The Muscular System: Structural and Functional Organization

A set of vocabulary flashcards detailing the anatomical structure, connective tissue coverings, functional classifications, gross anatomy, and lever systems of skeletal muscles.

Sphincters

Internal muscular rings that control the movement of food, blood, and other materials through body openings and passageways.

Antigravity muscles

Muscles, predominantly leg extensors, that maintain posture and prevent the body from falling over.

Glycemic control

The physiological regulation of blood sugar concentration within normal range achieved by muscles absorbing and storing glucose.

Smooth muscle

A class of muscle tissue characterized by nonstriated, involuntary, fusiform (spindle-shaped) cells.

Cardiac muscle

An involuntary, striated muscle tissue composed of branched cells called cardiocytes that contain intercalated discs.

Skeletal muscle

A striated, voluntary muscle tissue consisting of elongated, multinucleated cells known as myofibers.

Fascicle

A bundle of skeletal muscle fibers (myofibers) surrounded by perimysium.

Myofiber

An individual muscle cell.

Myofibril

A cluster of proteins situated inside a muscle cell.

Myofilament

Contractile protein threads inside a myofibril that slide past each other during muscle contraction.

Superficial fascia

The connective tissue layer located between muscles and the overlying skin, also termed the hypodermis or tela subcutanea.

Deep fascia

A dense connective tissue layer situated between neighboring muscles.

Epimysium

The outermost dense irregular connective tissue layer wrapping an entire muscle, continuous with the tendon.

Perimysium

The connective tissue sheath that surrounds each muscle fascicle.

Endomysium

A loose connective tissue sleeve that envelops each individual myofiber (muscle cell).

Parallel muscle

A muscle with a straplike fascicle arrangement running parallel to the long axis, such as the sternocleidomastoid.

Fusiform muscle

A muscle that is spindle-shaped, featuring a thick middle belly and tapered ends, such as the biceps brachii.

Triangular (convergent) muscle

A fan-shaped muscle whose fascicles spread widely at one end and converge onto a single tendon, such as the pectoralis major.

Unipennate muscle

A feather-shaped muscle where fascicles approach and attach to the tendon from only one side, such as the flexor pollicis longus.

Bipennate muscle

A feather-shaped muscle where fascicles approach and attach to a central tendon from both sides, such as the rectus femoris.

Multipennate muscle

A muscle arrangement resembling bunches of feathers converging onto a single tendon, such as the deltoid.

Circular muscle

A ring-like fascicle arrangement that forms sphincters surrounding body openings, such as the orbicularis oris.

Muscle compartment

A group of functionally related muscles enclosed by fascia, which also contains their supplying blood vessels and nerves.

Intermuscular septa

Very thick layers of fascia that physically separate distinct muscle compartments.

Aponeurosis

A broad, flat sheet-like tendon that connects muscle to bone or other tissues, such as the palmar aponeurosis.

Retinaculum

A band of connective tissue under which tendons from separate muscles pass.

Origin

The stationary skeletal muscle attachment site that remains relatively fixed during contraction.

Insertion

The mobile skeletal muscle attachment site on a structure that is pulled closer to the origin during contraction.

Belly

The thicker, fleshy portion of a skeletal muscle situated between its tendons of origin and insertion.

Action

The movement or resistance to movement produced by the contraction of a muscle.

Prime mover (agonist)

The muscle that produces the primary force and is mainly responsible for a specific joint action (e.g., the brachialis during elbow flexion).

Antagonist

A muscle that opposes the prime mover, yielding to it and relaxing to give it control or contracting to prevent excessive movement (e.g., the triceps brachii during elbow flexion).

Synergist

A muscle that aids the prime mover by stabilizing joints or preventing unwanted secondary movements (e.g., the biceps brachii during elbow flexion).

Fixator

A synergist muscle that holds a bone or origin firmly in place (e.g., the rhomboids fixing the scapula during elbow flexion).

Lever

A rigid bar (represented by a bone) that rotates about a fixed point to enhance the speed or power of bodily movement.

Fulcrum

The fixed pivot point around which a lever rotates, represented in the human musculoskeletal system by a joint.

Effort

The force applied to move a load, represented in the musculoskeletal system by muscular contraction.

Resistance

The load or object being moved that opposes the effort in a lever system.

First-class lever

A lever system where the fulcrum lies in the middle between the effort and the resistance ($\text{E-F-R}$), such as the atlanto-occipital joint.

Second-class lever

A lever system where the resistance is located in the middle between the fulcrum and the effort ($\text{F-R-E}$), such as bouncing a baby on one's knee or operating a wheelbarrow.

Third-class lever

A lever system where the effort is applied in the middle between the fulcrum and the resistance ($\text{F-E-R}$), which describes most joints in the human body (e.g., forearm flexion during a biceps curl).