There are two major divisions
Central Nervous System
Peripheral Nervous System
The brain and spinal cord make up the central nervous system (CNS). Those are all the parts that make it up. What you will see as we continue through this unit is that most of the cells of the entire nervous system lie within the central nervous system. These cells are responsible for most of the functions I described on the Unit 10 page. Some of these cells are motor neurons, that you learned about in the muscle pages (since they innervate muscle fibers); motor neurons actually lie within the central nervous system, but send out a very thin process all the way out to the muscle fibers they control. This thin process, the axon, leaves the central nervous system to get to the muscle.
As you will see, there are many types of neurons, and these neurons have a couple types of processes (axons are just one type). If you consider what you already know about the motor neuron... that it sends information from the CNS to the muscle, on a one-way path... you should start to understand the difficulty in having trillions of these neurons, each with directions to their connections, all within the CNS. If you compare this to a highway system, and imagine a highway system with trillions of roads all going in specific directions, yet interconnected, you should realize that a lot of organization of the roads is going to be necessary to prevent highway accidents! Well, like that, the CNS is extremely organized, having many well-defined areas within it.
We will spend the rest of this week's web pages on the cells within the CNS and the PNS, but then will return to learn about the highly ordered and regionalized CNS as we explore the spinal cord and brain over the remaining weeks.
This is made up of the nervous tissue that lies outside of the brain and spinal cord. What's outside of the brain and spinal cord?
|Nerves: Nerves carry the processes of neurons (the axons) that have to extend out of the CNS and toward the periphery. For example, motor neuron axons cannot run all by themselves toward their muscles-- they are so thin that they would break if they had to run alone. So they bundle up with other axons and form a nerve. We will be studying nerves this semester.|
|Autonomic Nervous System: Something that is autonomic can run on its own, like automatic. The ANS is made up of nervous tissue that controls involuntary organ and bodily function. For example, it is your autonomic nervous system that commands smooth muscle to contract in blood vessels so that the vessels constrict. We will NOT be studying the ANS this semester, but we will pick up with it to start off next semester.|
Why should we care about the PNS? Well, for one reason, the PNS is more exposed. The CNS is all protected within the skull and vertebral column. But the PNS has to leave this protected area and run throughout the body, into regions that offer much less protection. Because of this, the PNS is a bit more likely to be damaged than the CNS (like if you cut yourself deeply and cut through a nerve).
Another reason is because the PNS has to carry our commands out to our body for movement, and has to bring in our sensory information to know what's going on in our world. So any damage to this nervous system division would limit our interactions with our world.
One last reason is because the glia of the PNS differ from the glia of the CNS. This difference leads to the differing ability of the PNS and CNS to recover after injury, and also explains why some diseases may affect only one of these two divisions. Understanding the differences between the PNS and CNS will then help you to understand specific clinical diseases.
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