The Nervous System

• 10-1 What are the functions of the nervous system’s main divisions, and what are the three main types of neurons?

Neurons communicating with neurotransmitters make up our body’s nervous system, a communication network that takes in information from the world and the body’s tissues, makes decisions, and sends back information and orders to the body’s tissues (Figure 10.1).

nervous system

the body’s speedy, electrochemical communication network, consisting of all the nerve cells of the peripheral and central nervous systems.

Figure 10.1 The functional divisions of the human nervous system

To the left of the image, there is a human figure depicting the peripheral nervous system. These nerves, which connect the central nervous system to the rest of the body, do not include the brain or the spinal cord. Highlighted in orange in the image, they are shown to run through the neck, shoulders, arms, and legs, as well as the chest and stomach. The peripheral nervous system also connects to the sensory organs, such as the eyes and ears. The central nervous system, highlighted in green on the human figure, consists of the brain and the spinal cord only. In the middle of the image, the tree diagram is as follows: the first box, out of which the rest of the elements flow, is the nervous system and is colored blue. It then splits into two boxes: the peripheral (colored orange) and central (brain and spinal cord) (colored green) nervous systems. There are no further branches flowing from the central nervous system. The diagram continues through the peripheral nervous system box. It splits into two branches: to the left, autonomic (controls self-regulated action of internal organs and glands; and to the right, somatic. There are two further branches flowing from the somatic box: to the left is sensor input and to the right motor output (controls skeletal muscles. From the Autonomic box, there are two further branches: to the left, the sympathetic (arousing); and to the right, the parasympathetic (calming).

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 from 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 from the eye to the brain (Mason & Kandel, 1991).

central nervous system (CNS)

the brain and spinal cord.

peripheral nervous system (PNS)

the sensory and motor neurons that connect the central nervous system (CNS) to the rest of the body.

nerves

bundled axons that form neural cables connecting the central nervous system with muscles, glands, and sense organs.

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 outward to the body’s muscles and glands. Between the sensory input and motor output, information is processed via the interneurons of the spinal cord and brain. 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.

sensory (afferent) neurons

neurons that carry incoming information from the body’s tissues and sensory receptors to the brain and spinal cord.

motor (efferent) neurons

neurons that carry outgoing information from the brain and spinal cord to the muscles and glands.

interneurons

neurons within the brain and spinal cord; they communicate internally and process information between the sensory inputs and motor outputs.

The Peripheral Nervous System

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 turn the page and read on.

somatic nervous system

the division of the peripheral nervous system that controls the body’s skeletal muscles. Also called the skeletal nervous system.

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 operates on its own (autonomously).

autonomic [aw-tuh-NAHM-ik] nervous system (ANS)

the part of the peripheral nervous system that controls the glands and the muscles of the internal organs (such as the heart). Its sympathetic division arouses; its parasympathetic division calms.

The autonomic nervous system’s subdivisions serve two important functions (Figure 10.2). The sympathetic nervous system arouses and expends energy (think “fight or flight”). If something alarms or challenges you (such as taking the AP^® Psychology exam or being stuffed in an MRI machine), 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 AP^® exam or MRI is over), your parasympathetic nervous system will produce the opposite effects, conserving energy as it calms you (think “rest and digest”). The sympathetic and parasympathetic nervous systems work together to keep us in a steady internal state called homeostasis (more on this in Module 37).

Figure 10.2 The dual functions of the autonomic nervous system

The autonomic nervous system controls the more autonomous (or self-regulating) internal functions. Its sympathetic division arouses and expends energy. Its parasympathetic division calms and conserves energy, allowing routine maintenance activity. For example, sympathetic stimulation accelerates heartbeat, whereas parasympathetic stimulation slows it.

