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©VL Pisaane, 2012 Neurovestibular - 1
HUMAN SPACEFLIGHT Effects of Spaceflight on the Human Body
Neurovestibular Adaptation
by
L. E. Wehren, MD
V. L. Pisacane, PhD
©VL Pisaane, 2012 Neurovestibular - 2
NEUROVESTIBULAR ADAPTATION Topics
Introduction
Control Mechanisms
Vestibular Apparatus
Neurovestibular System
Spatial Disorientation During Aircraft Flight
Neurological Disorientation from Spaceflight
Definitions
©VL Pisaane, 2012 Neurovestibular - 3
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 - 4
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 -
CONTROL MECHANISMS Introduction
Feedback regulation can occur at different levels – Anatomical – Physiological – Biochemical
Response to change occurs to correct deviation by either enhancing it with positive feedback or depressing it with negative feedback
5
Effectors
Effectors
©VL Pisaane, 2012 Neurovestibular -
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 – 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),
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
6
Effectors
Effectors
©VL Pisaane, 2012 Neurovestibular -
CONTROL MECHANISMS Positive Feedback
Positive feedback is response that amplifies change in variable that can often result in destabilizing effect, further departure from 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 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
7
Effectors
Effectors
©VL Pisaane, 2012 Neurovestibular -
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
8
©VL Pisaane, 2012 Neurovestibular -
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
9
©VL Pisaane, 2012 Neurovestibular -
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
10
©VL Pisaane, 2012 Neurovestibular -
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
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
11
A
B
©VL Pisaane, 2012 Neurovestibular -
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
12
©VL Pisaane, 2012 Neurovestibular - 13
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 - 14
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 - 15
VESTIBULAR APPARATUS Semicircular Canals
False sense of rotation From:NASA,http://weboflife.nasa.gov/learningResources
/vestibularbrief.htm
©VL Pisaane, 2012 Neurovestibular - 16
VESTIBULAR APPARATUS Sensing Linear Acceleration and Gravity
Otoliths have greater specific gravity than surrounding tissue and, thus, provide inertia
Gravity or linear acceleration forces otolith to bend, 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
saccule
utricle
Otolith Organ (saccule or utricle); senses linear acceleration
From:NASA,http://weboflife.nasa.gov/learningResources/vestibularbrief.htm
©VL Pisaane, 2012 Neurovestibular - 18
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 -
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
19
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 -
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 – VFR rated 69 or 83% – IMC rated 14 or 17%
Weather Conditions – IMC = Instrument Meteorological Conditions – VMC = Visual Meteorological Conditions
Flight Ratings – VFR = Visual Flight Rules – IFR = Instrument Flight Rules
20
Figures from: Air Safety
Foundation, Safety Advisor, Physiology No.
1, 2004
©VL Pisaane, 2012 Neurovestibular -
SPATIAL DISORIENTATION DURING AIRCRAFT FLIGHT False Sense of Tilt
Figure A shows upright head with nominal resting frequency stimulated by position of hair cells
Figure B shows tilted head, that alters resting frequency
Figure C shows forward linear acceleration, which cannot be 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
21
A
B
C
Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel
©VL Pisaane, 2012 Neurovestibular -
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
22
©VL Pisaane, 2012 Neurovestibular -
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 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
23
Figure From: T R Czarnik, Artificial gravity: current concerns and design considerations, March 1999
Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel
©VL Pisaane, 2012 Neurovestibular -
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 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
24
Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel
From: Instrument Flying Handbook By Federal Aviation Administration
©VL Pisaane, 2012 Neurovestibular -
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
– 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
stick, which would reduce airspeed
25
Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel
Courtesy of NASA
©VL Pisaane, 2012 Neurovestibular -
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
26