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What is the Spinal Cord
 
Wise Young, MD, PhD 


This may seem to be silly question but, until people get spinal cord injury or know somebody who is, most pay little attention to their spinal cords.  Most people don't know the different parts of the spinal cord, what each part does, and how the spinal cord transmits sensory and motor information. Many think that the spinal cord conducts information like a telephone wire and the spinal cord can be fixed by reconnecting it.  Some people mistakenly believe that the spinal cord is the vertebral column. While almost everybody knows that spinal cord injury causes paralysis, many are not aware that the spinal cord also controls the bladder and bowel, sexual function, blood pressure, skin blood flow, sweating, and temperature regulation. 

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The spinal cord connects the brain to the body. The spinal cord resides in a bony spinal or vertebral column that has 24 segments.  Seven vertebra in the neck are called cervical (C1-C7), twelve chest or thoracic (T1-T12) segments form the rib cage, five segments for the lower back or lumbar (L1-L5), and five segments form the tail or sacral (S1-S5) vertebra.  The vertebral bodies are in the front of the spinal column.  Spinal discs are located between the vertebral bodies.  The front of the spinal cord is referred to as anterior while the back is referred to as posterior.  The sides of the spinal cord are called lateral.  Note that in animals that walk on four legs, posterior is dorsal and anterior is ventral.
spinalcord2.jpgEach segment has four spinal roots (left and right, posterior and anterior) that send and receive information from each side of the body.  Posterior roots receive sensation while anterior roots send motor signals to muscles.  For example, the C1-C3 segments send and receive information from the back of the head and neck, C4 covers the shoulder and deltoid muscles, C5 the biceps, C6 the wrist extensors, C7 the triceps, C8 the wrist flexors, and T1 the intrinsic muscles of the hand.  The spinal roots leave the vertebral column between the bony segments through openings in the vertebral column called foramina.  Note that there are only 7 cervical vertebra but 8 sets of cervical roots because the C1 roots are between the skull and C1.

The spinal cord is shorter than the vertebral column and occupies the spinal canal from the C1 to L1 vertebral levels.  In general, the bony vertebral segments are lower than the spinal cord levels. The spinal roots exit through the spinal column through openings between vertebral segments called foramina.  The spinal cord stops just below the L1 vertebral level and only spinal roots are present from L1 to S5 vertebral spinal column.  The end of the cord is called the conus.  Spinal roots below the conus are called the cauda equina because they resemble a horse's tail.   


How does the spinal cord work?


Neurons (nerve cells) in the brain, spinal cord, and peripheral nerve send axons (nerve fibers) up and down the spinal cord in spinal tracts. These spinal tracts are called white matter because axons are coated with a membrane called myelin and myelin appears white.  In the spinal cord, white matter is usually situated close to the surface of the cord, arranged into several columns called the anterior, posterior, and lateral columns.  The spinal cord contains neurons located in the middle part of the spinal cord.  The areas of the spinal cord that contain neurons is called gray matter.  The gray matter is most abundant in the parts of the spinal cord that connect to the arms and legs, called the cervical and lumbosacral enlargements.


The spinal cord transmits signals for sensations and to control movement, as well as breathing, bladder, bowel, sweating, blood pressure, sexual, and other essential functions of the body. The spinal cord contains neuronal circuitry for reflexes that control all these functions.  Over 20 million axons ascend and descend in the human spinal cord, organized into spinal tracts named according to their source and destination.  For example, the spinal tract that sends axons from the cerebral cortex to the spinal cord is called the corticospinal tract.  Likewise, the tract that sends axons from the red nucleus in the midbrain to the spinal cord is called the rubrospinal tract.  The sensory tract that transmits pain and temperature sensation from the spinal cord to the thalamus is called the spinothalamic tract.  Some tracts, however, are named by their position.  For example, the posterior column transmits sensory information from the spinal roots to the brainstem. 


Neurons that send axons to muscles are called motoneurons while neurons that send axons to other neurons are called interneurons.  Motoneurons and interneurons receive information from descending axons and sensory axons.  When you activate sensory input to the spinal cord by tapping a tendon, the activity turns on motoneurons that cause the muscle of that tendon to contract.  This is called a monosynaptic reflex.  To signal the muscles to move, the brain sends information directly to motoneurons or indirectly through interneurons that can either excite or inhibit other neurons. 


Sensory neurons send axons from the spinal cord to the brain.  Some sensory axons go from peripheral nerve neurons in posterior sensory ganglia located just outside of the spinal column.  Posterior sensory ganglion neurons send a T-shaped axon to the body where it collects information like touch and movement while the other end goes into the spinal cord and branches.  One branch goes into the gray matter where it activates motoneurons and the other end goes up the posterior column all the way to the brainstem. 


To read more about spinal cord injury:

What is the Spinal Cord

What is Spinal Cord Injury

Spinal Cord Injury Levels and Classification

Acute Spinal Cord Injury

Chronic Problems of Spinal Cord Injury

Recovery and Treatment

Recovery from Spinal Cord Injury

Spinal Cord Injury and Family

Ten Frequently Asked Questions Concerning Cure of Spinal Cord Injury

 
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