What is a spinal cord injury?

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Glia cells are "nerve-helper" cells that provide structural support, nourishment and protection for neurons.

Each neuron is made up of a cell body, which houses the nucleus. Axons and dendrites form extensions from the cell body.

Glia cells are "nerve-helper" cells that provide structural support, nourishment and protection for neurons.

Astrocytes , a kind of glial cell, are the primary support cells of the brain and spinal cord. They make and secrete proteins called neurotrophic factors, which assist in the maintenance of nerve cells in the central nervous system. They also break down and remove proteins or chemicals that might be harmful to neurons (for example, glutamate, a neurotransmitter that in excess causes cells to become overexcited and die by a process called excitotoxicity ).

Astrocytes aren't always beneficial: After injury, they divide to make new cells that surround the injury site, forming a glial scar that is a barrier to regenerating axons.

Microglia are immune cells for the spinal cord. After injury, they migrate to the site of injury to help clear away dead and dying cells. They can also produce small molecules called cytokines that trigger cells of the immune system to respond to the injury site. This clean-up process is likely to play an important role in recovery of function following a spinal injury.

Oligodendrocytes are glial cells that produce a fatty substance called myelin which wraps around axons in layers. Axon fibers insulated by myelin can carry electrical messages (also called action potentials) at a speed of 100 meters per second, while fibers without myelin can only carry messages at a speed of one meter per second. A spinal cord injury can compromise or destroy myelin.

Messages are passed from neuron to neuron through synapses, small gaps between the cells, with the help of chemicals called neurotransmitters. To transmit a message across a synapse, neurotransmitter molecules are released from one neuron (the "pre-synaptic" neuron) and cross the gap to the next neuron (the "post-synaptic" neuron).

The process continues until the message reaches its destination. There are millions and millions of connections between neurons within the spinal cord alone. These connections are made during development, using positive and negative (inhibitory proteins) signals to fine-tune them. Amazingly, a single axon can form synapses with as many as 1,000 other neurons.

There is a logical and physical topographical organization of the anatomy of the central nervous system, which is an elaborate web of closely connected neural pathways. This ordered relationship means that different segmental levels of the cord control different things and injury to a particular part of the cord will impact neighboring parts of the body.

Paralysis occurs when communication between the brain and spinal cord fails. This can result from injury to neurons in the brain (a stroke), or in the spinal cord. Trauma to the spinal cord affects only the areas below the level of injury. On the other hand, poliomyelitis (a viral infection) or Lou Gehrig's disease (ALS, amyotrophic lateral sclerosis) can affect neurons in the entire spinal cord.

Each segment of the spinal cord receives sensory input from a particular region of the body. Scientists have mapped these areas and determined the "receptive" fields for each level of the spinal cord.

Neurons in the motor cortex, the region of the brain that controls voluntary movement, send their axons through the corticospinal tract to connect with motor neurons in the spinal cord. The spinal motor neurons project out of the cord to the correct muscles via the ventral root. These connections control conscious movements like writing and running.

Information also flows in the opposite direction, resulting in involuntary movement. Sensory neurons provide feedback to the brain via the dorsal root. Some of the sensory information is conveyed directly to lower motor neurons before it reaches the brain, resulting in involuntary or reflex movements. The remaining sensory information travels back to the cortex.

The spinal cord is divided into five sections: the cervical, thoracic, lumbar, sacral, and coccygeal regions. The level of injury determines the extent of paralysis and/or loss of sensation. No two injuries are alike.

In addition to the control of voluntary movement, the central nervous system contains the sympathetic and parasympathetic pathways that control the "fight or flight" response to danger and regulation of bodily functions. These include hormone release, movement of food through the stomach and intestines, and the sensations from and muscular control to all internal organs.

A common set of biological events take place following spinal cord injury:

1. Cells from the immune system migrate to the injury site, causing additional damage to some neurons that survived the initial trauma, and death to others.

2. The death of oligodendrocytes causes axons to lose their myelin, which greatly impairs the conduction of messages or renders the remaining connections useless. The neuronal information highway is further disrupted because many axons are severed, cutting off the lines of communication between the brain and muscles and between the body's sensory systems and the brain.

3. Within several weeks of the initial injury, the area of tissue damage has been cleared away by microglia , and a fluid-filled cavity surrounded by a glilal scar is left behind. Molecules that inhibit regrowth of severed axons are now produced at or near this site. The cavitation is called a syrinx , which acts as a barrier to the reconnection of the two sides of the damaged spinal cord.

Although spinal cord injury causes complex damage, a surprising amount of the basic circuitry to control movement and process information can remain intact. This is because the spinal cord is arranged in layers of circuitry. Many of the connections and neuronal cell bodies forming this circuitry above and below the site of injury survive the trauma. An important question to research scientists is how much do these surviving neurons "know"? Can they regenerate and make new, correct connections?

Research points to a multiplicity of possible interventions to promote recovery from a spinal injury. Some would be delivered immediately following the injury; others are less time-specific and involve rebuilding and reconnecting the injured cord. Clearly, both approaches are important: Limiting degeneration will enhance the probability of greater recovery, while stimulating regeneration will build upon the remaining system to restore lost connectivity, and perhaps to prevent further degeneration.

To cure the paralysis and loss of function that spinal cord injuries cause, doctors will need a carefully orchestrated series of interventions. Treatments will begin in the emergency room and continue for months. Even new forms of rehabilitation will be part of the therapeutic package. To speed the day when this regimen is available, the Sam Schmidt Paralysis Foundation supports research on a variety of fronts. Individual grants encourage a multi-disciplinary approach to solving the complex medical problems that result from spinal cord injuries, in both the acute and chronic stage.

" Source: The Reeve Foundation "

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