Table of Contents

Life Sciences

STS-78 life science studies were divided into two fields - human physiology and space biology. The five areas of human physiology are musculoskeletal, metabolic, pulmonary, human behaviour and performance, and neuroscience. Three space biology experiments studied the growth of pine saplings, development of fish embryos and bone changes in laboratory rats.

Human Physiology

Musculoskeletal experiments-- The skeletal muscles, those that are attached to bones and make movement possible, were the focus of the LMS musculoskeletal investigations. During spaceflight, the human body is no longer under gravity's influence, and the function of the musculoskeletal system changes in response to the altered demands imposed upon it. Because astronauts float within the orbiting spacecraft, their leg and back muscles are freed of the load-bearing stresses experienced on Earth as they support the weight of the body. Although the skeletal muscles continue to control and move the body, muscles fibers become smaller (atrophy) in the absence of gravity.

Six LMS investigations continued the work of characterizing the effects of weightlessness on skeletal muscle function, performance and biochemistry. Each investigation concentrated on a different aspect of muscular adaptation. Some experiments tested the right limbs; others, the left. Certain experiments investigated performance of the arms, others studied the legs. Investigations of the functional characteristics of whole muscles were complemented by those examining individual fibers from the same muscle. The data gathered on LMS provided a detailed picture of how and why the performance of the limb movements are affected by immediate and more prolonged exposure to microgravity. Spin-offs from these studies on muscle degradation could be very helpful on Earth in improving rehabilitation programs following injury or prolonged bedrest.

Metabolic experiments -- Spaceflight and adaptation to microgravity place a variety of stresses on the human body and result in changes to normal biochemistry. Investigations have shown that extra calcium leaves the bones of astronauts and enters the bloodstream almost immediately as they reach orbit. Preliminary studies indicate that the calcium balance is altered in microgravity by an increase in the rate of bone resorption (removal of calcium from bone into the bloodstream). This loss of calcium from the bones is of major concern to mission planners considering long-duration stays in space. Investigators of this mission wanted to know what triggers the response, whether the process is cumulative and/or reversible after extended space travel, and whether recovery upon return to Earth is complete. In addition, since the aberration is similar to osteoporosis, physiologists hoped the information obtained from the space studies would provide valuable clues to the cause and treatment for this debilitating disease. Other biochemical processes that may be altered in microgravity include protein metabolism and energy balance. To quantify and understand these biochemical changes better, LMS investigators performed two experiments related to the physiological equilibrium of the human body and the adaptations experienced as a result of microgravity.

Pulmonary experiment-- The human lung is very sensitive to gravity; consequently, on Earth there are large differences in gas flow, blood flow and gas exchange between the top and bottom areas of the lung. Scientists once believed that the differences were primarily the result of the pull of Earth's gravity. Comprehensive studies on earlier Spacelab missions indicated, however, that much of the imbalance in the lungs between blood and gases was maintained in the microgravity environment. These results show that, while gravity plays a dominant role in the unevenness of lung functions in humans, there are significant non-gravitational effects as well. For this mission, investigators had modified tests performed on Spacelab-2 to obtain further information on this fundamental process.

Human behaviour and performance experiments -- The ability of the human body to respond quickly to changes in the external environment as well as to physiological challenges is crucial for life on Earth and in space. This capability requires a highly organized communication system, which is not only receptive to various stimuli (light, sound, chemicals or pressure) but also possesses an effective means of relaying this information to centers of recognition. On Earth, such mental acuity is affected by a number of factors, including fatigue, sickness, sleep, and changes in work/rest cycles and social conditions. In space, however, the effects of these catalysts may be magnified or inhibited, confounding normal physiological processes and diminishing effective behaviours. Most organisms have behaviour patterns that correspond to 24-hour cycles. These circadian rhythms are generated by internal biological clocks, which are regulated by external influences such as day/night cycles, seasonal changes, gravity and Earth's rotation. For the astronauts orbiting Earth, there are 16 sunrises and sunsets in a 24-hour period, Earth's rotation and seasonal effects are eliminated, and there is virtually no gravity. In this environment, scientists can examine circadian rhythms in the absence of ordinary external cues. LMS investigations were designed to determine whether human behaviour and performance are degraded by stress encountered during spaceflight and whether changes in social and work conditions negatively affect normal sleep cycles and waking performance.

Neuroscience experiment-- One of the most common symptoms of adjusting to weightlessness is space adaptation syndrome, which affects approximately two-thirds of all astronauts and produces symptoms similar to motion sickness on Earth. Conflicting signals from an environment that suddenly has no "up" or "down" may produce confusion in the vestibular system, which is located in the inner ear and provides a sense of balance and bodily orientation. Pressure sensors in the muscles and skin are also involved, and the eyes contribute by sensing the body's relationship to other objects. In weightlessness, information sent to the brain from the inner ear and other sense organs no longer corresponds to the cues experienced in a 1-g environment.

The goals of the neuroscience investigations were to document the changes that occur in the neurovestibular system, to investigate the mechanisms involved in these changes, and to identify countermeasures to alleviate the effects of space motion sickness. The TRE experiment described above was one of two neuroscience investigations on this mission.

Space Biology

Data from space biology research can lead to a better understanding of many of the basic mechanisms of both animal and plant physiology. From development and growth at the cellular level to the development and growth of entire organisms, the mechanisms that control various processes can be identified, studied and perhaps eventually controlled. For example, in-flight blood samples from astronauts have revealed increased levels of corticosteroids - hormones normally produced in response to stress. Their overabundance inhibits the growth of bones and leads to loss of bone mass. On this mission, an experiment involving 24 male rats studied the influence of this increase in blood steroids. This could have important spin-offs on Earth for people treated with corticosteroids over long periods of time and who experience side-effects such as osteoporosis.

Researchers on the LMS mission conducted three experiments to provide answers to this and similar questions. The Animal Enclosure Module housed the laboratory rats for bone loss study; the Space Tissue Loss Module allowed observation of the embryonic development of fish; and the Plant Growth Facility supported an investigation about the effects of microgravity on the cell walls of conifer seedlings.