A Textbook of Neuroanatomy

Maria A. Patestas, Leslie P. Gartner

• Publisher: Wiley-Blackwell
• Year: 2013
• Print ISBN:
  9781405103404
• Edition: 1
• eBook ISBN:
  9781118687741
• Topic: Biological Sciences
• Subtopic: Neuroscience
• Index: Biological Sciences

This complete, yet concise text is designed to help students easily master the anatomy and basic physiology of the nervous system. Accessible and clear, the text highlights interrelationships between systems, structures and the rest of the body as it moves through various regions of the brain. The first nine chapters introduce the main principles and terms in neuroanatomy, and the remaining chapters then use this information to describe the anatomy and function of the various pathways and discrete systems.

• Navigates students through the general principles and integrative components of the Nervous System
  
• Highlights interrelationships between systems, structures, and the rest of the body
  
• Emphasizes clinical relevance through clinical cases, questions, and follow-up discussions in each chapter
  
• Indicates medical conditions relevant to each chapter in the Clinical Considerations
  
• Features an accompanying website, www.blackwellpublishing.com/patestas, which includes all the illustrations, along with animations of key processes; also available on CD-ROM. Please contact our Higher Education team at [email protected] for more information.

PART 1

General Principles of the Nervous System

CHAPTER 1

Introduction to the Nervous System

CELLS OF THE NERVOUS SYSTEM

CENTRAL NERVOUS SYSTEM

PERIPHERAL NERVOUS SYSTEM

QUESTIONS TO PONDER

The human nervous system is an extremely efficient, compact, fast, and reliable computing system, yet it weighs substantially less than most computers and performs at an incredibly greater capacity. It has the capability of performing tasks that are far beyond the abilities of any computer yet devised. The present textbook deals mostly with the anatomy of the central nervous system, and in case the reader wonders why we study neuroanatomy, we should remember that it is our central nervous system more than anything else about us that makes us what we are, human beings.

The nervous system is subdivided, morphologically, into two compartments, the central nervous system (CNS), the brain and the spinal cord, and the peripheral nervous system (PNS), which emanates from and is a physical extension of the CNS. The PNS is composed of cranial and spinal nerve fibers and ganglia. Functionally, the nervous system is also subdivided into two components, the somatic nervous system, which is under the individual’s conscious control, and the autonomic nervous system, which controls the myriad of activities in conjunction with the voluntary nervous system. The autonomic nervous system is a tripartite organization, in that it has a sympathetic, a parasympathetic, and an entreric component. Simply stated, the first initiates the “flight or fight” response, the second is concerned with the body’s vegetative activities, whereas the enteric nervous system is involved in regulating the process of digestion. It must be understood, however, that the interplay of these three systems maintains homeostasis. The autonomic nervous system acts upon three cell types to perform its functions, these are cells of glands, smooth muscle, and cardiac muscle. Moreover, the nervous system has two other functional components, sensory and motor. The sensory component collects information and transmits it to the CNS (and is therefore called afferent), where the information is sorted, analyzed, and processed. Generally speaking, the motor component delivers the results of the analysis away from the CNS (and is therefore called efferent) to the effector organs, i.e., muscles and glands, resulting in a response to the stimulus.

Discussion of the topics of neuroanatomy requires that the student be familiar with some of the specialized terminology of the subject matter. One of the problems that students have in studying neuroanatomy is that there is a plethora of terms applied to the same or similar structures. It is important, therefore, to begin the discussion of this subject matter by listing and defining in Table 1.1 some of the terminology the student will encounter.

CELLS OF THE NERVOUS SYSTEM

Neurons

Neurons are the functional units of the central nervous system

The functional unit of the nervous system is the neuron. There are several types of neurons (detailed in Chapter 3) but they all have similar structures and functions. Neurons are capable of receiving, conducting, and transmitting impulses to each other as well as to muscle cells and cells of glands. Usually, neurons receive information at processes known as dendrites and transmit information along their single axon. Thus dendrites conduct information toward the cell body, whereas axons conduct information away from the cell body. Neurons usually communicate with each other as well as with other cells at synapses, where neurotransmitter substances are released from the axon terminal of the first neuron and bind to receptor molecules on the surface of the second neuron (or muscle/gland cell). Neurons may also communicate with each other via gap junctions, intercellular pores that permit the movement of small secondary messenger molecules from the cytoplasm of one cell into the cytoplasm of the neighboring cell, initiating a requisite response in the target cell.

