23.3: The vertebrate nervous system consists of the peripheral and central nervous systems.

In most animals, including all vertebrates, the nervous system is divided into two components: the peripheral nervous system and the central nervous system (FIGURE 23-5). The peripheral nervous system (PNS) is the network of (1) sensory cells modified to receive information from the environment and (2) motor pathways that transmit signals to effectors, the muscles and glands that are capable of responding to that stimulus. But the information received by the body’s sensory cells does not generally go straight to the cells that control the muscles and glands. First it passes through the central nervous system (CNS), which is made up of the spinal cord and brain. Not directly connected to sensory organs or to muscles, the central nervous system processes information that it receives from sensory cells about the organism’s surroundings and sends out instructions to other nervous tissue to act in response to that sensory information. Later in this chapter, we discuss the elaborate specializations—from memory to language and abstract reasoning—that have evolved in the most complex vertebrate brains.

The motor pathways of the peripheral nervous system have two major divisions: the somatic and the autonomic nervous systems. The somatic nervous system relays signals to your skeletal muscles and enables you to contract those muscles and move your limbs consciously, such as when you pick up a fork. The autonomic nervous system relays signals to your glands and your smooth muscle tissue and cardiac muscle. These signals, which are not under conscious control, cause the muscles surrounding your stomach and intestine, for example, to contract and to move food through the digestive tract. The central nervous system controls both the somatic and the autonomic nervous systems.

Because it enables you to initiate movements consciously, the somatic nervous system is commonly referred to as the voluntary nervous system. When a sensory neuron in the peripheral nervous system—say, in your hand—senses an irritation (such as a mosquito landing on your skin), it sends a signal to convey this information to the spinal cord, where the sensory neuron connects with an interneuron that extends to the brain (FIGURE 23-6). The signal is interpreted and a response is determined (probably that you want to make the mosquito go away), and this information is sent through another interneuron back down your spinal cord to a motor neuron that is attached to a muscle. The signal from the brain to the muscle causes you to swat at the mosquito—removing the source of irritation. This whole process takes only a tiny fraction of a second.

Within the somatic nervous system, some signals can bypass the brain, with sensory information stimulating the motor neuron through the spinal cord. This kind of direct sensory-motor response is called a reflex. Reflexes enable an organism to respond faster to an imminent danger (FIGURE 23-7). Your brain learns about it just a bit later.

In contrast to the somatic nervous system, the autonomic nervous system—which can be thought of as the “automatic” or involuntary nervous system—carries the signals by which the central nervous system regulates the heartbeat, breathing, glandular secretions, the muscles surrounding blood vessels that regulate circulation, and the movement of food through the digestive tract. It is through the autonomic nervous system, in large part, that the central nervous system regulates homeostasis.

Two components make up the autonomic nervous system: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is responsible for coordinating the body’s fight-or-flight response to stress, including increasing the heart and breathing rates and providing muscles with additional blood flow. The parasympathetic nervous system, in contrast, is responsible for controlling activities related to digesting food and eliminating waste, tending to slow the heart and breathing rates as these processes occur. Although the two systems tend to coordinate opposing effects, most muscles, organs, and glands receive input from both the sympathetic and the parasympathetic nervous systems.