Myofilaments, Sarcomeres & Organization

Vocabulary practice flashcards covering muscle tissue types, fiber structure, sarcomere components, sliding-filament mechanism, electrophysiology, crossbridge cycle, energy pathways, and whole muscle mechanics.

Thick Filaments

Myofilaments composed of bundles of the contractile protein myosin, each featuring intertwining polypeptide tails and two globular heads that bind actin.

Thin Filaments

Myofilaments composed of three proteins: the contractile protein actin and the regulatory proteins tropomyosin and troponin.

Actin

The primary contractile protein of thin filaments, consisting of subunits that each possess an active site capable of binding a myosin head.

Tropomyosin

A long, rope-like regulatory protein that twists around actin to cover up its active sites in a resting muscle fiber.

Troponin

A small globular regulatory protein that holds tropomyosin in place over actin active sites and binds calcium to initiate contraction.

Elastic Filaments (Titin)

Structural myofilaments composed of the massive, spring-like protein titin that runs through the core of thick filaments to hold them in place, stabilize myofibrils, and confer elasticity.

Sarcomere

The functional unit of contraction where muscle tension is produced, extending from one Z-disc to the next Z-disc.

A Band

The dark band of a sarcomere spanning the entire length of the thick filaments, containing both thick and thin filaments in the zone of overlap.

I Band

The light band of a sarcomere that contains only thin filaments and is bisected by the Z-disc.

H Zone

The central portion of the A band that contains exclusively thick filaments.

M Line

The structural line in the center of the A band consisting of proteins that hold thick filaments in place and anchor elastic filaments.

Z-disc

A zigzag protein line in the middle of each I band that anchors thin and elastic filaments and attaches adjacent myofibrils to one another.

Sliding-Filament Mechanism

The mechanism of muscle contraction whereby thin filaments slide past thick filaments toward the M line without either filament changing length, shortening the sarcomere.

Fascicle

A distinct bundle of individual muscle fibers enclosed by a connective tissue layer called perimysium.

Perimysium

A connective tissue sheath that surrounds and bundles muscle fibers together into a fascicle.

Epimysium

A dense connective tissue sheath that encloses the whole muscle, surrounding all fascicles.

Muscle Fascia

The most superficial layer of thick connective tissue continuous with the epimysium that anchors skeletal muscles to surrounding structures and binds muscle groups together.