Temporal and spatial summation

How do neurons integrate incoming signals?

Neuronal summation is the algebraic addition of graded potentials that converge at the trigger zone (the axon hillock and initial segment), where the neuron determines whether to fire an action potential. Nerve cells create synapses between them, which contribute to their continuous communication. For every single synapse, there is a presynaptic neuron sending information and a postsynaptic neuron receiving the information. The activation of a typical synapse, formed between an axon terminal and a dendrite, results either in the depolarization of the postsynaptic membrane creating an excitatory postsynaptic potential (EPSP) or its hyperpolarization creating an inhibitory postsynaptic potential (IPSP). These are graded potentials that depend on the type of receptors of the postsynaptic membrane activated by the neurotransmitter released by the presynaptic membrane. The graded potentials arising on different parts of the neuronal dendrites spread to the surrounding area of the dendritic tree, but they decay exponentially to the time they were generated and the distance from the point they were generated.

On the other hand, neurons can create action potentials on their axons that follow the all-or-none rule and do not decay, but these potentials start only at the axon hillock and its initial segment (together called the trigger zone), and require a depolarisation that crosses the cell's threshold. The potentials arriving at the dendrites and the cell body are usually subthreshold when reaching the axon hillock, meaning that a single stimulus cannot fire an action potential as it ranges below the threshold. At any given moment multiple EPSPs and IPSPs are created on the cell membrane of a neuron simultaneously. They spread to the dendrite and the cell body and the neuron proceeds into an algebraic summation of all inputs that finally reach the trigger zone. Depending on the result of summating inputs from multiple nerve cells (spatial summation) and from a single neuron (temporal summation) an action potential may or may not be elicited.

Upon activation of a synapse, the potential of the postsynaptic membrane is changing causing either depolarization or hyperpolarization. This depends on the interaction between the neurotransmitter released by the presynaptic neuron and the receptors located on the postsynaptic membrane.

Excitatory postsynaptic potential (EPSP)

The signals that travel from a presynaptic to a postsynaptic neuron and cause the depolarization of the postsynaptic membrane are called excitatory, because they increase the potential bringing it closer to the threshold for the axon hillock, making it more likely to fire an action potential. The neurotransmitter effect on the postsynaptic receptors, activates most usually sodium channels, increasing the permeability to this ion. Thus the rapid entry of the sodium ions into the cell causes a slight depolarization, a positive elevation of the potential. That is, if the resting potential in a given neuron is stabilized at -70mV an EPSP causes it to rise, e.g. -60mV.

Inhibitory postsynaptic potentials (IPSP)

These are stimuli that cause hyperpolarization of the cell’s membrane. In this case, the inputs result in a decrease in the potential of the postsynaptic membrane, as the neurotransmitter often activates potassium or chloride channels. The transmission of potassium ions outside of the cell leads to an increase in its negativity, for example, if the resting potential of a neuron is -70 mV an IPSP can reduce it to -80mV. Chloride channels work by a different route: when they open, chloride ions flow into the cell, bringing negative charge with them and driving the membrane potential toward the chloride equilibrium potential (approximately −70 mV). This holds the membrane near its resting value and resists any depolarising input.

What is spatial summation?

Spatial summation occurs when graded potentials from multiple synapses arrive at the trigger zone simultaneously and their amplitudes combine. The synaptic stimuli may generate multiple EPSPs or IPSPs on the postsynaptic membrane. These graded potentials are traveling along the length of the dendrites reaching the cell body with decrement. Inputs that are closely spaced on the dendrites are spatially summated, otherwise they decay. When they finally reach the cell body, they are being added algebraically, since the ultimate summation is taking place at the trigger zone. If there is a depolarization over the threshold an action potential initiates traveling down the axon.

What is temporal summation?

Temporal summation occurs when the same presynaptic axon terminal fires repeatedly and each successive stimulus arrives before the previous graded potential has fully decayed. This particular type of summation is mainly related to the frequency of the stimuli. Every graded potential generated on the postsynaptic membrane gradually decays. But, if another postsynaptic potential is triggered from the same synapse, before the first potential completely fades away, then the two potentials are added and this is called temporal summation. Thus, when stimuli are sent repeatedly in one synapse, the postsynaptic potentials are algebraically summated. Higher frequency of excitatory stimuli, lead to larger depolarization of the postsynaptic membrane and increase the probability of depolarization at the axon hillock of the postsynaptic neuron above the threshold for firing an action potential.

Temporal and spatial summation produce the same outcome (an action potential firing or suppression) but differ in stimulus source and timing. The table below compares them.