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Assessment of Programs in
Space Biology and Medicine 1991
3
Human Physiology
Physiology is the science of normal vital processes of animal organisms.
It is a fundamental science in its own right, but it is also the bridge to the practical
problems of human health and performance in space. Problems produced by
disordered physiology in space include a steady loss of bone and muscle, the
cause of which has yet to be determined; abnormalities of the cardiovascular
system related to pooling of blood in the chest and head; and motion sickness
and the difficulties it causes not only in terms of astronaut discomfort but also in
their performance in handling spacecraft. The committee has had, over the years,
access to anecdotal data from the Soviet space program. This anecdotal
information is, while interesting, not sufficiently reliable for drawing conclusions or
in planning the U.S. program for a number of reasons. There are differences in
experimental protocols and controls in laboratory equipment, and the Soviets do
not publish their results in refereed scientific journals. However, increased recent
cooperative activities between the Soviets and the United States suggest promise
for the future in standardized experimental procedures and data exchange: In
some areas of physiology, there has been appreciable progress.
Much remains to be done before we can be certain that humans can stay
healthy and perform well for extended periods in space. The physiological
problems of concern are divided into three main topics in order of priority
regarding their importance to extended human space travel. First and of greatest
concern is bone, muscle, and mineral metabolism; second, cardiovascular and
homeostatic functions; and third; sensorimotor integration.
BONE, MUSCLE, AND MINERAL METABOLISM
Status of the Discipline
The bone and muscle atrophy that occurs in the microgravity environment
is a severe hurdle to an extended human presence in space. Although astronauts
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are subject to elevated urinary calcium and increased risk of kidney stones, the
most significant risk to the musculoskeletal system may only be realized on return
to normal gravitational fields. As emphasized in both the 1987 and 1988 CSBM
reports, very little regarding the etiology of space-induced osteopenia (bone loss)
or muscle atrophy is understood. While countermeasures to inhibit or prevent this
bone loss are imperative, this goal will only be realized following an improved
understanding of the basic cellular mechanisms responsible for the maintenance
of muscle and bone mass. As importantly, the interaction of microgravity with
various risk factors (e.g., age, gender, race, nutrition) must be established to
ensure that the musculoskeletal integrity of payload specialists of diverse
backgrounds will not be compromised by extended spaceflight.
Major Goals
As recommended in the Goldberg Strategy, eight major goals have been
defined for the science of bone and mineral metabolism. In brief, they are as
follows: (1) determine the temporal sequence of bone remodeling in response to
microgravity; (2) establish the reversibility of this process on return to 1 g; (3)
establish the relationship between muscle activity and bone function; (4)
investigate countermeasures to prevent bone loss; (5) study the cellular
mechanisms responsible for bone loss; (6) study the interdependence of calcium
homeostasis and bone remodeling; (7) determine the etiology of pathologic
calcification; and (8) establish the biomechanics of the skeleton under
microgravity conditions. Given these goals, it is clear that it will be difficult to
separate operational considerations (astronaut health, safety, and performance)
from the basic scientific questions.
In light of these goals, it must be emphasized that understanding the
etiology of osteopenia is the focus of an enormous research program within the
National Institutes of Health. The extent of NIH programs, which include Centers
of Excellence (COE) and Specialized Centers of Research (SCOR), underscores
our nation's commitment to improving an understanding of these disease
processes. Multidisciplinary studies address issues such as promotion of bone
formation, inhibition of resorption, biomechanics of skeletal modeling and
remodeling. Also under development are treatment modalities such as
diphosphonates, fluorides, estrogen, calcitonin, calcium, electrical and
mechanical intervention, and exercise, with the aim of inhibiting or reversing bone
destruction. These studies range from the structural organization of the bone
mineral to human clinical trials of these novel treatment prophylaxes, and have
had a great impact on the way we perceive the musculoskeletal problems that
parallel aging, menopause, hyperparathyroidism, a diabetes, poor nutrition,
immobilization, and extended bedrest. Every effort should be made to ensure that
the goal-oriented NASA mission concerning musculoskeletal science exploits the
knowledge base derived from the scientific advances of other agencies.
