Photo courtesy National Library of
Medicine Muscles of the human
body
Muscles are one of those things that most of us take
completely for granted, but they are incredibly important for
two key reasons:
Muscles are the "engine" that your body uses to propel
itself. Although they work differently than a car
engine or an electric
motor, muscles do the same thing -- they turn energy
into motion.
It would be impossible for you to do anything without
your muscles. Absolutely everything that you conceive of
with your brain is
expressed as muscular motion. The only ways for you to
express an idea are with the muscles of your larynx,
mouth and tongue (spoken words), with the muscles of your
fingers (written words or "talking with your hands") or with
the skeletal muscles (body language, dancing, running,
building or fighting, to name a few).
Because
muscles are so crucial to any animal, they are incredibly
sophisticated. They are efficient at turning fuel into motion,
they are long-lasting, they are self-healing and they are able
to grow stronger with practice. They do everything from
allowing you to walk to keeping your blood
flowing!
In this edition of HowStuffWorks,
we will look at the different types of muscles in your body
and the amazing technology that allows them to work so well.
Types of Muscle When most people think of
"muscles," they think about the muscles that we can see. For
example, most of us know about the biceps muscles in our arms.
But there are three unique kinds of muscle in any mammal's
body:
Skeletal muscle is the type of muscle that we can
see and feel. When a body builder works out to increase
muscle mass, skeletal muscle is what is being exercised.
Skeletal muscles attach to the skeleton and come in
pairs -- one muscle to move the bone in one direction
and another to move it back the other way. These muscles
usually contract voluntarily, meaning that you think
about contracting them and your nervous system tells them to
do so. They can do a short, single contraction
(twitch) or a long, sustained contraction
(tetanus).
Smooth muscle is found in your digestive system,
blood
vessels, bladder, airways and, in a female, the uterus.
Smooth muscle has the ability to stretch and
maintain tension for long periods of time. It
contracts involuntarily, meaning that you do not have
to think about contracting it because your nervous system
controls it automatically. For example, your stomach and
intestines do their muscular thing all day long, and, for
the most part, you never know what's going on in there.
Cardiac muscle is found only in your heart, and
its big features are endurance and
consistency. It can stretch in a limited way, like
smooth muscle, and contract with the force of a skeletal
muscle. It is a twitch muscle only and contracts
involuntarily.
In this article, we will focus
on skeletal muscle. The basic molecular processes are
the same in all three types.
Inside a Muscle Cell Skeletal muscle is also
called striated muscle, because when it is viewed under
polarized
light or stained with an indicator, you can see
alternating stripes of light and dark.
Cross section of a skeletal muscle (200x)
showing the muscle fibers (red) and the fat cells
(white)
Skeletal muscle has a complex structure that is essential
to how it contracts. We will tease apart a skeletal muscle,
starting with the largest structures and working our way to
the smaller ones.
Parts of a skeletal-muscle
fiber
Parts of a Skeletal
Muscle The basic action of any muscle is
contraction. For example, when you think about moving
your arm using your biceps muscle, your brain sends a signal
down a nerve cell telling your biceps muscle to contract. The
amount of force that the muscle creates varies -- the muscle
can contract a little or a lot depending on the signal that
the nerve sends. All that any muscle can do is create
contraction force.
A muscle is a bundle of many cells called fibers.
You can think of muscle fibers as long cylinders, and
compared to other cells in your
body, muscle fibers are quite big. They are from about 1 to 40
microns long and 10 to 100 microns in diameter. For
comparison, a strand of hair is about 100 microns in diameter,
and a typical cell in your body is about 10 microns in
diameter.
A muscle fiber contains many myofibrils, which are
cylinders of muscle proteins.
These proteins allow a muscle cell to contract. Myofibrils
contain two types of filaments that run along the long
axis of the fiber, and these filaments are arranged in
hexagonal patterns. There are thick and thin filaments.
Each thick filament is surrounded by six thin filaments.
Thick and thin filaments are attached to another structure
called the Z-disk or Z-line, which runs
perpendicular to the long axis of the fiber (the myofibril
that runs from one Z-line to another is called a
sarcomere). Running vertically down the Z-line is a
small tube called the transverse or T-tubule,
which is actually part of the cell membrane
that extends deep inside the fiber. Inside the fiber,
stretching along the long axis between T-tubules, is a
membrane system called the sarcoplasmic reticulum,
which stores and releases the calcium ions that trigger muscle
contraction.
Contracting a Muscle The thick and thin
filaments do the actual work of a muscle, and the way they do
this is incredibly interesting. Thick filaments are made of a
protein called myosin. At the molecular level, a thick
filament is a shaft of myosin molecules arranged in a
cylinder. Thin filaments are made of another protein called
actin. The thin filaments look like two strands of
pearls twisted around each other.
