biology and biotechnology

Effects of Altered Gravity on Spinal Cord Excitability (H-Reflex)

Research Area: Nervous and Vestibular Systems

Expedition(s): 2-4

Principal Investigator(s): ● Douglas Watt, MD, PhD, McGill University, Montreal,

Québec, Canada

RESEARCH OBJECTIVES

Effects of Altered Gravity on Spinal Cord Excitability (H-Reflex) will study the prolonged loss of

muscle strength, muscle volume, and bone density, due to the weightlessness environment.

These conditions can cause reduced spinal cord excitability, which can lead to loss of loco-

motor function in the legs. Spinal cord excitability was isolated and measured to study possible

ways to reverse the process while still in flight. Reversal of this process will result in a healthier

crew following long-duration spaceflight.

EARTH BENEFITS

The information gained by this investigation may help

researchers develop countermeasures to overcome

decreases in spinal excitability. There are many

disorders that involve decreases in nerve impulses and

loss of sensitivity. Countermeasures that are used in

orbit may lead to advances on Earth in treatment of

various nerve disorders and spinal injuries.

SPACE BENEFITS

The most basic unit of exercise is the contraction of a

small group of muscle fibers innervated by the

projections of a single nerve cell. If this and other nerve

cells in the spinal cord become less excitable during

spaceflight, it would be more difficult to make muscle

fibers contract. As a result, more effort would be required to produce the same level of

exercise, or if the same apparent effort were maintained, the actual level of exercise would

decrease. If present after landing, it would be more difficult to stand and walk. However,

depending on the underlying mechanisms, it may be possible to reverse the process while still

in flight.

RESULTS

Spinal Cord Excitability (SCE) was measured with the H-reflex (Hoffmann reflex) method: 60

shocks of varying lengths were applied to the posterior tibial nerve and the electromyography

response detected at the soleus. H-Reflex was conducted repeatedly before, during, and after

flight. There was high variation between subjects but the H-reflex was observed to decrease by

about 35 percent after 5 days in space and remain at the new level for the duration of the

flight. Upon return to 1 g, full recovery took about 10 days. This indicates a reduction of spinal

cord excitability in microgravity due to a partial response to signals from the nervous system.

The consequence is that muscle mass decreases at 0 g even with exercise and implies there

JSC2001E34377 – Expedition 3
commander Culbertson sits in the shuttle
middeck performing the Effects of Altered
Gravity on the Spinal Cord Excitability
experiment.