Malignant Neoplasm A tumor or neoplasm that has the ability to metastasize or invade into surrounding tissues. The same as cancer. Malignant Tumor Same as malignant neoplasm. Medulla Oblongata The segment of the brain that is attached to the spinal cord.  It relays sensory information to the rest of the brain and regulates autonomic function, including heart rate and respiration. Metaplasia The conversion from one type of mature cell to a different type of mature cell.  It is a cellular replacement process.  An example is cirrhosis of the liver. Metastasis The ability to establish secondary tumor growth at a new location away from the original site. Metastatic Foci Secondary tumors in an organ different from the original site of cancer development. Microglia A type of glial cell. The microglia are small, mobile, phagocytic cells that function in defense against invading organisms and xenobiotics. Midbrain The area of the brain between the cerebrum and brain stem.  It contains the centers that process auditory and visual data and generates involuntary motor responses. Mitochondria Oval organelles bound by a double membrane with inner folds enclosing important metabolic enzymes.  They produce nearly all (95%) of the ATP and energy required by the cell. Motor Neurons The neurons that relay information from the CNS to other organs, terminating at the effectors.  Motor neurons are the efferent neurons of both the somatic and autonomic nervous systems.  They are also referred to as effector neurons. Multiple Sclerosis A disease in which the myelin around nerves is lost causing paralysis and loss of sensory and motor function. Muscular System The organ system involved with movement or locomotion and heat production.  The main organs are the skeletal muscles and tendons. Muscular Tissue One of the four types of tissue.  It is specialized for an ability to contract.  Muscle cells are elongated and referred to as muscle fibers.  When a stimulus is received at one end of a muscle cell, a wave of excitation is conducted through the entire cell so that all parts contract in harmony. Myelin Protein layers that surround neurons and serves like insulation.  Myelinated neurons usually transmit impulses at high speed, such as needed in motor neurons. Loss of myelination allows interruption of the action potential (like leakage) and causes a dysfunction of these cells.  This can cause paralysis and loss of sensory and motor function.