Biology of Humans
8. The Nervous System
In the previous chapter we considered the structure and function of neurons, the cells of the nervous system that communicate with one another and with muscles or glands. In this chapter, we explore the organization of the nervous system and the structures responsible for its many functions. We also discuss some disorders of the brain and spinal cord and their effects on the human body and mind.
Organization of the Nervous System
If you were to view the nervous system apart from the rest of the body, you would see a dense mass of neural tissue where the head should be, with a cord of neural tissue extending downward from it where the middle of the back should be. These structures are the brain and spinal cord, respectively, and they constitute the central nervous system (CNS), which integrates and coordinates all voluntary and involuntary nervous functions (Figure 8.1). Connected to the brain and spinal cord are many communication "cables"—the nerves that carry messages to and from the CNS. The nerves branch extensively, forming a vast network. Some of their cell bodies are grouped together in small clusters called ganglia (singular, ganglion). The nerves and ganglia are located outside of the CNS and make up the peripheral nervous system (PNS). The PNS keeps the CNS in continuous contact with almost every part of the body.
FIGURE 8.1. An overview of the nervous system. The various parts of the system have special functions but work together as an integrated whole.
The peripheral nervous system can be further subdivided on the basis of function into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that carry information to and from the CNS, resulting in sensations and voluntary movement. The autonomic nervous system, on the other hand, governs the involuntary, subconscious activities that keep the body functioning properly. The autonomic nervous system has two parts that generally produce opposite effects on the muscles or glands they control. One, the sympathetic nervous system, is in charge during stressful or emergency conditions. The other, the parasympathetic nervous system, adjusts bodily function so that energy is conserved during nonstressful times.
· The nervous system integrates sensory and motor information with memories, which allows us to perform highly skilled activities.
Although we have described the nervous system as having different parts and divisions, remember that all the parts function as a coordinated whole. Imagine for a moment that you are meditating in the park; your eyes are closed, and you are resting. While you are relaxing, the parasympathetic nervous system is ensuring that your life-sustaining bodily activities continue. Suddenly, someone grasps your hand. Sensory receptors in the skin (which are part of the somatic nervous system) respond to the pressure and warmth of the hand by sending messages over sensory nerves to the spinal cord. Neurons within the spinal cord relay the messages to the brain. The brain integrates incoming sensory information and "decides" on an appropriate response. For example, the brain may generate messages that cause your eyes to open. If the sight of the person holding your hand produces strong emotion, the sympathetic nervous system may speed up your heartbeat and perhaps even your breathing.
The Central Nervous System
The central nervous system includes the brain and spinal cord, which are made up of many closely packed neurons. Neurons are very fragile, and most cannot divide and produce new cells. Therefore, with few exceptions, a neuron that is damaged or dies cannot be replaced.
Protection of the Central Nervous System
The brain and spinal cord are protected by bony cases (the skull and vertebral column), membranes (the meninges), and a fluid cushion (cerebrospinal fluid).
The meninges. The meninges are three protective connective tissue coverings of the brain and spinal cord (Figure 8.2). The outermost layer, the dura mater, is tough and leathery. Beneath the dura mater is the arachnoid (Latin, meaning "like a cobweb"). The arachnoid is anchored to the next-lower layer of meninges by thin, threadlike extensions that resemble a spider's web (hence the name of the layer). The innermost layer, the pia mater, is molded around the brain. Fitting like a leotard, the pia mater dips into every indentation on the brain's surface.
FIGURE 8.2. The central nervous system is protected by the meninges, the cerebrospinal fluid, and the bones of the skull and vertebral column.
Meningitis is an inflammation of the meninges. All cases of meningitis must be taken seriously because the infection can spread to the underlying nervous tissue and cause encephalitis (inflammation of the brain), which can be deadly. Many types of bacteria and certain viruses can cause meningitis. If bacteria are the cause, the person is treated with antibiotics. If a virus is the cause, treatment includes medicines to alleviate pain and fever while the body's immune system fights the virus.
Freshmen college students housed in dormitories are at increased risk of getting bacterial meningitis because of their close living quarters. Part of the reason is the means by which the bacteria are spread. People can carry the bacteria in their throat without having any symptoms of illness and can spread the infection through coughing, sneezing, or intimate kissing. Upperclassmen are less susceptible, perhaps because they have built up immune defenses against the bacteria. Vaccines are available against most, but not all, forms of meningitis. Some colleges are now requiring that incoming freshmen be vaccinated against some of the most common forms of meningitis.
Cerebrospinal fluid. The cerebrospinal fluid fills the space between layers of the meninges as well as the internal cavities of the brain, called ventricles, and the cavity within the spinal cord, called the central canal. This fluid is formed in the ventricles and circulates from them through the central canal. Eventually, cerebrospinal fluid is reabsorbed into the blood.
Cerebrospinal fluid has several important functions:
• Shock absorption. Just as an air bag protects the driver of a car by preventing impact with the steering wheel, the cerebrospinal fluid protects the brain by cushioning its impact with the skull during blows or other head trauma.
• Support. Because the brain floats in the cerebrospinal fluid, it is not crushed under its own weight.
• Nourishment and waste removal. The cerebrospinal fluid delivers nutrients and chemical messengers and removes waste products.
The brain is, indeed, protected by the skull, meninges, and cerebrospinal fluid. Nonetheless, 7 million brain injuries occur annually in the United States. We discuss brain injury further in the Health Issue essay, Brain Injury: A Silent Epidemic.
