To live is to take in information from the world and the body’s tissues, to make decisions, and to send back information and orders to the body’s tissues. All this happens thanks to your body’s nervous system (FIGURE 2.5). Your brain and spinal cord form the central nervous system (CNS), your body’s decision maker. Your peripheral nervous system (PNS) gathers information from other body parts and transmits CNS decisions to the rest of your body.

Nerves are electrical cables formed from bundles of axons. They link your central nervous system with your body’s sensory receptors, muscles, and glands. Your optic nerve, for example, bundles a million axons into a single cable carrying messages from each eye to your brain (Mason & Kandel, 1991). Information travels in your nervous system through three types of neurons.

Your complexity resides mostly in your interneuron systems. Your nervous system has a few million sensory neurons, a few million motor neurons, and billions and billions of interneurons.

The peripheral nervous system has two parts—somatic and autonomic. Your somatic nervous system monitors sensory input and triggers motor output, controlling your skeletal muscles (which is why it is also called the skeletal nervous system). As you reach the end of this page, your somatic nervous system will report the information to your brain and then carry back instructions that will trigger your hand to reveal the next page. Your autonomic nervous system (ANS) controls your glands and the muscles of your internal organs, including those of your heart and digestive system. Like an automatic pilot, this system may be consciously overridden, but usually it operates on its own (autonomously).

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Within your autonomic nervous system, two subdivisions help you cope with challenges (FIGURE 2.6). If something alarms or challenges you (perhaps giving a speech), your sympathetic nervous system will arouse you, making you more alert, energetic, and ready for action. It will increase your heartbeat, blood pressure, and blood-sugar level. It will also slow your digestion and cool you with perspiration. When the stress dies down (the speech is over), your parasympathetic nervous system will calm you, conserving your energy as it decreases your heartbeat, lowers your blood sugar, and so on. In everyday situations, the sympathetic and parasympathetic divisions work together to steady our internal state.

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I [DM] recently experienced my ANS in action. Before sending me into an MRI (magnetic resonance imaging) machine for a shoulder scan, the technician asked if I had ever had claustrophobia (panic feelings when confined). “No, I’m fine,” I assured her, with perhaps a hint of macho swagger. Moments later, my sympathetic nervous system had a different idea. I found myself on my back, stuck deep inside a coffin-sized box and unable to move. Claustrophobia overtook me. My heart began pounding and I felt a desperate urge to escape. Just as I was about to cry out for release, I felt my calming parasympathetic nervous system kick in. My heart rate slowed and my body relaxed, though my arousal surged again before the 20-minute confinement ended. “You did well!” the technician said, unaware of my ANS roller-coaster ride.

From neurons “talking” to other neurons arises the complexity of the central nervous system’s brain and spinal cord.

It is the brain that enables our humanity—our thinking, feeling, and acting. Tens of billions of neurons, each communicating with thousands of other neurons, yield an ever-changing wiring diagram. By one estimate, based on small tissue samples, our brain has some 86 billion neurons (Azevedo et al., 2009; Herculano-Houzel, 2012).

The brain’s neurons cluster into work groups called neural networks, much as people cluster into cities rather than spreading themselves evenly across the nation (Kosslyn & Koenig, 1992). Neurons network with close neighbors by means of short, fast connections. Learning—to play a guitar, speak a foreign language, solve a math problem—occurs as experience strengthens those connections. Neurons that fire together wire together.

The other part of the central nervous system, the spinal cord, is a two-way highway connecting the peripheral nervous system and the brain. Some nerve fibers carry incoming information from your senses to your brain, while others carry outgoing motor-control information to your body parts. The neural pathways governing our reflexes, our automatic responses to stimuli, illustrate the spinal cord’s work. A simple spinal reflex pathway is composed of a single sensory neuron and a single motor neuron. These often communicate through an interneuron. The knee-jerk response, for example, involves one such simple pathway. A headless warm body could do it (FIGURE 2.7).

When people suffer damage to the top of their spinal cord, their brain is truly out of touch with their body. They lose all sensation and voluntary movement in body regions that connect to the spinal cord below its injury. Given a doctor’s knee-reflex test, their foot would respond with a jerk, but they would not feel the doctor’s tap. Men paralyzed below the waist may be capable of an erection (a simple reflex) if their genitals are stimulated (Goldstein, 2000). Women who are similarly paralyzed may respond with vaginal lubrication. But, depending on where and how completely their spinal cord is severed, people may have no genital responses to erotic images and no genital feeling (Kennedy & Over, 1990; Sipski et al., 1999). To produce physical pain or pleasure, sensory information must reach the brain.