Table 1.1 Common terms in neuroanatomy.

Neuroglia

Neuroglia constitute several categories of non-neuronal cells, namely microglia, macroglia, and ependymal cells.

Additional cells, known as neuroglia, constitute several categories of non-neuronal supporting cells. Those in the central nervous system are known as macroglia, ependymal cells, and microglia. The first two are derived from cells of the neural tube, whereas microglia are macrophages whose origins are monocyte precursors of the bone marrow.

Ependymal cells form a simple cuboidal epithelium that lines the central canal of the spinal cord and the ventricles of the brain. Additionally, these cells also participate in the formation of the choroid plexus, vascular tufts of tissue that manufacture cerebrospinal fluid. Macroglia is a collective term for the protoplasmic astrocytes, fibrous astrocytes, and oligodendroglia. Protoplasmic astrocytes support neurons in the gray matter, form a subpial barrier, and envelop capillaries of the CNS. Fibrous astrocytes are located in the white matter and appear to function in a similar fashion to protoplasmic astrocytes. Astrocytes also function in scavenging ions and neurotransmitter substances from the extracellular spaces. Oligodendroglia form myelin sheaths around axons and also surround dendrites and cell bodies of neurons in the CNS. Schwann cells are located in the PNS and they function in forming myelin around axons of the PNS. They also envelop unmyelinated axons.

CENTRAL NERVOUS SYSTEM

The central nervous system is composed of the large, anteriorly situated brain and smaller, cylindrically shaped spinal cord.

The central nervous system is a complex, hollow tube, whose rostral end, the brain, is enlarged and folded in an elaborate manner, whereas its caudal end, the spinal cord, is a long, tubular structure (Fig. 1.1). The brain is housed in the cranial cavity and at the foramen magnum is continuous with the spinal cord, housed in the vertebral canal. The dorsal surface of the spinal cord is closer to the spinous processes of the vertebrae, whereas its ventral surface is closer to the bodies of the vertebrae. Since the CNS, as well as most of the body, is bilaterally symmetric, the sagittal (midsagittal, according to some) plane bisects it into right and left halves. Positioning toward the sagittal plane is considered to be the medial direction and away from the sagittal plane is the lateral direction.

Brain

The brain is subdivided into five regions: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

The brain is subdivided into five major regions, the largest being the telencephalon, which is composed of the cerebral hemispheres; the other regions are: the diencephalon, whose component parts are the epithalamus, thalamus, hypothalamus and subthalamus; the mesencephalon, consisting of the cerebral peduncles (tegmentum and crus cerebri) and the tectum (superior and inferior colliculi); the metencephalon, including the pons and cerebellum; and the myelencephalon (medulla oblongata). Frequently the medulla oblongata, mesencephalon, and the pons are collectively termed the brainstem. The lumen of the CNS is a narrow slit, the central canal, in the spinal cord, but is expanded into a system of ventricles in the brain and is filled with cerebrospinal fluid. Twelve pairs of cranial nerves emerge from the brain to supply motor, sensory, and parasympathetic innervation for the head and neck and much of the viscera of the body.

Spinal cord

The spinal cord is a cylindrical structure whose neurons are arranged in such a fashion that the motor functions are ventrally positioned and the sensory functions dorsally positioned.

The spinal cord (Fig. 1.2) is a cylindrical aggregate of nervous tissue, where white matter surrounds a central cylinder of gray matter. The neurons of the spinal cord are arranged in such a fashion that those concerned with somatic motor function are located in the ventral horn and their axons leave via the ventral rootlets. These are accompanied by axons of the preganglionic sympathetic neurons, located in the lateral horn of the spinal cord in the thoracic and upper lumbar regions, and axons of preganglionic parasympathetic neurons located in the lateral horn of the sacral spinal cord. The dorsal horn of the spinal cord is the location where central processes of unipolar neurons of dorsal root ganglia enter the spinal cord via dorsal rootlets bringing sensory information to the CNS. Interneurons connect two neurons to each other (e.g., unipolar sensory neurons of the dorsal root ganglia to motor neurons of the ventral horn). Thus, interneurons have the capability of facilitating or inhibiting a motor response to a sensory stimulus. For example, if you prick your finger the reflex response is to pull the finger away from the offending stimulus; however, if a health professional sticks your finger for a blood test, the interneuron inhibits the withdrawal of the finger.

