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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

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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

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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

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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

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Effectors

Effectors

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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

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Effectors

Effectors

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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

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Effectors

Effectors

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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

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©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

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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

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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

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A

B

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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

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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

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VESTIBULAR APPARATUS Constituents

Six Semicircular Canals Otoliths in two Saccules Otoliths in two Utricles

From: NASA,http://weboflife.nasa.gov/learningResources/vestibularbrief.htm

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VESTIBULAR APPARATUS Semicircular Canals

False sense of rotation From:NASA,http://weboflife.nasa.gov/learningResources

/vestibularbrief.htm

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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

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NEUROVESTIBULAR SYSTEM Neurovestibular Control System

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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

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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

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From: M Schieppatia, E. Tacchini, A Nardone, J Tarantoola, S Corna, Subjective perception of body sway, J Neurol Neurosurg Psychiatry,1999;66:313-322

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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

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Figures from: Air Safety

Foundation, Safety Advisor, Physiology No.

1, 2004

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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

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A

B

C

Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel

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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

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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

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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

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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

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Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel

From: Instrument Flying Handbook By Federal Aviation Administration

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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

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Figures from: Department of the Army FM 3-04.301 Aeromedical Training for Flight Personnel

Courtesy of NASA

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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

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