3.2 Interactions of Synapses
Motor end plate: extreme type of synapse
the muscle fiber has only one end plate
each impulse in the motor axon generates a suprathreshold
end-plate potential--> twitch of the muscle

in CNS, at most synapses: individual synaptic potentials are far below threshold, often smaller than 1 mV.
(motor neuron: 0.2-0.4 mV, threshold about 10 mV)
postsynaptic cell: many synapses---> summation
presynaptic elements usually come from many different cells-->
converging (inhibitory, excitatory) on the postsynaptic cell
(a motor neuron: as many as 10,000 different presynaptic endings)
: integrative action of the nervous system
: brain's ability to choose between competing alternatives
: decision-making capability

2 different types of synaptic apposition

Synaptic summation
Fig. 3.10


summation between two synapses
Spatial summation
current flows in(EPSC)-->
local potential change (EPSP, electrotonuc potential)
part of the current spread to the axon hillock
two synaptic potentials summate---> enlarged EPSP
대부분의 세포:
cell body & dendrites either cannot generate AP
or threshold is high (30 mV: -65 to -35 mV).
Axon: readily excitable, axon hillock에서 AP 생성 (high density of
voltage-dependent Na+ channels, threshold 10 mV: -65 mV to -55 mV)

Some cortical neurons: one or more addtional trigger zones within
the dendritic tree: voltage-dependent Ca2+ channels,
generation of local AP, electrotonus
long length constant 를 가진 cell에서 frequent

Temporal summation
EPSC는 없어졌으나 EPSP는 아직 남아있다. 여기에 새로운 EPSP가
다달으면 depolarization is added.
실제: spatial & temporal summation occur simultaneously
time constant가 긴 cell에 frequent


Synapses onto a Single Central Neuron Are Grouped
According to Function

axo-axonic, axosomatic, axodendritic
(dendrodendritic, somasomatic: rare)
axosomatic is more effective than axodendritic synapse
1) Synapses on cell body are often Inhibitory
2) synapses on dendritic spines are often excitatory
3) synapses on axon terminals are often modulatory
--->controlling the amount of transmitter released
4)excitatory and inhibitory synapses have distintive ultrastructure
excitatory: synaptic vesicles are round shape
synaptic cleft: 약 30 nm
larger active zone (1-2 um2)
extensive postsynaptic dense region

inhibitory: oval or flattened vesicles
synaptic cleft: 약 20 nm
active zone: smaller
dense projection less obvious
little or no basment membrane



Postsynaptic inhibition
postsynaptic cell에서 inhibitory synapse에 의한 EPSP의 short-circuit
IPSP---> hyperpolarizing the membrane: preventing depolrarization
to the action-potential threshold
IPSP, IPSC: temporal & spatial summation with EPSP
---> ultimately determine axonal AP frequency
inhibitory synapse는 soma의 axon hillock 부분에 많다.
---> control how many EPSPs (originating mainly on the dendrites)
can have a depolarizing influence on the axon

Presynaptic inhibition

Fig. 3.11
axon-axonal synapse
Ia fiber's excitatory input to motor neuron
interneurons makes axo-axonal inhibitory synapse (GABAergic)
with the endings of the Ia fibers.
if interneurons are excited few msec before the Ia fibers,
EPSP inhibited (duration some hundred msecs)
ventral root fiber에서 monosynaptic reflex에 의한 compound
action potential (CAP) 측정---> inhibition의 time course
undesirable information can be suppressed before it reaches the
site of integration, the cell body of the neuron
inhibitory synapse: GABA transmitter--->
large primary afferent depolarization (PAD)-->
inactivates the excitatory Na+ channels in the Ia-fiber terminals.
(inhibition can be blocked by bicucullin, GABA antagonist)

Fig. 3. 12
one excitatory synapse + one inhibitory synaspe
EPSP+ EPSC produced by the excitatory synapse
if 2 ms before inhibitory nerve fiber is stimulated--->
both EPSP & EPSC are reduced
Extracellular recording from fiber
---> triphasic excitatory nerve terminal potential(ENTP)
followed by a much larger EPSC
Inhibitory synapse가 activation 되면
---> INTP present, reduction of ENTP & abolition of EPSC
GABA의 release에 의한 Cl- conductance의 증가

Heterosynaptic facilitation
learning:
presynaptic facilitation: short-term learning
medium-term learning: coactivation of 2 synapses
(one modulates to facilitates the effectiveness of
the other for a relatively long time)
Postsynaptic facilitation in sympathetic ganglia
1) synaptic potential
2) slow EPSPs mediated by ACh (5-100 ms long)
3) dopaminergic input (increase the amplitude of sEPSP few hours)

Presynaptic facilitation in mollusks and insects
activation of serotonin-releasing nerve fiber--->
block of a K+ channels in the presynaptic terminal membrane
---> repolarization delayed---> remain depolarized longer-->
more transmitter released.

Posttetanic potentiation:
In many cells activity can affect synaptic effectiveness. In particular, a train of high-frequency action potentials will be followed by a period during which each subsequent spike will produce a larger EPSP than normal. This enhancement usually lasts several minutesbut it can persit for 1 hr or more in some cells.










BRAIN FACTS