Neural integration: EPSP and IPSP; rate coding, temporal coding, phase coding and burst coding.
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A typical neuron receives not one, but many synaptic inputs at a time. Moreover, these inputs may have similar or opposing effects on the receiving neuron. Neural integration refers to the process in which a neuron integrates all input information and “decides” whether or not to produce action potentials.
Neurons are polarized, meaning there are different electrical charges inside and outside the cell. A typical neuron has a resting membrane potential of about -70mV. The negative value means the cell is more negative on the inside. In order to generate an action potential, membrane potential must reach the threshold of about -55mV, meaning the inside of the cell must become less negative – the cell becomes less polarized, or depolarized. Any voltage change in that direction is called excitatory postsynaptic potential, EPSP, because it makes the neuron more likely to fire. EPSPs are brought about by excitatory neurotransmitters. On the other hand, inhibitory neurotransmitters change the membrane voltage in the opposite direction, producing inhibitory postsynaptic potential, IPSP, making the cell HYPER-polarized and less likely to generate action potentials.
A neuron can receive both excitatory and inhibitory inputs simultaneously. It’s the balance between EPSPs and IPSPs that decides whether or not a neuron will fire.
A typical EPSP produced by a single action potential in the presynaptic neuron is around 1mV, and lasts about 15 to 20 msec. To reach the threshold, the postsynaptic neuron must receive multiple EPSPs nearly at once. There are 2 ways this can happen:
– Temporal summation: when a single presynaptic neuron fires repeatedly at short intervals, producing EPSPs so closely to each other that each EPSP occurs before the previous one decays, allowing their effects to add up and reach the threshold.
– and spatial summation: when multiple presynaptic neurons fire nearly synchronously to produce multiple EPSPs in the postsynaptic neuron.
All action potentials produced by a particular neuron are identical, and yet, they underlie the ability of our brain to perceive all the vast and complex information in and around our body. Just like Morse code, which uses simple dots and dashes to encode an unlimited number of messages, our brain uses action potentials to encode complex sensory information, a process known as neural coding.
Firstly, the brain differentiates between different stimuli by activating different sets of neurons. For example: different odors activate different sets of olfactory neurons in the olfactory nerve; sounds with different frequencies stimulate different nerve fibers in the auditory nerve; different taste sensations excite different taste cells.
Secondly, information about the intensity of a stimulus can be conveyed by activating receptors with different sensitivities; stimulating a smaller or larger number of neurons; or producing a lower or higher firing frequency in one neuron.
Thirdly, aside from the firing rate, other characteristics of the action potential pattern may also encode information. Various coding methods have been proposed, including:
– temporal coding, which emphasizes the precise timing for each action potential spike, in particular the first spikes produced by a population of neurons;
– phase coding, in which neurons transmit information by firing at different phases with respect to a background cycle;
– and burst coding, where neurons communicate in a burst of spikes with characteristic spike numbers and intervals between them.