Neural facilitation

Neural facilitation, also known as paired pulse facilitation (PPF), is a phenomenon in neuroscience in which postsynaptic potentials (PSPs) (EPPs or EPSPs) evoked by an impulse are increased when that impulse closely follows a prior impulse. PPF is thus a form of short-term synaptic plasticity. The mechanisms underlying neural facilitation are exclusively pre-synaptic; broadly speaking, PPF arises due to increased presynaptic Ca²⁺ concentration leading to a greater release of neurotransmitter-containing synaptic vesicles. Neural facilitation may be involved in several neuronal tasks, including simple learning, information processing, and sound-source localization.

Ca²⁺ plays a significant role in transmitting signals at chemical synapses. Voltage-gated Ca²⁺ channels are located within the presynaptic terminal. When an action potential invades the presynaptic membrane, these channels open and Ca²⁺ enters. A higher concentration of Ca²⁺ enables synaptic vesicles to fuse to the presynaptic membrane and release their contents (neurotransmitters) into the synaptic cleft to ultimately contact receptors in the postsynaptic membrane. The amount of neurotransmitter released is correlated with the amount of Ca²⁺ influx. Therefore, short-term facilitation (STF) results from a build up of Ca²⁺ within the presynaptic terminal when action potentials propagate close together in time.

Facilitation of excitatory post-synaptic current (EPSC) can be quantified as a ratio of subsequent EPSC strengths. Each EPSC is triggered by pre-synaptic calcium concentrations and can be approximated by:

EPSC = k([Ca²⁺]_presynaptic)⁴ = k([Ca²⁺]_rest + [Ca²⁺]_influx + [Ca²⁺]_residual)⁴

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