In complex multicellular animals, neurons form signaling circuits composed of three basic types of nerve cells: afferent neurons, interneurons, and efferent neurons. In circuits that relay information between the peripheral and central nervous systems, afferent neurons, also known as sensory or receptor neurons, carry nerve impulses from receptors or sense organs toward the central nervous system (i.e., the brain and spinal cord). These neurons report an event that has happened, like the arrival of a flash of light or the movement of a muscle. A touch or a painful stimulus creates a sensation in the brain only after information about the stimulus travels there via afferent nerve pathways. Efferent neurons, also known as effector neurons, carry nerve impulses away from the central nervous system to generate a response. A motor neuron, for example, carries a signal to a muscle to stimulate its contraction (see Figure 22-1b); other effector neurons stimulate hormone secretion by endocrine cells. Interneurons, the largest group, relay signals from afferent to efferent neurons and to other interneurons as part of a neural pathway. An interneuron can bridge multiple neurons, allowing integration or divergence of signals and sometimes extending the reach of a signal. In a simple type of circuit called a reflex arc, interneurons connect multiple sensory and motor neurons, allowing one sensory neuron to affect multiple motor neurons and one motor neuron to be affected by multiple sensory neurons; in this way interneurons integrate and enhance reflexes. For example, the knee-jerk reflex in humans, illustrated in Figure 22-4, involves a complex reflex arc in which one muscle is stimulated to contract while another is inhibited from contracting. The reflex also sends information to the brain to announce what happened. Such circuits allow an organism to respond to a sensory input by the coordinated action of sets of muscles that together achieve a single purpose.

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These simple signaling circuits, however, do not directly explain higher-order brain functions such as reasoning, computation, and memory development. Typical neurons in the brain receive signals from up to a thousand other neurons and, in turn, can direct chemical signals to many other neurons. The output of the nervous system depends on its circuit properties—the amount of wiring, or interconnections, between neurons and the strength of these interconnections. As complex and diverse as neural circuits are, they are comprised of a few basic patterns. These include divergence, in which one presynaptic neuron makes connections with many postsynaptic neurons; convergence, in which one postsynaptic neuron receives inputs from many presynaptic neurons; and feedback, in which the output of a postsynaptic neuron feeds back onto a presynaptic neuron or even onto itself (Figure 22-5). Feedback circuits form what are known as closed loops, in which the output of a system is used as the input. In a positive feedback circuit, the output sustains or increases the activity of the initial input. In a negative feedback circuit, the output inhibits the activity of the initial input.