The blood-brain barrier. The CNS is also protected by the blood-brain barrier, a mechanism that selects the substances permitted to enter the cerebrospinal fluid from the blood. This barrier is formed by the tight junctions between the cells of the capillary walls that supply blood to the brain and spinal cord. Because the cells are held together much more tightly than are cells in capillaries in the rest of the body, substances in the blood are forced to pass through the cells of the capillaries instead of between the cells. Thus, the membranes of the capillary cells filter and adjust the composition of the filtrate by selecting the substances that can leave the blood. The plasma membranes of the capillary walls are largely lipid. So, lipid-soluble substances, including oxygen and carbon dioxide, can pass through easily. Certain drugs, including caffeine and alcohol, are lipid soluble, explaining why they can have a rapid effect on the brain. However, the blood-brain barrier prevents many potentially life-saving, infection-fighting, or tumor-suppressing drugs that are not lipid soluble from reaching brain tissue, which frustrates physicians.
Brain: Command Center
In a sense, your brain is more "you" than is any other part of your body, because it holds your emotions and the keys to your personality. Yet, if you were to look at your brain, you might not recognize it as yourself. The brain is the consistency of soft cheese and weighs less than 1600 g (3 lb), which is probably less than 3% of your body weight. Nevertheless, it is the origin of your secret thoughts and desires; it remembers your most embarrassing moment; and it keeps all your body systems functioning harmoniously while your conscious mind concentrates on other activities. Let's look at how its many circuits are organized to perform these amazing feats.
Cerebrum. The cerebrum is the largest and most prominent part of the brain. It is, quite literally, your "thinking cap." Accounting for 83% of the total brain weight, the cerebrum gives you most of your human characteristics.
The many ridges and grooves on the surface of the cerebrum make it appear wrinkled. Some furrows are deeper than others. The deepest indentation is in the center and runs from front to back. This groove, called the longitudinal fissure, separates the cerebrum into two hemispheres. Each hemisphere receives sensory information from and directs the movements of the opposite side of the body. In addition, the hemispheres process information in slightly different ways and are, therefore, specialized for slightly different mental functions (Figure 8.3).
FIGURE 8.3. A section through the brain from front to back, indicating the functions of selected structures
How do the left and right cerebral hemispheres communicate with one another?
The corpus callosum.
The thin outer layer of each hemisphere is called the cerebral cortex. (Cortex means "bark" or "rind.") The cerebral cortex consists of billions of neuroglial cells, nerve cell bodies, and unmyelinated axons and is described as gray matter. Although the cerebral cortex is only about 2.5 mm (about 1/8 in.) thick, it is highly folded. These folds, or convolutions, triple the surface area of the cortex.
Beneath the cortex is the cerebral white matter, which appears white because it consists primarily of myelinated axons. Recall from Chapter 7 that myelin sheaths increase the rate of conduction along axons and are, therefore, found on axons that conduct information over long distances. The axons of the cerebral white matter allow various regions of the brain to communicate with one another and with the spinal cord. A very important band of white matter, called the corpus callosum, connects the two cerebral hemispheres so they can communicate with one another.
Other grooves on the surface of the brain mark the boundaries of four lobes on each hemisphere: the frontal, parietal, temporal, and occipital lobes (Figure 8.4). Each of these lobes has its own specializations. Although the assignment of a specific function to a particular region of the cerebral cortex is imprecise, it is generally agreed that there are three types of functional areas: sensory, motor, and association.
FIGURE 8.4. The cerebral cortex, (a) The cerebral cortex has four lobes. Some of the functions associated with each lobe are indicated, (b) These PET scans of the brain show regions of increased blood flow during different mental activities. The increased flow of blood shows which region becomes active when the cerebrum is engaged in hearing words, seeing words, speaking words, and reading words. Notice the relation between active regions of the cerebral cortex during these tasks and the cortical areas for various language skills shown in part (a).
Sensory Areas. Our awareness of sensations depends on the sensory areas of the cerebral cortex. The various sensory receptors of the body send information to the cortex, where each sense is processed in a different region. If you stand on a street corner watching a parade go by, you hear the band play because information from your ears is sent to the auditory area in the temporal lobe. You see the flags wave because information from your eyes is sent to the visual area in the occipital lobe. When you catch a whiff of popcorn, information is sent from the olfactory (smell) receptors in your nose to the olfactory area in the temporal lobe of the cortex. As you eat that popcorn, you know it is too salty because information from the taste receptors is sent to gustatory areas in the parietal lobe.
Still watching the parade, you know that you are standing in the hot sun and that your belt is too tight because information from touch, pain, and temperature receptors in the skin and from receptors in the joints and skeletal muscles is sent to the primary somatosensory area. This region forms a band in the parietal lobes that stretches over the cortex from ear to ear (Figure 8.5). Sensations from different parts of the body are represented in different regions of the primary somatosensory area (of the hemisphere on the opposite side of the body). The greater the degree of sensitivity, the greater the area of cortex devoted to that body part. Thus, your most sensitive body parts, such as the tongue, hands, face, and genitals, have more of the cortex devoted to them than do less sensitive areas, such as the forearm.
Motor Areas. If you decide to join the parade, the primary motor area (Figure 8.5) of the cerebral cortex will send messages to your skeletal muscles. This motor area controls voluntary movement. It also forms a band in the frontal lobe that stretches over the cortex, just anterior to the primary somatosensory area. The motor area is organized in a manner similar to the somatosensory area. Each point on its surface corresponds to the movement of a different part of the body. The parts of the body we have finer control over, such as the tongue and fingers, have greater representation on the motor cortex than do regions with less dexterity, such as the trunk of the body.
FIGURE 8.5. The primary motor and the primary somatosensory regions of the cerebral cortex are organized in such a way that each location on their surface corresponds to a particular part of the body. The general arrangement is similar in the two regions.
