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Information processing occurs at its most basic level within individual neurons.
Neuronal dendrites are designed to receive neurotransmitter ( $\text{NT}$ ) messages simultaneously from multiple sources.
Neurotransmitters are categorized based on their physiological impact on the postsynaptic cell:
- Some neurotransmitters are excitatory.
- Some neurotransmitters are inhibitory.
Stimuli from these neurotransmitters are integrated through continuous interactions between postsynaptic potentials.
The net effect of these interacting stimuli is consolidated at the axon hillock, which is the specific anatomical area located between the soma (cell body) and the axon.
The cumulative electrical state at the axon hillock determines whether an action potential is produced and propagated down the axon.

Postsynaptic potentials are graded potentials that develop within a postsynaptic cell specifically in response to the binding of neurotransmitters.
There are two primary types of postsynaptic potentials:
- Excitatory Postsynaptic Potentials ( $\text{EPSP}$ ): These lead to depolarization.
- Inhibitory Postsynaptic Potentials ( $\text{IPSP}$ ): These lead to hyperpolarization.

An $\text{EPSP}$ is defined as a graded depolarization of the postsynaptic membrane.
These potentials result from the opening of membrane channels that are chemically regulated.
A primary example of an $\text{EPSP}$ is the depolarization produced by the binding of Acetylcholine ( $\text{ACh}$ ) to its receptors.
Because an $\text{EPSP}$ is a graded potential and not an all-or-nothing event, its influence is confined to the immediate area surrounding the synapse where the neurotransmitter bound.

An $\text{IPSP}$ is defined as a graded hyperpolarization of the postsynaptic membrane.
This hyperpolarization moves the membrane potential further away from the threshold, making an action potential less likely.
$\text{IPSPs}$ may occur when potassium channels ( $K^+$ ) are stimulated to open, allowing positive ions to leave the cell and increasing the internal negativity.

A neuron is considered inhibited from producing an action potential when it receives a significant number of $\text{IPSPs}$ .
The mechanism of inhibition involves an increase in the amount of excitatory stimulation required to reach the threshold potential.
This increase is a direct result of the hyperpolarization of the membrane, driven by the opening of potassium ( $K^+$ ) channels.

To trigger a full action potential, a single $\text{EPSP}$ is typically insufficient to reach the threshold.
Neurons must combine multiple $\text{EPSPs}$ (and factor in $\text{IPSPs}$ ) through a process known as summation.
Two distinct forms of summation facilitate the reaching of the threshold:
1. Temporal Summation.
2. Spatial Summation.

Temporal summation occurs when multiple stimuli arrive at the same synapse at different times in rapid succession.
According to Figure $12-19\text{a}$ , the process follows a specific sequence:
- First Stimulus: Arrives at the initial segment of the membrane.
- Second Stimulus: Arrives very shortly after and its effects are added to those of the first stimulus.
- Threshold Reached: When the cumulative effect of these repeated, rapid stimuli from the same source reaches the threshold, an action potential is generated.
- Propagation: The action potential is then propagated along the neuron.

Spatial summation occurs when stimuli arrive simultaneously but at multiple different locations (different synapses).
According to Figure $12-19\text{b}$ , the process follows a specific sequence:
- Simultaneous Stimuli: Two or more stimuli arrive at the membrane at the same time at different points.
- Local Current Flow: Local currents spread the depolarizing effects across the membrane surface.
- Overlap: Areas of the membrane that experience an overlap of these local currents exhibit the combined effects of all simultaneous stimuli.
- Action Potential: If the combined effects at the axon hillock reach the threshold, an action potential is generated and propagated.

Depolarizing Stimuli: Applied at $\text{Time 1}$ or $\text{Time 3}$ , these create $\text{EPSPs}$ . Upon removal of the stimulus, the membrane returns to its resting potential.
Hyperpolarizing Stimuli: Applied at $\text{Time 2}$ or $\text{Time 3}$ , these create $\text{IPSPs}$ . Upon removal of the stimulus, the membrane returns to its resting potential.
The net change in the resting potential at any given moment is the result of the integrated total of all active $\text{EPSPs}$ and $\text{IPSPs}$ .