Final answer:
Spatial summation is the type of summation that has multiple stimuli in the same space that are additive. It involves the combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential.
Step-by-step explanation:
Spatial summation is the type of summation that has multiple stimuli in the same space that are additive.
It involves the combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential. For example, if multiple presynaptic neurons release neurotransmitters onto a postsynaptic neuron, the graded potentials from each input can sum up at the postsynaptic neuron's axon hillock. If the combined graded potentials reach the threshold, an action potential will be generated.