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.
The entire body can be mapped to see which spinal nerves supply
which area 
of the body. Such a map is shown here, and
was taken from Figure 11.31 of your book.
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:
