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SCI Life Archive
Spring/Summer 1997

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The Human Nervous System:
Master Organizer

By Lisa Hudgins, PVA Associate Director of Research

It's easy to ask, "Why don't scientist find a cure?" However, the body's complexity presents a wide spectrum of avenues researchers have to painstakingly check out. Like babies who must crawl before they can walk, you must first understand how the body works before you can repair it.

In an interview with Barbara Walters on ABC-TV's 20/20, Christopher Reeve talked candidly about his quadriplegia. He said he now has a greater need to understand how his body works. Like most people, Reeve, before injury, took his body's functions for granted. But that has changed. He (and all people with SCI) must pay close attention to his body. Due to the risk of secondary complications, he has to know what is "normal" and what is not.

The following article, the first in a four-part series, discusses the spinal cord and nervous system's anatomy and physiology. This basic information will help you understand more about Part II of the series, which discusses the different types of SCI. Part III will examine research strategies for repair and restoration of function, and Part IV will look at the dilemma of spinal cord research funding in this country.

Anatomy & Physiology of the Spinal Cord and Nervous System

The human nervous system is responsible for sending, receiving, and monitoring nerve impulses or signals. These electrical and chemical signals are required to organize everything we do -- from thinking about a problem, to digesting a meal, to throwing a baseball, to sweating when hot. Anatomically, the nervous system is divided into two main sections -- the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, the main processor of information, includes the brain and spinal cord.

The PNS involves those parts of the nervous system outside the brain and spinal cord, and it connects the CNS to the body's organs and extemities. The PNS is responsible for executing cammands issued by the CNS and relaying information from the body and the outside world back to the brain and spinal cord.

The nervous system also has two functional divisions: the somatic and the autonomic nervous systems. These systems are predominately located within the PNS. The somatic nervous system is involved in the control of mostly voluntary activities, such as tapping your foot to music. The autonomic nervous system (ANS) connects the CNS to the internal organs and glands of the body and is involved in regulating involuntary functions such as heartbeat.

The ANS has two subdivisions; the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system mobilizes energy and resources during times of stress and arousal, while the parasympathetic conserves energy and resources during relaxed states.

It may sound confusing to have so many different divisions in the nervous system, each called a "system." The main thing to remember is that the nervous system has divisions based on where the nerves are (CNS and PNS) and what they do (somatic and autonomic). The sympathetic and parasympathetic systems are part of the ANS.

As noted, the CNS is made up of the brain and spinal cord, which are connected. The spinal cord contains bundles of nerves that extend from the brain down the back and serve as a communications cable relaying information to and from the brain and the rest of the body. It is encased in a series of membranes called the meninges (when they become infected, the condition is called meningitis). The membrane attached directly to the spinal cord -- the pia mater -- contains the cord's blood supply. Surrounding the pia mater is a liquid called cerebrospinal fluid (CSF), which acts to cushion the spinal cord. CSF is held in place by a second membrane -- the arachnoid. The last, outer membrane, the dura mater; is tough and fibrous.

A Bony Tunnel
Although it is a critical part of the nervous system, the spinal cord is relatively small (about 18 inches long and the width of your little finger) and fairly fragile.

To prevent it from being easily damaged, it is housed inside a bony tunnel called the spinal or vertebral canal. Twenty-nine vertebrae or back bones stack on top of each other to make up the spine or vertebral column. Each of these oddly shaped vertebra has a hole it it. When the bones are stacked on top of each other, the vertebral foramen [opening, orifice, or short passage] of each one lines up to form the vertebral or spinal canal through which the spinal cord runs.

When stacked, the spaces form a tunnel that protects the spinal cord. Because of all the bending, and lifting people's backs must do, each vertebra is cushioned from the next one by a spongy cartilage disc that acts as a shock absorber. Ligaments connect all the vertebrae to one another so that the bones of the spine can remain properly aligned and move in a coordinated fashion.

