For example, rheumatoid arthritis is more likely to inflame the wrist joint than the hip joint. It also is unknown why some types of arthritis, such as rheumatoid, affect both sides of the body simultaneously. The explanation of these mysteries may lie in the number and types of nerves that go to particular joints and the chemical messengers that they produce. Several chemical messengers are formed and stored in nerve endings. These messengers contribute to the pain and inflammation of arthritis.
Various stimuli cause the release of substance P from nerve endings lying next to the joints.
Substance P causes blood vessels to dilate and also acts upon nearby cells to make them release mediators of inflammation, such as prostaglandins and collagenase, ultimately causing joint inflammation. The joints most likely to be inflamed by arthritis are the same joints that have the most nerves containing substance P. Communication between the joints and the nervous system also moves in the other direction. Therapies are being investigated that may improve arthritis by interrupting the connection between the nervous system and joint inflammation. Although no treatment is currently available that interrupts the connection between the nervous system and inflammation, physicians can try to reduce the consequences of stress with various psychological interventions. Relaxation techniques, behavior modification, biofeedback and patient support groups all have been used to improve feelings of health and well-being.