ANALGESIA

loss of sensation of pain that results from an interruption in the nervous pathway between sense organ and brain. Different forms of sensation (e.g., touch, temperature, and pain) from one area of skin travel to the spinal cord by different nerve fibres in the same nerve bundle. Hence, any injury or disease affecting such a nerve would abolish all forms of sensation in the area supplied by it. When the sensory nerves reach the spinal cord, however, their fibres separate and pursue different courses on their way upward to the brain. Thus, it is possible for certain forms of sensation to be lost, while others are preserved, in diseases that affect only certain areas in the spinal cord. Because some of the sensations of pain and temperature travel the same path, they may be lost together. Diseases of the cord that may cause analgesia without loss of the sensation of touch are tabes dorsalis, syringomyelia, and tumours of the cord. Analgesia may also be a manifestation of hysteria. The term is also used for pain relief induced by the action of such drugs as aspirin, codeine, and morphine.