ASEN 5016 Lecture 15: Neurovestibular Response to Space

Objectives

1. Describe normal function of vestibular system

2. Identify short term effects of low-g

3. Identify long term effects of low-g

4. SMS: Theories and countermeasures

5. Readaptation to gravity

Sensory Inputs

Visual

Tactile / Proprioceptive

Auditory

Taste

Smell

*Vestibular (balance)

Orientation cues stem from hearing, vision, touch and vestibular

Conflicting inputs can lead to vertigo or motion sickness

1. Normal Function of the Vestibular System

Labyrinth

5 Sensors on each side of the head

2 à linear accelerometers (otoliths)

utricle ~ horizontal (x-y)

saccule ~ vertical (y-z) planes

3 à rotational accelerometers (semi-circular canals)

horizontal ~30° off plane

anterior 90°

posterior 90°

Otoliths

Structure – stereocilia, kinocilium, statoconia, endolymph, hair cells

Process

- Bend toward kinocilium – depolarization (excitation)

- Bend away from kinocilium – hyperpolarization (inhibition)

* baseline: 100 Hz, min: 0 Hz, max: 400 Hz

Function

- Gravity-dependent – tilt

- Gravity-independent – linear acceleration

· Sensitivity to linear acceleration is ~ 1/5000 g

· Sensitivity to tilt is ~ 0.5°

Semicircular Canals

Structure – stereocilia, kinocilium, ampulla, cupula, crista, ampullaris, endolymph, hair cells

Process

- Bend toward kinocilium – depolarization (excitation)

- Bend away from kinocilium – hyperpolarization (inhibition)

* baseline: 100 Hz, min: 0 Hz, max: 400 Hz

Function

- Gravity-dependent – none (?)

- Gravity-independent – angular acceleration

Canals sensitive to accelerations of ~ 0.1° / sec²

Endolymph

· Surrounds maculae of otoliths

· Fills SCCs

· Derived from plasma, K+ rich, Na+ & Ca2+ poor

- disruption of homeostasis by ionic, osmotic, metabolic imbalance may displace vestibular organs – affect function

Vestibular Reflexes

Vestibulo-Ocular Reflexes (VOR)

- Rotational (head/paper shaking demonstration) - nystagmus

- Translational

- Ocular counter-rolling

· Vestibulospinal Reflex

- Vestibulocollic (neck)

Motion Sickness (terrestrial)

- normal response to movement (actual or perceived) to which the individual is not adapted

Correlations

Age – susceptibility increases with age until puberty, then decreases
Sex – women are generally more susceptible than men
Personality

Causes

Sensory Conflict (mainly between visual and vestibular)
Response to eye movements controlled by vestibular nuclei (?)

· Neck muscles stimulate vagus nerve (?)

Effects of Space Flight on Neurovestibular System

2. Short Term Effects

No effect on semi-circular canal response to acceleration

In normal gravity, eyes counter-rotate based on otolith response to tilt - this doesn’t occur in 0-g

à Resting position in 0-g gives sensation of “leaning back” because normal upright position is tilted slightly forward

Onset of Space Motion Sickness occurs during the first 24-48 hours, resolves within ~ 4 days (more on this later…)

SMS not correlated with any prior motion sickness
~80% of 1^st-time flyers, ~60% of repeat flyers develop symptoms

3. Long Term Effects

Increase in hair cell synapses to try to make the system become more sensitive (data from rats)

Over time, the brain ignores the “bad” signal and establishes a new baseline

Visual system become dominant orientation sense

What is the concern?

Space Motion Sickness (SMS)

· Vomiting during EVA could be deadly

respirating stomach acid breaks down surface tension in alveoli and allows plasma to cross membrane

· Reduced crew effectiveness due to nausea

· Readaptation to 1-g

Vection

à illusion that you are moving and visual field is fixed

Rotating Dome Experiment

Time lag to vection onset decreased with increasing MET

à - the longer time spent in space,

- the more dominant visual stimulation becomes

Vestibular Reflexes

· Vestibulo-Ocular Reflexes (VOR)

- only ocular counter-rolling seems affected (object tracking)

· Vestibulospinal Reflex (VSR)

- balance has no meaning in space

- posture is altered (fetal position)

- proprioceptive cues diminished

Hoffman Reflex – anticipatory reflex to falling unexpectedly

à by MET Day 6 response system no longer “prepares” for fall

à but post flight response was hyper-sensitive

4. Space Motion Sickness (SMS) Theories and Countermeasures

1) Fluid-Shift Theory

2) Sensory-Conflict Theory

visual system no real change

vestibular system big change

tactile big change

proprioceptive big change

auditory no real change (but certain frequencies can induce motion sickness)

Evolutionary protection from environmental poison hypothesis (Treisman)
Otolith-Asymmetry hypothesis (von Baumgarten)
Otolith Tilt-Translation Reinterpretation hypothesis (Young)

Pharmacological Countermeasures

Oral medications at onset may not help due to GI tract shutdown - to avoid problems, administer prophylactically

Scopolamine-Dextroamphetamine used to be used… but side effects – elevated BP and HR restlessness, dizzy, insomnia/euphoria

Intramuscular medications avoid this, and act more quickly

Phenergan (Promethazine)

Non-pharmacological Countermeasures

Simply let SMS run its course!

Sensory Conflict Training

Russian Space Program

- Rotating Chair

US Space Program

- Device for Orientation and Motion Environments (DOME)

- Tilt Translation Device (TTD)

Autogenic Feedback Training

Mechanical/Electrical Device Countermeasures

- Pressurized spring loaded insoles (Cupola SAND-501)

- Neck Pneumatic Shock Absorber (NPSA)

- Elastic Load Suits

- Body (Hip) Pneumatic Occlusive Cuff

- Electrical Stimulation (not tested in spaceflight)

Mastoid - forehead stimulation

Electro-acupuncture

5. Readaptation to gravity (Earth, or moon or Mars?)

Readaptation to gravity with hyper-sensitivity

“Giant hand” Syndrome
general hypersensitivity to minimal head movements

Summary

Otolith organs are gravity-dependent - head tilt is no longer sensed in micro-g

Sensory conflict between functioning receptors (semicircular canals, eyes, ears) and non/reduced functioning receptors (otoliths, proprioceptors) may induce SMS

SMS is important to consider and reduce/eliminate for crew safety and effectiveness