Our lips are more sensitive than is the skin on our forearm. We also have greater motor control of our lips than we do of our forearm. How is this difference in sensitivity and motor control represented on the cerebral cortex?
Just in front of the motor cortex is the premotor cortex. It coordinates learned motor skills that are patterned or repetitive, such as typing or playing a musical instrument. The premotor cortex coordinates the movement of several muscle groups at the same time. When a pattern of movement is repeated many times, the proper pattern of stimulation is stored in the premotor cortex. For example, as a guitar player practices playing a particular song many times, the pattern of stimulation needed to play that song is stored in the premotor cortex. Then, each time the song is played, the premotor cortex will stimulate the primary cortex in the pattern needed to play that song, without requiring the musician to think about where on the strings the fingers should be placed.
Association Areas. Next to each primary sensory area is an association area. These communicate with the sensory and motor areas, and with other parts of the brain, to analyze and act on sensory input. In particular, each sensory association area communicates with the general interpretation area, to recognize what the sensory receptors are sensing. The general interpretation area assigns meaning to sensory information by integrating the input from sensory association areas with stored sensory memories. For example, on a dark night your eyes may detect a small, moving object. If the object then rubs against your legs and purrs, your general interpretation area will assist you in recognizing it as the neighbor's friendly cat. However, if the object turns away from you and raises its tail, you will recognize it as a skunk.
Once the sensory input has been interpreted, the information is sent to the most complicated of all association areas, the prefrontal cortex. This most anterior part of the frontal lobe predicts the consequences of various possible responses to the information it receives and decides which response will be best for you in your current situation. The prefrontal cortex enables us to reason, plan for the long term, and think about abstract concepts. It also plays a key role in determining our personality.
Thalamus. The cerebral hemispheres sit comfortably over the thalamus (see Figure 8.3). The thalamus is often described as the gateway to the cerebral cortex because all messages to the cerebral cortex must pass through the thalamus first. The thalamus functions in sensory experience, motor activity, stimulation of the cerebral cortex, and memory. Sensory input from every sense except smell and from all parts of the body is delivered to the thalamus. The thalamus sorts the information by function and relays it to appropriate regions of the cortex for processing. Some regions of the thalamus also integrate information from different sources rather than just relaying it. At the thalamic level of processing, you have a general impression of whether the sensation is pleasant or unpleasant. If you step on a tack, for instance, you may experience pain by the time the messages reach the thalamus; however, you will not know where you hurt until after the message is directed to the cerebral cortex.
Hypothalamus. Below the thalamus is the hypothalamus (hypo, under), a small region of the brain that is largely responsible for homeostasis—the body's maintenance of a stable environment for its cells (discussed in Chapter 4). The hypothalamus, shown in Figure 8.3, coordinates the activities of the nervous and endocrine (hormonal) systems through its influence on the pituitary gland. The hypothalamus also influences blood pressure, heart rate, digestive activity, breathing rate, and many other vital physiological processes. It keeps body temperature near the set point, and it regulates hunger and thirst and therefore the intake of food. Moreover, because the hypothalamus receives input from the cerebral cortex, it can make your heart beat faster when you so much as see or think of something exciting or dangerous—a rattlesnake about to strike, for instance.
The hypothalamus is part of the limbic system (discussed later in this chapter), so it is also part of the circuitry for emotions. Specific regions of the hypothalamus play a role in the sex drive and in the perception of pain, pleasure, fear, and anger.
Brain Injury: A Silent Epidemic
Most of us take our brain for granted. We assume that this fragile control center is safe from harm, safely guarded by the thick bones of our skulls and cushioned by cerebrospinal fluid. The truth is that the brain is more vulnerable than we may think, and injuries to this vital organ are frighteningly common. Brain injury has been termed a “silent epidemic." It is silent because a brain-injured person doesn't have visible physical j symptoms. It is an epidemic because it is so common. One in every 220 people in the United States is suffering from a brain injury. A brain injury occurs every 16 seconds; a death from head injury c occurs every 12 minutes.
Brain injuries are categorized as either acquired or traumatic. Acquired brain injury (ABI) is caused by a disruption in oxygen flow to the brain. Examples of ABIs include strokes and aneurysms, heart attacks, brain tumors, anoxia, meningitis, seizure disorders, and substance abuse. There is a strong correlation between substance abuse and acquired brain injury because alcohol and other substances are neurotoxins that cause damage to the brain with repeated use. Furthermore, substance abuse is associated with poor nutrition, which can cause dehydration and ultimately wastes brain cells.
To reduce your risk of traumatic brain injury always wear a helmet when cycling.
Traumatic brain injury, or TBI, is caused by an external force. There are two types of TBI: open and closed. An open head injury is when the scalp is cut through and the skull is broken, damaging the brain underneath. A closed head injury happens when the head suddenly changes motion, forcing the brain to follow the movement, like when a car stops very suddenly. The brain is soft and jellylike, and it sits snugly within the skull. Sudden movement of the head can cause it to ricochet within the skull, damaging the millions of nerve fibers that run from one part of the brain to another. Also, the inside of the skull has many ridges and sharp edges that can cut or bruise the brain. Common causes of TBI include motor vehicle accidents, firearms, brawls, slip-and-fall accidents, and accidents related to sports such as skiing. Substance abuse is also associated with TBI, as the impairment caused by alcohol and drugs can lead to vehicular accidents and increase risk of falls and physical altercations. You can protect yourself against TBI by wearing a helmet when biking, skiing, or doing any other sport where a fall is likely. Since motor vehicle accidents cause nearly half of all head injuries, please buckle up!
