Temporal and Spatial Summation of Synaptic Potentials

Figure 9-la shows an experimental arrangement for recording the change in membrane potential of a motor neuron in response to action potentials in a single presynaptic sensory neuron. An intracellular microelectrode is placed inside the postsynaptic motor neuron, and presynaptic action potentials are triggered by electrical stimuli applied to the sensory nerve fiber. Figure 9-lb illustrates responses ofthe motor neuron to a single action potential in the sensory neuron and to a series of four action potentials. A single presynaptic action potential produces only a small depolarization of the motor neuron, called an excitatory postsynaptic potential (e.p.s.p.). A single e.p.s.p. is typically much too small to reach threshold for triggering a postsynaptic action potential. Figure 9-2 shows a recording of an e.p.s.p. in a motor neuron produced by an action potential in a single sensory neuron. In this experiment, an intracellular electrode was placed inside the sensory fiber to record the presynaptic membrane potential and to inject depolarizing current that elicited an action potential in the presynaptic fiber (upper recording trace). A second intracellular microelectrode in the motor neuron recorded the change in membrane potential of the postsynaptic cell. Note that the single e.p.s.p. is only about l mV in amplitude, which is much smaller than the 10-20 mV depolarization required to reach threshold. Thus, summation of e.p.s.p.'s is required to trigger a post-synaptic action potential in the motor neuron.

If a second action potential arrives at the presynaptic terminal before the postsynaptic effect produced by the first action potential has dissipated, the second e.p.s.p. will sum with the first to produce a larger peak postsynaptic depolarization. As shown in Figure 9-lb, the e.p.s.p.'s produced by a rapid series of presynaptic action potentials can add up sufficiently to reach threshold for triggering a postsynaptic action potential. This kind of summation of the sequential postsynaptic effects of an individual presynaptic input is called temporal summation. Temporal summation is an important mechanism that allows even a weak excitatory synaptic input to stimulate an action potential in a postsynaptic cell.

Motor Neuron Summation

Figure 9-1 Synaptic transmission at an excitatory synapse between two neurons. (a) The experimental arrangement for examining transmission between a sensory and a motor neuron in the patellar reflex loop. (b) Responses of the postsynaptic motor neuron to action potentials in the presynaptic sensory neuron. At the upward arrows, action potentials are triggered in the presynaptic neuron by an electrical stimulus.

Figure 9-1 Synaptic transmission at an excitatory synapse between two neurons. (a) The experimental arrangement for examining transmission between a sensory and a motor neuron in the patellar reflex loop. (b) Responses of the postsynaptic motor neuron to action potentials in the presynaptic sensory neuron. At the upward arrows, action potentials are triggered in the presynaptic neuron by an electrical stimulus.

Temporal summation of e.p.s.p.'s is illustrated in the intracellular recordings shown in Figure 9-3, which were obtained from a motor neuron of the sympathetic nervous system. Each set of traces in the figure consists of superimposed responses to three postsynaptic stimuli. In each case, one stimulus fails to activate the presynaptic input and so produces no postsynaptic response (trace a), one stimulus produces a postsynaptic response that fails to

Em of sensory fiber

Em of sensory fiber

Em of motor

40 mV

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0.5 msec reach threshold (trace b), and one stimulus produces a postsynaptic response that reaches threshold (trace c). Only if the successive e.p.s.p.'s summate sufficiently to reach threshold is an action potential triggered in the postsyn-aptic cell.

Another way that e.p.s.p.'s can sum to reach threshold is via the simultaneous firing ofaction potentials by several presynaptic neurons. A single neuron in the nervous system commonly receives synaptic inputs from hundreds or even thousands of presynaptic neurons. In the patellar reflex, for example, a single quadriceps motor neuron will receive excitatory synaptic connections from many stretch receptor sensory neurons, shown schematically in Figure 9-4a. An action potential in a single presynaptic cell produces only a small post-synaptic depolarization, as we have seen. If several presynaptic cells fire simultaneously, however, their postsynaptic effects sum together and can reach threshold (Figure 9-4b). This spatial summation of e.p.s.p.'s occurs when several spatially distinct synaptic inputs are active nearly simultaneously.

In the patellar reflex, both temporal and spatial summation are important in eliciting the reflexive response. In order to produce reflexive contraction of the quadriceps muscle, a tap to the patellar tendon must stretch the muscle sufficiently to fire a number of action potentials in each of a number of individual sensory neurons. Combined temporal summation ofthe effects of action potentials within the series and spatial summation of the effects of all of the individual sensory neurons ensure that postsynaptic motor neurons fire action potentials and trigger muscle contraction.

Figure 9-2 Simultaneous intracellular recordings from a single stretchsensitive sensory nerve fiber and a motor neuron receiving synaptic input from the sensory fiber. The upper trace shows an action potential triggered in the sensory fiber by passing a depolarizing electrical current through the intracellular electrode. After a brief delay, a small excitatory postsynaptic potential was evoked in the postsynaptic motor neuron (lower trace). Note the different voltage scales for the two traces. (Data provided by W. Collins, M. Honig, and L. Mendell of the State University of New York at Stony Brook.)

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