Space neuroscience orastroneuroscience is the scientific study of thecentral nervous system (CNS) functions duringspaceflight.Living systems can integrate the inputs from thesenses to navigate in their environment and to coordinateposture,locomotion, andeye movements.Gravity has a fundamental role in controlling these functions. Inweightlessness during spaceflight, integrating the sensory inputs and coordinating motor responses is harder to do because gravity is no longer sensed duringfree-fall. For example, theotolith organs of thevestibular system no longer signal head tilt relative to gravity whenstanding. However, they can still sense head translation during body motion. Ambiguities and changes in how the gravitational input is processed can lead to potential errors inperception, which affectsspatial orientation andmental representation. Dysfunctions of thevestibular system are common during and immediately after spaceflight, such asspace motion sickness in orbit andbalance disorders after return to Earth.[1]
Adaptation toweightlessness involves not just theSensory-motor coupling functions, but someautonomic nervous system functions as well.Sleep disorders andorthostatic intolerance are also common during and after spaceflight. There is nohydrostatic pressure in a weightless environment. As a result, the redistribution of body fluids toward the upper body causes a decrease in leg volume, which may affectmuscleviscosity andcompliance. An increase inintracranial pressure may also be responsible for a decrease in nearvisual acuity.[2] In addition, muscle mass and strength both decrease as a result of the reduced loading inweightlessness. Moreover, approximately 70% of astronauts experiencespace motion sickness to some degree during the first days.[3] The drugs commonly used to combat motion sickness, such asscopolamine andpromethazine, have soporific effects. These factors can lead to chronicfatigue. The challenge of integrativespace medicine and physiology is to investigate the adaptation of the human body to spaceflight as a whole, and not just as the sum of body parts because all body functions are connected and interact with each other.
To date, only three countries, theUnited States,Russia, andChina, have the capability to launch humans into orbit. However, 520astronauts from more than thirty different countries have flown in space and many of them have participated inspace neuroscience research. The launch of the first living animal in orbit onSputnik on November 3, 1957 marked the beginning of a rich history of unique scientific and technological achievements in spacelife sciences that have spanned more than fifty years to date.[4]
The first documentedspace neuroscience experiments were performed during the third human mission on board the RussianVostok spacecraft. These experiments began after the crew from previous missions complained fromnausea andspatial disorientation inweightlessness. Space neuroscience experiments typically addressed these operational issues until theSkylab andSalyut space stations were made available for more fundamental research on the effect of gravity on CNS functions. Approximately 400space neuroscience experiments have been performed from Vostok-3 in August 1962 to the Expedition-15 on board theInternational Space Station in October 2007.[5]
Sensory and sensorimotor disturbances when arriving in low Earth orbit are well documented, the most known of these beingspace motion sickness (SMS). Individual differences, spacecraft size, and body movements cause SMS symptoms. Typically lasting the first three or four days of weightlessness, symptoms range fromheadaches andfatigue tonausea andvomiting. The consequences vary from simple discomfort to possible incapacitation, creating potential problems duringextra-vehicular activity, re-entry, and emergency egress from the spacecraft. The body receives a variety of conflicting signals from the visual, somato-sensory, and vestibular organs in weightlessness. These conflicting inputs are thought to be the primary cause of SMS, but the precise mechanisms of the conflict are not well understood. Medications currently used to alleviate the symptoms produce undesirable side effects.[6]
Astronauts must remain alert and vigilant while operating complicated equipment. Therefore, getting enoughsleep is a crucial factor of mission success. Weightlessness, a confined and isolated environment, and busy schedules coupled with the absence of a regular 24-hour day makesleep difficult in space. Astronauts typically average only about six hours of sleep each night. Cumulative sleep loss and sleep disruption could lead to performance errors and accidents that pose significantrisk to mission success. Sleep andcircadian cycles also temporally modulate a broad range of physiological, hormonal, behavioral, and cognitive functions.
