Effector The body site where a response occurs which counters an initial stimulus and thus attempts to maintain homeostasis. Efferent Nerve A nerve that relays motor commands from the CNS to various muscles and glands. Element A chemical substance composed of only one atom, e.g., hydrogen, calcium, or singlet oxygen. Endocrine System The organ system that regulates body functions by use of chemicals, known as hormones.  Endocrine organs are the pituitary gland, parathyroid gland, thyroid gland, adrenal gland, thymus, pancreas, and gonads. Endoplasmic Reticulum A cell organelle, which provides an extensive network of membrane-like channels that, extends throughout the cytoplasm.  It synthesizes secretory products and is responsible for intracellular storage and transport. Enzymes A chemical (protein) that catalyses (accelerates) specific biochemical reactions without themselves being permanently changed. Ependymal Cells A type of glial cell in the CNS that produces a special fluid, known as the cerebral spinal fluid (CSF). Epithelial Tissue One of the four types of tissue in the body that is specialized to protect, absorb and secrete substances, as well as detect sensations.  It covers every exposed body surface, forms a barrier to the outside world, and controls absorption.