Placebo

An inert substance or inactive treatment given instead of a therapy that is being evaluated in a clinical trial.

Plasticity

The ability of neurons to modify the physical connections, thus changing the property of a neuronal circuit.

Progenitor cell

Any type of cell that spawns other cells.

Programmed cell death

Apoptosis (see above).

Proprioception

The unconscious perception relating to position, posture, equilibrium, or internal condition.

Proteases

Enzymes that degrade proteins. Proteases are important regulators of cell function, but the inappropriate activation of proteases resulting from trauma can be harmful.

Protein

Any of numerous naturally occurring, extremely complex substances (such as an enzyme or antibody) that consist of amino acid residues joined by peptide bonds. They are essential constituents of all living cells that are translated from the organism’s RNA.

Proteoglycans

Proteins on the extracellular side of the cell membrane to which sugar moieties are attached.

Quadriplegia

Paralysis of all four limbs, also called tetraplegia.

R01 grant

Federal research project grant that supports specific health-based research for 1 to 5 years. It can be investigator initiated or submitted in response to a request for application or program announcement.

R03 grant

Federal grant that supports small research projects for a limited period of time and with limited resources. Grants are awarded for up to 2 years with direct costs limited to $50,000 per year.

R21 grant

Federally supported exploratory or developmental research grant that supports the early development of an innovative project. Grants are awarded for up to 2 years, with total direct costs not to exceed $275,000 for the length of the project.

Receptors

Protein molecules, usually found on the surfaces of cells, that enable cells to respond to neurotransmitters, hormones, and other messenger molecules. Receptors may act directly by opening in the cell membrane ion channels that are part of the same receptor molecule, or they may indirectly by activating second messenger systems that go on to affect various processes in the cell. The term “receptor” also refers to specialized neuronal cells that receive sensory information, such as pain receptors and light receptors in the eye.

Ribonucleic acid (RNA)

Carries the code for a particular protein from DNA. Messenger RNA (mRNA) is translated by specialized proteins and ribosomes to form proteins.

Schwann cells

Glial cells in the peripheral nervous system that are primarily responsible for wrapping nerve fibers with myelin.

Second messenger system

Biochemical pathway within cells that is regulated by hormones or neurotransmitters, which bind to receptors on the