chapter 3 Structure of the Nervous System

Module 2 chapter 3 p630 wk1

chapter 3 Structure of the Nervous System

Outline

· ■  Basic Features of the Nervous System

An Overview

Meninges

The Ventricular System and Production of CSF

Section Summary

· ■  The Central Nervous System

Development of the Central Nervous System

The Forebrain

The Midbrain

The Hindbrain

The Spinal Cord

Section Summary

· ■  The Peripheral Nervous System

Spinal Nerves

Cranial Nerves

The Autonomic Nervous System

Section Summary

Ryan B., a college freshman, had suffered from occasional epileptic seizures since childhood. He had been taking drugs for his seizures for many years, but lately the medication wasn’t helping—his seizures were becoming more frequent. His neurologist increased the dose of the medication, but the seizures persisted, and the drug made it difficult for Ryan to concentrate on his studies. He was afraid that he would have to drop out of school.

He made an appointment with his neurologist and asked whether another drug was available that might work better and not affect his ability to concentrate. “No,” said the neurologist, “you’re taking the best medication we have right now. But I want to send you to Dr. L., a neurosurgeon at the medical school. I think you might be a good candidate for seizure surgery.”

Ryan had a focal-seizure disorder. His problems were caused by a localized region of the brain that contained some scar tissue. Periodically, this region would irritate the surrounding areas, triggering epileptic seizures—wild, sustained firing of cerebral neurons that result in cognitive disruption and, sometimes, uncontrolled movements. Ryan’s focus was probably a result of brain damage that occurred when he was born. Dr. L. ordered some tests that indicated that the seizure focus was located in the left side of his brain, in a region known as the medial temporal lobe.

Ryan was surprised to learn that he would remain awake during his surgery. In fact, he would be called on to provide information that the surgeon would need to remove a region of his brain that included the seizure focus. As you might expect, he was nervous when he was wheeled into the surgery, but after the anesthesiologist injected something through the tube in one of his veins, Ryan relaxed and thought to himself, “This won’t be too bad.”

Dr. L. marked something on his scalp, which had previously been shaved, and then made several injections of a local anesthetic. Then he cut the scalp and injected some more anesthetic. Finally, he used a drill and a saw to remove a piece of skull. He then cut and folded back the thick membrane that covers the brain, exposing the surface of the brain.

When removing a seizure focus, the surgeon wants to cut away all the abnormal tissue while sparing brain tissue that performs important functions, such as the comprehension and production of speech. For this reason, Dr. L. began stimulating parts of the brain to determine which regions he could safely remove. To do so, he placed a metal probe against the surface of Ryan’s brain and pressed a pedal that delivered a weak electrical current. The stimulation disrupts the firing patterns of the neurons located near the probe, preventing them from carrying out their normal functions. Dr. L. found that stimulation of parts of the temporal lobe disrupted Ryan’s ability to understand what he and his associates were saying. When he removed the part of the brain containing the seizure focus, he was careful not to damage these regions.

The operation was successful. Ryan continued to take his medication but at a much lower dose. His seizures disappeared, and he found it easier to concentrate in class. I met Ryan during his junior year, when he took a course I was teaching. I described seizure surgery to the class one day, and after the lecture he approached me and told me about his experience. He received the third highest grade in the class.

The goal of neuroscience research is to understand how the brain works. To understand the results of this research, you must be acquainted with the basic structure of the nervous system. The number of terms introduced in this chapter is kept to a minimum (but as you will see, the minimum is still a rather large number). With the framework you will receive from this chapter and from the features on MyPsychLab, you should have no trouble learning the material presented in subsequent chapters.

Basic Features of the Nervous System

FIGURE 3.1 Views of Alligator and Human

These side and frontal views show the terms used to denote anatomical directions.

Before beginning a description of the nervous system, I want to discuss the terms that are used to describe it. The gross anatomy of the brain was described long ago, and everything that could be seen without the aid of a microscope was given a name. Early anatomists named most brain structures according to their similarity to commonplace objects: amygdala, or “almond-shaped object”; hippocampus, or “sea horse”; genu, or “knee”; cortex, or “bark”; pons, or “bridge”; uncus, or “hook,” to give a few examples. Throughout this book I will translate the names of anatomical terms as I introduce them, because the translation makes the terms more memorable. For example, knowing that cortex means “bark” (like the bark of a tree) will help you to remember that the cortex is the outer layer of the brain.

When describing features of a structure as complex as the brain, we need to use terms denoting directions. Directions in the nervous system are normally described relative to the  neuraxis , an imaginary line drawn through the length of the central nervous system, from the lower end of the spinal cord up to the front of the brain. For simplicity’s sake, let us consider an animal with a straight neuraxis.  Figure 3.1 shows an alligator and two humans. This alligator is certainly laid out in a linear fashion; we can draw a straight line that starts between its eyes and continues down the center of its spinal cord. (See  Figure 3.1 . ) The front end is  anterior , and the tail is  posterior . The terms  rostral  (toward the beak) and  caudal  (toward the tail) are also employed, especially when referring specifically to the brain. The top of the head and the back are part of the  dorsal  surface, while the  ventral  (front) surface faces the ground. (Dorsum means “back,” and ventrum means “belly.”) These directions are somewhat more complicated in the human; because we stand upright, our neuraxis bends, so the top of the head is perpendicular to the back. (You will also encounter the terms superior and inferior. In referring to the brain, superior means “above,” and inferior means “below.” For example, the superior colliculi are located above the inferior colliculi.) The frontal views of both the alligator and the human illustrate the terms  lateral  and  medial : toward the side and toward the middle, respectively. (Look again at  Figure 3.1 . )

 lateral Toward the side of the body, away from the middle.