Flip It Video: Structure of the Nervous System

At the center of the image is a man shown from the back. His spinal cord and brain are shown in overlay and labeled. All the functions of the autonomic nervous system stem from the spinal cord. The nervous parasympathetic nervous system (calming) functions stem either from the top of the spine or the very bottom. The diagram shows how the nerves are connected to each organ for each specific function. At the top of the spine, connected through the cranial nerves, the parasympathetic nervous system has four functions. First, it contracts pupils through one nerve stemming from the spine connected to one nerve in the eye. Second, a single nerve stemming from the spinal cord splits to connect to the heart, stomach, and liver. Second, the parasympathetic system slows the heartbeat. The single nerve stemming from the spine has three branches in the heart. Third, it stimulates digestion. The nerve connects to each the stomach, the pancreas, and the intestine. Fourth, it stimulates the gallbladder. At the bottom of the spinal cord, connected through the sacral nerves, the parasympathetic nervous system has two further functions. First, a nerve stemming from the spine connects to two in the bladder and contracts the bladder. Second, at the very bottom of the spine, a single nerve splits into two to connect to each the penis and the testicle. The sympathetic nervous system (arousing) is connected through the “middle” of the spinal cord. First, it dilates pupils. Second, three nerves stemming from three vertebrae connect to the heart to accelerate heartbeat. Second, a nerve connected to the stomach and another to the pancreas, and two connected to the intestine inhibit digestion. Third, three nerves stemming from the spine connect to the liver. The image shows three nervous endings within the liver. Fourth, a single nerve stemming from the spine connects to the adrenal gland in the kidney to stimulate the secretion of epinephrine and norepinephrine. Three nerves stemming from the spinal cord connect to a single node and split into two nerves – one that connects to the bladder and one that connects to the sex organ. The first stem has two nerve endings in the bladder and its function is to relax the bladder. The second has a nerve ending in the penis and one in the testicle and its function is to stimulate ejaculation in males.

sympathetic nervous system

the division of the autonomic nervous system that arouses the body, mobilizing its energy.

parasympathetic nervous system

the division of the autonomic nervous system that calms the body, conserving its energy.

Check Your Understanding

Ask Yourself

• Have you ever felt your sympathetic nervous system kick in at a stressful moment? Were you able to sense your parasympathetic nervous system’s response when the experience ended?

Test Yourself

• Match the type of neuron to its description.

Type	Description
Motor neurons	carry incoming messages from sensory receptors to the CNS.
Sensory neurons	communicate within the CNS and process information between incoming and outgoing messages.
Interneurons	carry outgoing messages from the CNS to muscles and glands.

• What bodily changes does your ANS direct before and after you give an important speech?

Answers to the Test Yourself questions can be found in Appendix E at the end of the book.

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. 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 brain has some 86 billion neurons (Azevedo et al., 2009; Herculano-Houzel, 2012).

“The body is made up of millions and millions of crumbs.”

The student and teachers are drawn and sliced bread with arms and legs. The teacher, standing at the front of the class, is holding a pointer stick. On the blackboard, the text reads “Biology”. The sliced bread students are sitting at their desks and have books in front of them. The caption reads “The body is made up of millions and millions of crumbs.”

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; each layer’s cells connect with various cells in the neural network’s next layer. Learning—to play the violin, speak a foreign language, or solve a math problem—occurs as experience strengthens connections. Neurons that fire together wire together.

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.

reflex

a simple, automatic response to a sensory stimulus, such as the knee-jerk response.

Another neural circuit enables the pain reflex (Figure 10.3). 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.

Figure 10.3 A simple reflex

Flip It Video: Reflex Arc

The image shows a finger almost touching a flame. The first caption reads, “In this simple pain reflex, information is carried from skin receptors on the finger tip along a sensory neuron to the spinal cord. The path is marked in red with a red arrow flowing from the finger to the brain. From there, it is passed via interneurons to motor neurons that lead to the muscles in the hand and arm.” There is a blue arrow flowing back from the spinal cord to the hand and the muscle on the finger, indicating this flow of information. The sensory neuron, marked in red, deals with incoming information, while the motor neuron, marked in blue, takes outgoing information. These two are connected in the spinal cord with an interneuron. Because this reflex involves only the spinal cord, the hand jerks away from the candle flame even before information about the event has reached the brain, causing the experience of pain. The second caption explains, “Because this reflex involves only the spinal cord, the hand jerks away from the candle flame even before information about the event has reached the brain, causing the experience of pain.”

“ If the nervous system be cut off between the brain and other parts, the experiences of those other parts are nonexistent for the mind. The eye is blind, the ear deaf, the hand insensible and motionless.”

William James, The Principles of Psychology, 1890

Information travels to and from the brain by way of the spinal cord, unless that connection becomes broken (as in the case of full-body transplant volunteer Wang Huanming). 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. To produce bodily pain or pleasure, the sensory information must reach the brain.

Check Your Understanding

Ask Yourself

• Does our nervous system’s design—with its synaptic gaps that chemical messenger molecules cross in an imperceptibly brief instant—surprise you? Would you have designed yourself differently?

Test Yourself

• How does information flow through your nervous system as you pick up a fork? Can you summarize this process?

Answers to the Test Yourself questions can be found in Appendix E at the end of the book.