The Nervous System

• Author: Samuel D. Hodge, Jr./Jack E. Hubbard
• Profession: Skilled litigator, is chair of the department of legal studies at Temple University/Professor of Neurology at the University of Minnesota
• Pages: 257-345

The Nervous System

Activity of the nervous

system improves the

capacity for activity,

muscle makes

it stronger.

Dr. Ralph Gerard

(1900–1974)

5

Introduction

Why does a person with a herniated disc in the cervical spine occasionally complain of

pain or numbness in the arm or fingers? It is often because the protruding disc compresses

a nerve root coming off of the spinal cord.

The nerves that branch off of the spinal cord consist of 31 pairs; 8 cervical nerves, 12

thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. In turn, they

exit through holes in the vertebrae on the left and right sides called foramen in order to

transverse the body.1 These nerves form part of the nervous system, which is the wiring

for our body so that signals may be transmitted to and from the brain allowing the body

to move in a coordinated fashion.

Of the systems of the body, all varied and serving multiple functions, the nervous system

is the most fascinating, yet also the least understood. All of the other body systems function

to keep the nervous system intact and working. Or, as Thomas A. Edison put it, “The chief

function of the body is to carry the brain around.”2 Everything that an individual does,

says, hears, smells, touches, and thinks is accomplished by the nervous system. This system

determines who we are. As noted in Amorgianos v. National Railroad Passenger Corp.:

The human nervous system has two components: the central nervous system (CNS)

and the peripheral nervous system (PNS). The CNS, which consists of the brain and

the spinal cord, is the locus of cognition, behavior, memory, and emotion, as well as the

control of voluntary and reflex movements. The PNS consists of neurons and ganglia

that lie outside the brain and spinal cord, and is responsible for carrying sensory

information from the body to the CNS and for regulating the function of various

internal organs and other internal bodily processes.3

The prefix for the nervous system is neuro-; a neurologist and a neurosurgeon are the

medical and surgical specialists, respectively, who treat diseases and trauma to this body

system. More specifically, neurology involves the diagnosis and treatment of disorders

concerning the central, peripheral, and automatic nervous systems, along with their

coverings, blood vessels, and effector tissue, such as muscle.4

The nervous system detects changes within the body, known as the internal

environment, as well as outside the body, termed the external environment. It then analyzes

these alterations, incorporates them, decides on a plan of action, and then sends impulses

throughout the body for an appropriate response.5

This chapter explains the building blocks of the nervous system; how they are put

together to form the component parts of this unit; how a physician examines the nervous

system; how and why electrodiagnostic testing of the nervous system is performed; and

some of the more common disorders that affect this important body system. The second

part of the chapter explores some of the ways that the parts of the nervous system have

arisen in a law-related context.

258◆CHAPTER 5

An Overview

The easiest and most logical way to start an examination of the nervous system is to look

at it microscopically and then analyze how these building blocks are put together to form

the nervous system. Only two types of cells make up the nervous system—nerve cells,

known as neurons, and supporting cells called glial cells, also called neuroglia or sometimes

simply glia.6 Neurons, the basic structural and functioning units of the nervous system,

are organized in incredibly complex ways to form control centers that are interconnected

by defined pathways. When assembled together, neurons, along with the supporting glial

cells, form the two major divisions of the nervous system—the central nervous system

and the peripheral nervous system. The central nervous system (figure 5-1a) consists of the

brain and spinal cord and is encased and protected by bone. The brain is enclosed in the

skull, and the spinal cord lies within the vertebral column. The peripheral nervous system

is the way that the body is connected to the central nervous system. Figure 5-1b. The

cranial nerves that control the head and neck attach directly to the brain, and the spinal

nerves connect the rest of the body to the spinal cord.

The information processed by the nervous system is of three basic types: sensory,

integrative, and motor. The sensory system (also called afferent) brings in data regarding

the environment—sounds, sights, pain, touch, and smell—and allows individuals to

consciously perceive this information. These signals travel up the spinal nerves and spinal

cord on their journey to the brain. The integrative portions of the nervous system make

sense out of the sensory input, relying heavily upon memories stored within the brain. The

motor system (also called efferent) controls the muscles of the body and is responsible for

the way that humans act upon the sensory information as interpreted by the integrative

centers. These signals travel out from the brain and down the spine on their way to the

spinal nerves. Through the nervous system, people are uniquely capable of conceptual

thought, that is, the ability to record, store, and transmit data received and actions taken

as a source of learned experiences that can be applied to future environmental changes.

FIGURE 5-1.

The nervous system.

The nervous system is made of

the central nervous system (CNS)

and the peripheral nervous system

(PNS).

5-1a: The central nervous system,

composed of the brain and spinal

cord, are protected by bone; the

brain is encased within the skull,

and the spinal cord is located

within the vertebral column (spine).

5-1b: The peripheral nervous sys-

tem connects the central nervous

system with the rest of the body.

The cranial nerves are the connect

structures of the head to the brain;

the spinal nerves connect the rest

of the body to the spinal cord.

ba

THE NERVOUS SYSTEM◆259

A third system, the autonomic nervous system, regulates those bodily functions not typi-

cally under conscious control, such as blood pressure, heart rate, body temperature, and

bowel motility. The autonomic nervous system has controlling centers within the central

nervous system and connects to the organs of the body, such as the heart, blood vessels,

and intestines, via the peripheral nervous system. As stated in Smith, Kline & French Labo-

ratories v. Clark & Clark, the autonomic nervous system is that body part by which every

individual structure in the body is brought under dual control by the parasympathetic and

sympathetic nervous systems.7 Figure 5-2.

Building Blocks

The nervous system is constructed of two basic cell types: neurons and glial cells. Neurons

are the cells that combine to form nerves8 and create the functional units of the nervous

system, providing this system with the means to access sensory information, process that

information, and act accordingly.Glialcells, originally thought to play only a metabolic

and supportive function, are now understood to actively participate in neural functioning.

Neurons

Neurons are highly specialized cells that process and transmit information from one part

of the body to another.9 While they vary widely in size and shape, depending upon their

specific function, all neurons have three basic parts: (1) a cell body, which contains the

metabolic machinery to keep the cell alive; (2) dendrites, which, from their interconnections

with other neurons or sensory receptors, transmit impulses toward the cell body; and

(3) the axon, a single output from a neuron that conducts impulses away from the cell

body and connects with other neurons, muscle, or organs.10 Figure 5-3. A collection of

neuronal cell bodies within the nervous system is called a nucleus or a ganglion. There are

some key differences in the terminology used in describing the central and peripheral

nervous systems. For instance, nucleus is the name applied to a collection of neuronal cell

bodies in the central nervous system, while ganglion describes a collection of neuronal

FIGURE 5-2.

The autonomic nervous system.

The autonomic nervous system

(ANS) controls most organs of the

body with two different subsys-

tems, the sympathetic and para-

sympathetic systems, which exert

opposite actions. The sympathetic

system provides the “flight-or-

fight” responses such as increased

heart rate and blood pressure. The

parasympathetic system has a “chill-

out” effect, such as decreasing the

heart rate and blood pressure.