3-5 What are the functions of the nervous system’s main divisions, and what are the three main types of neurons?

All those neurons communicating with neurotransmitters make up our body’s nervous system (FIGURE 3.5). This communication network allows us 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. A quick overview: The brain and spinal cord form the central nervous system (CNS), the body’s decision maker. The peripheral nervous system (PNS) is responsible for gathering information and for transmitting CNS decisions to other body parts. Nerves, electrical cables formed of bundles of axons, link the CNS with the body’s sensory receptors, muscles, and glands. The optic nerve, for example, bundles a million axons into a single cable carrying the messages each eye sends to the brain (Mason & Kandel, 1991).

Information travels in the nervous system through three types of neurons. Sensory neurons carry messages from the body’s tissues and sensory receptors inward (thus, they are afferent) to the brain and spinal cord for processing. Motor neurons (which are efferent) carry instructions from the central nervous system out to the body’s muscles and glands. Between the sensory input and motor output, information is processed via the interneurons. Our complexity resides mostly in these interneurons. Our nervous system has a few million sensory neurons, a few million motor neurons, and billions and billions of interneurons.

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Our peripheral nervous system has two components—somatic and autonomic. Our somatic nervous system enables voluntary control of our skeletal muscles. As you reach the end of this page, your somatic nervous system will report to your brain the current state of your skeletal muscles and carry instructions back, triggering a response from your hand so you can read on.

Our autonomic nervous system (ANS) controls our glands and our internal organ muscles. The ANS influences functions such as glandular activity, heartbeat, and digestion. (Autonomic means “self-regulating.”) Like an automatic pilot, this system may be consciously overridden, but usually it operates on its own (autonomously).

The autonomic nervous system serves two important functions (FIGURE 3.6). The sympathetic nervous system arouses and expends energy. If something alarms or challenges you (such as a longed-for job interview), your sympathetic nervous system will accelerate your heartbeat, raise your blood pressure, slow your digestion, raise your blood sugar, and cool you with perspiration, making you alert and ready for action. When the stress subsides (the interview is over), your parasympathetic nervous system will produce the opposite effects, conserving energy as it calms you. The sympathetic and parasympathetic nervous systems work together to keep us in a steady internal state called homeostasis.

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I [DM] recently experienced my ANS in action. Before sending me into an MRI machine for a shoulder scan, the technician asked if I had issues with claustrophobia. “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. 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—projecting from neuron counts in small brain samples—our brains have some 86 billion neurons (Azevedo et al. 2009; Herculano-Houzel, 2012).

The brain’s neurons cluster into work groups called neural networks. To understand why, Stephen Kosslyn and Olivier Koenig (1992, p. 12) have invited us to “think about why cities exist; why don’t people distribute themselves more evenly across the countryside?” Like people networking with people, neurons network with nearby neurons with which they can have short, fast connections.

The other part of the CNS, the spinal cord, is a two-way information highway connecting the peripheral nervous system and the brain. Ascending neural fibers send up sensory information, and descending fibers send back motor-control information. 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.

Another neural circuit enables the pain reflex (FIGURE 3.7). When your finger touches a flame, neural activity (excited by the heat) travels via sensory neurons to interneurons in your spinal cord. These interneurons respond by activating motor neurons leading to the muscles in your arm. Because the simple pain-reflex pathway runs through the spinal cord and right back out, your hand jerks away from the candle’s flame before your brain receives and responds to the information that causes you to feel pain. That’s why it feels as if your hand jerks away not by your choice, but on its own.

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Information travels to and from the brain by way of the spinal cord. Were the top of your spinal cord severed, you would not feel pain from your paralyzed body below. Nor would you feel pleasure. With your brain literally out of touch with your body, you would lose all sensation and voluntary movement in body regions with sensory and motor connections to the spinal cord below its point of injury. You would exhibit the knee-jerk response without feeling the 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, they may be genitally unresponsive to erotic images and have no genital feeling (Kennedy & Over, 1990; Sipski & Alexander, 1999). To produce bodily pain or pleasure, the sensory information must reach the brain.