During contraction, the myosin thick filaments grab on to
the actin thin filaments by forming crossbridges. The
thick filaments pull the thin filaments past them, making the
sarcomere shorter. In a muscle fiber, the signal for
contraction is synchronized over the entire fiber so that all
of the myofibrils that make up the sarcomere shorten
simultaneously.
There are two structures in the grooves of each thin
filament that enable the thin filaments to slide along the
thick ones: a long rod-like protein called tropomyosin
and a shorter bead-like protein complex called
troponin. Troponin and tropomyosin are the molecular
switches that control the interaction of actin and myosin
during contraction.
During contraction, the thin filaments slide past
the thick filaments, shortening the sarcomere.
While the sliding of filaments explains how the muscle
shortens, it does not explain how the muscle creates the force
required for shortening. To understand how this force is
created, let's think about how you pull something up with a
rope:
You grab the rope with both hands, arms extended.
You loosen your grip with one hand, let's say the left
hand, and maintain your grip with the right.
With your right hand holding the rope, you change your
right arm's shape to shorten its reach and pull the rope
toward you.
You grab the rope with your extended left hand and
release your right hand's grip.
You change your left arm's shape to shorten it and pull
the rope, returning your right arm to its original extended
position so it can grab the rope.
You repeat steps two through five, alternating arms,
until you finish.
Muscles create force by cycling myosin
crossbridges.
To understand how muscle creates force, let's apply the
rope example.
Myosin molecules are golf-club shaped. For our example, the
myosin clubhead (along with the crossbridge it forms) is your
arm, and the actin filament is the rope:
During contraction, the myosin molecule forms a chemical
bond with an actin molecule on the thin filament (gripping
the rope). This chemical bond is the crossbridge. For
clarity, only one cross-bridge is shown in the figure above
(focusing on one arm).
As soon as the crossbridge is formed, the myosin head
bends (your arm shortening), thereby creating force and
sliding the actin filament past the myosin (pulling the
rope). This process is called the power stroke.
During the power stroke, myosin releases the ADP and
Pi.
Once ADP and Pi are
released, a molecule of adenosine
triphosphate (ATP) binds to the myosin. When the ATP
binds, the myosin releases the actin molecule (letting go of
the rope).
When the actin is released, the ATP molecule gets split
into ADP and Pi by the
myosin. The energy from the ATP resets the myosin head to
its original position (re-extending your arm).
The process is repeated. The actions of the myosin
molecules are not synchronized -- at any given moment, some
myosins are attaching to the actin filament (gripping the
rope), others are creating force (pulling the rope) and
others are releasing the actin filament (releasing the
rope).
Isotonic vs. Isometric
Contraction
The shortening
of the fibers creates mechanical force, or muscle
tension. Whether the muscle itself changes length
(same-force or isotonic contraction) or
not (same-length or isometric contraction)
depends upon the load attached to the muscle. For
example, your biceps muscle is attached to your shoulder
blade at one end and to your ulna in your forearm at the
other end. When the biceps contracts, it shortens and
pulls the ulna toward the shoulder blade (the ulna is
attached to the elbow joint). This movement allows you
to lift your forearm and a given load. In contrast, if
you are carrying a heavy load, such as a full suitcase,
that makes you unable to lift your forearm, then the
biceps does not shorten significantly. But the force
that the muscle generates is helping you carry the
suitcase.
Triggering Contraction The contractions of
all muscles are triggered by electrical impulses, whether
transmitted by nerve cells, created internally (as with a pacemaker)
or applied externally (as with an electrical-shock stimulus).
The electrical signal sets off a series of events that lead to
crossbridge cycling between myosin and actin, which generates
force. The series of events is slightly different between
skeletal, smooth and cardiac muscle. Let's describe the events
in skeletal muscle first.
The coupling process leading from electrical signal
(excitation) to contraction in skeletal
muscle
Let's take a look at what occurs within a skeletal muscle,
from excitation to contraction to relaxation:
An electrical signal (action potential) travels
down a nerve cell, causing it to release a chemical message
(neurotransmitter) into a small gap between the nerve
cell and muscle cell. This gap is called the synapse.
The neurotransmitter crosses the gap, binds to a protein
(receptor) on the muscle-cell membrane and causes an
action potential in the muscle cell.
The action potential rapidly spreads along the muscle
cell and enters the cell through the T-tubule.
The action potential opens gates in the muscle's calcium
store (sarcoplasmic reticulum).
Calcium ions flow into the cytoplasm,
which is where the actin and myosin filaments are.
