What is the general structure of synapses in the CNS and PNS

1
What is the general structure of synapses in
the CNS and PNS?
2
Synaptic Activity

• Action potentials (nerve impulses)
• are transmitted from presynaptic neuron
• to postsynaptic neuron (or other postsynaptic
  cell)
• across a synapse

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Synaptic Activity
Figure 1216 (Navigator)
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2 Types of Synapses

• Electrical synapses
• direct physical contact between cells
• Cells locked together at gap junctions
• Chemical synapses
• Cells not in direct contact
• signal transmitted across a cleft by chemical
  neurotransmitters

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2 Classes of Neurotransmitters

• Excitatory neurotransmitters
• cause depolarization of postsynaptic membranes
• promote action potentials
• Inhibitory neurotransmitters
• cause hyperpolarization of postsynaptic membranes
• suppress action potentials

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What events occur at a chemical synapse?
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Events at a Cholinergic Synapse

• Step 1 Action potential arrives, depolarizes
  synaptic knob

Figure 1216 (Step 1)
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• Step 2 Calcium ions enter synaptic knob, trigger
  exocytosis of ACh

Figure 1216 (Step 2)
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• Step 3 ACh binds to receptors, depolarizes
  postsynaptic membrane

Figure 1216 (Step 3)
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• Step 4 AChE breaks ACh into acetate and choline

Figure 1216 (Step 4)
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Delay of AP from Cell to Cell

• Synaptic delay
• 0.20.5 msec occurs between
• arrival of action potential at synaptic knob
• and effect on postsynaptic membrane
• Synaptic Fatigue
• Synapse inactive until ACh is replenished

12
What are the major types of neurotransmitters
and neuromodulators, and their effects on
postsynaptic membranes?
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Other Neurotransmitters

• At least 50 neurotransmitters other than ACh,
  including
• some amino acids
• peptides
• prostaglandins
• ATP
• some dissolved gases

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Important Neurotransmitters

• Other than acetylcholine
• norepinephrine (NE)
• dopamine
• serotonin

15

• Many drugs
• affect nervous system by stimulating receptors
  that respond to neurotransmitters
• can have complex effects on perception, motor
  control and emotional states

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How Neurotransmitters Work

• Direct effects on membrane channels
• e.g., ACh
• Indirect effects via G proteins
• e.g., NE, dopamine, seratonin
• Indirect effects via intracellular enzymes
• e.g., lipid soluble gases (NO, CO)

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Direct Effects
Figure 1217a
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Indirect Effects
Figure 1217b
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Indirect Effects
Figure 1217c
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Neuromodulators

• Other chemicals released by synaptic knobs
• Opioids
• Bind to the same receptors as opium or morphine
• Relieve pain
• Endorphins

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How is information processed in neural tissue?
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Information Processing
Figure 1218 (Navigator)
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Information Processing

• At the simplest level (individual neurons)
• many dendrites receive neurotransmitter messages
  simultaneously
• some excitatory, some inhibitory
• net effect on axon hillock determines if action
  potential is produced

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Postsynaptic Potentials

• Graded potentials developed in a postsynaptic
  cell
• in response to neurotransmitters

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2 Types of Postsynaptic Potentials

• Excitatory postsynaptic potential (EPSP)
• graded depolarization of postsynaptic membrane
• Inhibitory postsynaptic potential (IPSP)
• graded hyperpolarization of postsynaptic membrane

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Summation

• To trigger an action potential
• 1 EPSP is not enough
• EPSPs (and IPSPs) combine through summation
• temporal summation
• spatial summation

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Temporal Summation

• Multiple times
• Rapid, repeated stimuli at 1 synapse

Figure 1218a
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Spatial Summation

• Multiple locations
• Many stimuli, arrive at multiple synapses

Figure 1218b
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• In the nervous system
• a change in transmembrane potential that
  determines whether or not action potentials are
  generated is the simplest form of information
  processing

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SUMMARY

• Neural tissue and the neuron
• Anatomical divisions of the nervous system
• Central and peripheral nervous systems
• Nerves and axons

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• Functional divisions of the nervous system
• Somatic and autonomic nervous systems Afferent
  pathway and receptors
• Efferent pathway and effectors

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• Structure of neurons
• organelles of neuron
• neurofilaments, neurotubules, neurofibrils
• structures of axon
• axon hillock, initial segment, axoplasm
• synapse and neurotransmitters

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• Classification of neurons
• structural classifications
• anaxonic, bipolar, unipolar, and multipolar
• functional classifications
• sensory, motor, and interneurons

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• 4 types of neuroglia
• ependymal, astrocytes, and oligodendrocytes,
  microglia
• Ganglia and neurons of PNS
• satellite cells, Schwann cells

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• Transmembrane potential
• electrochemical gradient
• passive and active channels
• Gated channels
• chemically regulated, voltage-regulated,
  mechanically regulated

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• Action potentials
• threshold
• refractory period
• continuous and saltatory propagation
• 3 types of axons (A, B, and C fibers)

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• Transmission of nerve impulses across a synapse
• presynaptic and postsynaptic neurons
• electrical and chemical synapses
• excitatory and inhibitory neurotransmitters
• cholinergic synapses (ACh)
• other neurotransmitters (NE, dopamine, seratonin,
  GABA)

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• Graded potentials
• depolarization and hyperpolarization
• Neuromodulators
• Activity of neurotransmitters
• direct, indirect, and lipid-soluble gases

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• Information processing
• integration of postsynaptic potentials
• EPSPs and IPSPs
• spatial and temporal summation