Summation (neurophysiology)

Summation is a method of achieving or inhibiting action potential in the postsynaptic neuron, when one presynaptic neuron does not generate enough neurotransmitter.

Temporal summation
Temporal summation is an effect generated by a single neuron as a way of achieving action potential. Summation occurs when the time constant is sufficiently long and the frequency of rises in potential are high enough that a rise in potential begins before a previous one ends. The amplitude of the previous potential at the point where the second begins will algebraically summate, generating a potential that is overall larger than the individual potentials. This allows the potential to reach the threshold to generate an action potential.

Temporal summation is involved in vision. The inverse proportion of intensity and time (Bunsen-Roscoe law), applies as long as the stimulus is no greater than 0.1 second. For example, at 0.1 second, 130 quanta are absorbed, in any matter of provision, but when raised to one second there is a lesser rate of summation, needing 220 quanta to compensate for the decrease in intensity. As stated above, the frequency of vision is function of frequency of flashes, so the longer the stimulus, the better chance it can attain the number of quanta needed for vision.

Spatial summation
Spatial summation is a way of achieving action potential in a neuron which involves input from multiple cells. Spatial summation is the algebraic summation of potentials from different areas of input, usually on the dendrites. Summation of excitatory postsynaptic potentials allows the potential to reach the threshold to generate an action potential, whereas inhibitory postsynaptic potentials can prevent the cell from achieving action potential.

In the eye, within a small retinal angle of 10-15 minutes of arc, Ricco’s law states that intensity and area are inversely variable. Therefore, 100 quanta on one rod are equal to 1 quantum on 100 rods. This is due to the merging of signals of rods onto bipolars, in turn onto ganglion cells,