Gross Anatomy ?: Each of the following are found primarily within the vertebral canal, EXCEPT:?
B. Dorsal primary ramus of a spinal nerve
C. Dura mater
D. Cauda equina
E. All of the above are contained in the vertebral canal
I'm thinking it's B because the dorsal ramus is a branch that begins the exit through the intervertebral foramen but I'm not
sure. Please clarify :)
Answer:
D)Cauda equina (my interpretation is it's below the vertebral canal)
Realize that the spinal cord does not extend the entire length of the vertebral column in the adult. In the developing embryo the spinal cord extends all the way down to the sacral region, however, during growth and development the vertebral column grows faster than the spinal cord thus spinal levels and vertebral levels do not coincide.
1) realize that the spinal cord ends at approx. L3 in infants, and approx. L1-L2 in adults.
2) The spinal cord segments do not correspond to the vertebral levels.
Spinal nerves are made of dorsal and ventral roots.
Ventral roots leave the anterior (ventral) horn of the spinal cord and carry motor (efferent) fibers.
Dorsal roots enter the posterior (dorsal) horn of the spinal cord and carry sensory (afferent) fibers.
This means that the cell bodies whose axons make up the ventral root are found in the ventral horn of the spinal cord. Note that these are unipolar neurons.
The dorsal root, however, is comprised of axons from pseudounipolar neurons whose cell bodies are located in the dorsal root ganglion (spinal ganglion). The spinal ganglion is found in the intervertebral foramen, here it can lay on the pedicle of the lower vertebra.
Distal to the spinal ganglion, and immediately outside the vertebral foramen, the dorsal and ventral roots join to form a spinal nerve. This spinal nerve subsequently divides into a dorsal and ventral rami.
DO NOT confuse dorsal and ventral rami with dorsal and ventral roots.
Due to the differential growth of the spinal cord and the vertebra, we said the spinal nerves do not correspond with their vertebral levels.
Rationalize this phenomenon
Notice the spinal cord ending at approx. L1-L2 yet the dorsal and ventral roots continue down the vertebral canal to exit at their appropriate vertebral levels. This collection of dorsal and ventral roots is termed the cauda equina (horses tail).
b. dorsal primary ramus of spinal nerve
from the spinal cord there is the dorsal root [sensory](with spinal ganglion) and the ventral root [motor] that then merges to becomes the spinal nerve and exit the intervertebral foramen but then
spinal nerve which emerges from the intervertebral foramen then divides into a smaller dorsal ramus and a larger ventral ramus (both have sensory and motor neurons)
addition: the cauda equina is still found primarily in the vertebral canal (see image link)
--the dorsal primary ramus of spinal nerve exits as a periphery nerve
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