Synapses and Integration
• Junction between two presynaptic and postsynaptic neurons
• There are two types of synapse:
1. Electrical Synapses: Two neurons connected by gap junctions
2. Chemical Synapses: Chemical messenger is transmitted across the junction separating the two neurons
Fig. 4-13, p. 85
Myelinatedaxon
Cell body ofpostsynaptic neuron
Synaptic inputs(presynaptic axon terminals)
Axon terminalsCell body ofpostsynaptic neuron
Axonhillock
Dendrites
See Figure4-14
Synapses and Neuronal Integration
• Signal at synapse either excites or inhibits the postsynaptic neuron
• Two types of synapses
– Excitatory synapses
– Inhibitory synapses
Fig. 4-14, p. 86
Fig. 4-14, p. 86
Synaptic knob(presynapticaxon terminal)
Chemically gatedreceptor-channelfor Na+, K+, or Cl–
Neuro-transmittermolecule
Voltage-gatedCa2+ channel
Ca2+
Postsynaptic neuron
Subsynapticmembrane
Synapticcleft
Synapticvesicle
Axon ofpresynapticneuron
Receptor forneurotransmitter
1
2
3
4
5
3
4
5
Neurotransmitters
• Vary from synapse to synapse
• Same neurotransmitter is always released at a particular synapse
• Quickly removed from the synaptic cleft
Fig. 4-15a, p. 87
Thresholdpotential
35
(a) Excitatory synapse
Mem
bra
ne
po
ten
tial
(m
V)
in p
ost
syn
apti
c n
euro
n
Activation of synapse
EPSP
+30
5 15 45
Time (msec)
25
–70
–50
0
(b) Inhibitory synapse
Time (msec)
Mem
bra
ne
po
ten
tial
(m
V)
in p
ost
syn
apti
c n
euro
n
IPSP
Activation of synapseThresholdpotential
355 15 4525
+30
–70
–50
0
Fig. 4-15b, p. 87
Basic Synaptic Integration
• Post synaptic membrane membrane potential is affected by neurotransmitters from several presynaptic neurons simultaneously– Effects of many neurotransmitters are added together
• Adding = summation– Temporal summation: one single presynaptics increases
frequency of activity to affect post-synaptic membrane
– Spatial summation: more than one presynaptic active simultaneously
Summation: more than one pre-synaptic action potential influences the polarity of the post-synaptic neuron
Po
stsy
nap
tic
mem
bra
ne
po
ten
tial
(m
V)
(d) EPSP-IPSPcancellation
Time (msec)
–70
(a) No(a) Nosummationsummation
(b) Temporalsummation (c) Spatial
summation+30
0
–50
Fig. 4-16, p. 89
c?cb
A
Recordedat“A”
Cannot look at a membrane figure out which type of summation is occurring; can only say “summation to threshold” occurred so an action
potential was generated at point A
Any one of the could be affecting what is recorded at “A”
Basic Synaptic Integration
• Convergence and Divergence
Neuropeptides as Neuromodulators
• Neuropeptides are considered neuromodulators– don’t cause the formation of EPSP or IPSP, – bring about long term changes that subtly modulate, depress or
enhance the action of neurotransmitter at the synapse
• Large molecules consisting of from 2 to 40 amino acids– Synthesized in neuronal cell body in the endoplasmic reticulum
and Golgi complex– Packaged in large, dense-core vesicles present in axon terminal
Neuropeptides as Neuromodulators
– Release mechanism uncertain• Act as autocrine or paracrine agents
– Bind to presynaptic cell to affect amount of neurotransmitter released when an action potential occurs in that presynaptic
– Bind to post synaptic cell to affect number of receptors for the neurotransmitter released
Synaptic Drug Interactions
• Possible drug actions1. Altering the synthesis, axonal transport,
storage, or release of a neurotransmitter
2. Modifying neurotransmitter interaction with the postsynaptic receptor
3. Influencing neurotransmitter reuptake or destruction
4. Replacing a deficient neurotransmitter with a substitute transmitter
• Drug Examples– Cocaine: Blocks reuptake of neurotransmitter
dopamine at presynaptic terminals, so postsynaptic neuron is active longer
– Tetanus toxin: Prevents release of inhibitory neurotransmitter GABA, affecting skeletal muscles activation
– Strychnine: Competes with inhibitory neurotransmitter glycine at postsynaptic receptor site
Synaptic Drug Interactions
Presynaptic neuron involved in normal axon to dendrite synaptic transmission (a)
b. Presynaptic neuron involved in axon-axon transmission
Axon-axon transmission: neurotransmitter released from one presynaptic (b) binds with axon of another presynapticneuron.
Binding affects activity of recipient axon (a) next time it undergoes an action potential, normal post-synaptic neuron (c) is there by influenced
x
a
c
Axon-axon transmission• Presynaptic inhibition: normal axon is
inhibited from releasing neurotransmitter
• Presynaptic facilitation: normal axon releases more neurotransmitter