#temporal Vs spatial summation

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Let's Explore the differences between temporal and spatial summation. We will be discussing the mechanism, differences and comparisons of both summations to come to an understanding which is better. If you're medical science student, this blog will be really helpful for you. Visit My Assignment Services read more such blogs written by top Canadian Experts

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Tremblay et al.: Figures and Boxes

Figure 1: Diversity, Classification, and Properties of Neocortical GABAergic INs

Neocortical GABAergic IN: GABA-releasing (likely inhibitory) interneuron (connecting between perception/motor output?) within the fetal brain

all of these express one of [PV parvalbumin (relation to general albumin carrier in the blood?), somatostatin (insulin/glucagon balance regulator), 5HT3a serotonin receptor] and can thus be categorized by those markers

further classification of different neocortical GABA-ergic INs can be obtained via morphology, targeting biases, other biomarkers, and electrophysiological/synaptic properties

tree classification: 1 of 3 markers » morphology » targeting bias » anatomy » biophysical properties of firing (firing patterns, spiking, refractory) » synaptic properties » addt'l markers

Figure 2: Laminar Distribution of IN Groups

L1, 2, 3, 4, 5a, 5b, and 6 are all layers in the neocortex

distribution of the three major GABA-ergic IN types as distinguished by PV, Sst, and 5HT3a marker expression (5HT3a separated into VIP and non-VIP expressing cells) varies with neocortical layer

significance: varying compositional make-up of L layers signifies likely variance in roles they play in fetal brain function + later development of cognition

Box 1: PV FS Basket Cells Are Specialized for Speed, Efficiency, and Temporal Precision

physical properties of PV FS cells suit them for rapid, precise firing

machinery for fast EPSPs: AMPA receptors w/ GluR1 subunits only, low membrane resistance, very active dendrites

machinery for brief and highly repetitive APs: sub-threshold Kv1 channels that allow for quick repolarization, "Na+ channels with slower inactivation and faster recovery"

these components are all concentrated at the appropriate areas of the neuron to reach high efficacy

Figure 3: Cell-Specific Connectivity and Subcellular Domains Targeted by IN Subtypes

Sst martinotti cells synapse near the soma of the L2/3 cell

nonVIP 5HT3aR NGFCs synapse near the branches of the L2/3 dendrite/axon

in L5/6 pyramidal cells, NGFC inputs are broken up by different layers (?)

Figure 4: Circuit Motifs Involving INs

different functions of interneurons in modulating synaptic signaling are dependent on placement of IN and source of excitatory input

feedforward inhibition: distal excitation » IN and pyramidal cell; IN then also inhibits pyramidal cell to modulate the effects of the distal excitation

feedback inhibition: PC excitation » IN inhibits the original PC source of excitation to gradually modulate its signal » IN also inhibits same-layer proximal PCs to unify regional signaling pattern

disinhibition: IN » IN » PC so that the end result is the reduction of inhibition on the pyramidal cell's excitatory activity

Figure 5: Thalamocortical FFI by PV Neurons Imposes Coincidence Detection

FFI via PV neurons allows for temporal summation window in certain spaces/moments of time

this is achieved by increasing inhibition at all other time periods; now "near-synchornous inputs are required for efficient summation of EPSPs and to drive AP firing on the PC"

[I think that] NGFCs weaken the PV inhibition of PCs, allowing for a wider window of temporal summation and lateral signaling recruitment of other PCs?

FFI prevents saturation via PV cell recruitment; NGFC signaling weakens this mechanism. Thus, depending on how much FFI is needed in certain regions, the NGFC vs. PV cell population concentration ratios will vary

Figure 6: FBI and Differential Effect of PV and Sst IN-Mediated Inhibition

difference between PV and Sst IN inhibition: PVs show decreased response to repeated external inputs due to anatomical and synaptic features from Box 1; Ssts show increased response due to "opposing" or somewhat "antithetical" physical properties to PVs

thus PVs function to synchronize activity laterally across two or more pyramidal cells, i.e., spatial summation; Ssts function to amplify all inputs to a single PC, i.e., temporal summation

PVs have high permeability (low resistance) which means that EPSPs generated diffuse and disperse easily, so highly repetitive inputs do not build up and the cell is unable to undergo temporal summation; on the other hand, insensitivity to temporal summation means that the cell can 'detect' synchronized spatial summation » it's not just a LACK of temporal summation, it's that a "large amount of input at one time" must be present, which means it's uniquely suited to spatial summation

Ssts have low permeability (high resistance) which means they can essentially STORE charge and thus undergo temporal summation with highly repetitive inputs from external sources

Figure 7: Vip IN-Mediated Disinhibition

Vip INs selectively inhibit Sst INs that are targeting a single pyramidal cell, so that when Vips fire, PCs are disinhibited

area-wide excitability is increased broadly as Vips are recruited; or, if a single Vip is activated, so too are just a few selected PCs disinhibited