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Anatomy & Physiology (Open + Free)
Sy lla bu s
Unit 5:: Muscular System
Introduction
Module 17 /
Muscle Structures and
Functions
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Muscular Lev els of
Organization
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Microscopic Level—The Sarcomere
Describe how the parallel
organization of a sarcom ere
relates to force generation;
predict what effect changes in
filam ent ov erlap would hav e on
m uscle function.
The fundamental functional unit of muscle is called a sarcomere. One muscle may contain as many as
100,000 of the repeating sarcomere units. In the sarcomere, the myofilaments (thick filaments and thin
filaments) are organized into parallel units. Sarcomeres were first identified by imaging (histology), and the
nomenclature described below reflects their microscopic "appearance."
Molecular Organization of Muscle
Myofilaments are organized structures in muscle cells that contain the actin and myosin. The organized
globular proteins of actin in muscle cells form a thin filament, and bundles of over 200 myosin proteins
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form a thick filament. The thick filament myosin heads “walk” along the actin thin filaments. A single thin
filament is composed of 300-400 globular actins with 40-60 troponin and tropomyosin molecules. A single
thick filament is composed of more than 200 myosin molecules.
Actin filaments are thin, causing the actin "rope" to appear skinny. The myosin filament contains many
myosins bundled up with all of the head groups sticking out, so that it looks “fluffy.” That fluffiness makes it
look thick.
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Sarcomere Anatomy—H Zone, M Line, Z Disc, I Band and A Band
Histological sections of muscle show the anatomical features of the sarcomeres. Thick filaments, composed of
myosin, are visible as the A band of a sarcomere. Thin filaments, composed of actin, attach to a protein in
the Z disc (or Z line) called alpha-actinin, and they are present across the entire length of the I band and a
portion of the A band. The region where thick and thin filaments overlap has a dense appearance, as there is
little space between the filaments. This zone where thin and thick filaments overlap is very important to
muscle contraction, as it is the site where filament movement starts. Thin filaments do not extend completely
into the A bands, leaving a central region of the A band that only contains thick filaments. This central region
of the A band looks slightly lighter than the rest of the A band, and is called the H zone. The middle of the H
zone has a vertical line called the M line, where accessory proteins hold together thick filaments.
Microscopically Visible
Feature
Composition
A band
Region of thick-filament myosin proteins
H zone
Central region of the A band with no overlapping actin proteins when
muscle is relaxed
I band
Region of thin-filament actin proteins, with no myosin
M line
M line accessory proteins in center of myosin thick filament perpendicular
to the sarcomere
Z disc
Zig-zag line of Z line proteins and actin binding proteins perpendicular to
the sarcomere
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