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1. Bioastronautics: Space Exploration and its Effects on the
Human Body
Instructors:
L. E. Wehren, MD
V. L. Pisacane, PhD
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3. HUMAN SPACEFLIGHT
Effects of Spaceflight on the Human Body
Neurovestibular Adaptation
by
L. E. Wehren, MD
V. L. Pisacane, PhD
©VL Pisaane, 2012 Neurovestibular - 1

4. NEUROVESTIBULAR ADAPTATION
Topics
 Introduction
 Control Mechanisms
 Vestibular Apparatus
 Neurovestibular System
 Spatial Disorientation During Aircraft Flight
 Neurological Disorientation from Spaceflight
 Definitions
©VL Pisaane, 2012 Neurovestibular - 2

5. INTRODUCTION
Background
 First human space flights showed no significant sensory system
problems in space
 Many astronauts reported motion sickness of varying severity
 First space neurovestibular studies focused on causes and
consequences of space motion sickness
 Since Apollo missions, studies of neurovestibular system have
increased in complexity
 Weightless environment shown to provide different stimulus to otolith
organs of inner ear; therefore signals from otolith organs no longer
correspond with visual and other sensory signals sent to brain
 After a few days in space, astronaut begins to adapt to new neural
input
 On return to Earth gravity, astronaut confronted with undoing changes
in neurovestibular responses developed in space
©VL Pisaane, 2012 Neurovestibular - 3

6. INTRODUCTION
Problem
 Space environment
– Weightlessness alters function of some sensory organs
– Visual scene distorted in that objects float, no up or down, cabinets and drawers
on all four sides
 Effects
– Uncertain spatial orientation and illusion of motion of themselves or objects
– Space adaptation syndrome (SAS) leading to potential nausea and vomiting
– Disturbed hand-eye coordination
– Increased nystagmus and changes in its properties
 On return to gravity
– Problems with balance, orientation, and walking resolve in few days
©VL Pisaane, 2012 Neurovestibular - 4

7. CONTROL MECHANISMS
Introduction
 Feedback regulation can occur at
different levels Effectors
– Anatomical
– Physiological
– Biochemical
 Response to change occurs to correct
deviation by either enhancing it with
positive feedback or depressing it with Effectors
negative feedback
©VL Pisaane, 2012 Neurovestibular - 5

8. CONTROL MECHANISMS
Negative Feedback
 Negative feedback causes system to respond to
reverse direction of change tending to move
away from homeostasis
 Example: concentration of CO2 in body Effectors
– As CO2 increases, lungs signaled to increase
their activity to expel more CO2 and ingest
more air by deeper breaths and increased
rate of respiration
 Example: thermoregulation
– When body temperature rises (or falls), Effectors
receptors in skin and hypothalamus sense
change and cause brain to trigger commands
to decrease or (increase) body temperature
by several means, including shunting blood
to or from surface of body
©VL Pisaane, 2012 Neurovestibular - 6

9. CONTROL MECHANISMS
Positive Feedback
 Positive feedback is response that amplifies
change in variable that can often result in
destabilizing effect, further departure from
Effectors
homeostasis
 Positive feedback generally less common in
organic systems than negative feedback
 Example: in nerves, threshold electric potential
triggers the generation of much larger action Effectors
potential
 Example: blood clotting
– Injured tissue releases signal chemicals that
activate platelets in blood that in turn release
chemicals to activate more platelets, causing
rapid cascade and formation of blood clot
©VL Pisaane, 2012 Neurovestibular - 7

10. CONTROL MECHANISMS
Antagonistic Mechanisms
 Body uses strategy of antagonistic mechanisms to solve problem of maintenance of
body equilibrium or homeostasis
 Antagonistic mechanisms maintain equilibrium by means of alternating
compensatory mechanisms
– Some mechanisms
• Lower pH and others increase it
• Increase body temperature and others to lower it
• Some hormones reduce level of glucose in blood and others increase it
©VL Pisaane, 2012 Neurovestibular - 8

11. CONTROL MECHANISMS
Feedback and Feedforward Control 1/2
 Illustrated control loop controls dynamic
behavior of plant to maintain Set Point by
both negative feedback and feedforward
responses to changing conditions
 In feedback control sensed states
subtracted from Set Point to form Error
Signal that, with delay, affects plant
 Feedforward control is open loop strategy
that compensates for disturbances before
they affect controlled variable
 Feedforward control measures disturbance
variable, predicts its effect on plant, and
applies corrective action
 In feedforward control disturbances are
measured without reference to actual
system condition and results in much faster
response than feedback control system
©VL Pisaane, 2012 Neurovestibular - 9

12. CONTROL MECHANISMS
Feedback and Feedforward Control 2/2
 Feedforward control depends on set of
stored rules known to be successful in
given situational context
 As example, human upright posture is
inherently unstable
– To counter mechanical effect of large-
scale perturbation such as a slip, central
nervous system can make adaptive
adjustments in advance to improve
stability of body’s center-of-mass
– Such feedforward control relies on
accurate internal representation of
stability limits, which must be function
of anatomical, physiological, and
environmental constraints
 If one or more anatomical, physiological,
or environmental constraints should
change, feedforward control is disrupted
©VL Pisaane, 2012 Neurovestibular - 10

