Acetylcholine – A key chemical mediator involved in neuromuscular synaptic transmission

Action potential – An electrical impulse generated by neurons

Concentric contraction – The shorten- ing of a muscle during activation

Dynamic strength – The magnitude of isotonic or isokinetic contraction

Eccentric contraction – The lengthen- ing of a muscle during activation

Endomysium – The connective tissue surrounding a muscle cell

Epimysium – The connective tissue surrounding the entire muscle

Isokinetic – Literally “same speed”; when applied to muscle action, it implies constant velocity of shortening

Isometric – Literally “same length”; when applied to muscle action, it implies that the muscle length is held constant

Isotonic – When applied to muscle action, it implies that the load is constant

Motor end plate (neuromuscular junction) – The synapse between a motor neuron and a muscle fiber

Motor unit – The motor nerve axon and the myofibers with which it contacts

Musculotendinous junction – The area of interface between a skeletal muscle and its tendon

Myoblasts – The embryonic cells that develop into skeletal muscle cells

Myofibers – The fibers that constitute a muscle

Perimysium – The connective tissue surrounding a fascicle

Sarcolemma – Muscle-cell membrane and its associated basement membrane

Sarcomeres – The fundamental components of the contracting unit of the myofibri

Sarcopenia – The loss of muscle mass and strength as a result of aging

Sarcoplasmic reticulum – A continuous branching network of membrane, which is a specialized form of endoplasmic reticulum unique to muscle

Schwann cell – A specialized support cell that encases nerve fibers

Static strength – The magnitude of isometric contraction

Stem cells – Cells with the unlimited ability of self-renewal and regeneration; serve to regenerate tissue

Synapse –  specialized site at which an electrical signal is transmitted chemically across a junction to produce a similar electrical impulse on the opposite side

Tenocytes – The cells in tendons

Each muscle fiber contains multiple nuclei that lie immediately beneath the sarcolemma, which is its plasma membrane. A small proportion of the nuclei at the periphery of the myofiber are stem cells, also called satellite cells, which can repopulate damaged fibers after injury.5 The cytoplasm, called sarcoplasm, is similar to that of other cells. It contains a cellular matrix, organelles, and a variety of other molecules. Among the organelles, the Golgi apparatus and mitochondria are abundant and lie close to the nuclei. The sarcoplasmic reticulum is a continuous branching network of membrane, which is a specialized form of endoplasmic reticulum that is unique to muscle. Glycogen, lipid droplets, and myoglobin are among other cytoplasmic components.
Other cell types within muscle include fibroblasts, endothelial and smooth muscle cells constituting blood vessels, and Schwann cells around the sheath of nerve axons.
The cells in tendons are called tenocytes. These cells are typical fibroblasts with long cytoplasmic extensions. The cell density of tendons is similar to that of ligaments but lower than that of other tissues such as bone marrow or liver, conferring mechanical strength to these structures.
External to the sarcolemma is a basement membrane that merges with the extracellular matrix (ECM) to form the endomysium. The basement membrane is rich in protein and carbohydrate components, including collagen, laminin, fibronectin, and a variety of glycoproteins.
The ECM of tendons is composed of dense, parallel bundles of collagen fibers. These bundles are oriented along the line of tension between muscle and bone insertion for maximal transmission of load, and they have less crimp than the collagen bundles in ligaments. The collagen is almost 95% type I, with the remainder primarily type III col- lagen and proteoglycans. Tendons generally attach to bone via a specialized direct inser- tion site that has four zones: tendon, fibrocartilage, mineralized fibrocartilage, and bone. Sharpey’s fibers are collagen bundles that extend from the tendon or periosteum into the bone.
Blood vessels run parallel to the axis of myofiber in the connective tissue, often with several capillaries around each myofiber. They are arranged with enough redundancy to permit changes in length during the contraction-extension cycle of a muscle.