The human nervous system is responsible for sending, receiving, and monitoring all 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 extremities. The PNS is responsible for executing commands 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 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 sort of 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. The 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 in 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 vertebra in the back). In the lower back area are five lumbar vertebrae. Below these 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, also made up of fused vertebrae.

VERTEBRAE (Click on Illustration to Enlarge)

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 openings 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 innervetion 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 thoracic, 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 cauda equina (Latin for "horse's tail"). Because the spinal cord ends far above the tailbone, long extensions, collectively called the cauda equina, are required to reach the lower segments of the spine.

SIDE-VIEW OF SPINAL CORD IN VERTEBRAL COLUMN

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.

CROSS-SECTION OF SPINAL CORD

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 and structures 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.

NEURON

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.

GRAY & WHITE MANNER

Another class of cells called glia (Greek for "glue"), or glial cells, are nerve support cells found between neurons and the blood vessels supplying the 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.

In addition to providing myelin for neurons, glia may 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 themselves or to the glia can result in a loss of function in humans.

HOW PARALYSIS BEGINS

SCI can result from damage to the vertebral column or to the spinal cord itself. Most SCIs occur when trauma or injury to the vertebral column causes a fracture of bones or a tearing of ligaments. . Either can result in a displacement of the bones of the spine, which in turn can lead to SCI. Bone displacement can cause spinal-cord bruising (contusion), pinching (compression, stretching (distraction), or some combination of these.

In rare instances, injury to the vertebral column will result in an actual cutting or penetration of the spinal cord. Penetrating SCIs more often are due to gunshot or stab wounds that mayor may not damage surrounding vertebrae. SCI can also be caused by ischemia, a decrease or loss of blood flow to the cord. This can happen as a result of injury, disease, or certain surgical procedures, particularly those involving clamping the aorta. Fortunately, SCI from surgery is very rare.

Because the causes of SCI vary greatly, no two injuries are exactly alike. However, the basis of the resulting paralysis is the same: the death of neurons, and the disconnection and demyelination of axons. Thanks to research, scientists can generally describe the pathology (deviations from normal anatomy) and pathophysiology (abnormal biological and chemical processes) responsible for these changes, although they are still not completely understood.

At the time of injury, neurons, glia, and blood vessels in the injury zone experience an initial mechanical damage. Following this "primary" injury, several mechanisms become operative. This leads to further damage, called "secondary" injury, which is often responsible for more of the functional deficit people with SCI experience than is the initial trauma itself.

The progression of secondary injury can vary greatly depending on the severity of the initial trauma. Typically, after a moderately severe trauma, secondary injury begins within 30 minutes with hemorrhage or bleeding in the central gray matter of the spinal cord. Over the course of several hours, this hemorrhage radiates outward to include surrounding white matter as well. Within two hours of injury, a significant reduction of blood flow (ischemia) occurs in the region. Within six hours, edema (swelling) is visible in the area. The edema, hemorrhage, and ischemia all result in decreased oxygen supply (hypoxia) in that region of the spinal cord, which leads to the death (necrosis) of local tissue.

Also about two hours after injury, inflammatory cells begin their invasion. These immune-system cells protect us from disease and infection by killing harmful bacteria in our body and clearing away waste and debris. By the fourth hour, some of these inflammatory cells begin killing the damaged nerves in the spinal cord.

All of the events described above ultimately lead to further nerve damage in the spinal cord. If the initial trauma is severe, the secondary-injury process may begin immediately, and the entire injury zone can become filled with dead or necrotic tissue within 48 hours. However, immune-system cells eventually clean up the area by removing all the dead tissue. Several weeks after the initial trauma, only a cavity and/or scar tissue remain at the injury site. However, even in the most severe. injuries, surviving neurons cross the injury zone along the perimeter of the spinal cord. Although these neurons are intact, they are damaged, demyelinated, and nonfunctioning.

INJURY CLASSIFICATION

Each SCI is different, and each is described by its type ("complete" or "incomplete") and level. In general, "complete" means no voluntary movement or sensation exists below the injury level.  Some feeling or voluntary movement remains in an "incomplete" injury. A British neurologist, Dr. Frankel, developed a more detailed system of classification of neurological function. The American Spinal Injury Association (ASIA) subsequently refined this scale, which grades injuries from "A" (a complete injury) to "E" (recovery). The International Medical Society of Paraplegia (IMSOP) adopted the refined ASIA Impairment Scale, which is now the international standard for classification of neurological function.

ASIA and IMSOP have also developed standardized classifications for levels of SCI. According to these standards, the neurologic level of injury is defined as "the most caudal (lowest) segment of the spinal cord with normal sensory and motor function on both sides of the body." The generic way that level of injury is described is by the classifications "quadriplegia or tetraplegia," referring to injuries of the cervical regions, and “paraplegia," referring to injuries of the thoracic, lumbar, or sacral regions.

In general, after SCI the nerves above the level of injury continue to work normally; those below it are impaired. Consequently, the parts of the body innervated by the nerves below the level of injury don't function the way they used to. In fact, some parts may no longer operate at all.

Because the spinal cord connects with all the body's nerves, damage to it can alter every system. In addition to affecting a person's ability to move and feel, SCI can affect skin, breathing, bladder, bowel, sexual function, and subconsciously controlled phenomena like blood pressure and sweating.

Earlier, it was explained that spinal-cord nerves that control movement are called upper motor-neurons (UMNs); nerves that leave the spinal cord to connect with muscles are lower motor-neurons (LMNs). More specifically, axons from nerve-cell bodies in therain run inside the white matter of the spinal cord to connect with specific nerve-cell bodies (motor neurons) in the gray matter. The axons from these motor neurons then leave the cord to make connections with the muscles in the body. UMNs, which originate in the brain, regulate and control the movement stimulated by LMNs, the nerves that originate in the spinal cord.

In a UMN injury, control by the brain no longer exists because messages from the brain are cut off at the point of SCI. Therefore, LMNs react without limit or inhibition, causing uncontrolled muscle contractions. This is called spasticity, and an UMN injury is said to result in "spastic" paralysis. On the other hand, LMN injuries cause a "flaccid" paralysis because muscles of the limbs get cut off from nerves that supply them. This lack of innervation causes muscles to become limp or flaccid. Muscle spasms can either be "alternating" (producing twitching or shaking) or "sustained" (causing rigidity in the limbs). All spasticity represents the activity of uncontrolled reflexes of the LMNs and is the result of the brain's loss of control over the somatic nervous system.

Another example of uncontrolled reflexes is autonomic dysreflexia (AD) - the lack of the brain's control over the autonomic nervous system. AD is a serious condition that may occur in individuals with SCI at T6 or above. Before SCI, a stimulus below the level of injury would have signaled pain or discomfort from such things as a full bladder, sunburn or labor contractions. After SCI, These conditions can cause AD, which, if not treated promptly, can lead to stroke and is potentially life threatening.