Reflexes, spinal tracts, and spinal nerves

Now that you know how the individual cells of the nervous system work, it is time to take those cells and connect them up in appropriate ways. It is also time to use the information that you know about the structure of these neurons to understand how they fit into the nervous system.

We need to start by gaining a thorough understanding of the way the spinal cord is structured. Some of this you have done in lab. But, I'm going over it here to make sure that you truly understand it. Then we will go on to individual pages on reflexes, spinal tracts, and spinal nerves.

White matter versus gray matter

Gray matter contains

	cell bodies
	dendrites
	glia

White matter contains

	axons
	myelin

Where are these found in the spinal cord?

	Gray matter is in the middle (the butterfly)
	White matter surrounds the gray matter
	note: the organization is reversed in the brain

Synaptic connections occur in gray matter

Because the cell bodies and dendrites, the receptive areas of neurons, are found in the gray matter, that's where synapses occur. So, look within the butterfly shape to find synapses in the spinal cord.

What axons are found in the white matter?

If axons are on their way in or out of the spinal cord, they tend to run right in or out through the dorsal or ventral roots. These roots join to make the spinal nerve, and both the roots and the spinal nerves are considered parts of the peripheral nervous system (see the drawing below). Axons that run through the spinal nerve do not bother hanging out in the white matter of the CNS.

If axons are coming down from the brain, going up to the brain, or running up and down between body areas, only then will you find those axons in the white matter. Remember, when you look at a cross-section through the spinal cord, the axons of the white matter are cut... that means that they normally run up and down the spinal cord.

The dorsal half of the spinal cord tends to be sensory, while the ventral half of the spinal cord tends to be motor

The dorsal roots carry sensory information into the spinal cord. The sensory neurons themselves lie in the dorsal root ganglia just outside of the CNS. The sensory information enters through the dorsal roots and invades the dorsal horn of the gray matter.

The ventral horns, as you saw in lab, contain the large motor neuron cell bodies. The motor neuronal axons run out the ventral roots toward the muscles.

What lies between these two halves? Mostly interneurons. Interneurons can either:

	1: process sensory information locally and send it to local motor neurons for immediate action. If this occurs, and local motor neurons are activated, that is a reflex.
	2: they can take the sensory information and relay it to the brain for further processing. If this occurs, and information has to get sent to the brain, that information runs through the white matter spinal tracts.

Now you should have a better idea of what you will be learning about in this unit.

Here is a drawing that may help to put these ideas together for you:

spinalcord.JPG (13559 bytes)

Note that I have used a few terms in this drawing that you haven't learned yet in lab. For example, the dorsal and ventral "horns." Those terms are simply names for the wings of the gray matter (assuming that the gray matter looks like a butterfly). The only other terms that are unfamiliar are descending and ascending axons. You know what an axon is-- and part of understanding an axon is understanding that it acts as a one-way street. Information only flows one-way through an axon. Since this drawing is of the spinal cord, an ascending axon might be one that brings information up to the brain from the spinal cord (and the neuronal cell body is in the spinal cord). Likewise, a descending axon might be one that brings information down from the brain to the spinal cord (with the cell body located in the brain).