{M line}{}{Line in the center of the H zone made of delicate filaments that holds the myosin myofilaments in place in the sarcomere of muscle fibers.}

{matrix}{matrices}{() Intercellular material of connective tissue.}

{mechanoreceptors}{}{A sensory receptor that has the role of responding to mechanical pressures. Examples are pressure receptors in the carotid sinus or touch receptors in the skin.}

{medial}{}{Toward or near the midline.}

{mediastinum}{}{Tissues and organs of the thoracic cavity that form a septum between the lungs.}

{medulla oblongata}{}{Inferior portion of the brainstem that connects the spinal cord to the brain and contains autonomic centers controlling such functions as heart rate, respiration, and swallowing.}

{medullary cavity}{}{Large, marrow-filled cavity in the diaphysis of a long bone.}

{meiosis}{}{Process of cell division that results in the formation of gametes. Consists of two divisions that result in one [female] or four [male] gametes, each of which contains one-half the number of chromosomes in the parent cell.}

{Meissner's corpuscle}{}{Named for Georg Meissner, German histologist, 1829-1905; oval receptor found in the papillae of the dermis; responsible for fine, discriminative touch.}

{melanin}{}{A group of related molecules responsible for skin, hair, and eye color. Most melanins are brown to black pigments, some are yellowish or reddish.}

{melanocyte}{}{Cell found mainly in the stratum basale that produces the brown or black pigment melanin.}

{membrane}{}{Thin sheet of tissue usually surrounding a body cavity or surface.}

{meninges [meninx]}{}{Connective tissue membranes surrounding the brain.}

{Merkel's disk}{}{Named for Friedrich Merkel, German anatomist, 1845-1919; cuplike receptor found in the epidermis; responsible for light touch and superficial pressure.}

{merocrine}{}{Gland that secretes products with no loss of cellular material; an example is water-producing sweat glands; see also apocrine and holocrine.}

{mesoderm}{}{Middle primary germ layer.}

{mesothelium}{}{A single layer of flattened cells forming an epithelium that lines serous cavities; such as peritoneum, pleura, pericardium.}

{messenger RNA [mRNA]}{}{Type of RNA that moves out of the nucleus and into the cytoplasm where it is used as a template to determine the structure of proteins.}

{metabolism}{}{Sum of all the chemical reactions that take place in the body, consisting of anabolism and catabolism. Cellular metabolism refers specifically to the chemical reactions within cells.}

{metaphase}{}{Time during cell division when the chromosomes line up along the equator of the cell.}

{metastasis}{}{The shifting of a disease or its local manifestations, or the spread of a disease from one part of the body to another as in a malignant neoplasm.}

{microfilament}{}{Small fibril forming bundles, sheets, or networks in the cytoplasm of cells; provides structure to the cytoplasm and mechanical support for microvilli and stereocilia.}

{microtubule}{}{Hollow tube composed of tubulin, measuring approximately 25 nm in diameter and usually several micrometers long. Helps provide support to the cytoplasm of the cell and is a component of certain cell organelles such as centrioles, spindle fibers, cilia, and flagella.}

{microvillus [microvilli]}{}{Minute projection of the cell membrane that greatly increases the surface area.}

{midbrain}{}{Small region of brain stem between diencephalon and pons.}

{midsagittal plane}{}{Plane that separates the body into equal right and left portions.}

{mitochondrion [mitochondria]}{}{Small, spherical, rod-shaped or thin filamentous structure in the cytoplasm of cells that is a site of ATP production.}

{mitosis}{}{Cell division resulting in two daughter cells with exactly the same number and type of chromosomes as the mother cell.}

{molecule}{}{A substance composed of two or more atoms chemically combined to form a structure that behaves as an independent unit.}

{monomer}{}{Subunits of organic molecules that often form long chains called polymers.}

{monosaccharide}{}{Simple sugar carbohydrate that cannot form any simpler sugar by hydrolysis.}

{morula}{}{An embryonic stage characterized by a solid ball of cells.}

{motor [efferent] neuron}{}{Neuron that innervates skeletal, smooth, or cardiac muscle fibers.}

{motor area}{}{Region of brain from which impulses to muscles or glands originate.}

{motor end plate}{}{Specialized portion of muscle fiber membrane at neuromuscular junction.}

{motor unit}{}{Motor neuron and its associated muscle fibers.}

{mouth}{}{Anterior opening to the alimentary tract.}

{movement}{}{Self-initiated change in position of an organism or motion of internal parts.}

{mucosa [mucosae]}{}{Membrane that lines tubes and body cavities that open to outside of body; mucous membrane.}

{mucous membrane}{}{Thin sheet consisting of epithelium and connective tissue [lamina propria] that lines cavities that open to the outside of the body; many contain mucous glands that secrete mucus.}

{mucus}{}{Fluid secretion of mucous cells.}

{multipolar neuron}{}{One of three categories of neurons consisting of a neuron cell body, an axon, and two or more dendrites.}

{multiunit}{}{Smooth muscle cells that occur in the walls of blood vessels and the extrinsic and intrinsic muscles of the eye.}

{muscle fiber}{}{Muscle cell.}

{muscle spindle}{}{Three to 10 specialized muscle fibers supplied by gamma motor neurons and wrapped in sensory nerve endings; detects stretch of the muscle and is involved in maintaining muscle tone.}

{muscle tissue}{}{Contractile tissue consisting of filaments of actin and myosin, which slide past each other, shortening cells.}

{muscle twitch}{}{Contraction of a whole muscle in response to a stimulus that causes an action potential in one or more muscle fibers.}

{muscularis}{}{Layer of smooth muscle in the digestive system.}

{mutation}{}{A change in the number or kinds of nucleotides in the DNA of a gene.}

{myenteric plexus [plexi]}{}{Nerve fibers and parasympathetic cell bodies that lie between the muscle layers of the muscularis of the digestive system.}

{myosin myofilament}{}{Thick myofilament of muscle fibrils; composed of myosin molecules.}