(mŭs′ĕl) [L. musculus, diminutive of mus, mouse] A type of tissue composed of contractile cells. Each muscle cell is filled with parallel actin and myosin filaments. When activated by an internal release of calcium, the filaments use the energy in adenosine triphosphate to crawl along each other in opposite directions. This movement shortens the length of the cell, which then contracts.
The three classes of myocytes (muscle cells) are skeletal (striated), cardiac (striated), and smooth; most human muscle is skeletal. A typical muscle has a central portion (the belly) and two or more attachment ends with tendons. The more stationary of the attachments is called the origin; the more movable attachment is called the insertion. SEE: illus.
SELECTED MUSCLES OF THE BODY
MORPHOLOGICAL FORMS OF MUSCLE
abdominal m. A collective term for the external and internal abdominal oblique, cremaster, iliacus, psoas major (and minor if present), pyramidalis, quadratus lumborum, rectus abdominis, and transversus abdominis.
abductor digiti minimi m. An instrinsic muscle of the hand, located in the hypothenar eminence. Proximal attachment: pisiform. Distal attachment: base of proximal phalanx of digit 5. Innervation: ulnar (C8-T1). Action: abducts digit 5.
abductor pollicis brevis m. An intrinsic muscle of the hand, located in the thenar eminence. Proximal attachment: flexor retinaculum, scaphoid and trapezium. Distal attachment: lateral side of base of proximal phalanx of thumb. Innervation: median (C8-T1). Action: abducts thumb, aids in opposition with digit 5. SEE: arm for illus. (Muscles of the Arm).
adductor longus m. A muscle of the medial compartment of the thigh. Proximal attachment: pubis. Distal attachment: linea aspera of femur. Nerve: obturator (L2-L4). Action: adducts, flexes, and rotates thigh medially. SEE: leg for illus. (Muscles of the Leg).
adductor magnus m. A muscle of the medial and posterior compartments of the thigh. Proximal attachment: inferior ramus of pubis, ramus of ischium, ischial tuberosity. Distal attachment: linea aspera and adductor tubercle of femur. Nerve: obturator and sciatic. Action: adducts, flexes, and rotates thigh medially. SEE: leg for illus. (Muscles of the leg).
adductor pollicis m. An intrinsic muscle of the hand. Proximal attachment: metacarpals 2-3 and the capitate. Distal attachment: proximal phalanx of digit 1. Nerve: ulnar. Action: adducts thumb.
agonist m. A muscle whose action is opposed by another muscle (the antagonist muscle). Controlled movements involve two opposing muscles. The agonist muscle produces the main action; the antagonist muscle produces the opposite action to a lesser degree. The balance between agonist and antagonist muscles allows precise control of the final action.