countermeasure:

Any step taken to prevent or alleviate decrements in the physiological or psychological status of astronauts that may result from spaceflight. Measures may include in-flight protocols such as medication and exercise, as well as the screening of astronaut candidates using medical selection criteria.

cytokine:

Class of proteins that mediates immune (and other) responses.

dosimetry:

Experimental procedure used to determine dose (joules per kilogram) of ionizing radiation.

functional magnetic resonance imaging (fMRI):

Noninvasive scanning technology used to produce images of localized neural activity in the human brain. It is based on the fact that there are changes in local blood flow and blood oxygenation in response to neural activity.

hindlimb unloading:

Simulation of some aspects of microgravity effects on rodents by unloading of muscles and head-down tilt by employing various harnessing strategies to elevate the animal and prevent its hindlimbs from weight bearing on the floor.

histomorphometric:

Referring to structural changes in tissues observed and quantitated on biopsy specimens.

hypothalamus-pituitary-adrenal (HPA) axis:

Neuroendocrine axis that interacts with the immune response; mediates response to stress and other factors that could have a profound effect on immune response and resistance.

leukocyte:

White blood cell.

leukocyte distribution:

Distribution of subtypes of white blood cells (e.g., CD4+ and CD8+ lymphocyte distribution); indication of host potential for normal immune function.

leukocyte proliferation:

Response of lymphocytes and macrophages (white blood cells) to stimulation in which lymphocytes divide after receiving signals from macrophages; evidence of intact immune function.

locomotor:

Affecting or involving the organs that mediate active movements.

natural killer cells:

Immune cells found in the body under normal conditions that can destroy target virus-infected and tumor cells by an as-yet-unknown recognition mechanism.

neuroendocrine:

Referring to the complex integration of the central nervous system with pituitary hormone secretion.

Neurolab:

Shuttle mission (1998) dedicated to studies in neurosciences.

neuroplasticity:

Ability of differentiated neurons to alter structure or function in response to altered physiological demands.