How your muscular system works - Emma Bryce

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Each time you take a step, 200 muscles work in unison to lift your foot, propel it forward, and set it down. It’s just one of the many thousands of tasks performed by the muscular system: this network of over 650 muscles covers the body and is the reason we can blink, smile, run, jump, and stand upright. So how does it work? Emma Bryce takes you into the body to find out.

Lesson by Emma Bryce, directed by Viviane Leezer.

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Each time you take a step,

200 muscles work in unison
to lift your foot,

propel it forward,

and set it down.

It's just one of the many thousands
of tasks performed by the muscular system.

This network of over 650 muscles
covers the body

and is the reason we can blink,

smile,

run,

jump,

and stand upright.

It's even responsible for
the heart's dependable thump.

First, what exactly
is the muscular system?

It's made up of three main muscle types:

skeletal muscle, which attaches
via tendons to our bones,

cardiac muscle, which is only
found in the heart,

and smooth muscle, which lines
the blood vessels and certain organs,

like the intestine and uterus.

All three types are
made up of muscle cells,

also known as fibers,
bundled tightly together.

These bundles receive signals from
the nervous system

that contract the fibers, which in turn
generates force and motion.

This produces almost all
the movements we make.

Some of the only parts of the body

whose motions aren't governed
by the muscular system

are sperm cells,

the hair-like cilia in our airways,

and certain white blood cells.

Muscle contraction can be split
into three main types.

The first two, shortening muscle fibers
and lengthening them,

generate opposing forces.

So the biceps will shorten
while the triceps will lengthen or relax,

pulling up the arm and making it
bend at the elbow.

This allows us to, say, pick up a book,

or if the muscle relationship
is reversed, put it down.

This complementary partnership
exists throughout the muscular system.

The third type of contraction
creates a stabilizing force.

In these cases, the muscle fibers
don't change in length,

but instead keep the muscles rigid.

This allows us to grip a mug of coffee
or lean against a wall.

It also maintains our posture
by holding us upright.

Skeletal muscles form the bulk
of the muscular system,

make up about 30-40% of the body's weight,

and generate most of its motion.

Some muscles are familiar to us,
like the pectorals and the biceps,

Others may be less so,
like the buccinator,

a muscle that attaches your cheek
to your teeth,

or the body's tiniest skeletal muscle,

a one-millimeter-long tissue fragment
called the stapedius

that's nestled deep inside the ear.

Wherever they occur, skeletal muscles are
connected to the somatic nervous system,

which gives us almost complete control
over their movements.

This muscle group also contains
two types of muscle fibers

to refine our motions even further,
slow-twitch and fast-twitch.

Fast-twitch fibers react instantly
when triggered

but quickly use up their energy
and tire out.

Slow-twitch fibers, on the other hand,
are endurance cells.

They react and use energy slowly
so they can work for longer periods.

A sprinter will accumulate more
fast-twitch muscles in her legs

through continuous practice,

enabling her to quickly, if briefly,
pick up the pace,

whereas back muscles contain
more slow-twitch muscles

to maintain your posture all day.

Unlike the skeletal muscles, the body's
cardiac and smooth muscles

are managed by
the autonomic nervous system

beyond our direct control.

That makes your heart thump
roughly 3 billion times

over the course of your life,

which supplies the body
with blood and oxygen.

Autonomic control also contracts
and relaxes smooth muscle

in a rhythmic cycle.

That pumps blood through the smooth
internal walls of blood vessels,

enables the intestine to constrict and
push food through the digestive system,

and allows the uterus to contract
when a person is giving birth.

As muscles work, they also use energy
and produce an important byproduct, heat.

In fact, muscle provides
about 85% of your warmth,

which the heart and blood vessels
then spread evenly across the body

via the blood.

Without that, we couldn't maintain
the temperature necessary

for our survival.

The muscular system
may be largely invisible to us,

but it leaves its mark on almost
everything we do,

whether it's the blink of an eye
or a race to the finish line.