Figure 1.1 The brain, spinal cord, spinal nerves, and major somatic plexuses. Note that the back of the skull as well as the spinal processes of the vertebrae have been removed and that the dura mater and the arachnoid have been opened up so that the spinal cord may be viewed in its entire length.

Figure 1.2 The spinal cord, its meninges, spinal nerves, and sympathetic chain ganglia.

The white matter of the spinal cord is composed of ascending and descending tracts of nerve fibers that connect regions of the CNS to one another. Ventral and dorsal rootlets at each level of the spinal cord join each other to form the spinal nerves that leave the spinal cord at regular intervals, indicative of its segmentation. Attached to each dorsal root is a dorsal root ganglion, housing the soma of the unipolar (pseudounipolar) neurons.

Gray matter and white matter

Gray matter is composed of neuron cell bodies, clusters of which within the CNS are known as nuclei, whereas white matter is recognized by the presence of myelinated axons

The nerve cell bodies of the CNS are grouped into large aggregates, known as gray matter. Gray matter may be arranged in sheaths, as in the cerebral cortex, or as a smaller collection of nerve cell bodies, known as a nucleus (or occasionally, and technically incorrectly, a ganglion, e.g., basal ganglia). There are two major categories of neurons, those whose axons leave the CNS and interneurons, whose axons remain within the CNS. The first group, called principal cells by some neuroanatomists, are generally motoneurons (somatic or autonomic), whereas interneurons relay information from one (or one group) of neurons to a second (or second group) of neurons within the CNS (e.g., the interneuron of a reflex arc).

White matter is composed of processes of neurons, many of whose axons are wrapped in a myelin sheath, which in a living individual has a white color. These axons are collected into small bundles, known as fasciculi, or large bundles, called funiculi. Certain larger fiber bundles are named tracts or capsules, whereas axons that cross the midline to connect identical structures on opposing sides are known as commissures. Axons that travel up or down the CNS and cross the midline from one side to the other are said to decussate at the point of crossing over.

PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system is a continuation of the CNS; it is composed of clusters of nerve cell bodies, known as ganglia, as well as of bundles of axons and central processes, known as nerves

The peripheral nervous system is composed of cranial nerves, spinal nerves, their associated ganglia, and nerve fibers of the autonomic nervous system. It must be understood that the PNS is in physical continuity with the CNS, in fact cell bodies of many of the nerve fibers (axons) of the PNS are located in the CNS.

Somatic nervous system

The somatic nervous system is composed of the 12 pairs of cranial nerves and their ganglia as well as of the 31 pairs of spinal nerves and their dorsal root ganglia

There are 12 pairs of cranial nerves, identified both by name as well as by Roman numerals I through XII. All cranial nerves, with the exception of the vagus (CN X), serve structures in the head and neck only. The vagus nerve has responsibilities in the head and neck, but also serves many of the thoracic and abdominal viscera, e.g., the heart and alimentary tract. Those cranial nerves that have sensory components possess sensory ganglia housing the cell bodies of unipolar neurons whose single process bifurcates into a central and a peripheral process. The central process of a unipolar neuron enters the brain, whereas its peripheral process goes to a sensory receptor. There are no synapses occurring in these sensory ganglia.

There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal), attesting to the segmentation of the spinal cord (see Fig. 1.1). The cell bodies of sensory neurons (unipolar neurons) are located in the dorsal root ganglia (sensory ganglia). Again, it must be remembered that there are no synapses occurring in the dorsal root ganglia. The single process of each neuron bifurcates and the short central process joins other central processe...

Table of contents

• Cover
• Contents
• Dedication
• Title Page
• Copyright
• Preface
• PART 1: General Principles of the Nervous System
• PART 2: Integrative Components of the Nervous System
• Questions to ponder: answers to odd questions
• Index