TBI is getting a good deal of attention recently because nearly two-thirds of injured U.S. soldiers sent from Iraq to Walter Reed Medical Center have been diagnosed with traumatic brain injury. That percentage, thought to be higher than in any other past U.S. conflict, is said to be due to improved armor that allows soldiers to survive injuries that previously would have been fatal. Also, compared to other wars, fewer firearms and more improvised explosive devices (IEDs) are being used in combat; the intense vibrations from these explosives cause the brain to move within the soldiers' skulls.
Regardless of cause, no two brain injuries are the same. The symptoms are diverse and vary widely due to severity and location of injury as well as the individual's functioning before the accident. Symptoms frequently include cognitive and emotional limitations, including difficulties with memory, attention, and reasoning; depression; anxiety; and impulse control and anger management issues. Physical impairments are common and can range from weakness on one side of the body to paralysis.
Questions to Consider
• Many states have laws requiring motorcyclists and bicyclists to wear helmets. Do you think cyclists riding without helmets should be fined?
• Do you think that skiers and snowboarders should be allowed on the slopes without helmets?
Cerebellum. The cerebellum (see Figure 8.3) is the part of the brain responsible for sensory-motor coordination. It acts as an automatic pilot that produces smooth, well-timed voluntary movements and controls equilibrium and posture. Sensory information concerning the position of joints and the degree of tension in muscles and tendons is sent to the cerebellum from all parts of the body. By integrating this information with input from the eyes and the equilibrium receptors in the ears, the cerebellum knows the body's position and direction of movement at any given instant.
The coordination of sensory input and motor output by the cerebellum involves two important processes: comparison and prediction. During every move you make, the cerebellum continuously compares the actual position of each part of the body with where it ought to be at that moment (in relation to the intended movement) and makes the necessary corrections. Try to touch the tips of your two index fingers together above your head. You probably missed on the first attempt. However, the cerebellum makes the necessary corrections, and you will likely succeed on the next attempt. At the same time, the cerebellum calculates future positions of a body part during a movement. Then, just before that part reaches the intended position, the cerebellum sends messages to stop the movement at a specific point. Therefore, when you scratch an itch on your cheek, your hand stops before slapping your face!
Brain stem. The brain stem consists of the medulla oblongata, the midbrain, and the pons. The medulla oblongata is often called simply the medulla (see Figure 8.3). This marvelous inch of nervous tissue contains reflex centers for some of life's most vital physiological functions—including the pace of the basic breathing rhythm, the force and rate of heart contraction, and blood pressure. The medulla connects the spinal cord to the rest of the brain. Therefore, all sensory information going to the upper regions of the brain and all motor messages leaving the brain are carried by nerve pathways running through the medulla.
The midbrain processes information about sights and sounds and controls simple reflex responses to these stimuli. For example, when you hear an unexpected loud sound, your reflexive response is to turn your head and direct your eyes toward the source of the sound.
The pons, which means "bridge," connects lower portions of the CNS with higher brain structures. More specifically, it connects the spinal cord and cerebellum with the cerebrum, thalamus, and hypothalamus. In addition, the pons has a region that assists the medulla in regulating respiration.
Stop and think
Why would a brain tumor that destroyed the functioning of nerve cells in the medulla lead to death more quickly than a tumor of the same size on the cerebral cortex?
Limbic system. The limbic system is a collective term for a group of structures that help to produce emotions and memory (Figure 8.6). The limbic system is defined on the basis of function rather than anatomy, and it includes parts of several brain regions and the neural pathways that connect them.
FIGURE 8.6. The limbic system and reticular activating system. The diagram shows the limbic system in purple as a three-dimensional structure within the brain, viewed from the left side. The reticular activating system is shown in green. Note the upward arrows.
The limbic system is our emotional brain. It allows us to experience countless emotions, including rage, pain, fear, sorrow, joy, and sexual pleasure. Emotions are important because they motivate behavior that will increase the chance of survival. Fear, for example, may have evolved to focus the mind on the threats in the environment so it can prepare the body to face them.
Connections between the cerebrum and the limbic system allow us to have feelings about thoughts. As a result, you may become excited at the thought of winning the lottery. Such connections also allow us to have thoughts about feelings, thus keeping us from responding to emotions, such as rage, in ways that would be unwise. The limbic system includes the hypothalamus, as Figure 8.6 shows. In addition, it is connected to lower brain centers, such as the medulla, that control the activity of internal organs. Therefore, we also have "gut" responses to emotions.
You wouldn't be you without your memories, and the limbic system plays a role in forming them. Memory, the storage and retrieval of information, takes place in two stages. The first is short-term memory, which holds a small amount of information for a few seconds or minutes, as when you look up a phone number and remember it only long enough to place the call. The second stage, long-term memory, stores seemingly limitless amounts of information for hours, days, or years. Not all short-term memories get consolidated into long-term memories, but when they do, the hippocampus plays an essential role. The amygdala, another part of the limbic system that functions in long-term memory, has widespread connections to sensory areas as well as to emotion centers. It associates memories gathered through different senses and links them to emotional states.
The olfactory bulb transmits information about odors from the nose to the limbic system. Thus, the limbic system is a center where emotions, memory, and our sense of smell meet. As a result, we often have emotional responses to odors. The association between odor and emotion is the basis of aromatherapy as well as the perfume and scented candle industries. The interaction of emotion, sense of smell, and memory explains why odors can bring back memories. For example, the smell of cinnamon rolls may be pleasant because it reminds you of your grandmother baking special treats.
