Synaptic Integration

Abstract Neurons in the brain receive thousands of synaptic inputs from other neurons. Synaptic integration is the term used to describe how neurons ‘add up’ these inputs before the generation of a nerve impulse, or action potential. The ability of synaptic inputs to effect neuronal output is determined by a number of factors, including the size, shape and relative timing of electrical potentials generated by synaptic inputs, the geometric structure of the target neuron, the physical location of synaptic inputs within that structure, as well as the expression of voltage‐gated channels in different regions of the neuronal membrane. The process of synaptic integration is therefore modulated at multiple levels, contributing to the diverse and complex computational powers of the functioning brain. Key Concepts: Neurons within a neural network receive information from, and send information to, many other cells, at specialised junctions called synapses. Synaptic integration is the computational process by which an individual neuron processes its synaptic inputs and converts them into an output signal. Neurons are specialised for electrical signalling, with the main neuronal input signal (synaptic potentials) and the main neuronal output signal (action potentials) both involving transient changes in the size of the electrical potential across the neuronal membrane. Synaptic potentials occur when neurotransmitter binds to and opens ligand‐operated channels in the dendritic membrane, allowing ions to move into or out of the cell according to their electrochemical gradient. Synaptic potentials can be either excitatory (increasing the probability of action potential firing) or inhibitory (reducing the probability of action potential firing) depending on the direction and charge of ion movement. Action potentials occur if the summed synaptic inputs to a neuron reach a threshold level of depolarisation and trigger regenerative opening of voltage‐gated ion channels. Synaptic potentials are often brief and of small amplitude, therefore summation of inputs in time (temporal summation) or from multiple synaptic inputs (spatial summation) is usually required to reach action potential firing threshold. Nonlinear summation of synaptic potentials occurs when a synaptic potential changes the driving force for ion movement and therefore the amplitude of subsequent synaptic potentials. The impact of a synaptic input on neuronal output depends on its location within the dendritic tree, because synaptic potentials are attenuated as they spread passively through neuronal processes. Dendritic voltage‐gated ion channels may open or close in response to the membrane potential change during a synaptic potential, thereby altering (amplifying or attenuating) the potential's amplitude or time course.