FIGURE 22 Temporal and spatial summation. (A) Intracellular recordings are made from the two sensory neurons (SN1 and SN2) and a motor neuron (MN). (B) Temporal summation; a single action potential in SN1 produces a 1-mV EPSP in the MN. Two action potentials in quick succession produce a dual-component EPSP, the peak amplitude of which is approximately 2 mV. (C) Spatial summation; alternative firing of single action potentials in SN1 and SN2 produce 1-mV EPSPs in the MN. Simultaneous action potentials in SN1 and SN2 produce a summated EPSP, the amplitude of which is approximately 2 mV.
a dendrite and the flame of a cigarette lighter that will generate heat equivalent to a depolarization generated by a synaptic current at the synapse. Now consider the consequences of presenting the flame to one end of the rod on the temperature measured in the middle of the rod. Obviously, a temperature change would be produced; assume, for the sake of the example, that the temperature change is 1°C. If the flame is presented twice in quick succession, a greater temperature change would be recorded, because the heat generated would produce temperature changes that would summate because of the passive properties (heat capacity) of the rod. If the two flames were presented in quick succession, a temperature change of 2° C would be produced. If a long time elapses between the presentation of the two flames, there would be little or no summation, because the initial 1°C increase in temperature would have dissipated. Similarly, potentials in dendrites summate because of the passive properties of the nerve cell membrane. Specifically, the membrane has a capacitance and can store charge. Thus, the membrane can temporarily store the charge of the first EPSP, and the charge from the second EPSP can be added to that of the first.
Spatial Summation. Spatial summation (Fig. 22C) involves a consideration of more than one input to a postsynaptic neuron. An action potential in SN1 produces a 1-mV EPSP, just as it did in Fig. 22B. Similarly, an action potential in a second sensory neuron (SN2) by itself also produces a 1-mV EPSP. Now, consider the consequences of action potentials elicited simultaneously in SN1 and in SN2. The net EPSP is equal to the summation of the amplitudes of the individual EPSPs. Here the EPSP from SN1 is 1 mV, the EPSP from SN2 is 1 mV, and the summated EPSP is 2 mV (Fig. 22C). Thus, spatial summation is a mechanism by which synaptic potentials generated at different sites can summate. Spatial summation in nerve cells is determined by the space constant, which is the ability of a potential change produced in one region of a cell to spread passively to other regions of a cell (see Chapter 5).
Again, a thermal analog is useful to help understand this phenomenon. Consider the metal rod as an analogy for a dendrite. As before, temperature is recorded in the middle, but now heat sources can be delivered to either end separately or both ends at the same time. A flame presented to only one end of the metal rod produces a 1 °C increase in temperature at the middle of the metal rod; if a flame is presented simultaneously to each end of the metal rod, a 2°C increase in temperature is produced. The temperature changes induced by the two flames presented simultaneously will summate because of the ability of heat to spread passively along the rod.
In summary, whether a neuron fires in response to synaptic input is dependent, at least in part, on how many action potentials are produced in any one presynaptic excitatory pathway, as well as how many individual convergent excitatory input pathways are activated. The final behavior of the cell is also due to the summation of other kinds of synaptic inputs—specifi-cally, inhibitory synaptic inputs.
Some synaptic events decrease the probability of generating spikes in the postsynaptic cell. Potentials produced as a result of these events are called inhibitory postsynaptic potentials (IPSPs). Consider the inhibitory
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