CNS Anatomy: Spinal Cord

Overview

The goal in this exercise is to learn the anatomy of the spinal cord, and associated nerves and roots. The function of these different parts will be laid out in the web page about the Organization of the Peripheral Nervous System.

Recommended Reading

Read section 9.4 pp. 281-282 in Silverthorn (general organization of the spinal cord).

Spinal Cord Tissue

The most important terms of direction for studying spinal cord anatomy are ventral (which means "towards the stomach") and dorsal (which means "towards the back").  Note that in human anatomy, the ventral side of the spinal cord corresponds to the anterior side, and the dorsal side corresponds to the posterior side. 

The tissue in the spinal cord, like the brain, can be divided into gray matter, containing primarily neuronal cell bodies, and white matter containing axons, arranged into tracts.  The gray matter is located centrally and has roughly the shape of a butterfly. The white matter is around the outside.  The central canal is a space containing cerebrospinal fluid that links to the ventricles in the brain.  

The figure at the right depicts a cross-section from the thoracic region of the spinal cord. The ventral side can be identified by looking for the ventral median fissure.  As well, the ventral horn of the gray matter is rounded, while the dorsal horn is usually narrower and extends out to the edge of the spinal cord. The lateral horn, which is found mainly in the thoracic region of the spinal cord, contains the cell bodies of autonomic efferent neurons, specifically sympathetic preganglionic neurons.

The shape of the gray matter varies in different regions of the spinal cord. The figures below are low magnification views of histology slides of the spinal cord. The tissue has been treated with a black stain specific to myelin, so that the white matter appears dark, and the gray matter appears light.

Thoracic Spinal Cord

This section is from the thoracic region. This figure and the one below are oriented so that dorsal is at the top, and ventral is at the bottom.  The dorsal horn is narrow and extends out to the edge of the spinal cord, while the ventral horn is rounded.  There is also a lateral horn at this level of the spinal cord.

Sacral Spinal Cord

This section is from the sacral region. This part of the spinal cord contains efferent neurons that control the muscles of the legs, and so has many more somatic motor neurons in the ventral horn. Likewise, there is much more afferent input from the legs than from the trunk. Consequently, both the dorsal horn and ventral horn are much larger than in the thoracic segment.  The ventral median fissure can be seen in this section, but is harder to see in the thoracic section above.

Models of the Spinal Cord

There are two spinal cord models that we will study. When not being used in our class this week, these models are available on the third floor of the Health Sciences Library.  Pictures of the models are provided in the links here, and also in the lecture slides.

The first model depicts a cross-section of the spinal cord at the cervical level (in the neck). The spinal cord sits within the vertebral canal which is the passageway within the bones (vertebrae) of the spinal column.  This model depicts the three layers of the meninges (dura mater, arachnoid mater and pia mater). We will only identify the tough, thick outermost layer, known as the dura mater. The dura mater encloses a layer of cerebrospinal fluid (CSF) that cushions the brain and spinal cord. This space containing CSF is called the subarachnoid space because the arachnoid layer is attached just underneath the dura mater (see video 3.1.8 below).  In the head, the dura mater is attached to the skull bones, but in the spinal canal, there is an epidural space filled with fat and blood vessels that lies between the dura mater and the vertebral bones.

The dorsal root ganglion is the swelling that is found along the dorsal root.  The dorsal root ganglion contains the cell bodies of afferent neurons (see the next page on the organization of the peripheral nervous system).  The dorsal root ganglia are located just outside the dura matter.  Note that the dorsal root ganglion does not lie in a position that is dorsal to the spinal cord, but rather gets its name because it is a cluster of neuronal cell bodies ("ganglion") that surrounds the dorsal root. Distal to the dorsal root ganglion, the two roots fuse to form a spinal nerve.

The second model is a model of a longitudinal dissection of the spinal cord. All of the tissues ventral to the spinal cord are removed, thus, we are looking at the ventral surface of the spinal cord. On one side of the model, the roots are cut, revealing the dorsal roots; the ventral roots are visible on the surface on the undissected side.
Click here to see a full view of this model. 

The dorsal and ventral roots travel some distance caudally before they join to form a spinal nerve that exits the vertebral canal. These distances become longer for more caudal segments. The tissue of the spinal cord only extends to the beginning of the lumbar region, so the most caudal part of the vertebral canal is a group of long roots known as the cauda equina (because it resembles a horse's tail).  A white wavy line represents the dura mater on this model; thus the space adjacent to the spinal cord is the subarachnoid space.  A spinal tap is when a needle is inserted into the subarachnoid space in order to take a sample of the CSF.  This is done in the region of the cauda equina in order to reduce the risk of damage to the spinal cord. 

The dorsal root ganglia are small, subtle, and difficult to identify in these pictures or the model.  However, the spinal nerves are easy to see as the many white structures extending laterally away from the spinal column.  There are larger numbers of axons in the spinal nerves leaving from spinal segments that supply nerves that innervate the arms and the legs.  In these regions, the spinal nerves divide and rejoin to form a network called a plexus.  The more superior plexus whose nerves innervate the arms is called the brachial plexus; the lower one whose nerves innervate the legs is called the lumbosacral plexus.

On the model's right side, you can see the sympathetic chain ganglia (there are sympathetic chain ganglia on both sides of the body, but they are only modeled on the right side for this particular model). These ganglia contain the cell bodies of sympathetic postganglionic neurons.  In the model the sympathetic chain ganglia are bright pink and connected by yellow processes. 

Videos from Acland's Video Atlas of Human Anatomy

The Acland's Video Atlas of Anatomy provides narrated video demonstrations using high quality cadaver dissections.  These videos contain more detail and terminology than I expect you to learn for this class.  In each case, focus on being able to identify the terms given in blue boldface in the yellow boxes.  Dr. Acland speaks with a British accent, so you may want to utilize the closed captions.  A pdf transcript is also available for each video.

The first video (video 3.1.8) shows a cross section of the spinal column. This video shows very nicely the relationship of the three layers of meninges.

The second video (video 3.1.9) is a posterior (dorsal) view of the spinal cord.  This video shows the end of the spinal cord tissue and the cauda equina.  It also shows how the "filaments" (nerve rootlets) leaving the dorsal and ventral sides of the spinal cord coalesce to form the dorsal and ventral roots, and then how the roots come together to form a spinal nerve. 

Tips

I think the best views of the dorsal and ventral roots can be seen in the section "Dura around emerging spinal nerve". To put this in the best context, jump to the previous section "Spinal nerves: emergence from intervertebral foramen. Note: the intervertebral foramen is the passageway formed at the side when two vertebrae come together.  This passageway contains the nerve roots and the dorsal root ganglion.  Also note that the Acland's videos are looking at the spinal cord from the dorsal side, whereas the longitudinal dissection of the spinal cord model exposes the ventral surface of the spinal cord.

Video 3.1.10 (opens in a new tab) provides a review. Remember you will only be tested on the structures listed above.