Dissector Answers - Deep Back & Spinal Cord

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Identify and give the function of the significant parts of a typical vertebra and associated ligaments.
  2. Identify the specialized vertebrae.
  3. Describe the spine, its curvatures, and vertebral column movements.
  4. Identify the coverings and the supporting structures of the spinal cord. Give the point of the termination of the spinal cord and the dural sac.
  5. Identify the terminal specialties of the cord, their relation to lumbar puncture, the nerve rootlets, and blood supply.
  6. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.
  7. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord.
  8. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution.
  9. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation.
  10. Define and explain the significance of dermatomes.
  11. Explain the difference between superficial and deep (true) back muscles.

Learning Objectives and Explanations:

1. Identify and give the function of the significant parts of a typical vertebra and associated ligaments. (W&B 339-344, N 154A, 154B, 155, TG 1-02, 1-03A, 1-03D, 1-05A, 1-05B, 1-05D, 1-08A, 1-08B, 1-10, 1-11A, 1-11B) ) 2. Identify the specialized vertebrae. (W&B 343, N 17, 18, TG 1-03A, 1-03C, 1-08A) 3. Describe the spine, its curvatures and vertebral column movement. (W&B 344-345, N 153, TG 1-02) 4. Identify the coverings and the supporting structures of the spinal cord. Give the point of the termination of the spinal cord and the dural sac. (W&B 350-353, N 160, 161, 169, TG 1-18, 1-19, 1-20) 5. Identify the terminal specialties of the cord, their relation to lumbar puncture, the nerve rootlets, and blood supply. (W&B 350-353, N 160, 161, TG 1-11, 1-18, 1-20) 6. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury. 7. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord. (W&B 353-355, N 169, 170)

A cross-section of the spinal cord will show a butterfly-shaped area of gray matter surrounded by white matter. The butterfly has four "horns", two dorsal and two ventral. Sensory neurons originate in the dorsal horns and make up the dorsal roots, but their cell bodies are in the dorsal root ganglia, outside of the spinal cord. Motor neurons have their cell bodies within the spinal cord, in the ventral horn. (The ventral horn is "fatter", since those cell bodies take up space!) They unite to form the ventral roots. Distal to the dorsal root ganglia, the roots unite, forming the spinal nerve. The nerve quickly splits into two branches, or ventral and dorsal rami, which carry both sensory and motor fibers. (Latin, ramus = branch)

8. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution. (W&B 9-10, N 169, 170, TG 1-17) 9. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation. (W&B 364-368, N 180, 258, TG 1-17)
Both the nerves and the arteries are segmented along vertical axis of the thorax. The intercostal nerves come from the spinal nerves. The arteries are mostly segmental branches of the aorta. The veins, arteries, and nerves run in a groove under each rib with the veins most superior and the nerves most inferior (VAN). Each also gives off a smaller collateral branch that runs on top of the rib below.
10. Define and explain the significance of dermatomes. (W&B 123, 581 (limbs), N 164, TG 1-23A, 1-23B)
One dermatome is the region innervated by one spinal nerve. They typically overlap, with one nerve covering its own dermatome as well as half of the one above and below it.
11. Explain the difference between superficial and true back muscles. (W&B 89-93,335-338, N 174, 179, TG 1-12, 1-13, 1-14, 1-15)
Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless.

The Vertebral Column - General Characteristics of a Vertebra - The Cervical Vertebrae - The Thoracic Vertebrae -The Lumbar Vertebrae - The Sacral and Coccygeal Vertebrae - The Vertebral Column as a Whole - The Deep Muscles of the Back - The Spinal Cord - The Meninges of the Brain and Spinal Cord - The Spinal Nerves


Questions and Answers:

