Knowledge of the nervous system in general and of the brain and human behaviour in particular are of paramount importance to those who are dedicated to a safe and healthy environment. Work conditions, and exposures that directly affect the operations of the brain, influence the mind and behaviour. To evaluate information, to make decisions and to react in a consistent and reasonable manner to perceptions of the world require that the nervous system functions properly and that behaviour not be damaged by dangerous conditions, such as accidents (e.g., a fall from a poorly designed ladder) or exposure to hazardous levels of neurotoxic chemicals.
Damage to the nervous system can cause changes in sensory input (loss of vision, hearing, smell, etc.), can hinder the capacity to control movement and body functions and/or can affect the brain’s capacity to treat or store information. In addition, altered nervous system functioning can cause behavioural or psychological disorders. Mood and personality changes are a common occurrence following physical or organic damage to the brain. As our knowledge develops, we are learning more about the way in which nervous system processes are modified. Neurotoxic substances can cross the brain’s natural barrier and directly interfere with its intricate workings. Although some substances have a particular affinity to certain areas of the nervous system, most neurotoxins have widespread effects, targeting cell processes involved in membrane transport, internal cellular chemical reactions, liberation of secretory substances, and so on.
Damage to the various components of the nervous system can occur in different ways:
- direct physical injury from falling objects, collisions, blows or undue pressure on nerves
- changes in the internal environment, such as insufficient oxygen due to asphyxiants and heat exposure
- interference in the cellular processes through chemical action by substances, such as metals, organic solvents and pesticides
The insidious and multifaceted development of many nervous system disorders requires persons working in the field of occupational health to adopt different but complementary approaches to the study, understanding, prevention and treatment of the problem. Early alterations can be detected in groups of active, exposed workers using sensitive measures of impairment. Identification of initial dysfunction can lead to preventive actions. In the latter stages, a good clinical knowledge is required and differential diagnosis is essential to the adequate treatment and care of disabled workers.
Although chemical substances are mostly examined one by one, it should be remembered that in many workplaces mixtures of potentially neurotoxic chemicals are used, exposing workers to what can be called a “cocktail”. In processes such as printing, painting, cleaning, in poorly ventilated offices, in laboratories, pesticide application, microelectronics and many other sectors, workers are exposed to chemical mixtures. Although there may be information on each one of the substances separately, we have to consider the combined nocivity and possible additive or even synergistic effects on the nervous system. In some cases of multiple exposure, each particular chemical may be present in very small quantity, even below the detection level of exposure assessment techniques; however, when all are added together, the total concentration can be very high.
The reader should be aware of three major difficulties in reviewing facts about the nervous system within the scope of this Encyclopaedia.
First, the understanding of occupational diseases affecting the nervous system and behaviour has changed substantially as new approaches to viewing brain-behavioural relationships have developed. The main interest of characterization of gross morphological changes that occur due to mechanical trauma to the nervous system—particularly, but not exclusively to the brain—was followed by an interest in the absorption of neurotoxic agents by the nervous system; interest in the study of cellular mechanisms of nervous system pathology; and finally, the search for the molecular basis of these pathologic processes began to grow. These approaches coexist today and all contribute information for evaluating the working conditions affecting the brain, mind, and behaviour.
Second, the information generated by neuroscientists is staggering. The third edition of the book Principles of Neural Sciences edited by Kandel, Schwartz and Kessell which appeared in 1991—one of the most valuable reviews of the field—weighs 3.5 kg and is more than 1,000 pages long.
Third, it is very difficult to review knowledge about the functional organization of the nervous system as it applies to all niches of occupational health and safety. Until about 25 years ago, the theoretical views that gave support to those concerned health experts who specialize in the detection, monitoring, prevention, and the clinical treatment of a worker who has absorbed a neurotoxic agent sometimes did not overlap with theoretical views regarding workers’ brain trauma and the behavioural manifestations of minimal brain damage. Behavioural manifestations said to be the consequence of the disruption of specific chemical pathways in the brain were the exclusive territory of the neurotoxicologist; both structural tissue damage of specific regions of the brain, and distant neural structures linked to the area where the lesions occurred, were explanations invoked by neurologists. It is only in the past few years that converging views are appearing.
With this in mind, this chapter addresses issues important to the understanding of the nervous system and the effects of workplace conditions on its functioning. It begins with a description of the anatomy and physiology, followed by a section on neurotoxicity, which reviews exposure, outcomes and prevention.
Since the nervous system is central to the body’s well-being, many non-chemical hazards can likewise affect its normal functioning. Many of these are considered in different chapters dealing with these hazards. Traumatic head injuries are included in First Aid, heat stress is considered in the article “Effects of heat stress and work in the heat”, and decompression sickness is reviewed in the article “Gravitational stress”. Hand-arm vibration (“Hand-transmitted vibration”) and repetitive movement (“Chronic outcomes, musculoskeletal”) in the chapter Musculoskeletal System, which are risk factors for peripheral neuropathies, are likewise considered in these sections of the Encyclopaedia.
The chapter ends with a review of special issues and the outlook for future research avenues.