Gases and lipids - gasotransmitters
Nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide gases (H2S)
Bind with G protein–coupled receptors in the brain
Lipid soluble
Synthesized on demand
NO involved in learning and formation of new memories; brain damage in stroke patients, smooth muscle relaxation in intestine
H2S acts directly on ion channels to alter function
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42. Classification of Neurotransmitters: Chemical Structure
Endocannabinoids
Act at same receptors as THC (active ingredient in marijuana)
Most common G protein-linked receptors in brain
Lipid soluble
Synthesized on demand
Believed involved in learning and memory
May be involved in neuronal development, controlling appetite, and suppressing nausea
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43. Figure 11.22 Simple neuronal pool.
Presynaptic
(input) fiber
Facilitated zone
Discharge zone
Facilitated zone
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44. Types of Circuits Circuits Four types of circuits
Patterns of synaptic connections in neuronal pools
Four types of circuits
Diverging
Converging
Reverberating
Parallel after-discharge
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45. Figure 11.23a Types of circuits in neuronal pools.
Input
Diverging circuit
• One input, many outputs
• An amplifying circuit
• Example: A single neuron in the
brain can activate 100 or more motor
neurons in the spinal cord and
thousands of skeletal muscle fibers
Many outputs
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46. Figure 11.23b Types of circuits in neuronal pools.
Input 1
Converging circuit
• Many inputs, one output
• A concentrating circuit
• Example: Different sensory stimuli
can all elicit the same memory
Input 2
Input 3
Output
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47. Figure 11.23c Types of circuits in neuronal pools.
Input
Reverberating circuit
• Signal travels through a chain of
neurons, each feeding back to
previous neurons
• An oscillating circuit
• Controls rhythmic activity
• Example: Involved in
breathing, sleep-wake cycle,
and repetitive motor activities
such as walking
Output
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48. Figure 11.23d Types of circuits in neuronal pools.
Input
Parallel after-discharge
circuit
• Signal stimulates neurons
arranged in parallel arrays that
eventually converge on a
single output cell
• Impulses reach output cell at
different times, causing a burst
of impulses called an after-discharge
• Example: May be involved in
exacting mental processes
such as mathematical calculations
Output
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49. Patterns of Neural Processing: Serial Processing
Input travels along one pathway to a specific destination
System works in all-or-none manner to produce specific, anticipated response
Example – spinal reflexes
Rapid, automatic responses to stimuli
Particular stimulus always causes same response
Occur over pathways called reflex arcs
Five components: receptor, sensory neuron, CNS integration center, motor neuron, effector
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50. Figure 11.24 A simple reflex arc.
Stimulus
Interneuron 1
Receptor 2
Sensory neuron 3
Integration center 4
Motor neuron 5
Effector
Spinal cord (CNS)
Response
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51. Patterns of Neural Processing: Parallel Processing
Different parts of circuitry deal simultaneously with the information
One stimulus promotes numerous responses
Important for higher-level mental functioning
Example: a sensed smell may remind one of an odor and any associated experiences
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52. Developmental Aspects of Neurons
Nervous system originates from the neural tube and the ectoderm
The neural tube becomes CNS
Neuroepithelial cells of neural tube
Proliferate to form cells needed for development
Neuroblasts
Become neurons who are amitotic and migrate
Sprout axons to connect with targets
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53. Figure 11.25 A neuronal growth cone.
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54. About 2/3 of neurons die before birth
Cell Death
About 2/3 of neurons die before birth
If do not form synapse with target
Many cells also die due to apoptosis (programmed cell death) during development
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