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Progress
Significant progress has been achieved by NASA scientists over the past
five years. The initial mandates of the Goldberg committee have been addressed,
and the animal models developed by NASA personnel and their colleagues have
provided some insight into the skeletal tissue response to decreased functional
stimuli. Whole animal studies have been performed that demonstrate the
compromising effects of microgravity, and some information has been generated
regarding the temporal sequence of tissue response to disuse. Additionally,
human studies correlating inactivity (bedrest) to factors such as diminished bone
mass and increased urinary calcium have proven useful models to understanding
the risks of skeletal fracture or mineral disorders during extended spaceflight.
Lack of Progress
While significant work has been done, and NASA investigators have
established collaborations with superb musculoskeletal and connective tissue
scientists at a number of university centers, it must also be emphasized that the
directions for future programs are not yet clear. Aside from the general
hypothesis that gravity serves as a signal to the regulation of bone mass, it is not
clear if NASA investigators have constructed a series of basic testable
hypotheses regarding the cellular mechanisms that regulate this response. Nor is
it clear whether the models currently used will be appropriate to deal with the next
phase of study, i.e., countermeasures to prevent bone loss. Of the eight major
scientific goals of the Goldberg Strategy, only the first has been addressed.
Unfortunately, even this information is limited by the dissimilarities between the
animal model chosen and normal human physiology. Considering the current
focus on singular methodologies, it is unclear how NASA will investigate the
remaining seven research objectives.
Specifically, it is not clear if the principal animal model of NASA scientists,
the rat tail suspension model of osteopenia, has demonstrated either a
repeatable bone loss or how this skeletal response correlates to that experienced
in microgravity. Nor is it clear how this model will be developed to address issues
such as a minimum strain environment capable of retaining bone mass, or the
correlation of sites of bone formation and resorption to specific aspects of the
bone's mechanical environment (i.e., what are the changes in the lower limbs'
stress/strain environment following suspension?). Complementary
analytical/experimental models should be developed immediately which can test
(and validate) proposed hypotheses of bone remodeling. In addition, the
limitations of the rat as a model, which responds to microgravity by inhibited
growth, should be contrasted with the human condition of accelerated resorption.
It must be determined whether commitment to a singular animal model is an
efficient and effective means of addressing the range of scientific goals within the
Goldberg Strategy.
Simultaneously, a realistic and reasonable time frame for addressing the
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scientific issues of the Goldberg Strategy must be implemented. If certain
scientific criteria are not met within these temporal constraints, new avenues
should be established immediately; the risk of overcommitting to a single model
must be diminished. Indeed, despite the extreme efforts by NASA investigators
regarding their models for osteopenia, a great reluctance remains within the
general scientific community to accept the extrapolations to the human condition.
Importantly, the research program must move to beyond the phenomenologic
(i.e., bone tissue response to microgravity) to issues of mechanism. A
commitment to in vitro studies of bone cells, to study mechanisms of perception
and response to physical factors (e.g., mechanical/electrical), should be
implemented immediately. Additional alternative strategies and technologies (i.e.,
molecular biology and structural biology) within the musculoskeletal system
should also be considered. Before reasonable prophylaxes for bone loss can be
developed, the basic cell and subcellular mechanisms responsible for this
osteopenia must be understood. Finally, it is clear that this work will provide
important spinoff technology for the treatment of earth-based musculoskeletal
disorders.
CARDIOVASCULAR AND OTHER HOMEOSTATIC SYSTEMS
This section describes the problems, goals, and recommendations for
space-related and in-flight studies of four broad areas of adaptive and protective
physiology. The cardiovascular and neuroendocrine elements of the circulatory
system are addressed in two areas, focusing respectively on basic cardiovascular
functions and the influences of regulatory systems on these functions. The third
area includes immunology, hematopoiesis, and wound healing, which share
cellular constituents and the recognition of regulatory proteins. The fourth area,
nutrition and gastroentology, describes the lack of major problems and discusses
the absence of a relationship to serious problems with other systems.
Circulatory Adjustments
Cardiovascular physiology is a high-priority area for NASA. To a large;
extent, gravitational forces determine the distribution of intravascular and
intracardiac pressures and volumes. The cardiovascular system appears to
function normally during short-term exposure to microgravity. However, clinically
significant dysfunction is often apparent during readaptation to 1 g and is likely
amplified with prolonged spaceflight. In addition, prolonged exposure to the
altered loading conditions of microgravity is considered to be a potential cause of
irreversible functional and structural changes.
