What is a Central Nervous System and How Does it Compare to the Peripheral Nervous System?
More Importantly, Why Should I Be Aware or Care When Dementia is Involved?
by Teepa Snow, MS, OTR/L, FAOTA
First of all, let me say I am going to try and keep this really basic, but having said that, I am not sure that is possible. I hope to provide accurate information, and hopefully, helpful information that helps us understand how this applies or is related to the various forms of dementia. In this article, I am using a highway-road analogy combined with air traffic control to explain some of the relationships between the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
Hidden inside of each of us are the structures and wiring that allow us to be the human being we are. It is divided into two major categories and several functional units. The two major categories are core or central nervous system (CNS) and the edge or peripheral nervous system (PNS). The CNS takes data input, processes it, then determines what to do, and sends messages of output to the PNS to act on the message. The PNS is a more rural and regional system of connections that gather and send in information with an emergency reaction system and a life support system embedded, as well. The nerves of the PNS take back information that comes from the CNS and makes use of it at the end points or outer edges of the human body. In my simple picture, the CNS is represented as a blue circle and lines. The CNS includes the brain, twelve cranial nerves, and the spinal cord, and is in control of overall traffic in and out and deciding on traffic flow and holding patterns. The PNS is represented by orange lines. The PNS is composed of nerves that go from the spinal cord to organs or areas of the skin and muscles that are found throughout the body, as well as nerves that travel from end organs and skin locations to the spinal cord and send data in. There are two major groupings in the PNS, processing that is voluntary, done with purpose or on purpose to explore or investigate situations or the environment, and that which is involuntary or automatic. The automatic systems are divided into two categories. The first are those that happen to protect the human in reaction to emergency or problematic situations, quickly and spontaneously, such as pulling back from something sharp or hot. The second are those that occur routinely and rhythmically to sustain life such as peristalsis, heart rate, or blood pressure. In these situations, there is certainly input and interaction with the spinal cord portion of the CNS, but it may not go to the brain, central control center, at all. Ultimately, when these systems work together well, data from the PNS is used to help the human make use of all portions of the PNS and CNS to compliment, coordinate, or override single source input for mature and thoughtful function in the world.
An example would be: you are walking along the sidewalk and then come to a curb. The PNS for this activity would involve your body and your eyes; the irises (the muscles in the front of your eye that control pupil size and focal abilities), the six muscles around each of your eyeballs (that coordinate and direct your eyes to the curb to note the height change), and the muscles and nerves in your trunk, neck, arms, and legs (that modify what you were doing to shift into step down mode without losing your balance). The CNS portion is composed of the two cranial nerves called the optic nerves, the optic chiasm that is the nerve highway and interchange wiring that goes from the two retinae to both the primitive and cortical part of the brain, the occipital lobe that processes visual information, the amygdalae that senses dangerous or exciting situations, and the two additional cranial nerves called the vestibular nerves.
Additionally, in this situation there are messages that get sent to the prefrontal portion of CNS for determining the safety of stepping down given the color of the traffic light, as well as messaging coming and going to previous levels of function and ability and the hippocampal areas that have memories of past experiences of this event. Finally, we must add in the role and impact of the cerebellum to perform this familiar and rhythmic action without actively having to think it through, using motor memories and action patterns.
If this simple example hasn’t already overloaded your brain, then let’s add in the impact of dementia. We will need to consider the reality that dementia is individually unique, is changeable minute by minute, has various forms and types, and progresses throughout its course and in different environmental situations.
So, in our example, changes in either the CNS or the PNS would greatly affect the person’s ability to safely and effectively navigate what had been done without active consideration, but in a fairly unpredictable way. There would be some times when the person could proceed as they had done throughout their adult life. At other times, the very simple act of adapting to the curb could stop them in their tracks, or result in a dangerous or fatal incident. The risk that the two major systems will no longer be able to connect and function in a timely and consistent manner increases remarkably as the condition advances. Ultimately the CNS and the PNS are no longer able to work together to permit function. If the person has health conditions that affect the PNS, such as diabetic neuropathies, muscle weakness, or a bone fracture, the changes in the abilities of the CNS to take in and use that information may cause significant impairments or injuries to the person that otherwise would not occur. Similarly, as the CNS is being destroyed, the ability of the system to direct and provide the necessary support to the PNS to allow for actions and activities that were completed without effort or active attention, can now require intense focus or effort, or may not be possible.
Those of us who do live with neurodegeneration inside us, can only begin to imagine some of the frustration, surprise, sadness, embarrassment, irritation, joy, or pleasure that can be found when the two systems sometimes do their mature and functional thing and other times refuse to cooperate. As several friends living with dementia have described it, it is like living on an everchanging roller coaster with unexpected splash zones and sudden stops. Sometimes thrilling, sometimes chilling, and sometimes just normal. For the example I used, dementia can be extremely problematic in a condition known as Progressive Supranuclear Palsy (PSP), Posterior Cortical Atrophy (PCA), or Lewy Body Disease (LBD).
Over the course of this year, we will be exploring the various parts and portions of both the PNS and the CNS as they relate to dementia life and support options. My hope is that as we explore more, we will be better prepared and capable of supporting the common and unique changes that happen during the course of many, if not most dementias.
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Here are a few resources on information related to the CNS and PNS, you might find interesting or helpful: