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  University of Idaho
  All rights reserved.
  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

In general, the spinal cord has basically a central core of nonmyelinated neurons or what we call gray matter.  These are surrounded by myelinated neurons or axons (what we call white matter).  The white matter is basically pathways that are going from one structure to another major structure.

As you will see, the spinal cord gray matter is shaped like an H.  It’s shorter than the vertebral column, and it’s separated into regions like the vertebral column was as well.

On slide three, you see an overview of structures within the spinal cord.  As you can see, they’re broken into three major divisions; the dorsal funiculus, the lateral funiculus, and the ventral funiculus.  Each of these are going to correspond with different aspects of the body; the dorsal being to the back, ventral being to the front, and lateral being to the side.  Each of these contains a variety of different structures.  We’ll talk about these in some detail over the next couple slides.  And so let’s begin by looking at the structures within slide four. 

The first of these structures is what we call the dorsal horn.  The dorsal horn is the place where sensory information is going to enter.  So, it’s going to contain sensory nuclei.  What this structure does is receive information from outside the central nervous system. 

A second structure called the ventral horn is on the other side of the spinal cord.  It contains what we call motor nuclei.  This is where you receive information from the major motor pathways coming down from the brain.  Ultimately, this is going to synapse and send information to the muscles.  This structure also doesn’t contain clusters of nuclei like the dorsal horn.  Instead, they are arranged in columns that run the length of the spinal cord.

In addition, if you look in DeArmond et al. the spinal cord has different sizes depending on where you are in the spinal column.  For example, the ventral horn is larger and contains motor neurons that innervate the arms and legs.  Here you need lots and lots of neurons for where you’re going to be sending information to, which are basically the limbs.  In contrast, the dorsal horn is larger where sensory nerves from the limbs are entering the spinal cord.  Again, you need more neurons because you have more receptors.

In addition to that, we also have the internuncial neurons.  These are also in the gray matter.  Basically these neurons are going to moderate information flowing from the sensory neurons toward the brain and modulate neural activity within the motor system.  Again, as we’ve learned in the past internuncial neurons are also highly involved with reflexes.

Well, we’ve talked a little bit about the gray matter, what about the white matter.  Well the white matter is basically going to surround all the gray matter.  Again, these are divided into the three major columns, the dorsal, lateral, and ventral.  Each of these structures is going to contain bundles of ascending and descending axons that are going to and from the brain.

The dorsal column is going to lie between the two dorsal horns.  It’s basically sending information via axons up to the brain.  As you can see in DeArmond et al. they use the gracile fasiculus and cuneate fasiculus pathways to get that information to the brain.  You see on slide nine where those particular structures are located as you go up within the spinal column.

The lateral columns also send information to the brain.  It also sends information from the brain stem and the cortex to motor neurons.  Here you have a variety of different pathways, but one of the major ones is what is called the lateral corticospinal track (Cortical meaning where it starts, spinal where it’s ending).  So, it starts in the cortex and goes down to the spinal column.  This is the major pathway that’s going to be sending information to the final common pathway (which is the final neurons that go to the limbs).  Most information is going to be sent via this pathway in relation to motor activity.

The ventral columns also include ascending and descending axons.  Again, it sends information about pain and dermal sensation up the spinal cord to the brain, and descending pathways help to control axial muscles and posture.  So, you’re going to see a wide variety of structures leaving structures such as the basal ganglia and other locations.  This structure primarily uses the ventral corticospinal track and others.  Again, it’s going to be sending and receiving motor information as well as the lateral cortical spinal track.  Slide 12 kind of gives you a picture (it is also shown in Carlson) and shows you an overview of the different neurons, some of the different roots, etc., that are within the spinal cord. 

Well, what about the spinal cord in general.  Well, again, as within the vertebral column, it’s divided into four major regions; the cervical, thoracic, lumbar, and sacral.  Basically, all are related to body development segments and again the nerves enter and leave through the intervertebral foramen. 

The first of these nerves are the cervical nerves and they have eight segments.  The cervical nerves basically are involved with sensory perception and motor function in the back of the neck and the legs, the head and the arms.  So, it is a very, very major important structure.

The thoracic section has 12 segments.  Basically, it’s going to innervate the upper body, while the lumbar and sacral sections (as we see in slide 16) have 5 segments each, and innervate the lower body, the back and the legs.  Ultimately, the spinal cord terminates in the lumbar area.  That’s why when you go and get a lumbar puncture (spinal tap) to get cerebral spinal fluid you do so without damaging the nerves that are in the spinal cord.

There’s also another major structure that relates to the spinal cord and that is what is called the pyramidal descussation.  It is located in the medulla and is basically where the majority of spinal fibers cross over to the opposite side of the body.  The classic example relates to the cortical and ventral spinal tracks.  As you see in slide 17, 80 to 85% of motor fibers crossover to the contralateral side.  So, if you’re getting information from the motor structures that are in the brain, it’s going to cross over to the opposite side and then going to use the lateral corticalspinal track to send information down the spinal cord to whatever particular structure you’re going to innervate.  The remaining 15 to 20% remain on the same side or what we call ipsilateral and would then use the ventral cortical spinal track.

A couple of other things about the spinal column.  Few sensory axons enter at low levels such as the sacral level, but as you go up the spinal column, more and more sensory neurons enter.  Also, most descending axons terminate at cervical levels.  So as you can see here, you’re getting most of your intervation up from the upper body, etc., from very, very high levels, (in your neck).  As a result, if you damage that structure up very high and you damage spinal columns, or the spinal cord in the cervical region, oftentimes you have major amounts of paralysis throughout the body.

So, so as we see here, we have different levels of neurons and axons leaving and entering the different locations, thus the sacral level is going to have less white matter than the cervical areas. 

Well, this is the first section where we have talked about structures within the spinal cord.  As we continue on, we will be examining other structures that relate to the spinal column reflexes, and what happens when we have damage.  So until then, we hope you’re enjoying the class and hope you have a great day.