Status of the Discipline
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Many important features of cardiovascular adaptation in microgravity and
re-adaptation at I g have been identified during the past 20 years. There is an
early shift of body fluids toward the head, followed by a loss of intravascular and
interstitial fluid volume with decreased heart size. A fall in blood pressure on
changing from a supine to an erect posture with consequent light headedness or
fainting (orthostatic intolerance) and decreased exercise capacity are usually
evident after return to Earth. Simple descriptive observations have been made in
many subjects, but our understanding of the mechanisms responsible for
adaptation to microgravity and readaptation to 1 g is incomplete.
Major Goals
The 1988 Life Sciences Task Group report states very clearly that
biological and medical research in space should include different species and be
performed at all levels of integration. This approach requires the development of
a set of animal holding facilities, experiment modules, and work stations for
Shuttle use and a wide range of facilities for the Space Station, including a large
variable force centrifuge (VFC). This same report also makes the point that all
crew members should contribute to a life sciences/ space medicine data base by
serving as subjects in biomedical studies.
The report defines several important cardiovascular research areas
including (1) cardiovascular and systemic responses to the initial fluid shift and
their interactions; (2) mechanisms responsible for postflight orthostatic
intolerance; (3) the long-term, possibly irreversible, effects of the altered loading
conditions in microgravity, e.g., atrophy of heart muscle, chronic orthostatic
hypotension; and (4) maintenance of reflex control of blood pressure.
Specific high-priority areas of investigation include (1) the role of exercise
and physical fitness before, during, and after flight; (2) countermeasures against
cardiovascular abnormal function and rehabilitation after long flights; (3)
validation of ground-based models of microgravity for short-term and long-term
studies, and (4) characterization of drug actions and metabolism in microgravity.
The Goldberg Strategy also defines a series of scientific goals: (1) to
understand acute (0 to 2 weeks), medium-term (2 weeks to 3 months), and long-
term (3 months to several years) cardiovascular and pulmonary adaptation to
microgravity; (2) to examine the validity of ground-based models of microgravity
and to determine whether actual microgravity offers any unique advantages for
cardiovascular studies; and (3) to define measures that will hasten human
adaptation to microgravity and return to a 1-g environment.
Progress
Studies performed by Johnson Space Center investigators before, during,
and after several Shuttle flights have provided echocardiographic data on cardiac
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dimensions and function and also explored the use of oral saline loading as a
countermeasure against postflight orthostatic hypotension. Studies in progress
deal with reflex regulation of blood pressure (baroreceptor reflex).
A series of more detailed and comprehensive cardiovascular and
pulmonary experiments will occur on future Spacelab flights to meet many of the
objectives listed in the Goldberg Strategy for short-term flights, ranging from
direct measurement of central venous pressure and cardiac output during
maximal and minimal exercise, characterization of the baroreceptor reflex, and
detailed study of the effects of microgravity on body fluid volumes and their
regulation.
The validity of head-down tilt as a method of simulation of microgravity
will also be explored. SLS-1 will also initiate the use of animals (rats) for the study
of cardiovascular physiology. The SLS-1 experiments are also an important
component in a unique NASA pilot program in which experiments planned for a
Spacelab life sciences (SLS) flight will be discussed as a means of enhancing
elementary and secondary school science classes.
With joint participation by NASA, ESA, and the German Space Program,
the D-2 flight will extend the range of measurements and interventions with
detailed observations during intravenous fluid load and lower body negative
pressure.
Lack of Progress
The number of subjects available for physiological and medical studies is
generally fewer than would be desired. The Life Sciences Task Group of the
Space Science of the Twenty-First Century study noted that increased
participation by the crew in biomedical studies during all flights is an important
objective.
A better understanding of cardiovascular and pulmonary physiology at all
levels of investigation is still a major science goal and a requirement for a
significant rational approach to space medicine, including the development of
scientifically sound countermeasures against, and adaptation to, changing
gravitational forces.