The spine has four main sections. The first seven bones, cervical vertebrae, make up the neck. The next 12, the thoracic vertebrae, extend to about the waist (each of the 12 ribs is attached to a thoracic vertebrae in the back). In the lower back area are five lumbar vertebrae. Below the lumbar region is the sacrum, a flat V-shaped bone (made of five fused vertebrae) that anchors the spine to the pelvis or hip bones. At the very end of these four main sections is a small tailbone, the coccyx, made up of fused vertebrae.

Control System
At each vertebral level, spinal nerves project off the left and right sides of the spinal cord to every part of the body through opeings in the vertebral column.

At every level, spinal nerves branch off both sides of the spinal cord to supply innervation [distribution of nerves] to the entire body. There are 31 pairs of spinal nerves in all. Each pair provides innervation to the left and right sides of a segment of the body.

Like the vertebrae, the spinal nerves are named according to level: 8 cervical spInal nerves, 12 throracic, 5 lumbar, 5 sacral, and 1 coccygeal. Because the spinal cord itself is shorter than the spine (ending far above the tailbone), the lumbar, sacral, and coccygeal spinal nerves develop long extensions to exit the corresponding level of the vertebral column. These long spinal nerve extensions are distinctive in appearance and are collectively called the caudia equina (Latin for "horse's tail").

Each spinal nerve is attached to the cord by structures called dorsal and ventral roots. On each side, a dorsal root carrying sensory information to the CNS and a ventral root carrying motor information from the CNS connect to form a spinal nerve. Ventral roots leaving the cord contain motor (pertaining to movement) fibers; whereas, dorsal roots entering the cord contain sensory (pertaining to feeling) fibers.

These spinal roots mark the beginning of the PNS. At every level, one ventral and one dorsal root on each side of the body come together to form a spinal nerve. Each spinal nerve then divides repeatedly like the branches of a tree until the entire body is innervated.

Nerves within the spinal cord that are involved in controlling movement are called upper motor neurons (UMNs), whereas nerves that leave the spinal cord to connect to muscles are called lower motor neurons (LMNs). Each "nerve" in the body is not a single nerve but rather a collection of many individual sensory and motor nerve cells or neurons. There are many different types of neurons with many different shapes and structures.

The typical neuron has three main regions; a cell body, dendrites, and an axon. The cell body is the metabolic or manufacturing center of the neuron. It is responsible for making the nutrients necessary for the neuron to live and function.

Dendrites are fine tubular extensions that radiate from the cell body like antennae and are the major receptors of information from other cells. The axon (also called the nerve fiber) is a long stem that extends away from the cell body. It conducts neuronal signals from the nerve cell to distant targets in the body, such as muscles, organs, or other nerves. Neuronal signals are transmitted from one cell to another at junctions called synapses.

Most larger neurons make use of a special insulation called myelin to maximize the conduction of the nerve signal down their long axons. Myelin is insulation that surrounds the axon of many nerves. This myelin sheath helps the nerves conduct impulses and is required for proper function. The sheath wraps around the axon to prevent signal leakage and increases the speed and efficiency with which the signal is transmitted. Because myelin is white, the spinal cord appears two-toned in color when cut in half (cross-section). Gray matter, which looks somewhat like a butterfly, is found in the center of the cord and contains clusters of cell bodies. White matter surrounds the gray matter and contains bundles of myelinated axons. Specialized cells called oligodendrocytes and Schwann cells form myelin. Both of these are types of glia.

Glia (Greek for "glue"), or glial cells, are nerve support cells found between neurons and the blood vessels supplying ther nervous system; they outnumber nerve cells by at least ten to one. Although they do not generate electrical signals like neurons, glia provide important mechanical support for nerve cells and have other vital functions as well.

Glia also supply nutrients for the nerve cells, guide and direct axon outgrowth, maintain chemical balance in the environment surrounding the neurons, and clear out debris after neuronal death or injury. The main glial cell in the PNS is the Schwann cell, and the main glia in the CNS are the oligodendrocyte and the astrocyte. Damage or disease to either the nerve cells or to the glia can result in a loss of function.

Now we are ready to look at what happens when injury occurs. Part II of this series, which will appear in the next issue of SCI Life, will focus on the different ways in which the spinal cord can be injured and the consequences to the body.

Reprinted with the permission of Paraplegia News  
 

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