Methods to prevent sleep loss, reducehuman error, and optimize mental and physical performance during long-duration spaceflight are being investigated. Particular concerns include the effect of the space environment on higher-order cognitive processes likedecision-making and the impact of changing gravity on mental functions, which will be important ifartificial gravity is considered as acountermeasure for future interplanetary space missions.[7] It is also necessary to develop human-response measurement technologies to assess the crew's ability to perform flight-management tasks effectively. Simple and reliable behavioral and psycho-physiological response measurement systems are needed to assess mental loading,stress, task engagement, andsituation awareness during spaceflight.
All living organisms on Earth have the ability to sense and respond to changes in their internal and external environment. Organisms, including humans, must accurately sense before they can react, thus ensuringsurvival. The body senses the environment by specialized sensory organs. The CNS utilizes these sensations in order to coordinate and organize muscle activities, shift from uncomfortable positions, and adjustbalance properly. In common speech, five differentsenses are usually recognized:vision,hearing,smell,taste, andtouch. All these senses are somewhat affected by weightlessness.
In fact, the human body has seven sensory systems – not five. The sixth and seventh systems are the senses of motion located in the inner ear. The former signals the beginning and end of rotation and the latter signals body tilt relative to gravity as well as body translation. The seventh system no longer provides tilt information in weightlessness; however, it does continue to signal translation, so the afferent signals to the CNS are confusing. The experience of living and working in space alters the way the CNS interprets the otolith organ signals duringlinear acceleration. Although the perception is fairly accurate when subjects are exposed to angular acceleration inyaw in-flight, there are disturbances during angular rotation inpitch androll, and during linear acceleration along the body transversal and longitudinal axes. Perception of body motion is also altered during the same motion immediately after landing. There is an adaptation to weightlessness in orbit that carries over to post-flight reactions to linear acceleration.[8]
Exposure to weightlessness causes changes to the signals from the receptors totouch,pressure, andgravity, i.e., all information necessary for postural stability. Adaptive modifications in the central processing of sensory information take place to produce motor responses that are appropriate for the new gravitational environment. As a result, terrestrial motor strategies are progressively abandoned in weightlessness, as astronauts adapt to the weightless environment. This is particularly true for the major posturalmuscles found in the lower legs. The modifications in posture, movement, and locomotion acquired in reduced gravity are then inappropriate for Earth's gravity upon return. After landing,postural instability approaching clinicalataxia is manifested as a result of this in-flight neural reorganization.[9]
Difficulties withstanding,walking, turning corners, climbing stairs, and a slowing ofgait are experienced as astronauts re-adapt to Earth's gravity, until terrestrial motor strategies are fully re-acquired. Adaptation to spaceflight also induces a significant increase in the time required to traverse an obstacle course on landing day, and recovery offunctional mobility takes an average of two weeks.[10] These difficulties can have adverse consequences for an astronauts’ ability to stand up or escape from the vehicle during emergencies and to function effectively immediately after leaving the spacecraft after flight. Thus it is important to understand the cause of these profound impairments of posture and locomotion stability, and developcountermeasures.
The most significant sensorimotor problems astronauts will face during a stay on the Moon and Mars are likely to occur when walking around in theirspace suits. The suits are big and bulky and change the body'scenter of gravity. This along with the uneven terrain and limited field of view makes locomotion challenging.
The function of thevestibular system during spaceflight is by far the most carefully studied of all. This is especially true of the gravity-sensingotolith organs and their relationship toeye movements. The vestibularsemicircular canal function seems unchanged in weightlessness because the horizontal eye movements that compensated for headyaw rotation are not affected by spaceflight. The absence of gravity stimulation of the otoliths reduces the torsionalvestibulo-ocular reflex during headroll rotation in microgravity. This deficit is absent when astronauts are exposed tocentrifugal forces, suggesting that the adaptive CNS changes are taking place centrally rather than peripherally.[11]
During the first days in orbit, the asymmetry of vertical eye movements in response to moving visual scenes is inverted. A return to symmetry of thevestibulo-ocular andoptokinetic reflexes is then observed. Some studies have shown increased latencies and decreased peak velocities ofsaccades, while others have found just the opposite. It is possible that these conflicting results depend on when the measures were obtained during the mission. There is also a serious disruption ofsmooth pursuit eye movements, especially in the vertical plane.[12]
Human missions to Mars will include several transitions between different gravitational environments. These changes will eventually affect the reflex eye movements. A key question is whether astronauts can have different sets of reflexes among which they can rapidly switch based on the gravitational environment. Determination of the dual-adaptive capabilities of reflex eye movements in such circumstances is vitally important so that it can be determined to what extent theSensory-motor coupling skills acquired in one-g environment will transfer to others.
