Recall from Chapter 13 that the nervous system is said to be composed of two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The reason for designating two distinct divisions is to make the study of the nervous system easier. In this chapter, we discuss the part of the nervous system that lies at the center of the regulatory process: the central nervous system. Comprising both the brain and the spinal cord, the central nervous system is the principal integrator of sensory input and motor output. Thus the central nervous system is capable of evaluating incoming information and formulating responses to changes that threaten our homeostatic balance.

This chapter begins with a description of the protective coverings of the brain and spinal cord. After that, we briefly discuss the watery cerebrospinal fluid (CSF) and the spaces in which it is found. We then outline the overall structure and function of the major organs of the central nervous system, beginning at the bottom with the spinal cord; this is the simplest and least complex part of the CNS. Then our focus moves upward to the more complex brain, beginning first with the narrow brainstem (Figure 14-1) and the roughly spherical cerebellum attached to its dorsal surface. Again shifting our attention upward, we describe the structure and function of the diencephalon and then move on to a discussion of the cerebrum. As we move up the central nervous system, the complexity of both structure and function increases. The spinal cord mediates simple reflexes, whereas the brainstem and diencephalon are involved in the regulation of the more complex maintenance functions, such as regulation of heart rate and breathing. The cerebral hemispheres, which together form the largest part of the brain, perform complex integrative functions such as conscious thought, learning, memory, language, and problem solving. We end the chapter with a discussion of the somatic sensory pathways and the somatic motor pathways. This prepares us for Chapter 15, which covers the peripheral nervous system, Chapter 16, which covers autonomic regulation of vital functions, and Chapter 17, which covers the sense organs.

FIGURE 14-1 **The central nervous system.** Details of both the brain and the spinal cord are easily seen in this figure.

COVERINGS OF THE BRAIN AND SPINAL CORD

Because the brain and spinal cord are both delicate and vital, nature has provided them with two protective coverings. The outer covering consists of bone: cranial bones encase the brain; vertebrae encase the spinal cord. The inner covering consists of membranes known as meninges. Three distinct layers compose the meninges:

1. Dura mater

2. Arachnoid mater

3. Pia mater

Observe their respective locations in Figures 14-2 and 14-3. The dura mater, made of strong white fibrous tissue, serves as the outer layer of the meninges and also as the inner periosteum of the cranial bones. The arachnoid mater, a delicate, spiderweb-like layer, lies between the dura mater and the pia mater, or innermost layer of the meninges. The transparent pia mater adheres to the outer surface of the brain and spinal cord and contains blood vessels.

FIGURE 14-2 **Coverings of the brain. A**, Frontal section of the superior portion of the head, as viewed from the front. Both the bony and the membranous coverings of the brain can be seen. B, Sagittal section of the skull, viewed from the left. The dura mater has been retained in this specimen to show how it lines the inner roof of the cranium and the falx cerebri extending inward.

The dura mater has three important inward extensions:

1. Falx cerebri. The falx cerebri projects downward into the longitudinal fissure to form a kind of partition between the two cerebral hemispheres. The Latin word falx means “sickle” and refers to the curving sickle shape of this partition as it extends from the roof of the cranial cavity (see Figure 14-2, B).

2. Falx cerebelli. The falx cerebelli is a sickle-shaped extension that separates the two halves, or hemispheres, of the cerebellum.

3. Tentorium cerebelli. The tentorium cerebelli separates the cerebellum from the cerebrum. It is called a tentorium (meaning “tent”) because it forms a tentlike covering over the cerebellum.

Figure 14-2 shows a large space within the dura, where the falx cerebri begins to descend between the left and right cerebral hemispheres. This space, called the superior sagittal sinus, is one of several dural sinuses. Dural sinuses function as venous reservoirs, collecting blood from brain tissues for the return trip to the heart.

A number of spaces lie between and around the meninges (see Figure 14-2). Three of these spaces are the following:

1. Epidural space. The epidural (“on the dura”) space is immediately outside the dura mater but inside the bony coverings of the spinal cord. It contains a supporting cushion of fat and other connective tissues. Around the brain, because the dura mater is continuous with the periosteum on the inside face of the cranial bones, no epidural space is normally present.

2. Subdural space. The subdural (“under the dura”) space is between the dura mater and arachnoid mater. The subdural space contains a small amount of lubricating serous fluid.

