BIOL 4510/KINE 4510
Some Basic Cell Biology ± Morphology of Muscle Cells
There are 3 different muscle types:
smooth muscle skeletal, and cardiac
Although all 3 are capable of contracting, their morphology is quite different.
- VNHOHWDO▯ DQG▯ FDUGLDF▯ PXVFOH▯ DUH▯ FDOOHG▯ VWULDWHG▯ PXVFOH«▯WKH\▯ KDYH▯ D▯ VWULDWHG▯
appearance due to the repeat of highly organized structures called sarcomeres along
the length of these cells. Sarcomeres are made up of sarcoplasmic reticulum and
contractile proteins that are arranged into very organized structures (discussed later)
The morphology of skeletal muscle is more
complex (more organized) than the other 2 muscle
Skeletal muscle has 3 layers of connective tissue
Outer epimysium (epi ± on; mys ± muscle)
Central perimysium (peri ± around)
Collagen fibers and elastic fibers; blood vessels
1 Inner endomysium (endo ± inside)
The collagen fibers of the epimysium, perimysium and
endomysium come together at the end of muscle to form
the tendon or aponeurosis (broad sheet of tendon).
The muscle is divided into bundles (fascicles) which are
surrounded by the perimysium
Each muscle fascicle is divided into muscle fibers.
Each muscle fiber is divided into myofibrils.
Each skeletal muscle contains 100s ± 1000s of
Each myofibril is 0.5-2 µm diameter and can be as long
as the entire length of the muscle (30 cm or more)
Typical Muscle Fiber comprised of:
Myofibrils ± contractile proteins;
Mitochondria ± located on the periphery
for fast twitch fibers and around individual
myofibrils for slow twitch fibers
Sarcolemma ± plasma membrane
The sarcolemma forms specialized
invaginations called transverse tubules or
Sarcoplasm - cytoplasm
Sarcoplasmic reticulum (SR)
is an internal organelle system
comparable to the endoplasmic reticulum.
The SR comes in close apposition with the t-tubules
(terminal cisternae) forming dyads or triad.
2 Sarcoplasmic Reticulum
The main function of the SR is to store Ca
required for muscle contraction.
It is in close proximity to the t-tubule system
because of the role of sarcolemma and SR ion
channels involved in muscle contraction:
The myofibrils are comprised of myofilaments (contractile proteins), primarily:
Myosin - thick filaments
Actin ± thin filaments
Actin and myosin form a unique repeated pattern
known as the sarcomere.
Each myofibril contains ~10,000 sarcomeres end
Each sarcomere is ~ 1.6 ± 2.6 µm. Sarcomere
length varies with the length of the skeletal muscle
or the volume of the ventricle. The relevance of
sarcomere length on muscle function will be
discussed in a later lecture. But for now, it is worth
mentioning the ³force-sarcomere length relationship
40 C ardiac
Tension (% Max) (Starling)
1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8
Sarcomere Length (P m)
3 In electron micrographs, the sarcomere appears as
a dark band (A band; anisotropic) and a light band
(I band; isotropic).
The A band is comprised of mainly the thick
filaments (myosin), whereas the I band is made up
of the thin filaments (actin) and titin.
Titin keeps the myosin and actin aligned. Largest
protein in the body
M line (middle) ± center portion of each thick
filament attached to its neighbor contains many
proteins that hold thick filaments together in
H zone (Heller ± bright) ± contains thick filaments
but no thin filaments.
Z disc (Zwischen ± between) contains many
proteins and lots of actinin, which connects the thin
filaments and titin to adjacent sarcomeres or to the
extracellular matrix via integrins/adherins/dystrophin.
Each sarcomere lies between 2 Z discs.
Each sarcomere is encircled by the t-tubules.
The triad lies at the border of the
A band and I band (where the thick and thin
From the cross section of the myofibril at different locations of the sarcomere, one can
observe the coordinated pattern of the thick and thin filaments.
This patterning is important in force generation, which is accomplished by the
4 Due to the cross-striped pattern observed in the skeletal muscle formed by the
myofilaments, skeletal muscle is referred to as striated muscle. This striated pattern is
observed in both light and electron micrographs.
Light micrograph Electron micrograph
Skeletal muscle is multinucleated, and have their nuclei located on the periphery.
Skeletal muscle develops from mono-nucleated embryonic myoblast cells which fuse to
form myotubes and eventually muscle.
Some myoblast cells do not fuse, and form satellite cells. These cells can fuse later and
differentiate with existing skeletal muscle. They are involved in normal muscle growth
and regeneration of damaged muscle.
5 Skeletal Muscle: Fast, Intermediate and Slow Twitch
Skeletal muscle is classified into fast, intermediate or slow twitch muscle, based on their
speed of contraction.
This is primarily dependent on the type of myosin ATPase expressed in these
muscle types. Myosin ATPase hydrolyzes ATP required for force generation.
There are a number of biochemical and morphological differences between these 3