 medial Toward the middle of the body, away from the side.

 neuraxis An imaginary line drawn through the center of the length of the central nervous system, from the bottom of the spinal cord to the front of the forebrain.

 anterior With respect to the central nervous system, located near or toward the head.

 posterior With respect to the central nervous system, located near or toward the tail.

 rostral “Toward the beak”; with respect to the central nervous system, in a direction along the neuraxis toward the front of the face.

 caudal “Toward the tail”; with respect to the central nervous system, in a direction along the neuraxis away from the front of the face.

 dorsal “Toward the back”; with respect to the central nervous system, in a direction perpendicular to the neuraxis toward the top of the head or the back.

 ventral “Toward the belly”; with respect to the central nervous system, in a direction perpendicular to the neuraxis toward the bottom of the skull or the front surface of the body.

Two other useful terms are ipsilateral and contralateral.  Ipsilateral  refers to structures on the same side of the body. If we say that the olfactory bulb sends axons to the ipsilateral hemisphere, we mean that the left olfactory bulb sends axons to the left hemisphere and the right olfactory bulb sends axons to the right hemisphere.  Contralateral  refers to structures on opposite sides of the body. If we say that a particular region of the left cerebral cortex controls movements of the contralateral hand, we mean that the region controls movements of the right hand.

 ipsilateral Located on the same side of the body.

 contralateral Located on the opposite side of the body.

To see what is in the nervous system, we have to cut it open; to be able to convey information about what we find, we slice it in a standard way.  Figure 3.2  shows a human nervous system. We can slice the nervous system in three ways:

· 1. Transversely, like a salami, giving us  cross sections  (also known as  frontal sections  when referring to the brain)

 frontal section A slice through the brain parallel to the forehead.

 cross section With respect to the central nervous system, a slice taken at right angles to the neuraxis.

· 2. Parallel to the ground, giving us  horizontal sections

 horizontal section A slice through the brain parallel to the ground.

· 3. Perpendicular to the ground and parallel to the neuraxis, giving us  sagittal sections . The  midsagittal plane  divides the brain into two symmetrical halves. The sagittal section in  Figure 3.2  lies in the midsagittal plane.

 sagittal section (sadj  i tul ) A slice through the brain parallel to the neuraxis and perpendicular to the ground.

 midsagittal plane The plane through the neuraxis perpendicular to the ground; divides the brain into two symmetrical halves.

Note that because of our upright posture, cross sections of the spinal cord are parallel to the ground. (See  Figure 3.2 . )

FIGURE 3.2 Brain Slices and Planes

This figure shows planes of section as they pertain to the human central nervous system.

An Overview

The nervous system consists of the brain and spinal cord, which make up the central nervous system (CNS), and the cranial nerves, spinal nerves, and peripheral ganglia, which constitute the peripheral nervous system (PNS). The CNS is encased in bone: The brain is covered by the skull, and the spinal cord is encased by the vertebral column. (See  Table 3.1 . )

Figure 3.3  illustrates the relationship of the brain and spinal cord to the rest of the body. Do not be concerned with unfamiliar labels on this figure; these structures will be described later. (See  Figure 3.3 . ) The brain is a large mass of neurons, glia, and other supporting cells. It is the most protected organ of the body, encased in a tough, bony skull and floating in a pool of cerebrospinal fluid. The brain receives a copious supply of blood and is chemically guarded by the blood–brain barrier.

The brain receives approximately 20 percent of the blood flow from the heart, and it receives it continuously. Other parts of the body, such as the skeletal muscles or digestive system, receive varying quantities of blood, depending on their needs, relative to those of other regions. But the brain always receives its share. The brain can store only a small amount of its fuel (primarily glucose), and it cannot temporarily extract energy without oxygen, as the muscles can; therefore, a consistent blood supply is essential. A 1-second interruption of the blood flow to the brain uses up much of the dissolved oxygen; a 6-second interruption produces unconsciousness. Permanent damage begins within a few minutes.

Meninges

The entire nervous system—brain, spinal cord, cranial and spinal nerves, and peripheral ganglia—is covered by tough connective tissue. The protective sheaths around the brain and spinal cord are referred to as the  meninges  (singular: meninx, the Greek word for “membrane”). The meninges consist of three layers, which are shown in  Figure 3.3 . The outer layer is thick, tough, and flexible but unstretchable; its name,  dura mater , means “hard mother.” The middle layer of the meninges, the  arachnoid membrane , gets its name from the weblike appearance of the arachnoid trabeculae that protrude from it (from the Greek arachne, meaning “spider”; trabecula means “track”). The arachnoid membrane, soft and spongy, lies beneath the dura mater. Closely attached to the brain and spinal cord, and following every surface convolution, is the  pia mater  (“pious mother”). The smaller surface blood vessels of the brain and spinal cord are contained within this layer. Between the pia mater and arachnoid membrane is a gap called the  subarachnoid space . This space is filled with a liquid called  cerebrospinal fluid (CSF) . (Look again at  Figure 3.3 . )

 meninges (singular: meninx) ( men  in  jees ) The three layers of tissue that encase the central nervous system: the dura mater, arachnoid membrane, and pia mater.

 dura mater The outermost of the meninges; tough and flexible.

 arachnoid membrane ( a  rak  noyd