35.1 NERVOUS SYSTEM FUNCTION AND EVOLUTION

Except for sponges, all multicellular animals possess a nervous system, a network of many interconnected nerve cells. This network allows an animal to sense and respond to the environment, coordinate the action of muscles, and control the internal function of its body. Nerve cells, or neurons, are the basic functional units of nervous systems.

Nervous systems first evolved in simple animals. These early nervous systems could sense basic features of the environment, such as light, temperature, chemical odors, and physical forces. Guided by these cues, early nervous systems could trigger movements useful for obtaining food, finding a mate, or choosing a suitable habitat. As animals evolved more complex bodies with increasingly specialized organ systems for respiration, circulation, digestion, and reproduction, their nervous systems also became more complex. These more complex nervous systems could regulate internal body functions in response to sensory cues received from the environment. They also made possible more sophisticated behaviors that relied on improved decision making, learning, and memory. These abilities, possessed by many invertebrate and vertebrate animals, increased their fitness by improving their ability to survive, find and select a mate, and reproduce.

35.1.1 Animal nervous systems have three types of nerve cell.

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.

Most nerve cells have fiberlike extensions that receive information and other extensions 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.

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 mediate a broad array of behaviors, as well as to learn and retain memories of past experiences.

35.1.2 Nervous systems range from simple to complex.

Even single-celled animals have cell-surface receptors that enable them to sense and respond to their environment. By contrast, most multicellular animals have a nervous system. In general, the organization and complexity of an animal’s nervous system reflects its lifestyle. Sessile, or immobile, animals, like sea anemones or clams, have relatively simple sense organs and nervous systems. In contrast, active animals such as arthropods (which include insects, spiders, and crustaceans), squid, and vertebrates have more sophisticated sense organs linked to a brain and more complex nervous systems.

The sponges are the only multicellular animals that lack a nervous system (Fig. 35.2). Their simple body plan means that a nervous system is not required to coordinate the functions of different body regions. Instead, groups of cells within sponges respond to local chemical and physical cues.

Among multicellular organisms, the simplest nervous systems are found in cnidarians, which are radially symmetric animals such as jellyfish and sea anemones (Fig. 35.2). The nervous system of these animals has relatively few neurons and no ganglia or central brain (Fig. 35.3a). In these animals, the nerve cells are arranged like a net. The nerve net of a sea anemone is organized around its body cavity. At one end of its body is a mouth that opens into a central body cavity where food is digested. Sensory neurons sensitive to touch signal when one of the anemone’s tentacles has captured food prey, then motor neurons react to coordinate the tentacle’s movement toward the mouth for feeding and digestion. Sensory neurons responding to a noxious stimulus signal the sea anemone to retract or detach from its substrate.

More complex nerve connections develop in the head end of flatworms, segmented annelid worms, insects, and cephalopod mollusks, such as squid and octopus. These animals are all bilaterians, animals with symmetrical right and left sides and a distinct front and back end. Flatworms are representatives of one of the earliest branching groups of bilaterians, and their bodies are well adapted for forward locomotion. Sensory neurons are most numerous in the animal’s front end, so the flatworm can perceive the environment ahead of it as it moves forward. The flatworm’s nervous system can compare inputs from different sensory neurons and then send signals to the appropriate muscles to move toward a target. Paired ganglia (one on each side of the animal) in the head end of flatworms handle this processing task (Fig. 35.3b).

In animals with an organized nervous system, information is transmitted to different regions along the length of the animal’s body by distinct nerve cords and nerves (Fig. 35.3). These are bundles of the long fiberlike extensions from multiple nerve cells.

In earthworms, paired ganglia, each made up of many interneurons, are located in each of the body’s segments (Fig. 35.3c). These segmental ganglia help to regulate the muscles within each body segment that move the earthworm’s body. Paired ganglia near the head regulate the activity of motor neurons that control movements of the mouth for feeding. Ganglia serve to regulate key processes in local regions and organs of the animal’s body. For example, local ganglia assist in processing information from the eyes (Chapter 36) or control the digestive state of an animal’s gut (Chapter 40).

Centralized collections of neurons forming a brain are present in annelid worms, insects, and mollusks (Figs. 35.3c–35.3e). Sense organs also developed in the head region, from eyespots in flatworms that respond to light to more sophisticated image-forming eyes in octopus and squid. The evolution of a brain in these invertebrate animals enabled them to learn and perform complex behaviors. Many of these nervous system capabilities are shared with vertebrate animals.

The evolution of a brain is also a hallmark of vertebrate animals (Fig. 35.3f). It enabled them to evolve complex behaviors that rely on learning and memory and, in certain vertebrates, the ability to reason. At the same time, animal brains became increasingly complex in the number of nerve cells they contain (fruit fly, about 0.25 million; cockroach, about 1 million; mouse, about 75 million; humans, about 100 billion), in the number of connections between neurons, and in the variety of nerve cell features.

Although animal nervous systems differ in organization and complexity, their nerve cells have fundamentally similar molecular and cellular features and use fundamentally the same mechanisms to communicate with neighboring cells. These mechanisms have been retained over the course of evolution to code and reliably transmit information. The ability to receive sensory information from the environment, and transmit and process this information within a nervous system, is therefore shared across a wide diversity of animal life, contributing to the success of multicellular animals.

35.1.3 What body features arose as adaptations for successful predation?

A notable feature of most animal nervous systems is that nervous system tissue, including specialized sense organs such as eyes, becomes concentrated at one end of the body. For example, the paired ganglia and eyespot of the flatworm are located at one end of its body, as are the brain and sense organs of the earthworm, squid, and insect, as shown in Fig. 35.3. The concentration of nervous system components at one end of the body, defined as the “front,” is referred to as cephalization. Cephalization is a key feature of the body plan of most multicellular animals.

Cephalization is thought be an adaptation for forward locomotion because it allows animals to take in sensory information from the environment ahead of them as they move forward. As the quality and amount of sensory information increased, brain size and complexity increased. Cephalization is also considered to be an adaptation for predation, allowing animals to better detect and capture prey.

Cephalization evolved independently multiple times in different animal groups and is therefore thought to confer certain advantages. These advantages likely include the ability to sense environmental stimuli encountered by forward motion and to process this information quickly to enable a suitable behavioral response. Although cephalization is a feature of many animals, it has been particularly well studied in vertebrates. In vertebrates, the brain, many sense organs, and mouth are all located in the head region. Vertebrates also evolved several novel features, including a jaw, teeth, and tongue. These are all thought to be adaptations for successful predation, or more generally the acquisition and processing of food.

As a result of evolutionary selection for enhanced sensory perception and the ability to respond to important cues in the environment, the brain, sensory organs, and nervous system of many animals are complex in their organization. These are linked to more sophisticated decision-making abilities that allow for a broad range of behaviors, as well as to improved memory and learning. These abilities are critical to the success of both predators and prey and underlie the complex behavioral interactions that occur among members of a species when they mate, reproduce and disperse, and care for their young.