Inweightlessness, astronauts must rely much more on vision to maintain theirspatial orientation, because theotolith organs can no longer signal the “down” direction. During prolonged exposure, however, reliance seems to shift toward an intrinsic, body vertical reference. The erroneousillusions of self-motion during head movements performed during and after return to Earth gravity are presumably due to a re-interpretation ofvestibular inputs. Ground-based studies suggest that the CNS resolves the “tilt-translation”ambiguity based on the frequency content of thelinear acceleration detected by theotolith organs, with low frequency indicating “tilt” and high frequency indicating “translation”. A crossover exists at about 0.3 Hz where the otolith signals are then ambiguous. Exposure to weightlessness presumably results in a shift of this crossover frequency, which could then contribute tospatial disorientation and SMS.[13]
Although investigations of higher cognitive processes, such asnavigation andmental rotation are limited,[14] the astronauts frequently report that the spacecraft interiors look longer and higher than they actually are, and a reduction in the perceived height of three-dimensional objects is observed in-flight compared with pre-flight, suggesting an alteration in themental representation of three-dimensional cues in weightlessness.Perception is a model of the brain, a hypothesis about the world that presupposes theNewton's laws of motion. These laws change in weightlessness and, therefore, one could expect changes in the mental representation of objects’ shape and distance during spaceflight.[15] The rare investigations carried out in space so far have not demonstrated drastic changes, probably because the CNS continues to use aninternal model of gravity, at least for a short while.[16] It can be speculated that the way of processing three dimensions will be more developed after a long absence of a gravitational reference.
Further investigations carried out in space will perhaps reveal that other higher cortical functions are impaired in weightless conditions. The combination ofvirtual reality with the measurement ofevoked potentials andbrain mapping on board the International Space Station should provide exciting results on the adaptive mechanisms of cerebral functions in weightlessness.
FromVoskhod to theInternational Space Station, spacecraft have improved in size and comfort and have allowed more and more people traveling into orbit. However, even with all of the human spaceflight experience gained over the past fifty years, no single completely effective countermeasure, or combination of countermeasures, exists against the negative effects of long-duration exposure to weightlessness. If a crew of astronauts were to embark on a six-month journey to Mars today, the countermeasures currently employed would presumably leave them less operational after landing.[1]
Many believe that physiological adaptation to Mars gravity (0.38 G) and re-adaptation to Earth gravity (1 G) would be enhanced by frequent exposure toartificial gravity on board the spacecraft en route to and from Mars. This would require an on-board human-ratedcentrifuge or spacecraft rotation to produce acentrifugal force similar to gravity. This solution, while potentially effective, raises a number of operational, engineering, and physiological issues that will need to be addressed. The human physiological responses to long-duration exposure to anything other than zero-gravity or Earth's gravity are unknown. Research is needed to identify the minimum level, duration, and frequency of gravity level required to maintain normal CNS functions, as well as the importance of agravity gradient across the body.[17]
The complex functioning of the CNS, even in the 1-G environment of Earth, has not revealed all its secrets. The most basicspace neuroscience questions must be answered to minimize risks and optimize crew performance during transit and planetary operations. The results of this research will certainly find other applications inmedicine andbiotechnology. Our ability to understand how Earth's gravitational environment has shaped the evolution of sensory and motor systems can give us a clearer understanding of the fundamental mechanisms of CNS functions. Knowledge of the effects of gravity on CNS functions in humans, as well as elucidation of the basic mechanisms by which these effects occur, will be of direct benefit to understanding the impact of, and providing countermeasures for, long-term exposure of humans to theweightlessness of spaceflight and the partial gravity ofMoon andMars bases.
| Biology |
|  | ||||
|---|---|---|---|---|---|---|
| Environment | ||||||
| Society | ||||||
| Technology |
| |||||
