ANSWERS TO FOCUS QUESTIONS: DEEP BACK AND SPINAL CORD
1. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.
· Herniation Focal protrusion of disc material secondary to rupture of annulus fibrosus. This most commonly occurs in the lumbar region, and can result in compression of spinal nerves (which usually leads to the clinical presentation).
· Dislocation of vertebrae: Like any other dislocation, this is a case of the bone slipping out of place, most commonly in the cervical region. The spinal canal is relatively small and these injuries very often result in damage to the spinal cord.
· Fracture of the dens: Due to the relative strength of the ligaments compared to the dens in this region, stress often results in its fracture. In effect, it is "the first thing to go".
· Cord injuries: The spinal cord can be injured in various ways, all of which are extremely serious and can result in significant impairment of function.
2. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord.
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 (right and left) dorsal and two (right and left) ventral. The axons of sensory neurons make up the dorsal roots and some of these axons terminate in the dorsal horn. The cell bodies of sensory neurons 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!) The axons of motor neurons in the ventral horns unite to form the ventral roots. Distal to the dorsal root ganglia, the dorsal and ventral roots unite, forming the spinal nerve. The nerve quickly splits into two branches, or the ventral and dorsal rami, which carry both sensory and motor fibers. (Latin, ramus = branch)
3. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution.
· Parts (proximal to distal):
o ROOTS: dorsal and ventral. Roots are made of several rootlets
o SPINAL NERVE: where the two roots come together briefly
o PRIMARY RAMI: dorsal and ventral. These are the first branches off the spinal nerve.
o Cutaneous branches: posterior, lateral and anterior.
· Functional components:
o Sensory: Afferent, either somatic or visceral.
o Motor: Efferent, either somatic or visceral.
§ Somatic motor: to voluntary muscle.
§ Visceral motor (autonomic): to involuntary muscle and glands.
o White ramus communicans: connection between sympathetic trunk and ventral primary ramus. These are only found between T1 and L2. (More on these later.)
o Gray ramus communicans: connection between sympathetic trunk and ventral primary ramus. Found in all spinal nerves (31 pairs). (More on these later.)
4. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation.
Both the nerves and the arteries are segmented along the 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.
5. Define and explain the significance of dermatomes.
One dermatome is the region of skin 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.
6. Explain the difference between superficial and true back muscles.
· SUPERFICIAL BACK MUSCLES control upper limb movements as well as aid in respiration. They mostly receive their nerve supply from the ventral rami of cervical nerves. These include the:
o Trapezius: innervated by the accessory nerve (CN XI).
o Latissimus dorsi: innervated by the thoracodorsal nerve, made from branches of ventral primary rami of C7 & C8.
o Rhomboideus major & minor: innervated by the dorsal scapular nerve, a branch of the ventral primary ramus of C5.
o Levator scapulae: innervated by the dorsal scapular nerve and branches of the ventral primary rami of C3 & C4.
o Serratus posterior (superior and inferior): innervated by intercostal nerves, first four and last four respectively. (Latin, serratus = to saw)
· DEEP OR TRUE BACK MUSCLES (e.g., the erector spinae muscles) specifically act on the vertebral column, producing its movements and maintaining posture. They are innervated by dorsal rami of spinal nerves. The fascia covering them constitutes the thoracolumbar fascia.
7. What is the coccygeal ligament?
At the level of termination of the dura sac, S2, the dura continues below as the coccygeal ligament to attach to the coccyx.
8. 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.
9. What does the subarachnoid space usually contain?
The subarachnoid space usually contains cerebrospinal fluid (CSF).
10. 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.
11. Where does the filum terminale lie?
The filum terminale extends inferiorly from the conus medullaris, as part of the cauda equina in the lumbar cistern, to become enclosed within the coccygeal ligament.
12. Where does the filum terminale attach?
The filum terminale attaches to the coccyx .
13. What are the relations of the denticulate ligaments to the roots of spinal nerves?
The denticulations separate the dorsal and ventral roots by lying between them.
14. How many denticulate ligaments are there?
There are 2 denticulate ligaments, each with 21 “teeth”.
15. Where do the denticulate ligaments attach?
They pierce the arachnoid mater to attach to the inner surface of the dura mater.
16. At what vertebral level is the conus medullaris?
The conus medullaris is at L1-2.
17. Where is the dura mater in relation to the intervertebral foramen?
Within the intervertebral foramen, attached to the periosteum surrounding it.
18. 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.
19. 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.
20. What is the functional difference between dorsal and ventral roots?
Ventral roots carry motor fibers, both somatic and visceral. Dorsal roots carry sensory fibers.
21. 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.
22. 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.
23. 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 (more on this in Brain, Mind, and Behavior).
24. What reinforces the spinal arteries?
Aorta or vertebral aa. --> segmental aa. --> posterior branch --> spinal branches --> radicular branches --> anterior and posterior spinal arteries via anastomoses.
25. What are radicular arteries?
Radicular arteries are branches of spinal branches of segmental arteries that run along the dorsal and ventral spinal nerve roots.
26. 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.
27. 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.
28. What is the significance of the arrangement of gray matter and white matter of the cord?
Gray matter is located on the inside of the spinal cord and is "butterfly-shaped." 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.
29. If you were to expose the posterior longitudinal ligament in cervical and lumbar regions, would you be able to 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.
30. 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 annulus fibrosis. However, because the ligament thins laterally, most disc herniations occur in a posterolateral direction.