Understanding Pain Pathways: A Nursing Approach

Let's start off by talking about A delta fibers and how they transmit pain. A Delta fibers are lightly myelenated fibers meaning they conduct impulses faster than a nonmyelenated fiber but less than a heavily myelenated one. Think of this how an insulated wire might transmit electricity; those that are more heavily insulated can transmit currents over longer distances than those that are not. A delta fibers are responsible for the process of recognizing pain. These fibers are activated when you touch something hot or something cold. They tell you, hey get your hand off of this or there is gonna be trouble. The reason these fibers can do this is because of where they end in the brain. The thalamus is where A delta fibers mainly congregate and it is this location that provides us with how we understand what it is we are experiencing. Think of having a splinter or stepping on something sharp. We might not be able to see the object causing the pain but we know exactly where the pain is. This is your thalamus telling you via A delta fibers. An experiment you can do to test A delta fiber transmission is to poke your partner with a fork after they fall asleep (make sure they are out), they might twitch and move the area but they should not wake up (I wonder how many people will test this).

The second type of fibers we will talk about is type C fibers. Type C fibers are unmyelinated and conduct pain impulses more slowly than that of A delta. These fibers are responsible for noxious stimuli in the body as a result of some type of physiological alteration. Burns, Cuts, sepsis, inflammation, etc; these are all things that travel via c fiber pathways. So if we burn ourselves, A delta tells us to move our hand, c fibers then bring the more deep rooted pain, the aching, the burning, after the damage has been done. These fibers end in a few places, some in the brainstem near the reticular formation and other in higher brain areas like the thalamus. Think of pain patients have when having a gallbladder attack, they can't really pinpoint the area they just have a general idea of where the pain is. It's somewhat local but maybe we have pain radiation into the back or the side, this is all because of c fiber pain. Some oft he information is going to the thalamus but the majority of it is firing in lower areas in the brainstem. These processes you might also see referred to as nociception. Please see the figure 2-1 for how these signals are transmitted: Mechanisms of Pain - Recognition and Alleviation of Pain in Laboratory Animals - NCBI Bookshelf

One special thing to note about c fiber pain is that since the pain fires in the reticular complex, it makes sleeping very difficult for patients with intense pain of this nature. The reticular complex is the sleep/wake center of the brain and it is constantly stimulated with c fiber transmissions if a person is experiencing pain. So how does this translate into how we treat patients and how do we assume that enough pain medication is enough for a patient? Let's talk about a few important concepts in how we can address pain: Transmission vs Transduction

Think of these two concepts like the cars on a train. There is an engine up front and a caboose in the back (we hope, they are so cool!). Depending on what medicine we give refers to how we will treat the pain and what part of the train we will be working with. Lets start with transduction.

Transduction is mediated at the source of pain. Have you ever seen an advil commercial where they talk about blocking pain at the source? This is how transduction works by blocking pain signals from firing through the use of blocking chemical mediators. By doing this we have basically stopped pain from being transmitted all together. We are blocking the pain at the source of where the pain is being produced. So the engine doesn't even get to pull all those cars in between, were stopping it in its tracks!

Transmission is on the back end of things, or the caboose. Pain signals have already been transmitted but we need to suppress where they are transmitted to. Most notably this is how opiates work; we suppress the CNS enough to decrease the brainstems perception of pain. Think about this concept for a minute and how it can relate to breathing as well. If we suppress brainstem activity, not only do we help pain but we also suppress the body's ability to want to breathe since those pathways must travel through the brainstem. So when opiates are given, were blocking the back end of pain, not at the source but at the end of the line.

How does this all translate into how we treat pain? This is a good question. One thing we did not talk about is neuroleptic pain and that's a whole ball of wax in its own (which I can also discuss if anyone is interested).

Nursing considerations when giving narcotics or nsaids revolve around patient response to the medications given. If a patient just had surgery, it is expected that they will have some A delta fiber activation and a lot of c fiber activation. Usually, they can tell you where all the incisions are but are also followed by some type of more deep rooted pain (the c fibers). So how do we know if enough is enough? Well since we know that c fiber pain terminates in the sleep wake center one way to evaluate successful pain managementis a sleeping patient. How does this work? Well we know that the majority of c fibers terminate in the sleep wake center (reticular complex). If a patient is having pain that is severe or even moderate, they physiologically will not be able to sleep due to the constant firing of impulses in this area(constant stimulation). A patient may be able to rest their eyes but they cannot sleep. When we give morphine or dilaudid we are suppressing these fibers below the level of the reticular complex. By doing so, the fibers are no longer firing in that area of the brain, subsequently the patient is able to sleep. Pain is a subjective analysis, it is perceived by the patient and communicated to the nurse. If a patient is sleeping, they are unconscious and unable to perceive pain to give us an experience. This does not mean that pain is absent, it means pain is controlled to a level where the patient is able to rest and recover. This is the ultimate goal of a nurse, to allow the patient to rest and heal. There is also a fine line, too much we suppress the CNS too much and cause all sorts of problems, too little and the patient will never be able to rest. It's crucial to find the middle ground and to get patients to understand that pain management is a two way street.

Pain Sensitivity and Modulation in Primary Insomnia

See also: Principles and Practice of Sleep Medicine 6th Edition chapters 23 and 24