13. CONTROL MECHANISMS
Examples of Feedforward Control
 Feedforward control can be described as
learned anticipatory responses to known
cues
 Example: when one tries to maintain
constant speed while driving there is
tendency to depress gas pedal as one
begins to climb hill even before car slows A
 Figure A contrasts feedback and
feedforward control to maintain hot
water at constant temperature
 Figure B illustrates feedforward control
based on input to plant together with
feedback control based on plant’s
response to changed input
B
©VL Pisaane, 2012 Neurovestibular - 11

14. CONTROL MECHANISMS
Virtual Cliff Experiment
 When brain and central nervous system
involved, feedforward response often plays
important role
 Experiment by Gibson and Walk in 1960
illustrated that some of nominal rule-based
feedforward response may be inherited
 They set up “virtual cliff experiment” for
babies, as illustrated
 Young babies have no problem crawling over
the elevated region
 Reluctant to crawl over “cliff” even when
their mothers encourage them to do so
 See internet video at:
– http://cognitivepsychologyisfun.blogspot.c
om/2009/10/virtual-cliff-experiment.html
©VL Pisaane, 2012 Neurovestibular - 12

15. VESTIBULAR APPARATUS
Introduction
 Aircraft and spacecraft maintain positions based on information from sensors as
part of control systems
 Similarly, neurovestibular system responsible for sensing body’s movements
and interfacing with brain to
– Sense orientation
– Maintain balance
– Coordinate body motions
 Relevant sensors
– Brain
– Eyes
– Vestibular organ or apparatus
• Otolith detects linear acceleration and gravity
• Semicircular canals detect rotational acceleration
– Proprioceptors
• Sensory nerve terminals that give information concerning
movements and position of body
• Located primarily in muscles and tendons
©VL Pisaane, 2012 Neurovestibular - 13

16. VESTIBULAR APPARATUS
Constituents
 Six Semicircular Canals
 Otoliths in two Saccules
 Otoliths in two Utricles
From: NASA,http://weboflife.nasa.gov/learningResources/vestibularbrief.htm
©VL Pisaane, 2012 Neurovestibular - 14

17. VESTIBULAR APPARATUS
Semicircular Canals
False sense of rotation From:NASA,http://weboflife.nasa.gov/learningResources
/vestibularbrief.htm
©VL Pisaane, 2012 Neurovestibular - 15

18. VESTIBULAR APPARATUS
Sensing Linear Acceleration and Gravity
 Otoliths have greater specific
gravity than surrounding tissue
and, thus, provide inertia
saccule
 Gravity or linear acceleration
forces otolith to bend, utricle
producing a force on hair cell
 Utricle essentially horizontal
and primarily registers
accelerations in horizontal
plane of head
 Saccule essentially vertical and
mostly registers accelerations
in vertical plane of head
Otolith Organ (saccule or utricle); senses linear acceleration
From:NASA,http://weboflife.nasa.gov/learningResources/vestibularbrief.htm
©VL Pisaane, 2012 Neurovestibular - 16

19. NEUROVESTIBULAR SYSTEM
Neurovestibular Control System
©VL Pisaane, 2012 Neurovestibular - 17

20. NEUROVESTIBULAR SYSTEM
Example: Posture Control
 Posture control example of
function of neurovestibular
system
 If one shuts his eyes, has
greater tendency to sway
than when eyes are open
 Posture control is learned
capability; babies must learn
to stand; sway increases
among elderly
 For example, if knocked out,
we fall but keep breathing,
which is innate capability
©VL Pisaane, 2012 Neurovestibular - 18

21. NEUROVESTIBULAR SYSTEM
Posture Control with Eyes Shut
 Figures show influence of stance and eyes on sway in 20 young adults
 Note increase in sway with eyes closed
– L-L = lateral –lateral, A-P (anterior-posterior)
– Score = 0 -10 rating by subject
– Sway area measured in mm2
From: M Schieppatia, E. Tacchini, A Nardone, J Tarantoola, S Corna, Subjective
perception of body sway, J Neurol Neurosurg Psychiatry,1999;66:313-322
©VL Pisaane, 2012 Neurovestibular - 19

22. SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT
Introduction
 Spatial disorientation defined as failure of pilot to
correctly sense attitude or motion of aircraft or of
him or herself, resulting from inadequate or
erroneous sensory information
 Aeronautical Information Manual ranks spatial
disorientation among most frequently cited
contributing factors to fatal aircraft accidents
 During period of 1994-2003, 184 of 202 spatial
disorientation accidents (91%) were fatal
 Spatial disorientation makes only modest
contribution to overall accident rate, but is
responsible for high percentage of its fatalities
 VFR flight into IMC is number one cause of spatial
disorientation accidents
Figures from:
– VFR rated 69 or 83% Air Safety
– IMC rated 14 or 17% Foundation,
Safety Advisor,
 Weather Conditions Physiology No.
– IMC = Instrument Meteorological Conditions 1, 2004
– VMC = Visual Meteorological Conditions
 Flight Ratings
– VFR = Visual Flight Rules
– IFR = Instrument Flight Rules
©VL Pisaane, 2012 Neurovestibular - 20

23. SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT
False Sense of Tilt
 Figure A shows upright head with
nominal resting frequency stimulated
by position of hair cells A
 Figure B shows tilted head, that alters
resting frequency
 Figure C shows forward linear
acceleration, which cannot be B
distinguished from head tilt
 When adequate visual reference is not
available, aircrew members may
experience illusion of backward tilt
during increase in acceleration and
forward tilt during decrease in
acceleration
C
Figures from: Department of the Army FM 3-04.301
Aeromedical Training for Flight Personnel
©VL Pisaane, 2012 Neurovestibular - 21

24. SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT
False Sense of Flying Level
 A problem arises when an aircraft accelerates about
its roll axis below pilot's vestibular sensory threshold
 If pilot's attention is diverted during this time, when
he shifts his attention back to attitude indicator, he
will find display portraying unexpected attitude
 That will result in conflict between his vestibular
sensations, which tell him he is flying straight and
level, and his visual sensations, which tell him he is in
banked attitude
 Should he initiate a sharp control movement to
correct undesired attitude shown on display, he will
feel as though he is rotating from wings-level
attitude into bank, when just the opposite is the case
 In other words, his eyes will tell him positively that
he is correcting an undesirable situation, while his
inner ear (vestibular sensations) will tell him
positively that he is moving into one
©VL Pisaane, 2012 Neurovestibular - 22

25. SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT
Coriolis Illusion of Rotation
 Coriolis illusion is most dangerous of all vestibular
illusions for aircraft
 Can cause overwhelming disorientation
 Occurs whenever prolonged turn is initiated and
the pilot makes head motion in different
geometrical plane; illustrated in first figure
 For example, if pilot initiates head movement in a
geometrical plane other than that of turn, fluid Figure From: T R Czarnik, Artificial gravity: current
concerns and design considerations, March 1999
stabilizing in original canal and simultaneously
moves in new canals stimulating two other
cupulas
 Combined effect of deflection in all three cupulas
creates perception of motion in three different
planes of rotation: yaw, pitch, and roll
 Result is that pilot experiences overwhelming
head-over-heels tumbling sensation
Figures from: Department of the Army FM 3-04.301
Aeromedical Training for Flight Personnel
©VL Pisaane, 2012 Neurovestibular - 23

26. SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT
Graveyard Spin
 Illusion that leads to graveyard spin usually
occurs in fixed-wing aircraft
 For example, pilot enters spin and remains
in it for several seconds
 Pilot’s semicircular canals reach
equilibrium; no motion is perceived
 Upon recovering from spin, pilot From: Instrument Flying Handbook By Federal Aviation Administration
undergoes deceleration, sensed by
semicircular canals
 Pilot has sensation of being in spin in
opposite direction even if flight
instruments contradict that perception
 If deprived of external visual references,
pilot may disregard instrumentation and
make control corrections against falsely
perceived spin
 If so, aircraft will then re-enter spin in
original direction
Figures from: Department of the Army FM 3-04.301
Aeromedical Training for Flight Personnel
©VL Pisaane, 2012 Neurovestibular - 24

27. SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT
Proprioceptive Illusions
 Proprioceptive system responsible for
illusions including
– False perception of true vertical
– Drunken walk of sailor who feels ship as
steady and dry land as heaving
– Person viewing wide field of objects or
stripes moving at constant speed will
eventually see display as fixed and
person as moving
Courtesy of NASA
– Rotary motion can evoke illusions of
body tilt
 Properly executed turn vectors gravity
and centrifugal force through vertical axis
of aircraft such that pilot may falsely
interpret it as climbing in altitude
 Recovering from turns lightens pressure
on seat and creates illusion of descending
– This may cause the pilot to pull back on Figures from: Department of the Army FM 3-04.301
stick, which would reduce airspeed Aeromedical Training for Flight Personnel
©VL Pisaane, 2012 Neurovestibular - 25

28. NEUROLOGICAL DISORIENTATION FROM SPACEFLIGHT
Introduction
 Astronauts frequently experience momentary disorientation when entering or
returning from space
 Many astronauts report some symptoms of space adaptation syndrome (SAS)
(motion sickness) during first days in orbit
 Nystagmus (jerky eye movements) observed in some astronauts early in flight
 Adaptation to weightlessness often leads, upon return to Earth, to
– Disorientation
– Postural instability
– Motion sickness
– Modifications in body segment motion
– Increased response latencies to external perturbations
 These effects would impede astronaut’s response if emergency occurred
 Longer the time in weightlessness, more pronounced symptoms upon return
©VL Pisaane, 2012 Neurovestibular - 26

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