Reticular Activating System. The reticular activating system (RAS) is an extensive network of neurons that runs through the medulla and projects to the cerebral cortex (shown in Figure 8.6 in green). The RAS functions as a net, or filter, for sensory input. Our brain is constantly flooded with tremendous amounts of sensory information, about 100 million impulses each second, most of them trivial. The RAS filters out repetitive, familiar stimuli—the sound of street traffic, paper rustling, the coughing of the person next to you, or the pressure of clothing. However, infrequent or important stimuli pass through the RAS to the cerebral cortex and, therefore, reach our consciousness. Because of the RAS, you can fall asleep with the television on but wake up when someone whispers your name.
In addition, the RAS is an activating center. Unless inhibited by other brain regions, the RAS activates the cerebral cortex, keeping it alert and "awake." Consciousness occurs only while the RAS stimulates the cerebral cortex. When sleep centers in other regions of the brain inhibit activity in the RAS, we sleep. In essence, then, the cerebrum "sleeps" whenever it is not stimulated by the RAS. Sensory input to the RAS results in stimulation of the cerebral cortex and an increase in consciousness, which explains why it is usually easier to sleep in a dark, quiet room than in an airport terminal. Conscious activity in the cerebral cortex can also stimulate the RAS, which in turn will stimulate the cerebral cortex. Therefore, thinking about a problem may keep you awake all night.
Stop and think
When a boxer is hit very hard in the jaw, his head—containing his medulla and RAS—is twisted sharply. Why might this twisting result in a knockout, in which the boxer loses consciousness?
Spinal Cord: Message Transmission and Reflex Center
The other major component of the central nervous system besides the brain is the spinal cord. The spinal cord is a tube of neural tissue that is continuous with the medulla at the base of the brain and extends about 45 cm (17 in.) to just below the last rib. For most of its length, the spinal cord is about the diameter of your little finger. It becomes slightly thicker in two regions, just below the neck and at the end of the cord, because of the large group of nerves connecting these regions of the cord with the arms and legs. The central canal, filled with cerebrospinal fluid, runs the length of the spinal cord.
The spinal cord is encased in and protected by the stacked bones of the vertebral column (Figure 8.7). Pairs of spinal nerves (considered part of the peripheral nervous system) extend from the spinal cord through openings between the vertebrae to serve different parts of the body. The vertebrae are separated by disks of cartilage that act as cushions.
FIGURE 8.7. The spinal cord is a column of neural tissue that runs from the base of the brain to just below the last rib. It is protected by the bones of the vertebral column.
The spinal cord has two functions: (1) to transmit messages to and from the brain and (2) to serve as a reflex center. The transmission of messages is performed primarily by white matter, found toward the outer surface of the spinal cord. White matter in the spinal cord consists of myelinated axons grouped into tracts. Ascending tracts carry sensory information up to the brain. Descending tracts carry motor information from the brain to a nerve leaving the spinal cord.
The second function of the spinal cord is to serve as a reflex center. A reflex is an automatic response to a stimulus, prewired in a circuit of neurons called a reflex arc. The circuit consists of a receptor, a sensory neuron (which brings information from the receptors toward the CNS), usually at least one interneuron, a motor neuron (which brings information from the CNS toward an effector), and an effector (a muscle or a gland). The gray matter, which is located in the central region of the spinal cord, houses the interneurons and the cell bodies of motor neurons involved in reflexes.
Spinal reflexes are essentially "decisions" made by the spinal cord. They are beneficial when a speedy reaction is important to a person's safety. Consider, for example, the withdrawal reflex. When you step on a piece of broken glass, impulses speed toward the spinal cord over sensory nerves (Figure 8.8). Within the gray matter of the spinal cord, the sensory neuron synapses with an interneuron. The interneuron, in turn, synapses with a motor neuron that sends a message to the appropriate muscle to contract and lift your foot off the glass.
FIGURE 8.8. A reflex arc consists of a sensory receptor, a sensory neuron, usually at least one interneuron, a motor neuron, and an effector.
While the spinal reflexes were removing the foot from the glass, pain messages from the cut foot were sent to the brain through ascending tracts in the spinal cord. However, it takes longer to get a message to the brain than it does to get one to the spinal cord, because the distance and number of synapses to be crossed are greater. Therefore, by the time pain messages reach the brain, you have already withdrawn your foot. Nonetheless, once the sensory information reaches the conscious brain, decisions can be made about how to care for the wound.
The Peripheral Nervous System
The nerves and ganglia of the PNS carry information between the CNS and the rest of the body. The PNS consists of spinal nerves and cranial nerves.
The body has 31 pairs of spinal nerves, each of which originates in the spinal cord and services a specific region of the body. One member of each pair serves a part of the right side of the body, and the other serves the corresponding part of the left side (Figure 8.9a). All spinal nerves carry both sensory and motor fibers. Fibers from the sensory neurons enter the spinal cord from the dorsal, or posterior, side, grouped into a bundle called the dorsal root. The cell bodies of these sensory neurons are located in a ganglion in the dorsal root. The axons of motor neurons leave the ventral (front side) of the spinal cord in a bundle called the ventral root. The cell bodies of motor neurons are located in the gray matter of the spinal cord. The dorsal and ventral roots join to form a single spinal nerve, which passes through the opening between the vertebrae.
The 12 pairs of cranial nerves (Figure 8.9b) arise from the brain and service the structures of the head and certain body parts, including the heart and diaphragm. Some cranial nerves carry only sensory fibers, others carry only motor fibers, and others carry both types of fibers.
FIGURE 8.9. (a) Spinal and (b) cranial nerves. The 12 pairs of cranial nerves can be seen in this view of the underside of the brain. Most cranial nerves service structures within the head, but some service organs lower in the body. The descriptions indicate whether the neuron carries sensory information (toward the brain) or motor information (away from the brain).
