INVISIBLE FORCES AND PHANTOM LIMBS

The somatosenses, which detect things like pressure, vibration and temperature, are normally grouped into the skin senses, the internal senses and the vestibular senses. It is important for us to know which way up we are, and how we are moving (especially how we are accelerating). This is achieved by a part of the inner ear called the semicircular canals, which contain small lumps immersed in a viscous fluid. When we move, these lumps move within the fluid. The lumps are in contact with hair cells (like those in the cochlea), and the motion of the lumps in the fluid bends the hair cells and results in neural messages, which are relayed to the vestibular nuclei in the brainstem. This type of sense is referred to as the vestibular system, [vestibular system located in the inner ear, this responds to acceleration and allows us to maintain body posture] and without it we could not walk without staggering. You can see impairment of this system in someone who has consumed too much alcohol. Motion sickness and dizziness are associated with unusual output from the vestibular system. In the skin senses, the transducers are nerve endings located around the surface of the body. There are also inner senses that tell us, for example, about muscle tension and joint position, which have detectors distributed in all the muscles and joints of the body. These work together with our vestibular system to coordinate our movements and maintain balance and posture. Many people who have had limb amputations report that they still feel that they have the amputated limb, and often say that this ‘phantom limb’ is causing them great pain. Research by Ramachandran and Blakesee (1999) on patients who have such feelings shows that the representation of touch by neurons in the brain can be changed, resulting in a remapping of the body’s tactile repr sentation of itself. So, for example, touching the cheek of a patient elicited an apparent sensation in the phantom thumb. The ‘motor homunculus’ (see chapter 3) shows the sensory representations o different parts of the body in the cortex. The proximity of the representations of the different parts of the body in this mapping, and the remapping of neurons within this after a limb amputation, is the probable reason for these remarkable effects.