Neural facilitation
Neural facilitation, also known as paired-pulse facilitation (PPF), is a phenomenon in neuroscience in which postsynaptic potentials (PSPs) (EPPs, EPSPs or IPSPs) 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 Where k is a constant. Facilitation = EPSC2 / EPSC1 = (1 + [Ca2+]residual / [Ca2+]influx)4 - 1 Early experiments by Del Castillo & Katz in 1954 and Dudel & Kuffler in 1968 showed that facilitation was possible at the neuromuscular junction even if transmitter release does not occur, indicating that facilitation is an exclusively presynaptic phenomenon.