3. Subarachnoid space. As its name suggests, the subarachnoid space is under the arachnoid and outside the pia mater. This space contains a significant amount of cerebrospinal fluid.

The meninges of the cord (see Figure 14-3) continue on down inside the spinal cavity for some distance below the end of the spinal cord. The pia mater forms a slender filament known as the filum terminale (see Figure 14-1). At the level of the third segment of the sacrum, the filum terminale blends with the dura mater to form a fibrous cord that disappears in the periosteum of the coccyx.

Infections of the meninges are discussed in Box 14-1.

Box 14-1

HEALTH matters

Meningitis

Infection or inflammation of the meninges is termed meningitis. It most often involves the arachnoid and pia mater, or the leptomeninges (“thin meninges”). Meningitis is most commonly caused by bacteria such as Neisseria meningitidis (meningococcus), Streptococcus pneumoniae, or Haemophilus influenzae. However, viral infections, mycoses (fungal infections), and tumors also may cause inflammation of the meninges. Individuals with meningitis usually complain of fever and severe headaches, as well as neck stiffness and pain. Signs also include avoidance of bright lights and loud sounds, lethargy, and confusion. Persons experiencing these symptoms, some of which mimic the flu, should seek medical help immediately. In infants and very young children, so-called projective vomiting is common. College campuses are often the sites for localized outbreaks of meningitis, and several bacterial forms are highly contagious.

Depending on the primary cause, meningitis may be mild and self-limiting or may progress to a severe, even fatal, condition. If only the spinal meninges are involved, the condition is called spinal meningitis. A lumbar puncture (“spinal tap”) can be diagnostic, especially for bacterial meningitis such as the type caused by H. influenzae. Spinal meningitis must be treated immediately to avoid an increase in intracranial pressure and damage to the nervous system. If not treated quickly, meningitis can lead to deafness, deficits in cognitive ability, and permanent brain disorders such as epilepsy. Meningitis caused by H. influenzae type B, pneumococci bacteria, or the mumps virus can be prevented by immunization.

CEREBROSPINAL FLUID

In addition to its bony and membranous coverings, nature has further protected the brain and spinal cord against injury by providing a cushion of fluid both around the organs and within them. This fluid is the cerebrospinal fluid (CSF). The CSF does more than simply provide a supportive, protective cushion, however. It is also a reservoir of circulating fluid that, along with blood, the brain monitors for changes in the internal environment. For example, changes in the carbon dioxide (CO₂) content of CSF trigger homeostatic responses in the respiratory control centers of the brainstem that help regulate the overall CO₂ content and pH of the body.

Fluid Spaces

Cerebrospinal fluid is found in the subarachnoid space around the brain and spinal cord and within the cavities and canals of the brain and spinal cord.

The four large, fluid-filled spaces within the brain are called ventricles. Two of them, the lateral (or first and second) ventricles, are located one in each hemisphere of the cerebrum. As you can see in Figure 14-4, the third ventricle is little more than a thin, vertical pocket of fluid below and medial to the lateral ventricles. The fourth ventricle is a tiny, diamond-shaped space where the cerebellum attaches to the back of the brainstem. Actually, the fourth ventricle is simply a slight expansion of the central canal extending up from the spinal cord.

FIGURE 14-4 **Fluid spaces of the brain. A**, Ventricles highlighted in blue within a translucent brain in a left lateral view. B, Ventricles as seen from above. C, Ventricles as seen from the front.

Formation and Circulation of Cerebrospinal Fluid

Formation of CSF occurs mainly by separation of fluid from blood in the choroid plexuses. Choroid plexuses are networks of capillaries that project from the pia mater into the lateral ventricles and into the roofs of the third and fourth ventricles. Each choroid plexus is covered with a sheet of a special type of ependymal (glial) cell that releases the CSF into the fluid spaces. From each lateral ventricle the fluid seeps through an opening, the interventricular foramen (of Monro), into the third ventricle, then through a narrow channel, the cerebral aqueduct (of Sylvius), into the fourth ventricle (Figure 14-5). Some of the fluid moves from the fourth ventricle directly into the central canal of the cord. Some of it moves out of the fourth ventricle through openings in its roof, two lateral foramina (of Luschka) and one median foramen (of Magendie). These openings allow CSF to move into the cisterna magna, a space behind the medulla that is continuous with the subarachnoid space around the brain and cord. The fluid circulates in the subarachnoid space and then is absorbed into venous blood through the arachnoid villi (fingerlike projections of the arachnoid mater into the brain’s venous sinuses). Briefly, here is the circulation route of cerebrospinal fluid: it is formed as fluid is separated from blood in the choroid plexuses, then it flows into the ventricles of the brain, circulates through the ventricles and into the central canal and subarachnoid spaces, and is then absorbed back into blood.

