How
Muscles Work
by Craig C. Freudenrich, Ph.D.
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.
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.
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:
1. You grab the rope
with both hands, arms extended.
2. You loosen your
grip with one hand, let's say the left hand, and maintain your grip with the
right.
3. With your right
hand holding the rope, you change your right arm's shape to shorten its reach
and pull the rope toward you.
4. You grab the rope
with your extended left hand and release your right hand's grip.
5. 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.
6. 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:
1. 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).
2. Initially, the
crossbridge is extended (your arm extending) with adenosine
diphosphate (ADP) and inorganic phosphate (Pi) attached to the myosin.
3. 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.
4. 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).
5. 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).
6. 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:
1. 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.
2. The
neurotransmitter crosses the gap, binds to a protein (receptor) on the muscle-cell
membrane and causes an action potential in the muscle cell.
3. The action
potential rapidly spreads along the muscle cell and enters the cell through the
T-tubule.
4. The action
potential opens gates in the muscle's calcium store (sarcoplasmic reticulum).
5. Calcium ions flow
into the cytoplasm, which is where the
actin and myosin filaments are.
6. 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.
7. Upon binding
calcium ions, troponin changes shape and slides tropomyosin out of the groove,
exposing the actin-myosin binding sites.
8. Myosin interacts
with actin by cycling crossbridges, as described previously. The muscle thereby
makes force, and shortens.
9. After the action
potential has passed, the calcium gates close, and calcium pumps located on the
sarcoplasmic reticulum remove calcium from the cytoplasm.
10. As the calcium gets pumped
back into the sarcoplasmic reticulum, calcium ions come off the troponin.
11. The troponin returns to its
normal shape and allows tropomyosin to cover the actin-myosin binding sites on
the actin filament.
12. 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
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:
1. breaks down creatine
phosphate,
adding the phosphate to ADP to create ATP
2. carries out anaerobic
respiration, by which glucose is broken down to lactic acid and ATP is formed
3. carries out aerobic
respiration, by which glucose, glycogen, fats and amino acids are broken down in the
presence of oxygen to produce ATP (See How Exercise Worksfor details.)
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:
1. Calcium ions come
from outside of the cell.
2. Calcium ions bind
to an enzyme complex on myosin, called
calmodulin-myosin light chain kinase.
3. The enzyme
complex breaks up ATP into ADP and transfers the Pi directly to myosin.
4. This Pi transfer activates
myosin.
5. Myosin forms
crossbridges with actin (as occurs in skeletal muscle).
6. When calcium is
pumped out of the cell, the Pi gets removed from myosin by another enzyme.
7. 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.