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Synaptic gating
Synaptic gating is the ability of neural circuits to gate inputs by either suppressing or facilitating specific synaptic activity. Selective inhibition of certain synapses has been studied thoroughly (see Gate theory of pain), and recent studies have supported the existence of permissively gated synaptic transmission. In general, synaptic gating involves a mechanism of central control over neuronal output. It includes a sort of gatekeeper neuron, which has the ability to influence transmission of information to selected targets independently of the parts of the synapse upon which it exerts its action (see also neuromodulation).
Bistable neurons have the ability to oscillate between a hyperpolarized (down state) and a depolarized (up state) resting membrane potential without firing an action potential. These neurons can thus be referred to as up/down neurons. According to one model, this ability is linked to the presence of NMDA and AMPA glutamate receptors. External stimulation of the NMDA receptors is responsible for moving the neuron from the down state to the up state, while the stimulation of AMPA receptors allows the neuron to reach and surpass the threshold potential. Neurons that have this bistable ability have the potential to be gated because outside gatekeeper neurons can modulate the membrane potential of the gated neuron by selectively shifting them from the up state to the down state. Such mechanisms have been observed in the nucleus accumbens, with gatekeepers originating in the cortex, thalamus and basal ganglia.〔
==Gated synapse model==
The model for gated synapses was originally derived from the model electronic circuit, in which the gatekeeper serves as a transistor in a circuit. In a circuit, a transistor can act as a switch that turns an electrical signal on or off. In addition, a transistor can serve to amplify an existing current in a circuit. In effect, the gatekeeper neuron acts as the transistor of a gated synapse by modulating the transmission of the signal between the pre-synaptic and post-synaptic neurons.
In a model gated synapse, the gate is either open or closed by default. The gatekeeper neuron, therefore, serves as an external switch to the gate at the synapse of two other neurons. One of these neurons provides the input signal and the other provides the output signal. It is the role of the gatekeeper neuron to regulate the transmission of the input to the output. When activated, the gatekeeper neuron alters the polarity of the presynaptic axon to either open or close the gate. If this neuron depolarizes the presynaptic axon, it allows the signal to be transmitted. Thus, the gate is open. Hyperpolarization of the presynaptic axon closes the gate. Just like in a transistor, the gatekeeper neuron turns the system on or off; it affects the output signal of the postsynaptic neuron. Whether it is turned on or off is dependent on the nature of the input signal (either excitatory or inhibitory) from the presynaptic neuron.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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