There are only limited Soviet data on the cardiovascular consequences of
prolonged exposure to microgravity. The U.S. data are limited to the Skylab
exposure, which produced data on medium-term exposure. Much more
information is needed on cardiovascular and pulmonary responses to the
changing loading conditions in space, and on the interactions between the
cardiovascular system and its control mechanisms.
Potential interactions between the space environment and cardiovascular
and pulmonary physiology modified by disease processes or pharmacological
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agents have not been addressed and also need attention.
Hormones and Stress
Hormones that affect the cardiovascular system are of great importance
to NASA and should be considered in the context of the cardiovascular changes
that occur in space cardiovascular deconditioning.
Progress
Recommended measurements of many hormones have already been
made in astronauts who made the 28-, 59-, and 84-day flights in Skylab in 1973
and 1974. Two comments are relevant in terms of these observations. First, it
would be beneficial to repeat them to increase the number of subjects studied in
order to get a better idea of biological variations. Second, it would be wise to
repeat them using the more refined methods that have been developed since
1974. A controlled study of blood and urinary levels of the relevant hormones is
planned for SLS-1. Experiments are planned for mission specialists who will be
concerned with laboratory experiments and will not be involved in flying the
spacecraft. Consequently, better data should be obtained.
Lack of Progress
The observations planned to be made on SLS-1 are still far from ideal.
Even if the best data are obtained, it remains only descriptive. It is important that
ongoing ground-based investigations of the fundamental mechanisms involved in
producing the hormonal responses observed in space be carried out in humans
and animals. One concern not specifically addressed in either the 1979 or the
1987 report is the hormonal response to stress. For instance, the adrenal
hormone, cortisol, was measured in Skylab and was found to be elevated from
time to time during spaceflight. It would be appropriate to carry out a more
detailed, better controlled study of the pituitary hormone ACTH (that stimulates
the release of cortisol from the adrenal cortex) and cortisol secretion on a future
SLS mission and on the Space Station. Measurements of ACTH and cortisol are
also relevant to investigation of circadian rhythms in space. This topic is covered
in detail in another part of this report (see Chapter 4 discussion regarding
circadian rhythms).
Many experiments that have examined physiological and behavioral
variables in space have failed to take into account the rhythmic nature of these
factors. For example, it is not possible to accurately determine the overall pattern
of adrenal cortisol secretion (a stress-related hormone) by simply taking a single
blood sample on a daily basis. However, NASA has carried out such studies with
the result that contradictory findings have been obtained within and between
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different missions. All experimental protocols that propose to use single point
measurements for variables that show pronounced pulsatile and/or circadian
variations need to be reexamined to insure that the data, which will be collected
at enormous costs, are valid and will not be meaningless to the scientific
community.
Immunology, Hematopoiesis, and Wound Healing
Status of Discipline
Immune cells in mammalian bone marrow and lymphoid organs, such as
the thymus and spleen, provide integrated and adaptive host defense against
infections, other external environmental challenges, and deviations in host
cellular growth and differentiation. Many of the cells and proteins of the immune
system, which initiate and regulate lymphocyte and antibody responses, also
control the production and functions of cells in the blood and connective tissues.
Major Problems and Goals
The importance of investigations of immunology and related systems in
space was suggested initially by the findings of abnormalities in human
lymphocytes, red blood cells, and other blood cells on Spacelab Mission D1, and
rat immunity on unmanned Soviet spaceflights, as well as by a few anecdotal
reports of an increased incidence of cutaneous, gastrointestinal, and renal
infections in humans on Russian and U.S. spaceflights.
Progress
Most studies of immunity in space have been directed to the detection of
abnormalities in human and animal lymphocyte numbers and morphology, their
proliferation and synthesis of immunological proteins in response to bacterial
products, and the serum concentrations of gamma globulins important in
immunity. Other human white blood cells prepared prior to launch also showed
impaired function after spaceflight, relative to ground controls, when studied after
landing.
Spaceflight results in significant reductions in both plasma volume and
red blood cell mass within days, and reversal of both after a few weeks. Although
some reports have cited minor abnormalities in red blood cell biochemistry, no
major primary metabolic or structural defects have been documented. There are
plans to measure aspects of iron uptake and storage in bone marrow.
