² Excerpt from [ref.130].
Neurophysiological studies concern the functions of the nervous
system—in particular the central nervous system (CNS)—under normal,
simulated, and actual flight conditions. Of paramount importance is the
maintenance of equilibrium and orientation in three-dimensional space.
The ability of man and his close relatives among the vertebrates to
maintain these functions depends on an integrated sensory input from the
vestibular organ; the eyes; the interoceptors of the muscles, tendons,
joints, and viscera; and the exteroceptors of the skin.
Certain parameters of the environmental and space-flight conditions
drastically affect man’s ability to maintain equilibrium and spatial
orientation. Centrifugal forces modify or reverse the directional vector
of gravity. Linear acceleration may increase enormously, as may angular
stimulation. The sensory organs listed above are unreliable under such
conditions. The very organ which is designed specifically to furnish
information on spatial orientation may malfunction in man while he is in
flight. Thus, with respect to sensory orientation, these labyrinthine
organs are by no means precision instruments.
The use of classical histological methods and the observation of
equilibrium disturbances resulting from operative interference with the
internal ear have in the past been the two principal sources of
knowledge concerning the structure and function of the labyrinth, but
the answers given to various questions vary considerably in their value.
The development of electrophysiological techniques and the refinement in
recent years of the ultrastructural analysis by means of the electron
microscope may allow more precise experimental studies of the
correlation of function and structure.
Before considering vestibular impulses in their bulbar and descending
spinal pathways, a recent study concerning the generation of impulses in
the labyrinth must be mentioned. Von Bekesy’s finding ([ref.131]) of the
direct current potentials in the cochlea aroused speculation about the
existence of similar labyrinthine potentials. Such dc potentials were
also detected in the semicircular canal of the guinea pig by Trincker
([ref.132]), who measured the potential changes in the endolymph,
surface of the cupula, or side of the crista during cupular deflection.
It seems likely, however, that the effects do not represent the
physicochemical changes in the cupula but the electrical potentials in
the nerve and nerve endings of the crista. Attempts at differentiating
these effects have failed so far. Great expectations are brought by the
advances of microchemistry, microphysiology, and physical chemistry with
regard to the excitatory processes, the generation of the nerve impulse.
Quite apart from a need to understand vestibular nerve discharges and
patterns more adequately in such terms, the analysis of the vestibular
system has in the past revealed general biological principles which were
not readily discernible through the examination of other tissues
([ref.133]).
The neural connections of the vestibular organ consist of numerous
chains of neurons, reciprocally linked in many ways and having their
synapses in various anatomical nuclei. All the chains work in intimate
collaboration, and the final pattern of reflex responses is attributable
largely to the highly complex integrating activity of the center. The
labyrinthine function is automatic, carried out in a reflex fashion: in
other words, mostly below the level of consciousness. The brain centers
through which the labyrinth elicits the various appropriate muscular
reactions of the head, body, limbs, and eyes—the righting, the postural,
and the ocular reflexes—represent an intricate mechanism. Before we can
hope for a satisfactory understanding of their functional organization,
we will have to know their anatomy in more detail. Thus, we are
confronted with a fruitful field for the exploration of basic mechanisms
of neuronal activity. Major advances dining the last years have provided
us with new information about the neuroanatomy of the vestibular system
(refs. [ref.134]-[ref.137]).
Vestibular impulses entering the brainstem ascend and descend the
neuroaxis and cross the midline. It was previously believed that the
vestibular apparatus had only subcortical projections. Recently,
however, it has been established by means of electrophysiological
methods that the organ is represented by a projection area in the
cerebral cortex of some animals (refs. [ref.138]-[ref.141]). The use of
brief electrical stimulation of the vestibular nerve in order to elicit
a cortical response has been of great value for the mapping of these
areas.
Among a great variety of sensory receptors, the vestibular ones are
capable of evoking the most widespread somatovisceral effects throughout
the body. Moreover, vestibular effects seem to be imperious and less
dependent upon the state of readiness of the nervous system. As a
consequence of the extensive distribution of vestibular effects, there
are many opportunities for central integration. Proprioceptive and
vestibular systems are both known to be active in posture and
locomotion; streams of impulses arising from the receptors in each of
these systems must converge to influence the activity of the final
common path. The state of the motor centers of the spinal cord, as
affected by vestibular stimulation, has been tested by dorsal root and
other sensory input interventions. These experiments have provided us
with insight into the mechanisms concerned with the vestibular control
of spinal reflexes (refs. [ref.142]-[ref.146]).
It has long been known that the vestibular apparatus is essential for
the development of motion sickness. Commonplace subjective experience of
nausea relates to visceral changes mediated through autonomic efferent
pathways and may ultimately involve rhythmic somatic nerve discharges to
skeletal muscles responsible for retching and vomiting. However, very
little is known about the central nervous mechanisms responsible for
elaboration of the whole syndrome. Since the maintenance of vestibular
bombardment for some length of time seems essential for the development
of motion sickness, one would presume this to be an instance of slow
temporal summation. Experimental findings demonstrate a powerful effect
of temporal summation upon somatic motor outflow during vestibular
stimulation ([ref.147]), and not upon parasympathetic outflow.
The practical implication of these studies is closely related to
physiological effects of weightlessness. Based on experimental evidence
from short weightless periods obtained in aircraft, it was concluded
that "when the exposure becomes longer, there may develop minor
physiologic disturbances which, if cumulative or irritating, may cause
or enhance psychiatric symptoms" ([ref.148]). Although the zero-g
condition, per se, does not cause spatial disorientation if visual cues
are provided, the astronauts reported a temporary loss of orientation
during the orbital flight while they were engaged in activities which
diverted their attention. However, no disturbing sensory inputs were
observed during the weightless period. Violent head maneuvers within the
limited mobility of the helmet were performed in every direction without
illusions or vertigo. The subjective sensations of "tumbling forward"
after sustainer engine cutoff reported by the Mercury astronauts, and
Titov’s motion sickness attacks, which were particularly dismaying
during head movements, were well within the entire range of
psychosomatic experiences already obtained during aerodynamic
trajectories ([ref.149]). Interestingly enough it now appears that the
otolithic output in mammals including man is the differential of linear
acceleration, and therefore unaffected by zero g.
Of interest in this connection are the problems which may be encountered
during and following long-term exposure to weightlessness. Although
there is no evidence of adverse effects on operative behavior, the
possibility of biological disturbances on a cellular or subcellular
level, which may cause a deterioration of the somatic basis, has been
repeatedly stressed. Whether effects of this sort will occur or whether
the organism will be able to adapt is still an open question. Since
motion sensitivity based on vestibular stimulation differs widely among
individuals, the selection of astronauts may solve the problem of zero-g
vestibular disturbance. Reports from the MA-8 (Sigma 7) and Vostok III
and IV flights seem to support this assumption. Moreover, experiments
are being made in the slow rotation room at the Naval School of Aviation
Medicine to study the Coriolis effects which arise when "artificial
gravity" is produced by angular acceleration. Since man can adapt to
wave motion on shipboard within a few days, a similar process may be
expected to occur in the case of long-term weightlessness ([ref.150]).
chapter 5
_Molecular Biology and Bioinstrumentation_
To support biological investigations in space and to accumulate baseline
data needed for manned space flight, NASA has conducted a program in
laboratory research and theory. A multidisciplinary approach has
included such fields as ecology, physiology, organic and biological
chemistry, engineering, electronics, and optics. Emphasis in this
program has been placed on qualitative and theoretical rather than
purely descriptive research, and the investigation of fundamental
biological phenomena at all levels, from the molecular to the total life
form.