The Muscular System

Parts of The Musculoskeletal System

Muscles! But, what are muscles really made of?

Largest to Smallest:

Muscle >

Fascicle (bundle of muscle fibers / cells) >

Muscle Fiber (muscle cell) >

Myofibril / Mitochondria

Functions of Muscle Tissue

Main Functions:

  • Producing Movements

  • Circulating Blood

  • Producing Heat (ex: shivering)

  • Digesting Food

  • Supporting the body

  • Joint Stability (control proper joint movement and realign correctly after movement)

Types of Muscle Tissue

Skeletal Muscle

  • In short, these are muscles attaching to your bones.

  • Skeletal muscles have black and white stripes called striations.

  • These types of muscles are voluntary. You have to think in order to contract these muscles. For example, in order to contract the skeletal muscles in your hand, you must send a command and willingly make a fist.

Cardiac Muscle

  • Cardiac muscle is found in the heart.

  • This type of muscle is striated (striped).

  • Cardiac muscle is involuntary. An example is your heart. Thinking is not required to make the heart beat.

Smooth Muscle

  • Smooth muscle is found mainly in the walls of hollow organs (ex: the stomach and the intestines), the urinary system, and the digestive system.

  • Smooth muscle does not contains striations.

  • This type of muscle is involuntary. It is not needed to think in order to use our smooth muscles in order to move food through the digestive system.

Muscle Contraction: Labeling a Myofibril

In order to know how a muscle contracts, we have to dive deeper. To be precise, we must take a look at a myofibril; see picture.

The Filaments:

Titin - allows for elasticity

Thick Filament(s):

Myosin - a thick filament with heads

Thin Filament(s):

Actin, Troponin, Tropomyosin


The Bands / Zones:

I Band - consists of only thin filaments

A Band - entire length of thick filament

H Zone - consists of only thick filament

Zone of Overlap - the entire length for which the thick and thin filaments overlap


The Lines:

Z Line: This marks the end of the sarcomere. The Z Line anchors the titin filament.

M Line: This marks the middle of the sarcomere. The M Line helps keep the myosin filament, whose middle is also marked by the M Line, in place.

Muscle Contraction: The Sliding Filament Theory

Essential Background Information:

The sliding filament theory of muscle contraction is a cycle (which means it will be a process which can repeat) which will be described in steps below.

1) Filaments are threadlike objects which are often found in animals and plants. There are 4 main filaments to know about in muscle contraction: myosin, tropomyosin, troponin, and actin. See picture!

2) When our body wants to contract a muscle, we send a "contract" signal through our nerves. Once the signal is sent, calcium is released from the sarcoplasmic reticulum (see bullet point for definition).

  • The sarcoplasmic reticulum is the endoplasmic reticulum of the muscle cell. It can also be defined as the transportation network of a muscle cell which helps transfer nerve signals and calcium.

3) (You might be wondering what calcium has to do with anything, but I will explain.) Calcium is sent in order to attach to troponin. Calcium attaching to troponin causes tropomyosin to expose places on the actin filament; previously, tropomyosin was covering those places.

4) Myosin at this time has ADP, phosphate (an element that is found in bones and teeth and is needed for cells to function), and energy. The energy is stored in the myosin's heads. Myosin's heads attaches itself to the now exposed places on actin. Phosphate is released once myosin's heads attach.

  • This position of myosin being attached to actin is called the cross-bridge.

  • Where did this ADP, phosphate, and energy come from? It came from ATP (an energy molecule) after it (ATP) goes through hydrolysis.

5) Remember, myosin may not have phosphate anymore; however, it still has ADP and energy.

6) The muscle contracting is myosin's heads pulling the actin filament (once it has attached to the specific sites / places on the actin filament). This movement of force is called the power stroke. Each power stroke brings the filaments 10 nanometers closer to the "m line." The myosin uses the energy it had to do a power stroke, so at the end of it, it does not have energy any longer. At the end of the power stroke, ADP is also then released. Myosin heads are still attached to actin at this point!

7) Myosin needs energy to do another power stroke and contract the muscle, but it does not have it. This is where ATP, an energy molecule comes in. ATP attaches to myosin which causes myosin to detach from actin. The ATP then becomes phosphate and ADP. Energy is released.


The cycle starts again (assuming there is enough calcium in order to attach to troponin in order to remove tropomyosin from the binding places of actin so myosin can do its job).