Lack of Progress
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The very uncommon occurrence of serious infections during spaceflight,
despite the apparently profound laboratory defects in some immune functions in
vitro, casts some doubt on the preliminary findings of alterations of immune
responses. However, the grave consequences of any attenuation of adaptive
host defense during spaceflight and the important roles of immune proteins as
regulators of non-immune blood cells and connective tissue cell generation and
functions, such as wound healing, mandate recommendations for more
investigations of immune system functions. Our current understanding of
immunity in space is not commensurate with the goals proposed in Space
Science in the Twenty-First Century—Imperatives for the Decades 1995-2015,
Life Sciences and earlier sets of recommendations.
Above all else in practical importance are in vivo studies designed to
elucidate any abnormalities in integrated immune, hematopoietic, and tissue-
healing responses of humans. The critical need for 1-g in-flight controls cannot be
overemphasized, if the resultant data are to be analyzed rigorously for specific
mechanisms and to provide unequivocal clinical guidelines for countering any
abnormalities. The availability now of stimuli and inhibitors of immune and bone
marrow cell functions, derived from genetic techniques, provides an opportunity
to detect and possibly correct any excesses or deficiencies of activity or any
regulatory abnormalities. Studies of the mechanisms for reduction in red blood
cell mass with spaceflight and of effective countermeasures for attenuated
compensation to bleeding and blood clot formation are recommended.
Gastrointestinal System and Nutrition
Major Goals
A Strategy for Space Biology and Medical Science for the 1980s and
1990s recommends studies of caloric needs, nitrogen balance, supplements to
the basic diet during spaceflight, and the energetic requirements of work in
space. Space Science in the Twenty-First Century—Life Sciences discusses
nutritional requirements in the context of integrated functions and closed
ecological life support systems (CELSS).
Progress
Appreciable data are available on energy expenditures and motor
performance. In experiments carried out to date, nitrogen balance in space has
been negative, probably because dietary intake was often inadequate in view of
space motion sickness and crowded schedules and because of muscle wasting.
The amounts of vitamins and minerals provided to the astronauts more than meet
the Recommended Dietary Allowances, (Tenth edition, National Academy Press,
Washington, D.C., 1991), and there is no evidence indicating any additional
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special needs in space. Little is known of possible minor changes in bowel, liver,
pancreatic, and salivary functions during spaceflight, although there are plans to
study gastric emptying and gastrointestinal motility and their relationship to
movement of body fluids.
Lack of Progress
Very few significant problems have been encountered in space with
nutrition or the gastrointestinal system to date. However, NASA has plans to
conduct investigations on areas that may be potential problems on future Shuttle
flights.
SENSORIMOTOR INTEGRATION
Status of Discipline
The changes in the gravito-inertial environment that inevitably occur
during a space mission lead to disturbances of sensorimotor function including
impaired spatial orientation, instability of posture and gaze, and motion sickness.
Fortunately the central nervous system (CNS) adapts to those environmental
changes within a few days so that the problems are of relatively short duration
provided a constant environment is maintained. There are two caveats to this
assessment of relative risk. One is that gravito-inertial changes occur at the most
critical parts of a mission—during takeoff or landing. For instance, sensorimotor
problems could impair crew effectiveness for several days after landing on Mars.
The second caveat is that the use of a spinning spacecraft to eliminate problems
of prolonged microgravity would entail repeated changes in gravito-inertial
environment when it was necessary to change spin rates or to service different
parts of the craft. To deal with these concerns, A Strategy for Space Biology and
Medical Science for the 1980s and 1990s recommends experimentation in five
areas: spatial orientation, postural mechanisms, vestibuloocular reflex (VOR),
neural processing in the vestibular system, and motion sickness. Both in-flight
and ground-based experiments were proposed in each area. Because the otolith
organs of the vestibular labyrinth are the primary sensors informing the brain of
the linear accelerations induced by gravity, the research recommended focuses
on neural mechanisms that transform vestibular sensory inputs (especially those
from otolith organs) into sensations, movements, and changes in homeostatic
state (i.e., space adaptation sickness).
