Muscle Tissue Terms

Actin: A contractile protein that is part of the thin filaments in muscle fibers.

Aponeurosis: A sheetlike tendon joining one muscle with another, or with bone.

Endomysium: Fine areolar connective tissue that surrounds each individual muscle fiber.

Epimysium: Sheath of dense irregular connective tissue that surrounds an entire muscle organ and attaches it either directly by fusing to the periosteum of bone or perichondrium of cartilage, or indirectly by extending beyond the muscle as a ropelike tendon or sheetlike aponeurosis.

Excitation-Contraction Coupling: A sequence of events by which transmission of an action potential along the sarcolemma leads to sliding of the myofilaments.

Isometric contraction: The production of tension in a muscle that is insufficient to move a load, thus the muscle does not shorten and no work is done.

Isotonic contraction: The shortening of a contracting muscle that moves a load and does work.

Motor unit: A motor neuron together with all of the muscle fibers that it stimulates.

Multiple-wave summation: When a muscle fiber is stimulated repeatedly, complete relaxation cannot occur between stimuli, and the tension produced is greater than that from a single stimulus, which results from the greater concentration of Ca2+ in the sarcoplasm, and the stretch of the elastic components of the muscle early in contraction.

Muscle: An organ composed of one of three types of muscle tissue (skeletal, cardiac, or smooth), specialized for contraction to produce voluntary or involuntary movement of parts of the body.

Muscle action potential: A stimulating impulse that propagates along the sarcolemma and transverse tubules; it is generated by acetylcholine, that binds to chemically-gated Na+ channel proteins that open and allow sodium ions to enter the cell, which causes depolarization.

Muscle fascicle: A bundle of muscle fibers within a muscle organ.

Muscle fatigue: Inability of a muscle to maintain its strength of contraction or tension; it may be related to insufficient oxygen, depletion of glycogen, and/or lactic acid buildup.

Muscle fiber: A muscle cell.

Muscle spindle: Encapsulated receptor found in skeletal muscle that is sensitive to stretch. Also called a neuromuscular spindle.

Muscle tension: The force exerted by a contracting muscle on some object.

Muscle tone: The continued mild or partial contraction of an entire muscle, even when at rest, which keeps muscles firm, healthy and ready to respond, and maintains posture.

Muscle twitch: The response of a muscle to a single, brief threshold stimulus, which gives the simplest contraction observable in a myogram.

Muscular dystrophy: A group of inherited muscle-destroying diseases.

Muscular system: The organ system consisting of the skeletal muscles of the body and their connective tissue attachments.

Muscular tissue: A basic tissue type that contracts upon stimulation to produce movement; its three varieties are skeletal, cardiac, and smooth muscle.

Myoblasts: Embryonic mesoderm cells from which all muscle fiber develops.

Myofibril: A bundle of myofilaments that form a threadlike structure with a banded appearance that runs the entire length of a muscle fiber.

Myofilaments: Filaments of contractile proteins found in muscle fibers, consisting mainly of thick filaments (myosin) and thin filaments (actin, troponin, and tropomyosin).

Myoglobin: The oxygen-binding, iron-containing protein present in the sarcoplasm of muscle fibers; it resembles a single subunit of hemoglobin and contributes the red color to muscle.

Myogram: A graphic recording of mechanical contractile activity produced by an apparatus that measures muscle contraction.

Myosin: The contractile protein that makes up the thick filaments of muscle fibers.

Neuromuscular junction: A synapse between an axon terminal of a motor neuron and the sarcolemma of a muscle fiber. Also called myoneural junction.

Perimysium: Fibrous connective tissue that surrounds each muscle fascicle.

Sarcolemma: The cell membrane of a muscle fiber, especially of a skeletal muscle fiber.

Sarcomere: The functional contractile unit of striated muscle, which is the segment of a myofibril between two adjacent Z discs.

Sarcoplasm: The cytoplasm of a muscle fiber.

Sarcoplasmic reticulum (SR): Elaborate network of smooth endoplasmic reticulum surrounding each myofibril in a muscle fiber; the enlarged tubules and each end are called terminal cisternae; the SR functions to reabsorb calcium ions during relaxation and to release them to cause contraction.

Sliding Filament Model: A model which states that during muscle contraction, the thin filaments slide past the thick filaments, overlap between the myofilaments increases, the sarcomere shortens, and the whole muscle shortens.

Transverse tubule (T tubule): Invagination of the sarcolemma of striated muscle fibers that encircles the myofibrils at each I Band-A band junction, and it conducts muscle action potentials.

Tendon: A white fibrous cord of dense regular connective tissue that attaches muscle to bone.