BIO 004 · Human Anatomy
The Spinal Cord
Block 5 · Module 3: The Central Nervous System, the Spinal Cord
A reference for the spinal cord video and lab. This page covers the external anatomy of the cord, its enlargements and terminations, the roots and rootlets that connect it to the spinal nerves, and the internal anatomy of its gray and white matter. The focus is on the structures and the job each one does.
How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet. Print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice: type the term, click Reveal to check yourself. The comparison grids respond to Study and Quiz too, with a Reveal button on each row.
The Foundations video gives you a complete foundational understanding of this topic, enough on its own for a foundational course. Learn it first, then move on to the Deep dive, which adds the majors-level depth for this course.
- Describe the external anatomy of the spinal cord, including its enlargements and terminations.
- Identify the roots, rootlets, and dorsal root ganglion.
- Describe the internal anatomy of the gray horns and the white columns and tracts.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
The spinal cord in place
Add a labeled posterior view showing the cervical and lumbar enlargements, the conus medullaris, and the cauda equina.
A spinal cord cross-section
Add a labeled cross-section with the gray horns, the white columns, and the roots.
The roots of a spinal nerve
Add a labeled view of the dorsal and ventral roots, the rootlets, and the dorsal root ganglion.
The Spinal Cord, an Overview
The spinal cord runs from the medulla oblongata down the vertebral canal. It carries signals between the brain and the body and is the center for spinal reflexes.
- Spinal cordthe cord of nervous tissue within the vertebral canal, continuous with the medulla above
- Lengthabout 16 to 18 inches in the adult, running from the medulla to the L1 to L2 level
- Diameternot uniform; widest in the cervical region and narrowest in the thoracic region
- Cord growththe cord stops lengthening at about 4 to 5 years of age, while the vertebral column keeps growing
- Functionsconducting sensory and motor signals between the brain and the body, and acting as the integration center for spinal reflexes
External Anatomy of the Spinal Cord
Two regions of the cord are visibly wider, and the cord ends well above the end of the vertebral column. Two grooves run its full length.
- Cervical enlargementa widening from C4 to T1 where the nerves of the upper limbs arise
- Lumbar enlargementa widening from T11 to S2 where the nerves of the lower limbs arise
- Conus medullaristhe tapered, cone-shaped end of the spinal cord, at the disc between L1 and L2
- Filum terminalea thread of pia mater extending from the conus medullaris that anchors the cord to the coccyx
- Cauda equinathe hair-like bundle of nerve roots that descends past the conus medullaris within the vertebral canal
- Anterior median fissurethe deep groove down the front of the cord
- Posterior median sulcusthe shallow groove down the back of the cord; with the fissure it divides the cord into right and left halves
The Roots and Rootlets
Each side of the cord connects to a spinal nerve through two roots. Tiny rootlets gather into a root, and the two roots join to form the mixed spinal nerve.
- Rootletsthe small bundles of axons that attach in a row along the cord and merge to form a root
- Roota bundle of axons that connects a spinal nerve to the cord; each spinal nerve has two
- Posterior (dorsal) rootthe sensory root; it carries sensory axons from receptors in the periphery into the cord
- Posterior (dorsal) root ganglionthe swelling on the posterior root that holds the cell bodies of the sensory neurons
- Anterior (ventral) rootthe motor root; it carries motor axons from the cord out to muscles and glands
- Spinal nervethe mixed nerve formed where the posterior and anterior roots join; it carries both sensory and motor axons
- Intervertebral foramenthe gap between adjacent vertebrae where the spinal nerve exits the vertebral canal
Internal Anatomy: The Gray Matter
A cross-section of the cord shows a butterfly of gray matter at the core, wrapped in white matter. This is the reverse of the brain, where gray matter is on the outside.
- Gray matterthe inner, butterfly-shaped region of neuron cell bodies
- Central canalthe small cerebrospinal fluid channel at the center of the cord, continuous with the fourth ventricle
- Gray commissurethe bridge of gray matter that connects the two sides, surrounding the central canal
- Anterior white commissurethe bridge of white matter that connects the two sides of the cord
The gray matter is shaped into projections called horns. Compare what each horn holds.
| Horn | Contents |
|---|---|
| Posterior (dorsal) horn | interneurons and the incoming axons of sensory neurons |
| Lateral horn | autonomic motor nuclei; present only in the thoracic and lumbar segments |
| Anterior (ventral) horn | somatic motor nuclei that drive skeletal muscle contraction |
Internal Anatomy: The White Matter
The white matter of the cord is sorted into columns, and each column holds tracts. A tract is a bundle of axons that share a common origin and destination.
- White matterthe outer region of the cord, made of myelinated axons
- Columnsthe anterior, posterior, and lateral white columns, the major divisions of the cord's white matter
- Tractsbundles of axons within the columns that share a common origin and destination
- Ascending tractssensory tracts that conduct impulses up toward the brain; also called afferent tracts
- Descending tractsmotor tracts that conduct impulses down away from the brain; also called efferent tracts
A cluster of cell bodies and a bundle of axons each have a different name in the CNS and the PNS. Compare the four terms.
| Location | Cluster of cell bodies | Bundle of axons |
|---|---|---|
| Central nervous system | nucleus | tract |
| Peripheral nervous system | ganglion | nerve |
Distinguishing the Cord Segments
A cross-section taken at different levels of the cord does not look the same. The proportions of gray and white matter, the shape, and the size all shift from top to bottom. Compare them.
| Segment | Distinguishing features |
|---|---|
| Cervical | large diameter with a large amount of white matter; oval in shape |
| Thoracic | small diameter with a small amount of gray matter; has a small lateral gray horn |
| Lumbar | almost circular; less white matter than the cervical segments |
| Sacral | relatively small, with more gray matter and little white matter |
| Coccygeal | the smallest; resembles the lower sacral segments |
See also: The Peripheral Nervous System for the spinal nerves these roots form, and The Brainstem and The Meninges and Cerebrospinal Fluid.
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Describe the external landmarks of the spinal cord: the enlargements, the conus medullaris, and the cauda equina.
- Compare the dorsal and ventral roots by the kind of information each one carries.
- Explain the cross-sectional organization of the cord using its gray horns and white columns.
- Contrast an ascending and a descending tract by the direction each carries information.
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.