The nervous system (see Figure 3.12), the electrical information highway of the body, is made up of nerves—bundles of interconnected neurons that fire in synchrony to carry messages. The central nervous system (CNS), made up of the brain and spinal cord, is the major controller of the body’s functions, charged with interpreting sensory information and responding to it with its own directives. The CNS interprets information coming in from the senses, formulates an appropriate reaction, and sends responses to the appropriate system to respond accordingly. Everything that we see, hear, smell, touch, and taste is conveyed to us from our sensory organs as neural impulses, and each of the commands that the brain sends to the body, both consciously and unconsciously, travels through this system as well.
Nerves are differentiated according to their function. A sensory (or afferent) neuron carries information from the sensory receptors, whereas a motor (or efferent) neuron transmits information to the muscles and glands. An interneuron, which is by far the most common type of neuron, is located primarily within the CNS and is responsible for communicating among the neurons. Interneurons allow the brain to combine the multiple sources of available information to create a coherent picture of the sensory information being conveyed.
The spinal cord is the long, thin, tubular bundle of nerves and supporting cells that extends down from the brain. It is the central throughway of information for the body. Within the spinal cord, ascending tracts of sensory neurons relay sensory information from the sense organs to the brain while descending tracts of motor neurons relay motor commands back to the body. When a quicker-than-usual response is required, the spinal cord can do its own processing, bypassing the brain altogether. A reflex is an involuntary and nearly instantaneous movement in response to a stimulus. Reflexes are triggered when sensory information is powerful enough to reach a given threshold and the interneurons in the spinal cord act to send a message back through the motor neurons without relaying the information to the brain (see Figure 3.13). When you touch a hot stove and immediately pull your hand back, or when you fumble your cell phone and instinctively reach to catch it before it falls, reflexes in your spinal cord order the appropriate responses before your brain even knows what is happening.
If the central nervous system is the command center of the body, the peripheral nervous system (PNS) represents the front line. The PNS links the CNS to the body’s sense receptors, muscles, and glands. As you can see in Figure 3.14, the peripheral nervous system is itself divided into two subsystems, one controlling internal responses and one controlling external responses.
The autonomic nervous system (ANS) is the division of the PNS that governs the internal activities of the human body, including heart rate, breathing, digestion, salivation, perspiration, urination, and sexual arousal. Many of the actions of the ANS, such as heart rate and digestion, are automatic and out of our conscious control, but others, such as breathing and sexual activity, can be controlled and influenced by conscious processes.
The somatic nervous system (SNS) is the division of the PNS that controls the external aspects of the body, including the skeletal muscles, skin, and sense organs. The somatic nervous system consists primarily of motor nerves responsible for sending brain signals for muscle contraction.
The autonomic nervous system itself can be further subdivided into the sympathetic and parasympathetic systems (see Figure 3.14). The sympathetic division of the ANS is involved in preparing the body for behavior, particularly in response to stress, by activating the organs and the glands in the endocrine system. The parasympathetic division of the ANS tends to calm the body by slowing the heart and breathing and by allowing the body to recover from the activities that the sympathetic system causes. The sympathetic and the parasympathetic divisions normally function in opposition to each other, such that the sympathetic division acts a bit like the accelerator pedal on a car and the parasympathetic division acts like the brake.
The autonomic nervous system has two divisions: The sympathetic division acts to energize the body, preparing it for action. The parasympathetic division acts to calm the body, allowing it to rest.
Our everyday activities are controlled by the interaction between the sympathetic and parasympathetic nervous systems. For example, when we get out of bed in the morning, we would experience a sharp drop in blood pressure if it were not for the action of the sympathetic system, which automatically increases blood flow through the body. Similarly, after we eat a big meal, the parasympathetic system automatically sends more blood to the stomach and intestines, allowing us to efficiently digest the food. And perhaps you’ve had the experience of not being at all hungry before a stressful event, such as a sports game or an exam (when the sympathetic division was primarily in action), but suddenly finding yourself starved afterward, as the parasympathetic takes over. The two systems work together to maintain vital bodily functions, resulting in homeostasis, the natural balance in the body’s systems.