Muscle tissue exists in three forms, each tailored to its location and function. Skeletal muscle is voluntary and striated, attached to bones and controlled consciously to produce movement. Smooth muscle is involuntary and non-striated, lining the walls of hollow organs and blood vessels to drive unconscious processes such as peristalsis and vasoconstriction. Cardiac muscle, found exclusively in the heart wall, is involuntary yet striated, with cells joined by intercalated discs that allow the heart to contract as a coordinated unit. Together these three types generate all movements and transport functions of the body.
Within a skeletal muscle, organization follows a clear hierarchy of nested connective tissue sheaths and contractile units. The whole muscle is wrapped in the epimysium and subdivided into fascicles, each surrounded by perimysium. Inside each fascicle, individual muscle fibers are wrapped by endomysium. Within each fiber are myofibrils patterned into repeating units called sarcomeres, built from the contractile proteins actin in thin filaments and myosin in thick filaments. When a motor neuron stimulates the fiber at the neuromuscular junction, releasing acetylcholine across the synaptic cleft, the muscle fiber depolarizes and the sarcomeres shorten to produce force. This arrangement allows the connective tissue sheaths to transmit force from the contractile proteins to the tendon and ultimately to the bone.
Muscles rarely work in isolation, and movement typically arises from coordinated groups classified by their role. The prime mover, or agonist, initiates an action, while synergists assist in the same motion. Antagonists oppose the action and balance it during controlled movements. The quadriceps femoris group on the anterior thigh illustrates a major prime mover: the rectus femoris along with the vastus lateralis, vastus intermedius, and vastus medialis converge through the patellar ligament to insert on the tibial tuberosity, powerfully extending the knee. Around the shoulder, the rotator cuff is a group of four muscles (the supraspinatus, infraspinatus, teres minor, and subscapularis) that stabilize the humeral head within the shallow glenoid cavity, with the supraspinatus initiating abduction and the others providing rotation. The hand's skeleton enables human dexterity through eight carpals in the wrist, five metacarpals in the palm, and fourteen phalanges in the fingers, with each finger having three phalanges and the thumb having two.