Download Microscopic Level—The Sarcomere

Microscopic Level—The Sarcomere
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 form a thick filament. The thick filament myosin
heads “walk” along the actin thin filaments. A single thin filament is composed of 300400 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.
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