Temporal summation

Explain how temporal summation and spatial summation take place... 

Temporal summation occurs when multiple EPSPs (or IPSPs) produced by a single presynaptic neuron in close sequence exert their effect on membrane potential of the postsynaptic neuron. For example, an action potential in the presynaptic neuron produces an EPSP and partial depolarization of the postsynaptic neuron (see Figure 5.2). While the postsynaptic neuron is still depolarized, a second action potential in the presynaptic neuron produces another EPSP in the postsynaptic neuron that adds to the first and further depolarizes this neuron. 

As more EPSPs add together, the membrane depolarizes closer to threshold until an action potential is generated. Although temporal summation is illustrated in Figure 5.2 with the summation of relatively few EPSPs, in actuality, addition of up to 50 EPSPs may be necessary to reach threshold. Because a presynaptic neuron may generate up to 500 action potentials per... 

Figure 5.2 Temporal summation. Multiple excitatory postsynaptic potentials (EPSPs) produced by a single presynaptic neuron in close sequence may add together to depolarize the postsynaptic neuron to threshold and generate an action potential.

As with temporal summation, this example has been simplified to illustrate the concept clearly. In actuality, a large number of excitatory inputs from different presynaptic neurons are necessary to depolarize the postsynaptic neuron to threshold. Because a typical neuronal cell body receives thousands of presynaptic inputs, spatial summation also occurs quite readily. The number of presynaptic neurons that are active simultaneously therefore influences the strength of the signal to the postsynaptic neuron. Under normal physiological conditions, temporal summation and spatial summation may occur concurrently. 

Graven-Nielsen, T., Kendall, S. A., Henrikksson, K. G., Bengtsson, M., Sorensen, J., Johnson, A., Gerdle, B., Arendt-Niesen, L. Ketamine reduces muscle pain, temporal summation, and referred pain in fibromyalgia patients, Pain 2000, 85, 483-491. 

Finally, the concentrations of all constituents were summated and compared with the gravimetric ally measured mass to establish the unaccounted mass denoted unknown. The assumption here is that the various data sets accurately describe the PM characteristics. The filtration devices employed at the sites were all equivalent to the reference method. However, corresponding studies use different procedures with respect to filter handling, data treatment and selection, analytical techniques, etc. In addition, temporal variation, local site characteristics, artefacts like volatilisation and particle-bound water as well as varying distances from major sources will affect... 

It should be pointed out that all these effects on response kinetics of the EAG could be caused by malfunction in a subset of ORNs rather than represent changes in kinetics of the receptor potentials. Since EAG and EPG represent summations of such potentials, and ORNs differ greatly in their temporal response dynamics, a deletion of a subset of ORNs with a minor contribution to final amplitude could change the steepness of rise or fall of the EAG. 

Fig. 3. Temporal summation of response. Although the initial (calmodulin-dependent, CaM) and the sustained (C-kinase) phases of All-induced aldosterone secretion are mediated by different protein kinases, their activities are integrated to produce a monotonic rather than a biphasic secretory response.

Excitation can also result from a summation of chemical neurotransmitters released from several presy-naptic neurons that terminate on one postsynaptic neuron. In addition to such spatial control mechanisms, there are mechanisms that are time-dependent (temporal controls). Because neurotransmitters remain bound to their receptors for a time, excitation can also result from an increased rate of release of neurotransmitter from the presynaptic neuron. 

Both temporal and spatial summation of subthreshold potentials are possible, (a) Subthreshold, no summation. (b) Temporal summation, (c) Spatial summation, (d) Spatial summation of EPSP and IPSP. (From Campbell, N.A. et al.. Biology, 5th edn., Addison Wesley Longman, Menlo Park, CA, 1999. With permission.)... 

This chapter will focus on practicable methods to perform both the model specification and model estimation tasks for systems/models that are static or dynamic and linear or nonlinear. Only the stationary case win be detailed here, although the potential use of nonstationary methods will be also discussed briefly when appropriate. In aU cases, the models will take deterministic form, except for the presence of additive error terms (model residuals). Note that stochastic experimental inputs (and, consequently, outputs) may stiU be used in connection with deterministic models. The cases of multiple inputs and/or outputs (including multidimensional inputs/outputs, e.g., spatio-temporal) as well as lumped or distributed systems, will not be addressed in the interest of brevity. It will also be assumed that the data (single input and single output) are in the form of evenly sampled time-series, and the employed models are in discretetime form (e.g., difference equations instead of differential equations, discrete summations instead of integrals). 

Motor imit potential (MUP) Spatial and temporal summation of aU single-fiber potentials innervated by the same motor neuron. Also referred to as the motor unit action potential (MUAP). 

In the nerve system, there is summation—both spatial and temporal. This is easily illustrated with skin receptors. If a variahle skin contact area electrode is used for the examination of electric current perception, it is found that the current density threshold is dependent on die contact area. As the area is increased, the current density threshold is lower because of die summation effect of the receptor responses in the skin (Martinsen et al., 2004). Because of the temporal summation, the excitatory signals must be synchronized in order to trigger the neuron. However, slow changes of the presynaptic DC levels may also make a cell more or less excitable. Such level shifts are according to the Nernst concept and are also under hormonal control. 

Fang, J., Shahani, B., and Graupe, D. (1997). Motor unit number estimation by spatial-temporal summation of single motor unit potentials, Muscle and Nerve, 20 461-468. 

Now let us assume that 2n + 1 modes are oscillating with the same amplitude Eq and that the phase of one consecutive modes differ by a constant value, i.e. (Pk (Pk-i = > where we use k to indicate the kth mode. The latter means that the phases are correlated, or locked to each other. Then the total electric field E t) is the summation of these locked modes the result is a temporal intensity distribution, as shown schematically in Figure 3.13c. The repetition period in the pattern is associated with the round-trip time in the resonator. 

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