All animal nervous systems are made up of three types of nerve cell: sensory neurons, interneurons, and motor neurons. Each type of neuron has a different function. Sensory neurons receive and transmit information about an animal’s environment or its internal physiological state. These neurons respond to physical features such as temperature, light, and touch or to chemical signals such as odor and taste. Interneurons process the information received by sensory neurons and transmit it to different body regions, communicating with motor neurons at the end of the pathway to produce suitable responses. For example, a motor neuron may stimulate a muscle to contract to produce movement. Other motor neurons may adjust an animal’s internal physiology, constricting blood vessels to adjust blood flow or causing wavelike contractions of the gut to aid digestion. As a result, nervous system function is fundamental to homeostasis, the ability of animals, organs, and cells to actively regulate and maintain a stable internal state (Chapter 5).

Most nerve cells have fiberlike extensions, some of which receive information and others that transit information. The result is a network of interconnected nerve cells that form a circuit. These circuits allow information to be received, processed, and delivered. In most animals, interneurons form specialized circuits that may consist of a diverse array of nerve cells having different shapes, sizes, and chemical properties.

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The ability to process information first evolved with the formation of ganglia (singular, ganglion). Ganglia are groups of nerve cell bodies that process sensory information received from a local, nearby region, resulting in a signal to motor neurons that control some physiological function of the animal. They can be thought of as relay stations or processing points in nerve cell circuits. Eventually, the centralized concentration of neurons that we call a brain evolved, and this organ was able to process increasingly complex sensory stimuli received from the environment or anywhere in the body. Neurons in the brain form complex circuits, allowing the brain to regulate a broad array of behaviors, as well as to learn and retain memories of past experiences.