Use of the term circuit for a group of neurons and their synaptic interconnections is an analogy to electronic circuits, in which an arrangement of components (e.g., resistors, capacitors, transistors, and their connecting wires) accomplishes a particular function. Electrical or electronic circuits can represent signals in either analog or digital ways—that is, in terms of continuously varying values or in terms of integers. Neurons similarly feature two kinds of processes: analog-like signals that vary in strength (such as graded potentials at synapses) and digital-like “all-or-nothing” signals (such as action potentials) that vary in frequency. The nervous system comprises many different types of neural circuits to accomplish basic functions in cognition, emotion, and action—all the categories of behavior and experience.
The simplest neural circuit that is routinely encountered in the nervous system is the neural chain, a straightforward linking of a series of neurons. From the seventeenth century until well into the twentieth century, most attempts to understand behavior in neural terms were based on chains of neurons, which do indeed account for some behaviors. For example, the basic circuit for the stretch reflex, such as the knee-jerk reflex we discussed at the start of Chapter 3 in the textbook, consists of a sensory neuron, a motor neuron, and a single synapse where the sensory neuron communicates with the motor neuron.
For some purposes the afferent (input) parts of the visual system can be represented as a neural chain (see Figure 1a) (in reality, however, the retina contains many kinds of neural circuits, which we discuss in Chapter 7 in the textbook). A more accurate schematic diagram of the visual system (see Figure 1b) highlights two other features that are common to many kinds of neural circuits: convergence and divergence.
In many parts of the nervous system, the axons from large numbers of neurons converge on certain cells. In the human eye, about 100 million receptor cells concentrate their information down on about 1 million ganglion cells; these ganglion cells convey the information from the eye to the brain (see Figure 1b). Higher in the visual system there is much divergence: the 1 million axons of the optic nerve communicate to billions of neurons in several different specialized regions of the cerebral cortex.