Calcium ions bind to troponin-tropomyosin molecules
located in the grooves of the actin filaments. Normally, the
rod-like tropomyosin molecule covers the sites on actin
where myosin can form crossbridges.
Upon binding calcium ions, troponin changes shape and
slides tropomyosin out of the groove, exposing the
actin-myosin binding sites.
Myosin interacts with actin by cycling crossbridges, as
described previously. The muscle thereby makes force, and
shortens.
After the action potential has passed, the calcium gates
close, and calcium pumps located on the sarcoplasmic
reticulum remove calcium from the cytoplasm.
As the calcium gets pumped back into the sarcoplasmic
reticulum, calcium ions come off the troponin.
The troponin returns to its normal shape and allows
tropomyosin to cover the actin-myosin binding sites on the
actin filament.
Because no binding sites are available now, no
crossbridges can form, and the muscle relaxes.
As
you can see, muscle contraction is regulated by the level of
calcium ions in the cytoplasm. In skeletal muscle, calcium
ions work at the level of actin (actin-regulated
contraction). They move the troponin-tropomyosin complex
off the binding sites, allowing actin and myosin to interact.
Energy for Muscle Contraction
Rigor
Mortis
After death, calcium
levels inside the muscle cells rise and the body's level
of ATP drops. Inside the muscles, myosin binds to actin
and the muscles contract. However, with no ATP to reset
the crossbridges and release the myosin, all of the
muscles remain contracted and stiff -- this state is
called rigor
mortis.
Muscles use
energy in the form of ATP. The energy from ATP is used to
reset the myosin crossbridge head and release the actin
filament. To make ATP, the muscle does the following:
breaks down creatine phosphate, adding the
phosphate to ADP to create ATP
carries out anaerobic respiration, by which
glucose is broken down to lactic acid and ATP is formed
Muscles have a mixture of two basic types of fibers: fast
twitch and slow twitch. Fast-twitch fibers are capable
of developing greater forces, contracting faster and have
greater anaerobic capacity. In contrast, slow-twitch
fibers develop force slowly, can maintain contractions
longer and have higher aerobic capacity. Training
can increase muscle mass, probably by changing the size and
number of muscle fibers rather than the types of fibers. Some
athletes also use performance-enhancing
drugs, specifically anabolic steroids, to build muscle,
although this practice is dangerous and is banned in most
athletic competitions.
Other Muscle Cells Compared to skeletal
muscle, smooth-muscle cells are small. They are
spindle-shaped, about 50 to 200 microns long and only 2 to 10
microns in diameter. They have no striations or sarcomeres.
Instead, they have bundles of thin and thick filaments (as
opposed to well-developed bands) that correspond to
myofibrils. In smooth-muscle cells, intermediate
filaments are interlaced through the cell much like the
threads in a pair of "fish-net" stockings. The intermediate
filaments anchor the thin filaments and correspond to the
Z-disks of skeletal muscle. Unlike skeletal-muscle cells,
smooth-muscle cells have no troponin, tropomyosin or organized
sarcoplasmic reticulum.
As in skeletal-muscle cells, contraction in a smooth-muscle
cell involves the forming of crossbridges and thin filaments
sliding past thick filaments. However, because smooth muscle
is not as organized as skeletal muscle, shortening occurs in
all directions. During contraction, the smooth-muscle cell's
intermediate filaments help to draw the cell up, like the
closing a drawstring purse.
Calcium ions regulate contraction in smooth muscle, but
they do it in a slightly different way than in skeletal
muscle:
Calcium ions come from outside of the cell.
Calcium ions bind to an enzyme
complex on myosin, called calmodulin-myosin light chain
kinase.
The enzyme complex breaks up ATP into ADP and transfers
the Pi directly to myosin.
This Pi transfer
activates myosin.
Myosin forms crossbridges with actin (as occurs in
skeletal muscle).
When calcium is pumped out of the cell, the Pi gets removed from myosin by another
enzyme.
The myosin becomes inactive, and the muscle relaxes.
This process is called myosin-regulated
contraction.
The third type of muscle is cardiac muscle.
Cardiac-muscle cells are striated, and are a lot like
skeletal-muscle cells except that in cardiac muscle, the
fibers are interconnected. The sarcoplasmic reticulum of
cardiac-muscle cells is not as well-developed as that of
skeletal-muscle cells. Cardiac-muscle contraction is
actin-regulated, meaning that the calcium ions come from both
the sarcoplasmic reticulum (as in skeletal muscle) and the
outside of the cell (as in smooth muscle). Otherwise, the
chain of events that occurs in cardiac-muscle contraction is
similar to that of skeletal
muscle.
