Interaction of the Nervous System and the Immune System

In the past, little interaction was thought to occur between the nervous system and the immune system. However, recent studies demonstrate that there are ways in which the nervous system and the immune system communicate with each other. The placebo effects observed in people with MS and other diseases may be examples of this process. The field of study that examines immune system-nervous system interactions has been termed psychoneuroimmunology.

The brain may communicate with the immune system in many different ways. The brain influences the production of hormones, which then may affect the function of the immune system. In addition, nerve fibers have connections with immune organs. The chemical messengers used by the nervous and immune systems appear to be involved in cross-communication. Nerve cells communicate with each other by releasing chemicals known as neurotransmitters, whereas immune cells communicate with each other by secreting different chemicals, known as cytokines. Research studies have shown that cytokines may influence nerve-cell activity and that neurotransmitters may influence immune-cell function. Thus, the nervous system and the immune system do not appear to function independently but rather are components of a network in which they communicate and alter each others' activity.

Because there appear to be important interactions between the nervous system and the immune system, modifying brain activity may alter the immune system and immune diseases such as MS. For example, psychological stress and depression may influence the functioning of the immune system. One way to manage stress is to write about stressful life events. The act of writing about such events has been associated with improved immune function and improvement in two immune diseases, rheumatoid arthritis and asthma.

A variety of studies have evaluated the influence of the nervous system on MS activity. In animals, injury to one component of the peripheral nerves (nerves that are outside the brain and spinal cord)—the sympathetic nervous system—leads to altered immune function and worsening of experimental allergic encephalomyelitis (EAE), an animal model of MS.

Many studies have examined the possible influence of psychological stress on MS. In this area, two large questions exist. The first question is: Does stress cause MS attacks? For the first question, several individual studies (4) and a combined analysis of these studies (5), known as a meta-analysis, indicate that the risk of attacks increases after stressful life events. At this time, however, this is still a "chicken-or-the-egg" question. That is, it is possible that stress actually causes attacks. Alternatively, the earliest manifestation of an attack could be stress, in which case the attack causes stress. Interestingly, in the meta-analysis, the magnitude of the negative effect of stress (53 percent) was similar to the magnitude of the positive effect of the injectable MS medications (36 percent).

Another question in this area is whether stress worsens chronic MS symptoms. For example, if someone with MS has muscle stiffness on a daily basis, can times of increased stress be associated with increased stiffness? Limited studies in this area indicate that stress may indeed increase the severity of chronic symptoms.

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