Somatic Nervous System
The peripheral nervous system is subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system carries sensory messages that tell us about the world around us and within us, and it controls movement. Sensory messages carried by somatic nerves result in conscious sensations, including light, sound, and touch. The somatic nervous system also controls our voluntary movements, allowing us to smile, stamp a foot, sing a lullaby, or frown as we sign a check.
Autonomic Nervous System
As part of the body's system of homeostasis, the autonomic nervous system automatically adjusts the functioning of our body organs so that the proper internal conditions are maintained and the body is able to meet the demands of the world around it. The somatic nervous system sends information about conditions within the body to the autonomic nervous system. The autonomic nervous system then makes the appropriate adjustments. Its activities alter digestive activity, open or close blood vessels to shunt blood to areas that need it most, and alter heart rate and breathing rate.
Recall that the autonomic nervous system consists of two branches: the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system gears the body to face an emergency or stressful situation, such as fear, rage, or vigorous exercise. Thus, the sympathetic nervous system prepares the body for fight or flight. In contrast, the parasympathetic nervous system adjusts body function so that energy is conserved during relaxation.
Both the parasympathetic and the sympathetic nervous systems send nerve fibers to most, but not all, internal organs (Figure 8.10). When both systems send nerves to a given organ, they have opposite, or antagonistic, effects on its function. If one system stimulates, the other system inhibits. The antagonistic effects are brought about by different neurotransmitters. Whereas sympathetic neurons release mostly norepinephrine at their target organs, parasympathetic neurons release acetylcholine at their target organs.
FIGURE 8.10. Structure and function of the autonomic nervous system. Most organs are innervated by fibers from both the sympathetic and the parasympathetic nervous systems. When this dual innervation occurs, the two branches of the autonomic nervous system have opposite effects on the activity level of that organ. A chain of ganglia links the pathways of the sympathetic nervous system, which therefore usually acts as a unit, with all its effects occurring together. In contrast, the ganglia of the parasympathetic nervous system are each near the organ they service, so parasympathetic effects are more localized.
The sympathetic nervous system acts as a unified whole, bringing about all its effects at once. It is able to act in this way because its neurons are connected through a chain of ganglia. A unified response is exactly what is needed in an emergency. To meet a threat, the sympathetic nervous system increases breathing rate, heart rate, and blood pressure. It also increases the amount of glucose and oxygen delivered to body cells to fuel the response. In addition, it stimulates the adrenal glands to release two hormones, epinephrine and norepinephrine, into the bloodstream. These hormones back up and prolong the other effects of sympathetic stimulation. Lastly, the sympathetic nervous system inhibits digestive activity, because digesting the previous meal is hardly a priority during a crisis.
The effects of the parasympathetic nervous system occur more independently of one another. After the emergency, organ systems return to a relaxed state at their own pace. Organs can respond to the parasympathetic nervous system independently because the ganglia containing the parasympathetic neurons that stimulate each organ are located near the individual organs—not in a chain near the spinal cord, as they are in the sympathetic nervous system.
Disorders of the Nervous System
Disorders of the nervous system vary tremendously in severity and impact on the body. Some disorders, such as a mild headache, are often more of a nuisance than a health problem. Others, such as insufficient sleep, can cause more problems than a person might expect. Still other disorders, such as stroke, coma, and spinal cord injury, can have devastating effects on a person's well-being.
Excessive exercise may make your muscles hurt. However, thinking too much cannot cause a headache. The brain has no pain receptors, so a headache is not a brain ache. Headaches can occur for almost any reason: they can be caused by stress or by relaxation, by hunger or by eating the wrong food, or by too much or too little sleep. The most common type of headache is a tension headache, affecting some 60% to 80% of people who suffer from frequent headaches. In response to stress, most of us unconsciously contract the muscles of our head, face, and neck. Therefore, the pain of a tension headache is usually a dull, steady ache, often described as feeling like a tight band around the head. Migraine headaches are usually confined to one side of the head, often centered behind one eye. A migraine headache typically causes a throbbing pain that increases with each beat of the heart. It is sometimes called a sick headache because it may cause nausea and vomiting. Some migraine sufferers experience an aura, a group of sensory symptoms, different for different people, that occurs just before an attack. The aura may include visual disturbances (a blind spot, zigzag lines, flashing lights), auditory hallucinations, or numbness. Though the causes of migraines are not entirely understood, some researchers believe that migraines are set off by an imbalance in the brain's chemistry. Specifically, the level of one of the brain's chemical neurotransmitters, serotonin, is low. With too little serotonin, pain messages flood the brain.
What would you do?
In an experimental pain treatment for severe headaches, a tiny electrode is implanted in the skin and placed near the nerve responsible for the pain. The device, powered by a battery implanted near the collarbone, delivers continuous electric pulses intended to block the pain signals in the nerve and stop the pain. If you suffered from severely painful headaches, would you opt for this treatment? What criteria would you use to decide?
A stroke, also called a cerebrovascular accident, is the death of nerve cells caused by an interruption of blood flow to a region of the brain. Neurons have a high demand for both oxygen and glucose. Therefore, when the blood supply to a portion of the brain is shut off, the affected neurons begin to die within minutes. The extent and location of the mental or physical impairment caused by a stroke depend on the region of the brain involved. If the left side of the brain is affected, the person may lose sensations in or the ability to move parts of the right side of his or her body because motor nerve pathways cross from one side of the brain to the other in the lower brain. Because the language centers are usually in the left hemisphere, the person may also have difficulty speaking. When the stroke damages the right rear of the brain, some people show what is called the neglect syndrome and behave as if the left side of things, even their own bodies, does not exist. The person may comb only the hair on the right side of the head or eat only the food on the right side of the plate.
