E6 Synaptic integration (spatial and temporal summation)

Topic

The neuron continuously integrates the excitatory and inhibitory synaptic inputs it receives—whether from chemical synapses or gap junctions—by summing them in two distinct ways. Temporally, the neuron performs temporal summation, which occurs when successive synaptic inputs overlap before the first one has finished decaying. Spatially, the neuron performs spatial summation, which occurs when simultaneous synaptic inputs at different points along the dendrite converge at the neuron's soma.

As the synaptic signal travels along the dendrite toward the soma, it undergoes passive attenuation, governed by the same length constant defined for passive propagation along the axon: the larger the dendritic length constant, the less the synaptic signal attenuates while traveling a given distance toward the soma.

When the net sum of the current received by the neuron—resulting from either temporal or spatial summation—exceeds a threshold, the neuron fires an action potential. This behavior is modeled in a simplified manner using an "integrate-and-fire" scheme, in which the neuron's voltage resets immediately after each firing; it returns to the initial voltage level from which it began integrating its synaptic inputs, prior to commencing the integration of subsequent inputs.

Inhibition received by the neuron can act in two distinct ways: subtractively, by directly hyperpolarizing the neuron and subtracting current from the net sum; or divisively, by reducing the neuron's gain without significantly altering its membrane potential—that is, causing the neuron to respond proportionally less to its excitatory inputs without necessarily hyperpolarizing it. This capacity of the neuron to undergo subtractive or divisive inhibition is a key mechanism for controlling the excitability of the cortical network to which it belongs. Ultimately, it is this very neuron—by integrating excitatory and inhibitory synaptic inputs across both time and space, attenuating them along the dendrite, and comparing their net sum against a firing threshold—that determines when a new action potential is fired and how it contributes, through its two modes of inhibition, to the excitability of the entire cortical network.

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