January 17, 2010

The first questions sent to me was how often will I update this blog. Well, probably every week or so.

Two readers asked how the brain adapts to the loss of a limb or organ and/or how this change is perceived by the afflicted person. It’s an interesting question, one that has been the focus of much research. About 95% of amputees experience sensations in areas where the missing limb used to be. So if you lost, say your left leg, you might feel your left foot itching. This is called Phantom Limb Phenomenon. The perceived sensations vary from person to person ranging from vague feelings of warmth to shooting pain. The phenomenon may occur fairly soon after losing the limb and continue on through the person’s life. Interestingly, a researcher at McGill University, Ronald Melzack, discovered that phantom limb may occur in children born without legs.

Okay, so what causes these a person to feel these sensations in a body part that is no longer there?

Well, the neurologic basis for this isn’t known for certain and scientists debate whether it’s due to changes at the level of the spinal cord or higher in the nervous system - perhaps even on the surface of the brain, the cortex. (My money is on areas higher up than the spinal cord). Regardless, it’s likely due to changes in nerve cell behavior that result from the loss of connections. If you lose a leg, you lose the nerves that transmit sensations (such as pain, vibration, touch) from your muscles and skin to the spinal cord on up to the cortex. It is the processing of this information within the cortex that actually “makes sense” out of it.

Each side of the brain has an organized map of sensation (called the homunculus) from the opposite side of the body. This map is laid down as the brain develops and is organized in pretty much the same way from person to person. It doesn’t change much during one’s life. So the leg area of the homunculus receives information from the nerves to the leg via the spinal cord and brain stem. If the nerves to the leg are cut off, as happens in an amputation, the nerves cells (neurons) in the cortex no longer receive any information from the leg.
Nerve cells communicate with each other through connections called synapses. If a neuron loses synapses it becomes hyperactive and may begin to produce signals to other neurons spontaneously. (Like the kid in school who threw spitballs when the teacher wasn’t looking.) This spontaneous activity from the brain areas that gave us conscious appreciation of the amputated body area is why phantom limb phenomenon happens. Because the brain area that used to make sense of the lost body part becomes spontaneously active, we perceive sensations for the missing part.