Spinal Tracts

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What are spinal tracts?

    These are communication pathways within the CNS... they are like cables of axons that run up and down the spinal cord in the white matter.  Some run up the spinal cord (cephalad), while others descend the spinal cord from the brain.

What tracts exist?

    There are many tracts.  Each tract carries specific types of information from one location to another.  So, if you want to carry painful information from the spinal cord to the brain, you use one particular tract (the spinothalamic tract), while if you want to carry voluntary movement commands from the brain to the spinal cord, you use a different tract (the lateral corticospinal tract).

    I am only holding you responsible for the tracts listed below.   Your book lists more than this, and many more exist than even your book describes.   But the major ones are the following ones.

Motor tracts:

bulletCorticospinal tracts soundicon.gif (538 bytes)
bulletLateral corticospinal tract
    This tract carries information about movement from the brain to the spinal cord.  That means that if your brain decides that it wants your legs to move, it has to send this voluntary movement command down to the region of the spinal cord that is responsible for leg movements.
    (The anterior corticospinal tract is much more minor, but it crosses at the spinal level of the motor neuron it synapses on... so it runs contralaterally.)

Sensory tracts:

bulletDorsal Columns:  this specific term is not in your book, but is a common term to describe the tracts that carry touch and proprioceptive information to your brain.   Therefore, light touch, deep pressure, and proprioceptive senses (carried out by muscle spindles and other sensory receptors) are carried through these pathways.
bulletFasciculus cuneatus soundicon.gif (538 bytes)
    This spinal tract carries this sensory information from your thoracic and cervical body areas up to the brain.  These areas include your arms.
bulletFasciculus gracilis soundicon.gif (538 bytes)
    This spinal tract carries this sensory information from your sacral and lumbar body areas up to the brain.  These areas include your legs.
bulletSpinothalamic tracts soundicon.gif (538 bytes):  Pain and temperature (the free nerve endings in skin) sensory information is carried by these tracts to your brain.
bulletlateral spinothalamic tract
bulletanterior spinothalamic tract

To see these tracts, go to this spinal cord cross-section and view #27 for F. cuneatus, #28 for F. gracilis, #25 for the lateral corticospinal tract, #24 for the lateral spinothalamic tract, and #18 for the anterior spinothalamic tract (called a tectospinal tract there).

Where do each of these tracts run?

Cross-sectional view:

    Figure 11.5 of your book also shows you these tracts.  If you look at the white matter, you will see that it can be considered to lie either dorsally (above the dorsal horn of gray matter), laterally (between the butterfly wings), or ventrally (below the ventral horn of the gray matter).  Each of these three regions is called a "funiculus."  The lateral tracts run within the lateral funiculus, the ventral tracts run within the ventral (or anterior) funiculus, and the dorsal column pathways run within the dorsal (or posterior) funiculus.

Tracing the tracts longitudinally-- where do they cross?

    Our brains contain information about the left side of our body on the right side of the brain.  And vice versa, too.  That means that if sensory information comes from your right hand, it has to end up in the left side of your brain.   Correspondingly, if you want to move your right hand, the left side of your brain has to command it.  So, it ends up that almost all information has to cross sides within the CNS.  The only exception to this rule has to do with information passage to one particular region of the brain, the cerebellum... but we won't get into that here.

    Why is this so?  I can't tell you why.  But all vertebrates have this condition.

    Where does the information cross.  That can be answered rather easily, and will be the topic of this section of the web page.

    Why should you care where the crossing occurs?  It turns out that understanding where the information crosses is a useful clinical tool.   If a person comes in to see you, their allied health care attendant, and has problems sensing pain on one side of the body as well as problems sensing touch on the contralateral side (but not ipsilateral side) of the body, then you can tell that their problem is in the spinal cord.  But if they are having problems sensing both pain and touch on the same side of the body, then their problem is located in their brain.   Knowing whether a problem lies in the spinal cord or the brain is an important step in diagnosis.  So, now I hope you care, and I'll tell you where each crosses.

bulletAscending Pathways to the brain:
bulletDorsal columns:
    Information from the dorsal columns crosses after it has entered the brain (remember, this is an ascending pathway).  It crosses in the region of the brain called the medulla (you'll learn about it next week).
    In this way, sensory information from the right side of the body runs up the right side of the spinal cord, but then runs through the left side of the brain.
bulletSpinothalamic tracts:
    The information that is going to head up to the brain in the spinothalamic tracts crosses before it enters the tract.  The pain SN axons enter the spinal cord, synapse on INs that cross to the other side immediately, and then go into the white matter to run up the spinal cord in the spinothalamic tract.
    In this way, information from the right side of the body runs up the left side of the spinal cord and remains on the left side throughout the brain.
bulletDescending Pathways from the brain:
bulletLateral corticospinal tract:
    The information coming down from the brain crosses over in the brain (again, in the medulla) before entering the lateral corticospinal tract.
    In this way, motor commands to control the right side of the body run through the left side of the brain, but through the right side of the spinal cord.

    Can you now understand what symptoms a person would display if they had damage to the right side of their spinal cord?  They would not be able to move or feel touch on the right, but they also would not be able to feel pain on the left.   These inabilities would exist all the way caudally from the point of damage.   So if the damage was at L1, the inabilities would be in all the levels from L1 through the sacral levels of the spinal cord.

 

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.