FIGURE 14-5 **Flow of cerebrospinal fluid.** The fluid produced by filtration of blood by the choroid plexus of each ventricle flows inferiorly through the lateral ventricles, inter-ventricular foramen, third ventricle, cerebral aqueduct, fourth ventricle, and subarachnoid space and to the blood.

The amount of CSF in the average adult is about 140 ml (about 23 ml in the ventricles and 117 ml in the subarachnoid space of brain and cord). Box 14-2 explains the diagnostic value of testing a patient’s cerebrospinal fluid.

Box 14-2

DIAGNOSTIC study

Lumbar Puncture

The meninges extends beyond the cord, which provides a convenient location for performing lumbar punctures without danger of injuring the spinal cord. A lumbar puncture is a withdrawal of some of the cerebrospinal fluid (CSF) from the subarachnoid space in the lumbar region of the vertebral column. The physician inserts a needle just above or below the fourth lumbar vertebra, knowing that the spinal cord ends 2 or more centimeters (about an inch) above that level (Figure 1). The fourth lumbar vertebra can be easily located because it lies on a line with the iliac crest. Placing a patient on his or her side with the knees and chest drawn together to arch the back separates the vertebrae sufficiently to create a space in which the needle can be inserted. As the needle enters the CSF, the thin nerve roots roll off the tip of the needle—thus allowing collection of CSF without damaging nerve tissue.

Cerebrospinal fluid removed through a lumbar puncture can be tested for the presence of blood cells, bacteria, or other abnormal characteristics that may indicate an injury or infection, such as meningitis (Figure 2). A sensor called a manometer is sometimes attached to the needle to determine the pressure of the CSF within the subarachnoid space. The lumbar puncture can also be used to introduce diagnostic agents, such as radiopaque dyes for x-ray photography, into the subarachnoid space.

A&P CONNECT

When the circulation of cerebrospinal fluid (CSF) is blocked, there can be dramatic effects on the structure of the brain. An example is hydrocephalus, a condition in which the CSF produces abnormal fluid pressure in the brain. A brief description of hydrocephalus and how it can be treated, along with some dramatic clinical images, can be found in Hydrocephalus online at A&P Connect.

QUICK CHECK

1. Name the three membranous coverings of the central nervous system in order, beginning with the outermost layer.

2. Trace the path of cerebrospinal fluid from its formation by a choroid plexus to its reabsorption into the blood.

SPINAL CORD

Structure of the Spinal Cord

The spinal cord lies within the spinal cavity, extending from the foramen magnum to the lower border of the first lumbar vertebra (Figure 14-6), a distance of about 45 cm (18 inches) in the average body. The spinal cord does not completely fill the spinal cavity—which also contains the meninges, CSF, a cushion of adipose tissue, and blood vessels.

FIGURE 14-6 **Spinal cord.** The inset illustrates a transverse section of the spinal cord shown in the broader view.

The spinal cord is an oval-shaped cylinder that tapers slightly as it descends and has two bulges, one in the cervical region and the other in the lumbar region (see Figure 14-6). Two deep grooves, the anterior median fissure and the posterior median sulcus, just miss dividing the cord into separate symmetrical halves. The anterior fissure is the deeper and the wider of the two grooves—a useful factor to remember when you examine spinal cord diagrams. It enables you to tell at a glance which part of the cord is anterior and which is posterior.