Major Goals
As indicated in the Goldberg Strategy, the neuronal mechanisms
underlying a sense of spatial orientation are complex, as yet relatively poorly
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understood, and directly relevant to assuring effective functioning of humans
involved in space missions. Accordingly, the strategy recommends a vigorous
program of both ground-based and flight research aimed at better understanding
these mechanisms as they operate on Earth, in space, and on return from low-
gravity to high-gravity environments.
The report also recommends that the adaptation of the posture control
system to microgravity be investigated by studying its input/output behavior
before, during, and after flight.
The VOR, which converts head motion into compensatory eye movement,
is both well understood and important in maintaining stable vision. However, it is
not known how this reflex is affected by microgravity. Accordingly, the 1987
strategy report recommends that it be studied in microgravity.
The Goldberg Strategy concludes that neural processing mechanisms in
the vestibular system must be investigated both in normal gravity and
microgravity if NASA is to achieve its goal of controlling for sensorimotor changes
and space adaptation sickness. These studies would focus on the vestibular
nuclei of the brainstem, which serve both as the point at which vestibular signals
from receptors of the vestibular labyrinth in the inner ear enter the brain and as a
primary processing center for all signals—vestibular, visual, and proprioceptive
(sense of body position)—that relate to orientation in space and to motor
responses that maintain a stable orientation. In addition to their potential for
solving operational problems, studies of vestibular nuclei in microgravity provide
unique opportunities for understanding basic vestibular function since
microgravity is the only environment in which certain aspects of semicircular
canal and otolith function can be studied in isolation.
The report further concludes that one focus of sensorimotor studies must
be upon the adaptive mechanisms that alter vestibular processing in response to
altered feedback from the environment. Present knowledge indicates that it is
these mechanisms that both allow an astronaut to adapt to microgravity
conditions and then raise problems during return to normal gravity when the
adapted responses are no longer appropriate. Further, space adaptation sickness
may be a side effect of the adaptive process.
The Goldberg Strategy reviews uncertainties about the etiology of motion
sickness and stresses the importance of vigorous research on this subject, with
the shorter term objective of mitigating associated operational difficulties. Longer-
term objectives include eliminating such difficulties as well as enhancing
understanding of the function of the nervous system.
Progress
Overall, NASA has made a concerted effort to stimulate appropriate,
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quality research in the sensorimotor area. Space adaptation sickness (SAS) has
clearly been perceived as an important problem that can only be solved by a
combination of basic and more applied research. In the area of basic research,
NASA has supported an array of external studies, which have included
contributions by a number of leading vestibular physiologists. These have
generated useful information in the areas of spatial sensation, vestibular
processing, posture and gaze control, and vestibular-autonomic interactions. A
promising development is the establishment of the Vestibular Research Facility
(VRF) at Ames Research Center (ARC), which is becoming a focal point for state-
of-the-art studies of the vestibular system in animals, involving investigators from
many universities and other research institutions. If this facility can become a
national resource and a site of projects with joint funding from NASA and NIH, its
impact will be even greater. Whereas in some cases microgravity is a singular
point so that behavior on-orbit cannot be extrapolated from measurements at
various g levels on the ground, it is likely that some of the vestibular changes in
going from 1 g to microgravity could be extrapolated from changes between a
range of gravitational forces. Therefore, parallel development of the ARC
centrifuge or other similar facilities as centers for vestibular studies could
contribute to solving some of the outstanding questions.
Given the limited number of flight opportunities, considerable progress
has also been made in studying human sensorimotor performance in
microgravity. A great deal of this progress has resulted from the well-planned
experiments conducted by groups at MIT as well as in Canada and Europe.
Tantalizing preliminary results and interesting new concepts have resulted from
their studies on the Spacelab and D-1 missions. Follow-up studies on SLS1, IML-
1 and D-2 missions should be very productive, but it must be emphasized that
extensive replications will be needed before any of these preliminary results can
be accepted with confidence.
As set forth in the Goldberg Strategy, what is needed now in
vestibuloocular studies are 3-dimensional analysis and modeling of the VOR to
account for changes in microgravity and investigation of the effects of conflict
between visual, vestibular, and proprioceptive stimuli on gaze stability and on the
adaptive systems that regulate the VOR. Studies of vestibuloocular and
optokinetic (visually induced) reflexes scheduled for SLS-1 will address these
issues as will planned ground-based studies of adaptive changes in otolith-ocular
reflex. Thus work in this area is on track.
