Synaptic Gating - Permissive Gating

Permissive Gating

The firing of an action potential, and consequently the release of neurotransmitters, occurs by this gating mechanism. In synaptic gating, in order for an action potential to occur, there must be more than one input to produce a single output in the neuron being gated. The interaction between these sets of neurons creates a biological AND gate. The neuron being gated is bistable and must be brought to the up state before it can fire an action potential. When this bistable neuron is in the up state, the gate is open. A gatekeeper neuron is responsible for stimulating the bistable neuron by shifting it from a down state to an up state and thus, opening the gate. Once the gate is open, an excitatory neuron can cause the bistable neuron to further depolarize and reach threshold causing and action potential to occur. If the gatekeeper does not shift the bistable neuron from down to up, the excitatory neuron will not be able to fire an action potential in the bistable neuron. Both the gatekeeper neuron and excitatory neuron are necessary to fire an action potential in the bistable neuron, but neither is sufficient to do so alone.

An example of this type of gating may occur in the nucleus accumbens. Studies have shown hippocampal neurons may gate the transmission of signals between the prefrontal cortex and the nucleus accumbens. Stimulation of the prefrontal cortical neurons has a very small chance of eliciting an action potential in the nucleus accumbens while those neurons are in the down state. Likewise, stimulation of hippocampal neurons fails to produce action potentials in the nucleus accumbens; however, these hippocampal neurons have been shown to switch their targets in the nucleus accumbens to the up state. As long as these neurons remain in the up state, stimulation of prefrontal cortical neurons has a much higher likelihood of producing action potentials in the nucleus accumbens. Thus, the hippocampus serves as the gatekeeper for information flow from the prefrontal cortex to the nucleus accumbens, such that its action permissively gates these synapses.

Synaptic gating involves a variety of mechanisms by which the efficacy of neuronal activity is modulated. Additional studies demonstrate the permissive properties of synaptic gating. In certain instances, membrane depolarization will cause an opening of the gates that previously had an inhibitory effect on the neuron they were gating. This permissive gating is more than a matter of simple summation, however. Summation is the convergence of many EPSPs at the axon hillock (either from a single neuron firing at a high frequency or from many neurons firing at once) that depolarizes the membrane potential to the point of threshold. The membrane depolarization caused by the opening of synaptic gates causes an additional increase in intracellular calcium that facilitates the release of neurotransmitters; thus, it is able to selectively distribute information from the presynaptic cell.