Synaptic Inhibition & Equilibrium potential for inhibition

Synaptic Inhibition

The signaling mechanism in the neuromuscular junction is simple
1. a muscle fiber innervated by one motor neuron.
2. muscle fiber receives only excitatory input
3. on neurotransmitter ACh binds to nicotinic ACh receptor
4. highly effective: each synaptic potential--> trigger AP

1. excitatory and inhibitory connections
2.from hundreds of other neurons with different chemical transmitter
3. different kinds of synaptic receptors-->control distinct ion 채널

EPSP(Excitatory postsynaptic potential)
Excitatory postsynaptic current
IPSPIPSP(Inhibitory postsynaptic potenial(small hyperpolarization))
Inhibitory postsynaptic current(outward current)
EPSP와 IPSP가 거의 동시에 발생하면---> summation
---> reduction of depolarization: 흥분전달의 감소 또는 억제
(sculpturing role of inhibition)
Inhibition: reduction or blockage of excitation

Equilibrium potential for inhibition
Fig. 3.5
motor neuron in the spinal cord, votlage clamp.
at resting potential(-74 mV), the IPSP는 hyperpolarizing.
막을 점차 hyperpolarizing---> IPSP가 약 -82 mV에서 reversed
이러한 reversal potential은 K+과 Cl-
concentration gradient에 의해 영향받는다.
(EK: -80 mV, ECl,-70 mV)
따라서 IPSC는 K+과 Cl-의 membrane conductance의 증가에 의해 발생.
[Cl-]o 150 mM >> inside (15 mM): Cl- flows in--->
negative charge inside of the cell
K+ efflux: removes positive charge.
따라서 positive, outward current, net hyperpolarization

Fig. 3.6
How does increased membrane conductance inhibit excitation?
current pulse injection into the cell--->
electrotonic potential 생성
(amplitude proportional to the membrane resistance)
Inhibitory synapse를 activation시키면--->
IPSP에 의한 hyperpolrization (few mV)
IPSP와 electrotonic potential과 같이 일으키면
electrotonic potetial이 감소 또는 억제됨

current-voltage(CV) relation
-70 mV, resting potential, positive current---> depolarize 막
GABA application---> CV curve shift to the direction of hyperpol.
increase slope (decrease in membrane resistance)
e.g.: control: 0.1 uA---> -25 mV,
inhibited: ---> -60 mv (insufficient to trigger AP)
the decrease in resistance short-circuits excitatory currents
---> prevent excitation
Effects of reducing [Cl-]o
control curve not affected, but CV curve during inhibition
shift to the right by 20 mV
during inhibition: at various synapses:
increaseed membrane conductance for Cl-, K+ or both
(ECl, EK near the RP)
---> stabilize the RP---> reduces excitatory depolarization