The Nervous System

2-5 What are the two major divisions of the nervous system, and what are their basic functions?

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 2.3). 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 of 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 the messages each eye sends to your brain (Mason & Kandel, 1991).

Information travels in your nervous system through three types of neurons.

• Sensory neurons carry messages from your body’s tissues and sensory receptors inward to your spinal cord and brain for processing.
• Motor neurons carry instructions from your central nervous system out to your body’s muscles and glands.
• Interneurons within your brain and spinal cord communicate with one another and process information between the sensory input and motor output.

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

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 trigger your hand to turn the 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).

The two subdivisions of your autonomic nervous system help you cope with challenges (Figure 2.4). If something alarms or challenges you (perhaps giving a speech or presentation), 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. It will conserve 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. I recently experienced my ANS in action. Before sending me into an MRI (magnetic resonance imaging) machine for a routine shoulder scan, the technician asked if I had claustrophobia issues (panic feelings when confined). “No, I’m fine,” I assured her, with perhaps a hint of macho swagger. Moments later, as I found myself on my back, stuck deep inside a coffin-sized box and unable to move, my sympathetic nervous system had a different idea. As 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 suddenly 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.

The Central Nervous System

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. According to one estimate based on human tissue samples, an adult male brain has about 86 billion neurons, give or take 8 billion (Azevedo et al., 2009). A grain-of-sand-sized speck of your brain contains some 100,000 neurons and one billion “talking” synapses—places where neurons meet and greet their neighbors (Ramachandran & Blakeslee, 1998). Operating this heart-of-your-smarts computing system takes energy. Your brain accounts for only about 2 percent of your body weight but uses 20 percent of your energy.

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 form networks 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 brain and the peripheral nervous system. 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.

What happens 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 similarly paralyzed may respond with vaginal lubrication. But, depending on where and how completely the spinal cord is severed, people may have no genital responses to erotic images and no genital feeling (Kennedy & Over, 1990; Sipski & Alexander, 1999). To produce physical pain or pleasure, the sensory information must reach the brain.