Taking off into the world of spinal nerves
Every nerve in the body can be described as either sensory, motor, or mixed. The spinal nerves are all mixed nerves. That is because the sensory dorsal roots and the motor ventral roots join together as the spinal nerves.
Figure 11.24 (on page 407) shows the structure of a peripheral nerve. We have talked about the fact that peripheral nerves contain the axons of neurons that are running to and from the CNS. Many of the axons are myelinated, but others are not. I have mentioned that these axons are bundled together to form a nerve. But what bundles them? Connective tissue, of course!
Just like you saw in muscles, there are levels of axonal bundling. The epineurium is the connective tissue that wraps around the entire nerve. Axons within the nerve are bundled into large fascicles held together by perineurium. And connective tissue that runs along individual myelinated axons or groups of unmyelinated axons is called endoneurium. What are these things again?
The rest of this page is material from pages 412 (but not table 11.9) through 415 (the details on the plexi are for lab).
There's a spinal nerve for every body "segment."
A spinal nerve runs out and in between adjacent vertebrae, all along the length of the vertebral column. Each spinal nerve can thus be considered to be either cervical, thoracic, lumbar, or sacral (or even coccygeal). Any other details of the spinal nerve numbering are left for lab.
Spinal nerves supply specific body regions.
The sensory information that enters your spinal cord from the skin around your belly button enters at the nearest entryway, which is via the T10 spinal nerve. Likewise, muscles that lie in that region of the body are innervated by motor neurons which exited the spinal cord via the T10 spinal nerve.
Actually, I should have scanned this image, but I got a CD with the book images from the last edition, and assumed they were all really good. This one looks bad. Sorry.
The point I'm making should still be evident, though. Each area of the body sends its sensory information into the spinal cord at the nearest level spinal nerve. Each of these body regions is called a dermatome.
The dermatomes are drawn as if they are perfectly separated regions in such a map. But, in fact, the borders are a little fuzzy, and there's some overlap between adjacent dermatomes for which spinal nerve supplies it.
This dermatome map is useful clinically. If a patient is having trouble sensing touch in a more specific region, lets say in the upper thighs and up by the hip, only on the left side, that could mean that the patient's left L2 spinal nerve could be pinched a bit somehow. That would mean that the patient's problem is restricted to a local spinal one, possibly problems with the L2 or L3 vertebrae being out of alignment.
Now that we have finished our discussion of the spinal cord, you can all look forward to next week's lab on the spinal cord! Also, you can start to get excited about learning about the brain in next week's web pages! (there's certainly a lot more that we could have done on the spinal cord, but we are running out of time...) I hope you're all looking forward to the brain! And remember:
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