Common causes of strokes include blood clots blocking a vessel, hemorrhage from the rupture of a blood vessel in one of the meninges, or the formation of fatty deposits that block a vessel. High blood pressure, heart disease, diabetes, smoking, obesity, and excessive alcohol intake increase the risk of stroke.
Although a comatose person seems to be asleep—with eyes closed and no recognizable speech—a coma is not deep sleep. A person in a coma is totally unresponsive to all sensory input and cannot be awakened. Although the cerebral cortex is most directly responsible for consciousness, damage to the cerebrum is rarely the cause of coma. Instead, coma is caused by trauma to neurons in regions of the brain responsible for stimulating the cerebrum, particularly those in the reticular activating system or thalamus. Coma can be caused by mechanical shock— as might be caused by a blow to the head—tumors, infections, drug overdose, or failure of the liver or kidney.
Spinal Cord Injury
The spinal cord is the pathway that allows the brain to communicate with the rest of the body. Therefore, damage to the spinal cord can impair sensation and motor control below the site of injury. The extent and location of the injury will determine how long these symptoms persist, as well as the degree of permanent damage. Depending on which nerve tracts are damaged, injury may result in paralysis, loss of sensation, or both. If the cord is completely severed, there is a complete loss of sensation and voluntary movement below the level of the cut.
Restoring the ability to function to people with spinal cord injuries is an active area of research. Some researchers are trying to reestablish neural connections by stimulating nerve growth through treatments with nerve growth factors. Others are exploring the potential use of stem cells for treatment (discussed in Chapter 19a). Stem cells retain the ability to develop into nerve cells, and they have been used successfully by researchers to restore some movement in laboratory mice with spinal cord injuries. Another approach to restoring the ability to move is to use computers to electronically stimulate specific muscles and muscle groups. The stimulation is delivered through wires that are either implanted under the skin or woven into the fabric of tight-fitting clothing. A small computer, usually worn at the wrist, directs the stimulation to the appropriate muscles. This technology has helped some people with a spinal cord injury to walk again. It has also helped some people by stimulating the diaphragm, a muscle important in breathing.
In Chapter 7, we learned that neurons communicate with one another using chemicals called neurotransmitters. Neurotransmitter molecules fit into receptors on the membrane of the receiving neuron and cause ion channels to open, either exciting or inhibiting the receiving neuron. Different neurotransmitters play roles in different behavioral systems.
In this chapter, we learned that different parts of the nervous system are specialized for different functions. The limbic system of the brain is a “pleasure center.” The sympathetic nervous system prepares the body for emergency situations.
Next, in Chapter 8a, “Special Topic: Drugs and the Mind,” we will consider psychoactive drugs—those that affect a person's mental state. We will see that psychoactive drugs work by increasing or decreasing the effects of specific neurotransmitters and therefore affecting specific regions of the brain.
Highlighting the Concepts
Organization of the Nervous System (pp. 129-130)
• The nervous system is divided into the central nervous system (CNS), which includes the brain and spinal cord; and the peripheral nervous system (PNS), which includes all the neural tissue outside the CNS. The peripheral nervous system can be further subdivided into the somatic nervous system and the autonomic nervous system.
The Central Nervous System (pp. 130-138)
• The brain and the spinal cord are protected by the bony cases of the skull and vertebral column, by membranes (the meninges), and by a fluid cushion (cerebrospinal fluid).
• The meninges are three protective layers of connective tissue that cover the brain and spinal cord. Bacteria and viruses can cause inflammation of the meninges, resulting in the condition called meningitis.
• The cerebrospinal fluid, located between layers of the meninges, serves as a shock absorber for the brain, supports the brain, and provides nourishment to and removes waste from the brain.
• The blood-brain barrier is a filter that allows only certain substances to enter the cerebrospinal fluid from the blood, thus protecting the brain and spinal cord from many potentially damaging substances.
• The brain serves as the body's central command center, coordinating and regulating the body's other systems.
• The cerebrum is the thinking, conscious part of the brain. It consists of two hemispheres. Each hemisphere receives sensory impressions from and directs the movements of the opposite side of the body. The cerebrum has an outer layer of gray matter called the cerebral cortex and an underlying layer of white matter consisting of myelinated nerve tracts that allow communication between various regions of the brain.
• The cerebral cortex has three types of functional areas: sensory, motor, and association. Our awareness of sensation depends on the sensory areas of the cerebral cortex. Motor areas of the brain control the movement of different parts of the body. Association areas communicate with the sensory and motor areas to analyze and act on sensory input.
• The thalamus is an important relay station for all sensory experience except smell. It also plays a role in motor activity, stimulation of the cerebral cortex, and memory.
• The hypothalamus is essential in maintaining a stable environment within the body. It regulates many vital physiological functions, such as blood pressure, heart rate, breathing rate, digestion, and body temperature. The hypothalamus also coordinates the activities of the nervous and endocrine systems through its connection to the pituitary gland. As part of the limbic system, the hypothalamus is a center for emotions.
• The primary function of the cerebellum is sensory-motor coordination. It integrates information from the motor cortex and sensory pathways to produce smooth movements.
• The medulla oblongata regulates breathing, heart rate, and blood pressure. It also serves as a pathway for all sensory messages to higher brain centers and for motor messages leaving the brain.
• The pons connects lower portions of the CNS with higher brain structures. It connects the spinal cord and cerebellum to the cerebrum, thalamus, and hypothalamus.
• The limbic system, which includes several brain structures, is largely responsible for emotions. The hippocampus, which is part of the limbic system, is essential to converting short-term memory to long-term memory.