Two bundles of nerve fibers called nerve roots project from each side of the spinal cord (see Figure 14-6). Fibers comprising the dorsal (posterior) nerve root carry sensory information into the spinal cord. Cell bodies of these unipolar, sensory neurons make up a small region of gray matter in the dorsal nerve root called the dorsal (posterior) root ganglion. Fibers of the ventral (anterior) nerve root carry motor information out of the spinal cord. Cell bodies of these multipolar, motor neurons are in the gray matter that composes the inner core of the spinal cord. Numerous interneurons are also located in the gray matter core of the spinal cord. On each side of the spinal cord, the dorsal and ventral nerve roots join together to form a single mixed nerve called, simply, a spinal nerve. Spinal nerves, components of the peripheral nervous system, are considered in more detail in the next chapter.

The spinal cord ends at vertebra L1 in a tapered cone called the conus medullaris. As you can see in Figure 14-7, many nerve roots extending from the conus medullaris form a sort of “horse tail” of spinal nerve roots called the cauda equina. Within the cauda equina the long cordlike filum terminale is formed from the spinal meninges.

FIGURE 14-7 **Cauda equina.** Photograph shows the inferior portion of the dura mater dissected posteriorly, revealing the cauda equine and nearby structures.

Although the gray matter core of the spinal cord looks like a flat letter H in transverse sections of the cord, it actually has three dimensions, because the gray matter extends the length of the cord. The H-shaped rod of gray matter is made up of anterior, lateral, and posterior gray columns. When viewed in a cross section, as in Figure 14-6, the columns forming the H appear to spread out like animal horns—and thus are also called anterior, posterior, and lateral gray horns. The left and right gray columns are joined in the middle by a band called the gray commissure. It is through the gray commissure that the central canal carries CSF through the spinal cord. The gray columns consist predominantly of cell bodies of interneurons and motor neurons.

White matter surrounding the gray matter is subdivided in each half of the cord into three white columns or funiculi: the anterior, posterior, and lateral white columns. Each white column, or funiculus, consists of a large bundle of nerve fibers (axons) divided into smaller bundles called spinal tracts, shown in Figure 14-8. The names of most spinal cord tracts indicate the white column in which the tract is located, the structure in which the axons that make up the tract originate, and the structure in which they terminate. For example, the lateral corticospinal tract is located in the lateral white column of the cord. The axons that compose it originate from neuron cell bodies in the spinal cortex (of the cerebrum) and terminate in the spinal cord. The anterior spinothalamic tract lies in the anterior white column. The axons that compose it originate from neuron cell bodies in the spinal cord and terminate in a portion of the brain called the thalamus.

FIGURE 14-8 **Major tracts of the spinal cord.** The major ascending (sensory) tracts are highlighted in blue. The major descending (motor) tracts are highlighted in red.

You may wish to refer to the atlas that accompanies this book, where you will find detailed photographs of a human spinal cord. How many structures can you identify in the atlas by sight?

Functions of the Spinal Cord

The spinal cord performs two general functions. Briefly, it provides conduction routes to and from the brain and serves as the integrator, or reflex center, for all spinal reflexes.

Spinal cord tracts provide conduction paths to and from the brain. Ascending tracts conduct sensory impulses up the cord to the brain. Descending tracts conduct motor impulses down the cord from the brain. Bundles of axons compose all tracts.

Tracts are both structural and functional organizations of the nerve fibers of the spinal cord. They are structural organizations in that all the axons of any one tract originate from neuron cell bodies located in the same area of the central nervous system, and all the axons terminate in a single structure elsewhere in the central nervous system. For example, all the fibers of the spinothalamic tract are axons originating from neuron cell bodies located in the spinal cord and terminating in the thalamus. Tracts are functional organizations in that all the axons that compose one tract serve one general function. For instance, fibers of the spinothalamic tracts serve a sensory function. They transmit impulses that produce the sensations of crude touch, pain, and temperature.

Because so many different tracts make up the white columns of the cord, we mention only a few of the more important ones. Locate each tract in Figure 14-8. Consult Tables 14-1 and 14-2 for a brief summary of these tracts.

TABLE 14-1

Major Ascending Tracts of Spinal Cord

NAME	FUNCTION	LOCATION	ORIGIN^*	TERMINATION^†
Lateral spinothalamic	Pain, temperature, and crude touch on opposite side	Lateral white columns	Posterior gray column on opposite side	Thalamus
Anterior spinothalamic	Crude touch and pressure	Anterior white columns	Posterior gray column on opposite side	Thalamus
Fasciculi gracilis and cuneatus	Discriminating touch and pressure sensations, including vibration, stereognosis, and two-point discrimination; also conscious kinesthesia	Posterior white columns	Spinal ganglia on same side	Medulla
Anterior and posterior spinocerebellar	Unconscious kinesthesia	Lateral white columns	Anterior or posterior gray column	Cerebellum
Spinotectal	Touch related to visual reflexes	Lateral white columns	Posterior gray columns	Superior colliculus (midbrain)

^*Location of cell bodies of neurons from which axons of tract arise.