12. What is the coccygeal ligament?
At the level of termination of the dura sac, S2, the dura continues below as the coccygeal ligament (or filum terminale externum) to attach to the coccyx. (N 160, TG 1-20)
13. What do the lateral continuities of the dura mater cover at and distal to the intervertebral foramen?
They cover the dorsal root ganglia and spinal nerves, eventually blending with the epineurium of the spinal nerves and their dorsal and ventral primary rami. (N 173, TG 1-19)
14. What does the subarachnoid space usually contain?
The subarachnoid space usually contains cerebrospinal fluid (CSF). (N 169A, 169B, TG 1-19, 1-20)
15. With what is the subarachnoid space continuous?
The subarachnoid space surrounding the spinal cord is continuous with the subarachnoid space surrounding the brain. This means that it is also continuous with the ventricular system within the brain, where CSF is produced. (N 109, TG 7-50)
16. Where does the filum terminale lie?
The filum terminale (internum) extends inferiorly from the conus medullaris, as part of the cauda equina in the lumbar cistern, to become enclosed within the coccygeal ligament (or filum terminale externum). (N 160A, 160B, 161, TG 1-18, 1-20)
17. Where does the filum terminale attach?
The filum terminale attaches to the coccyx . (N 159, TG 1-18, 1-20)
18. What are the relations of the denticulations and the denticulate ligament to the roots of spinal nerves?
The denticulations separate the dorsal and ventral roots by lying between them. (N 169, TG 1-19)
19. How many denticulations are there?
There are 21 denticulations. (N 169, TG 1-18)
20. Where do the denticulations attach?
They pierce the arachnoid mater to attach to the inner surface of the dura mater. (N 169, TG 1-19)
21. At what vertebral level is the conus medullaris?
The conus medullaris is at L1-2. (N 160A, 160B, 161, TG 1-18, 1-20)
22. Where is the dura mater in relation to the intervertebral foramen?
Within the intervertebral foramen, attached to the periosteum surrounding it. (N 173, TG 1-18, 1-19, 1-20)
23. Examine dorsal and ventral rootlets as they emerge from the cord. How is the segmental pattern created?
Rootlets coalesce as they enter the dural sleeve, lateral to their exit from the cord, thereby forming segmental roots. (N 169)
24. What is a root?
The roots are nerves which leave the gray matter through ventral or dorsal horns and unite to become the spinal nerve. (N 169, TG 1-17, 1-19)
25. What is the functional difference between dorsal and ventral roots?
Ventral roots carry motor fibers, both somatic and visceral. Dorsal roots carry sensory fibers. (N 169, TG 1-17)
26. Trace the roots to the point of union to form the spinal nerve. Are they in separate sheaths before uniting?
No, the spinal nerve roots are usually contained within a common sleeve of dura mater. (N 169, TG 1-17)
27. Where is the dorsal root ganglion?
The dorsal root ganglion is located in the intervertebral foramen on the dorsal root. There is one ganglion per spinal nerve. (N 169, 180, TG 1-17, 1-19)
28. What is the extent of the dura mater, arachnoid mater, and pia mater on the spinal nerves?
The pia mater fuses with the arachnoid mater at the spinal nerve, while the dura mater still provides a covering. (N 169, TG 1-17, 1-19)
29. Note posterior and anterior spinal arteries. What are their sources?
The spinal arteries arise within the skull. Anterior spinal arteries are branches of the vertebral arteries and posterior spinal arteries are branches of the posterior inferior cerebellar arteries. (N 171, 172A172B, TG 1-21, 7-56A, 7-56B)
30. What reinforces the spinal arteries?
Aorta or vertebral aa. --> segmental aa. --> posterior branch --> spinal branches --> radicular branches --> anterior and posterior spinal arteries via anastomoses. (N 171, TG 1-21)
31. What are radicular arteries?
Radicular arteries are branches of spinal branches of segmental arteries that run along the dorsal and ventral spinal nerve roots. (N 171, 172A172B, TG 1-19, 1-20, 1-21)
32. Are all radicular arteries the same?
Most radicular arteries are small, but there is often a rather large radicular artery, the great radicular artery, that may be found in the lower thoracic or upper lumbar levels, usually on the left side. The great radicular artery represents a major blood supply for the lower spinal cord. (N 171, 172A172B, TG 1-19, 1-20, 1-21)
33. What is the source for radicular arteries?
Radicular arteries arise from vertebral arteries in neck, segmental arteries (intercostal & lumbar), and lateral sacral arteries in the pelvis. (N 171, 172A172B, TG 1-19, 1-20, 1-21)
34. Observe arrangement of gray matter and white matter of the cord. What is their significance?
Gray matter is located on the inside of the spinal cord and makes up the "butterfly." It is composed of cell bodies and has dorsal, ventral and lateral horns. White matter, located on the outside of the "butterfly", is made up of myelinated nerve tracks. (W&B 33, N 169)
35. After exposing the posterior longitudinal ligament in cervical and lumbar regions, can you see the intervertebral discs?
You can not see the discs very well. The posterior longitudinal ligament spreads laterally at each disc to attach to it. (N 158, TG 1-11)
36. What is the significance of the attachment of the posterior longitudinal ligament to the discs?

The attachment of the posterior longitudinal ligament to the intervertebral disc helps to reinforce the posterior aspect of the anulus fibrosus. However, because the ligament thins laterally, most disc herniations occur in a posterolateral direction. (N 158, TG 1-11)

Updated: 07 Sep 2011