Download Synapses and Integration

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
Dendrites
Synaptic inputs
(presynaptic axon terminals)
See Figure
4-14
Cell body of
postsynaptic neuron
Cell body of
Axon terminals
postsynaptic neuron
Axon Myelinated
hillock axon
Fig. 4-13, p. 85
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
Axon of
presynaptic
neuron
1
Voltage-gated
Ca2+ channel
Synaptic knob
(presynaptic
axon terminal)
Synaptic
vesicle
Ca2+
2
Neurotransmitter
molecule
Synaptic
cleft
Subsynaptic
membrane
3
Chemically gated
receptor-channel
for Na+, K+, or Cl–
3
4
5
4
Receptor for
neurotransmitter
5
Postsynaptic neuron
Fig. 4-14, p. 86
Neurotransmitters
• Vary from synapse to synapse
• Same neurotransmitter is always released at a particular synapse
• Quickly removed from the synaptic cleft
Membrane potential (mV)
in postsynaptic neuron
+30
0
Activation of synapse
Threshold
potential
–50
EPSP
–70
5
15
35
25
Time (msec)
45
(a) Excitatory synapse
Membrane potential (mV)
in postsynaptic neuron
Fig. 4-15a, p. 87
+30
0
Activation of synapse
Threshold
potential
–50
–70
IPSP
5
15
(b) Inhibitory synapse
35
25
Time (msec)
45
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
Postsynaptic membrane potential (mV)
Summation: more than one pre-synaptic action potential
influences the polarity of the post-synaptic neuron
(b) Temporal
(a) No summation (c) Spatial (d) EPSP-IPSP
summation
summation cancellation
+30
0
–50
–70
Time (msec)
b
c
c?
Fig. 4-16, p. 89
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
A
Recorded
at
“A”
Any one of the  could be affecting what is recorded at “A”
Basic Synaptic Integration
• Convergence and Divergence
Fig. 4-17, p. 91
Advanced Neural Integration
• Neuromodulators
• Drugs
• Axon-axon transmission
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 actions
1. 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
Synaptic Drug Interactions
• 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
Presynaptic neuron involved in normal axon to dendrite synaptic
transmission (a)
a
x
c
b. Presynaptic neuron involved in axon-axon
transmission Axon-axon transmission: neurotransmitter released from one
presynaptic (b) binds with axon of another presynaptic
neuron.
Binding affects activity of recipient axon (a) next time it
undergoes an action potential, normal post-synaptic neuron
(c) is there by influenced
Axon-axon transmission
• Presynaptic inhibition: normal axon is
inhibited from releasing neurotransmitter
• Presynaptic facilitation: normal axon
releases more neurotransmitter
End of Basic Neuron Physiology
• On to chemical messengers in general
and then hormones in detail