^†Structure in which axons of tract terminate.

TABLE 14-2

Major Descending Tracts of Spinal Cord

NAME	FUNCTION	LOCATION	ORIGIN^*	TERMINATION^†
Lateral corticospinal (or crossed pyramidal)	Voluntary movement, contraction of individual or small groups of muscles, particularly those moving hands, fingers, feet, and toes of opposite side	Lateral white columns	Motor areas or cerebral cortex of opposite side from tract location in cord	Lateral or anterior gray columns
Anterior corticospinal (direct pyramidal)	Same as lateral corticospinal except mainly muscles of same side	Anterior white columns	Motor cortex but on same side as location in cord	Lateral or anterior gray columns
Reticulospinal	Maintain posture during movement	Anterior white columns	Reticular formation (midbrain, pons, medulla)	Anterior gray columns
Rubrospinal	Coordination of body movement and posture	Lateral white columns	Red nucleus (of midbrain)	Anterior gray columns
Tectospinal	Head and neck movement during visual reflexes	Anterior white columns	Superior colliculus (midbrain)	Medulla and anterior gray columns
Vestibulospinal	Coordination of posture/balance	Anterior white columns	Vestibular nucleus (pons, medulla)	Anterior gray columns

^*Location of cell bodies of neurons from which axons of tract arise.

^†Structure in which axons of tract terminate.

Five important ascending, or sensory, tracts and their functions, stated very briefly, are as follows:

1. Lateral spinothalamic tracts: crude touch, pain, and temperature

2. Anterior spinothalamic tracts: crude touch and pressure

3. Fasciculi gracilis and cuneatus tracts: discriminating touch and conscious sensation of position and movement of body parts (kinesthesia)

4. Spinocerebellar tracts: subconscious kinesthesia

5. Spinotectal tracts: touch that triggers visual reflexes

Further discussion of the sensory neural pathways may be found on pp. 506–507.

Six important descending, or motor, tracts and their functions described in brief are as follows:

1. Lateral corticospinal tracts: voluntary movement; contraction of individual or small groups of muscles, particularly those moving hands, fingers, feet, and toes on opposite side of body

2. Anterior corticospinal tracts: same as preceding except mainly muscles of same side of body

3. Reticulospinal tracts: help maintain posture during skeletal muscle movements

4. Rubrospinal tracts: transmit impulses that coordinate body movements and maintenance of posture

5. Tectospinal tracts: head and neck movement related to visual reflexes

6. Vestibulospinal tracts: coordination of posture and balance

Further discussion of motor neural pathways may be found on pp. 490–491.

The spinal cord also serves as the reflex center for all spinal reflexes. The term reflex center means the center of a reflex arc or the place in the arc where incoming sensory impulses become outgoing motor impulses. They are structures that switch impulses from afferent to efferent neurons. In two-neuron arcs, reflex centers are merely synapses between neurons. In all other arcs, reflex centers consist of interneurons interposed between afferent and efferent neurons. Spinal reflex centers are located in the gray matter of the cord.

A&P CONNECT

Reflex centers can act as pain control areas. Pain control areas can inhibit the pain information heading toward the conscious processing centers of the brain. Identifying pain control areas and how they work has lead to the development of transcutaneous electrical nerve stimulation (TENS) units and other therapies to reduce pain. To learn more about this check out Pain Control Areas online at A&P Connect.

QUICK CHECK

3. What are spinal nerve roots? How does the dorsal root differ from the ventral root?

4. Name the regions of the white and gray matter seen in a horizontal section of the spinal cord.

5. Contrast ascending tracts and descending tracts of the spinal cord. Can you give an example of each?

BRAIN

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Tags: Anthonys Textbook of Anatomy _ Physiology

May 25, 2016 | Posted by admin in ANATOMY | Comments Off

COVERINGS OF THE BRAIN AND SPINAL CORD