Progress in meeting the objectives set forth in the 1987 report regarding
neural processing can be summarized as follows:
Studies of structural changes in labyrinthine receptors in microgravity are
under way and will be continued on SLS-1. In the future, it will be necessary to
perform control experiments in which some animals are maintained at 1 g on orbit
using the centrifuge so that we can be confident that any changes observed are
due to exposure to microgravity rather than other factors.
Studies of vestibular afferent signals (i.e., those nerves that transmit
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impulses to the CNS) in microgravity and studies of the role of gravity in neonatal
development of sensorimotor functions will require periods of a month or more in
orbit. Plans should be formulated to perform these studies on Space Station
Freedom and prerequisite ground-based development should be initiated. The
CSBM is not aware of any such activities at the present time.
The Goldberg Strategy recommends "a vigorous, ground-based program
of research involving the use of modern physiological methods" to investigate the
central processing of vestibular sensory information, especially that from the
gravity-sensitive otolith organs. NASA is to be commended for establishing the
VRF Program and a good series of university-based studies to address this goal.
Further emphasis on single neuron recordings will be needed to attain the goal of
understanding those elements of vestibular processing that are related to
sensorimotor changes in microgravity. Even more important is the development
of neural models of the vestibular system that could both generate hypotheses for
experimental testing and assist in the interpretation of experimental results. If
properly managed, the nascent modeling effort at the ARC could provide a
national resource for such theoretical studies of vestibular systems.
As appropriate, NASA appears to have been pursuing an active
multidimensional research program in understanding motion sickness.
Lack of Progress
Within this generally positive context, NASA might consider some fine
tuning of its program related to spatial orientation along the following lines. The
Goldberg report stresses that vestibular signals are only one of the inputs
influencing spatial orientation, with other kinds of inputs certainly involved and
perhaps determinative under particular sets of circumstances. The issue of
whether observed instances of sensorimotor difficulties are, or are not, entirely a
function of disturbed vestibular processing has become of operational importance
in connection with ongoing and projected increases in mission duration (letter to
Administrator Truly, NASA, regarding extended duration orbiter medical program,
December 20, 1989). In this connection, both increased attention to possible
nonvestibular influences on spatial orientation, and improved monitoring of
astronaut performance (as recommended in that letter), would seem desirable. It
also appears that these and other observed disturbances may relate not so much
to steady-state conditions as to fluctuating environments, such as the changing
gravity loads associated with return from orbit. This suggests that in the ground-
based program increased attention might profitably be given to the program of
spatial orientation in fluctuating environments. Finally, the breadth of the problem
of spatial orientation is such that efforts in this area will probably be most
productive when considered in connection with those in possibly related areas,
including, for example, not only other sensorimotor subareas but also
chronobiology and human behavior generally. NASA might want to explore ways
to assure effective interactions along these lines, possibly by way of organizing a
meeting focused on the spatial orientation problem but involving investigators
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from a number of different relevant areas.
NASA should consider increasing its support of ground-based studies of
postural mechanisms to complement the flight studies since significant changes
in eye-hand coordination and in postural responses to applied perturbations are
being found. When a sufficient level of understanding has been reached, it may
become possible to train astronauts to adopt postural strategies that will be useful
in altered gravity environments as suggested by the Goldberg Strategy.
The problem of VOR has already been discussed and NASA appears to
be headed in the correct direction. Similarly, issues related to neural processing
are being dealt with properly, although a great deal of work remains to be done.
It remains the case that no single hypothesis as to the origin of space
motion sickness fits all existing observations, and no single therapeutic regime
with assured effectiveness has yet been developed. While existing lines of
research may yet achieve one or both of these objectives, it may be that the time
has come to take a fresh look at whether the syndrome, with individual variations
in the expression and intensity of components, is actually several distinguishable
syndromes. A recognition of this fact could accelerate both therapeutic and basic
research. When viewed in the light of resources required to develop new
therapeutic drugs, the amount of funding for this key area is clearly inadequate. If
NASA requires a solution to this problem, ways need to be found to mobilize a
much larger joint NASA/ university initiative in the area.
