PAIN

major theories

Sensations from an amputated limb can manifest themselves in many different ways, such as sensations of touch. For example, some amputees are able to feel their arm resting on a table, their fingers able to feel the texture of the table. Other times they can feel the absent limb in movement, possibly reaching for a glass. Some report that the limb is drastically deformed or forshortened, or that it remains rigid.

Sometimes, though, it is not merely a sensation that they feel, rather - pain. Frequent complaints are: tingling, prickling and shooting pains. Although this is not always the case. Some find themselves revistiting a pain they had previous to the amputation, such as an ingrown toenail. The pain is also not usually constant, some are reoccuring and others do not even begin until long after the surgery. There are many case histories to explain this in more detail on one of our other pages.

The purpose of this section is to give an adequate display of the PAIN PATHWAY, from the activation of nerve fibers, to its relay stations in the thalamus and cerebral cortex.

The popular notion is that a nerve is "something that hurts", this has only limited basis in fact. Most nerves do not hurt when they go into activity or are injured. But when you hurt yourself the basic disturbance consists of impulses in specific pain fibers and nerve centers. Embedded in our skin, for example, there are some three million "pain spots", regions which are a good deal more sensitive to pain than the surfaces immediately surrounding them.

The most minor mishap can start a series of events in the nervous system. When you stub your toe, the first thing that you seem to feel is a sharp pain that comes almost immediately. This is from the nerve impulses that travel along the fibers at amazing speeds, telling your brain to signal your foot to "pull away". So, you jerk your foot away without even thinking. In fact you react so quickly that the pain itself does not actually come until you have taken the emergency measures of removing your foot. In such cases you react first and feel afterwards.

Another sensation comes a bit later, the diffuse, burning kind of pain which may linger for a period of time. It is produced by a different, slower pathway. This sensation is reminding you to "take it easy for awhile", meaning favoring your toe for a bit. So what is it inside of us that can transmit all of these separate sensations?

Now that you know the structure of a neuron, imagine many of those bundled together, creating our nerves, and just as neurons have different forms, so do nerves.

There are two systems for the transmission of pain, either it is the fast system of the slow system. There are alternate nerve fibers that service each system, correspondingly the two nerve fibers are termed fast nerve fibers and slow nerve fibers.

These systems have separate structures, so, beginning with

--FAST NERVE FIBERS--

Fast fibers are relatively small, only about 3 - 20 microm. in size (micrometer = 0.00001 meter). Just as the neurons could be myelinated, so can the nerve fibers. Since myelin speeds up the transmission process, one might figure that these fast fibers are myelinated, which they are. They can conduct an impulse at the rate of 20 meters per second. They are one section of the fibers termed the A fibers.
These fibers have their receptors only in certain areas of the body, they service only the skin and mucous membranes, we will discuss these receptors in more detail later.

--SLOW NERVE FIBERS--

These are termed the C fibers and are tiny and unmyelinated. Since the electrical message is allowed to leak out of the cell, because of the lack of myelin, the transmission is much slower. These fibers are about 1 microm. in size but conduct at only about 1 meter per second. That is to say for example, if there was one of these fibers running from one of your feet, it would take the impulse two seconds to signal pain to the brain. Unlike the fast fibers that service only the skin and mucous membranes, these C fibers service all of the skin and all of the body tissue, except nervous tissue and the brain itself, which is insensitive to pain.

When there is a painful stimulus present, there are specialized receptors beneath the skin, our primary example, that can detect the certain stimulus. There are many different types of receptors, there are ones specialized just for light touch. Others are for deep pressure, temperature information, meaning heat or cold and even movement. Lastly, there are specialized receptors for pain, termed nociceptors, meaning that they detect noxious stimuli. They are free nerve endings that terminate just below the skin or other tissues. They service all the tissues of the body, so that three kinds of pain are recognized:

1. superficial or cutaneous pain ( cutaneous means, of the skin )
2. deep pain from muscles, tendons and joints
3. visceral pain

---THE PAIN PATHWAY---

Now that we have covered some of the basics, underlying this pathway, we can get right down to it! Throughout this explanation we will also be contrasting the pain pathway with the other pathway closely related, which is the pathway for non noxious stimuli. Once the painful stimulus has been detected by the nociceptors, they send their afferent message (afferent, meaning going TO the spinal cord ) by way of the nerve fibers, to the spinal cord. The pain messages do something different than other messages, they cross over to the other side of the spinal cord - right away, whereas the other sensations do not cross over right away. This is the first difference between the pathways; the pain and temperature pathway is termed, the SPINOTHALAMIC tract, the other sensations travel along the MEDIAL LEMNISCAL tract.

This Picture is of the spinothalamic tract, you may refer to it at any time, by clicking on it for a larger version. The spinal cord when looked at as a cross section, as in the illustration, one can see that it contains both white matter and grey matter. The white matter, around the outer edges, contains the ascending and descending fiber pathways. The grey matter on the other hand, contains many neuronal cell bodies and synapses. The ascending pathways, relay the messages they have received from the environment to the brain, while the descending pathways relay the brain's motor mesages to the muscles. The two main sensory systems that we are concentrating on, that enter the spinal cord are, pain-temperature, and light touch. The pain pathway, the spinothalamic tract, crosses over almost immediately after it has entered the spinal cord; this means within one or two vertebral segments. After it has crossed over it travels to the cerebral cortex, the outer part of the brain.

As mentioned above, the other pathway for light touch is the medial lemniscal pathway. This, unlike the pain pathway, waits until it reaches the brainstem, which is about at the top of your neck. Eventually they both do cross over and from there, send their information on to an area in the brain called the thalamus. The thalamus is kind of the main relay station for most information that enters the brain awaiting processing. The thalamus does perceive the pain but it is not able to localize it, it then sends that information on to the cerebral cortex.

On a certain area in the cerebral cortex is a map of the human body, called a Homunculus. So, each place on the body that receives a sensory stimulus, will project its message to its corresponding part on the sensory cortex area, the homunculus.

Sometimes it it very important for the body to remove itself or one of its limbs from the dangerous situation, therefore it must have a mechanism for drawing back. The message that has been processed in the brain is sent back down the spinal cord to the appropriate area, and into a muscle, where it acts; this pathway it the corticospinal pathway, named because the impulse originates in the cortex and travels down the spine.