Circuits of Neurons Process Information

In Chapter 3 we saw that neurons function in circuits to process information; the simplest such circuit is the neural chain. However, by adding in even very simple features, such as recursive loops and circuit elements that provide patterns of inhibition, we can begin to see how more-complex forms of integration and control are possible. Here we present two very straightforward examples of simple circuits: the feedback circuit and the oscillator circuit.

The Feedback Circuit Is a Regulator

In a feedback circuit, part of the output is fed back to the input. There are two types of feedback circuits: positive and negative. In positive feedback circuits, the effect of the output is to sustain or increase the activity of the initial input; in negative feedback circuits, the output inhibits the activity of the initial input. In some feedback circuits, a branch of the axon of a neuron loops back and contacts the same neuron (Figure 1a). In others, one or more intermediate neurons (interneurons) form the feedback loop.

Figure 1  Negative Feedback Circuits
(a) A circuit within a single cell. (b) A simple oscillator circuit, in which a continuous series of action potentials from neuron A is modulated by inhibitory output from cell C to cell B, which ultimately causes rhythmic output from cells B and C.

Feedback circuits were first discovered in the nervous system in the 1940s, and psychologist Donald O. Hebb (1949) pointed out their relevance for psychological and neuroscience theory. For example, a positive feedback circuit can be used to sustain neural activity, which can contribute to maintaining a motivational state or to forming the cellular basis of memory. Negative feedback circuits help regulate many body functions by maintaining relatively constant conditions.

A thermostat is an example of a negative feedback device; many neural circuits in the body function in the same basic manner. The stretch reflex (an example of which is the knee jerk reflex illustrated in textbook Figure 3.17) serves as part of a negative feedback system in maintaining posture as you stand. Swaying a little to the front causes muscles in the back of the leg to stretch; the leg muscles respond by contracting, bringing the body again to the vertical. Similarly, swaying to the back stretches muscles in the front, and again the stretch reflex brings about the necessary compensation.

The Oscillator Circuit Controls Rhythmic Behavior

Many kinds of behavior—from beating of the heart, breathing, walking, sleeping, and waking, to annual migration—are rhythmic, and their cycles differ in duration from short to long. Some neurons, mostly in invertebrates, show inherent spontaneous rhythmicity of activity: the frequency of neural impulses of such pacemaker cells waxes and wanes in regular alternation. Rhythmic wing beating in some insects is controlled by such oscillation. In both invertebrates and vertebrates, however, oscillatory activity usually depends on circuits of neurons. Part (b) of the figure shows a simple oscillator circuit.