Summation (neurophysiology)
Depending on the sum total of many individual inputs, summation may or may not reach the threshold voltage to trigger an action potential.
His main contributions to neurophysiology involved the study of the knee-jerk reflex and the inferences he made between the two reciprocal forces of excitation and inhibition.
[1] Much of today's knowledge of chemical synaptic transmission was gleaned from experiments analyzing the effects of acetylcholine release at neuromuscular junctions, also called end plates.
The pioneers in this area included Bernard Katz and Alan Hodgkin, who used the squid giant axon as an experimental model for the study of the nervous system.
The relatively large size of the neurons allowed the use of finely-tipped electrodes to monitor the electrophysiological changes that fluctuate across the membrane.
[3] One of Katz's seminal findings, in studies carried out with Paul Fatt in 1951, was that spontaneous changes in the potential of muscle-cell membrane occur even without the stimulation of the presynaptic motor neuron.
In 1954, the introduction of the first electron microscopic images of postsynaptic terminals revealed that these MEPPs were created by synaptic vesicles carrying neurotransmitters.
PSPs are deemed excitatory if they increase the probability that an action potential will occur, and inhibitory if they decrease the chances.
Glutamate then binds to AMPA receptors contained in the postsynaptic membrane causing the influx of positively charged sodium atoms.
[9] In contrast to glutamate, the neurotransmitter GABA mainly functions to trigger inhibitory postsynaptic potentials (IPSPs) in vertebrates.
In either case, the net effect is to add to the intracellular negativity and move the membrane potential farther away from the threshold for generating impulses.
[8][10] When EPSPs and IPSPs are generated simultaneously in the same cell, the output response will be determined by the relative strengths of the excitatory and inhibitory inputs.
The subsequent change in stimulation threshold of the postsynaptic membrane can be enhanced or inhibited, depending on the transmitter chemical involved and the ion permeabilities.
Thus the synapse acts as a decision point at which information converges, and it is modified by algebraic processing of EPSPs and IPSPs.
Spatial summation began to receive a lot of research attention when techniques were developed that allowed the simultaneous recording of multiple loci on a dendritic tree.
Modern studies of neural summation focus on the attenuation of postsynaptic potentials on the dendrites and the cell body of a neuron.
[8] Shunting inhibition is exhibited in the work of Michael Ariel and Naoki Kogo, who experimented with whole cell recording on the turtle basal optic nucleus.
