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.

Keywords: synapse; action potential; dendrite; neuron; postsynaptic potential

Figure 1.

Schematic representation of the basic structure and synaptic connectivity between a pre‐ and postsynaptic neuron.

Figure 2.

Charge transfer across the neuronal membrane (bottom panels) and resulting change in neuronal membrane potential (top panels) during generation of excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials.

Figure 3.

Action potential generation during summation of excitatory postsynaptic potentials (EPSPs).

Figure 4.

Nonlinear temporal summation of excitatory and inhibitory postsynaptic potentials, which occurs because the first of two overlapping synaptic potentials alters the electrical gradient for ion movement during the second synaptic potential.

Figure 5.

Attenuation of excitatory postsynaptic potentials (EPSPs) during passive spread from their dendritic site of generation to the soma.

Figure 6.

Nonuniform dendritic distribution of voltage‐gated ion channels (VGCs) modulates propagation of synaptic potentials towards the soma.

Figure 7.

Dendritic spikes, caused by regenerative opening of dendritic voltage‐gated ion channels, can boost the somatic impact of synchronous synaptic inputs.

Figure 8.

Mechanism of dendritic NMDA spikes. At the soma summation of synchronous synaptic potentials on different dendritic branches is usually close to linear (a); however, synchronous activation of multiple synapses on the same dendritic branch can lead generation of a large depolarisation known as an NMDA spike (b). NMDA receptors are jointly ligand‐ and voltage‐gated, meaning that NMDA‐receptor currents can be regenerative under certain conditions. This regenerative event is restricted to the activated region of the dendritic tree (c), but the associated depolarisation may spread to the soma.

Figure 9.

A backpropagating action potential occurs when depolarisation from an axonal action potential spreads back into the dendritic tree.

Figure 10.

The passive spread of subthreshold somatic depolarisation into the axon, known as analogue axonal signalling, can alter the amplitude and time course of the axonal action potential.

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Further Reading

Bear MF, Connors BW and Paradiso MA (2006) Neuroscience: Exploring the Brain, 3rd edn. Philadelphia: Lippincott.

Johnston D and Wu SM‐S (1995) Foundations of Cellular Neurophysiology. Cambridge, MA: MIT Press.

Kandel ER, Schwartz JH and Jessell TM (2000) Principles of Neural Science, 4th edn. New York: Elsevier.

Shepherd GM (1997) The Synaptic Organization of the Brain, 4th edn. Oxford: Oxford University Press.

Zigmond MJ, Bloom FE, Landis SC, Roberts JL and Squire LR (1999) Fundamental Neuroscience. San Diego, CA: Academic Press.

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How to Cite close
Etherington, Sarah J, Atkinson, Susan E, Stuart, Greg J, and Williams, Stephen R(May 2010) Synaptic Integration. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000208.pub2]