Neuronal Integration

In the nervous system, neurons receive both excitatory and inhibitory synaptic inputs. The decision of a postsynaptic neuron to fire an action potential is determined by only one factor: whether the threshold level of membrane potential has been reached. Reaching threshold is determined at any instant by the sum of all existing excitatory and inhibitory synaptic potentials. This process of summing up, or integrating, synaptic inputs is called neuronal integration.

Neuronal integration in the neural circuitry of the patellar reflex is shown in Figure 9-11. When the sensory neuron from the antagonistic muscle fires action potentials, e.p.s.p.'s are produced in the motor neurons that control the antagonist muscle. If there is sufficient temporal summation among the e.p.s.p.'s, an action potential is triggered (Figure 9-11b). If the inhibitory neuron is stimulated at the same time, however, the same series of excitatory inputs might be unable to reach threshold (Figure 9-11c). As shown in Figure 9-11d, this inhibitory effect can be overcome by increasing the strength of the excitatory input, which could be accomplished by increasing the number of presynaptic action potentials in the sensory neuron (temporal summation) or by increasing

Em of motor neuron (mV)

-100

Time

Time

Stimulate 4 action potentials in sensory neuron

Figure 9-11 The integration of excitatory and inhibitory synaptic inputs by a postsynaptic neuron. (a) A schematic diagram of the experimental arrangement for the measurements shown in (b), (c), and (d). (b) Stimulation of the excitatory presynaptic neuron (the sensory neuron) produces a postsynaptic action potential if temporal summation is sufficient to reach threshold.

the number of sensory neurons activated (spatial summation). The balance between the excitatory and inhibitory inputs dictates whether a postsynaptic action potential is generated.

The information-processing capacity of a single neuron is considerable. A typical neuron receives hundreds or thousands of synapses from hundreds or thousands of other neurons and makes synaptic connections itself with an equal number of postsynaptic neurons. This capacity is increased still further by the widely varying weights of different synaptic inputs to a cell. Some synapses produce large changes in postsynaptic membrane potential, while others cause only tiny changes. Furthermore, the weight given a particular input might vary with time, as in the case of presynaptic inhibition. A network of some 1010 of these sophisticated units, like the human brain, has staggering information-processing ability.

Figure 9-11 (cont'd)

(c) Stimulation of the inhibitory presynaptic neuron prevents the excitatory inputs in (b) from reaching threshold.

(d) The inhibitory effect can be overcome by increasing the amount of excitatory stimulation.

Em of motor neuron (mV)

Threshold

Threshold

Stimulate Stimulate 4 inhibitory action potentials in neuron sensory neuron

Stimulate Stimulate 4 inhibitory action potentials in neuron sensory neuron

Figure 9-11 (cont'd)

(c) Stimulation of the inhibitory presynaptic neuron prevents the excitatory inputs in (b) from reaching threshold.

(d) The inhibitory effect can be overcome by increasing the amount of excitatory stimulation.

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