• The reticular activating system is a complex network of neurons that filters sensory input and keeps the cerebral cortex in an alert state.
• The spinal cord is a cable of nerve tissue extending from the medulla to approximately the bottom of the rib cage. The spinal cord has two functions: to conduct messages between the brain and the body and to serve as a reflex center.
The Peripheral Nervous System (pp. 138-139)
• The PNS consists of spinal nerves, each originating in the spinal cord and serving a specific region of the body, and cranial nerves, each arising from the brain and serving the structures of the head and certain body parts such as the heart and diaphragm.
• The PNS is divided into the somatic nervous system, which governs conscious sensations and voluntary movements, and the autonomic nervous system, which helps regulate our unconscious, involuntary internal activities.
• The autonomic nervous system can be divided into the sympathetic and parasympathetic nervous systems, two branches with antagonistic actions. The sympathetic nervous system gears the body to face stressful or emergency situations. The parasympathetic nervous system adjusts body functioning so that energy is conserved during restful times.
Disorders of the Nervous System (pp. 139-141)
• Headaches can range from relatively mild (tension headaches) to severe (migraine).
• A stroke is caused by an interruption of blood flow leading to the death of nerve cells. The effects depend on the region of the brain affected.
• Coma, a condition in which a person is totally unresponsive to sensory input, can be caused by a blow to the head, tumors, infections, drugs, or failure of the liver or kidney.
• Because the spinal cord contains the pathways of communication between the brain and the rest of the body, damage to the spinal cord impairs functioning below the site of injury.
Reviewing the Concepts
1. Distinguish between the central nervous system and the peripheral nervous system. List two components of each. p. 129
2. Describe three features that protect the brain and spinal cord. pp. 130-131
3. What are the functions of cerebrospinal fluid? p. 131
4. What forms gray matter? What is its function? What forms white matter? What is its function? p. 132
5. Describe the three types of functional areas of the cerebral cortex. pp. 132-134
6. In what way is the organization of the primary somatosensory area and that of the primary motor area of the cerebral cortex similar? In what way do these areas differ? pp. 132-133
7. List five functions of the hypothalamus. pp. 134-135
8. What is the function of the cerebellum? p. 135
9. Which functional system of the brain is responsible for emotions? pp. 134-135
10. Describe the two functions of the reticular activating system. pp. 136-137
11. List the two functions of the spinal cord, and relate each function to the structure of the spinal cord. pp. 137-138
12. You are cooking dinner and carelessly touch the hot burner on the stove. You remove your hand before you are even aware of the pain. Using the anatomy of a spinal reflex arc, explain how you could react before you were aware of the pain. p. 138
13. What are the two divisions of the peripheral nervous system? What type of response does each control? pp. 138-139
14. Compare and contrast the functions of the sympathetic and parasympathetic nervous systems. p. 139
15. List some effects of sympathetic stimulation, and explain how these prepare the body for an emergency. p. 139
16. You are watching a football game with your friends. A wide receiver makes an incredible catch and then runs 20 yards, skillfully dodging defensive players to make a touchdown. Your friend Joe says, "Amazing! How does he do that?" The receiver's outstanding sensory-motor coordination is largely due to the actions of his
c. reticular activating system.
17. As you sit here studying, you are unlikely to be aware of the pressure of your clothes against your body and the rustling of paper as other students turn pages. The part of the brain that "decides" that these are unimportant stimuli is the
a. reticular activating system.
18. Belinda was riding a bicycle without a helmet and was struck by a car. She hit the back of her head very hard in the fall. The physician is quite concerned because the medulla is located at the base of the skull. She explains to Belinda's parents that injury to the medulla could result in
a. the loss of coordination so that the child may never regain the motor skills needed to ride a bicycle.
b. the loss of speech.
c. amnesia (the loss of memory).
d. death because many life-support systems are controlled here.
19. The neural center that regulates body temperature is the _____.____________________________
20. The region of the brain that regulates basic physiological processes such as breathing and heart rate is the _____.
21. The branch of the nervous system that prepares the body to respond to emergency situations is the _____.
22. The brain region responsible for intelligence and thinking is the _____.
Applying the Concepts
1. Joe and Henry were both in car accidents, and both suffered spinal cord damage. Joe's injury was in the lower back, and Henry's was in the neck region. The degree of injury to the spinal cord is similar in both Joe and Henry. Would the resulting problems be equal in severity? Explain. Describe some of the difficulties that you might expect Joe and Henry to have.
2. When you have a cold, you might take a decongestant to help you breathe. Some decongestants contain pseudoephedrine, which mimics the effects of the sympathetic nervous system. What side effects might you expect? Would you expect this medication to make you drowsy?
3. When you have dental work done, the dentist often administers a local anesthetic in the gums near the region that requires drilling. You are usually advised not to eat anything until the anesthetic wears off. This advice is given out of concern for your tongue, not your teeth. Why?
4. After Jorge's car accident he could remember events that took place before the accident, but he would quickly forget a conversation or a television show he just watched. What part of the brain was injured in the accident?
Becoming Information Literate
When a person has a head injury, physicians may induce a coma by administering a high dose of barbiturates or sedative drugs. The intent of medically induced coma is to allow the brain to rest. It prevents additional injury by reducing blood flow, which reduces swelling. There are success stories. However, there are also risks—pneumonia or a blood clot in the lung—that could be fatal.
Use at least three reliable sources (books, journals, or websites) to gather information that would help you decide whether you would want someone you loved, who had sustained a severe head injury, to be placed in a medically induced coma. Explain why you made your decision. List each source you considered, and explain why you chose the three sources you used.