Major Lab: A Movable Limb

These are the materials that were used in completing this lab. Not shown are an egg, another type of noodle, some permanent markers, the stapler, and the red exercise shirt that was cut up.



Introduction: This lab is introducing one to the basics of movement. Below you will see two pictures depicting the elbow joint.



Using paper towel rolls, a stapler, pipe cleaners, and a red stretchy shirt, I made a working model of a hinge joint, the elbow joint. This picture is the elbow extended out. As you can see, on the back of the tube is the triceps brachii. In this picture, it is contracted and the tendon is connected at the point of insertion, which is at the ulna. The fabric is loose to show that it is not stretched. On the top is the biceps brachii. This is muscle is relaxed. It is stretched out and the muscle is long. The tendons connect the muscle to the radius.


This picture is of the elbow joint bent. The muscles are working in pairs, as described in an early section. The tricep brachii now is relaxed and stretched out. The bicep is contracted, meaning the muscles have shortened. Under the skin, this is what it looks like, for a clearer picture.



Now that we have the basics, let's jump into one of the muscle cells and look at how actin-myosin fibers make the muscle contract. Remember, the WHOLE muscle contracts, we are just looking at one cell.


First an explanation of the different parts. The orange "zig-zag" on the side is called the Z line. It holds onto the actin filaments and will move in accordance. The pink strips are actin filaments. The blue lines are myosin filaments. The little blue lines coming off of the myosin are the cross-bridges which are vital in muscle movement. This picture is the muscle fiber relaxing. It needs to contract.


What happens are calcium (white puffs) come and attach to troponin on the actin filaments. It then pulls away the tropomyosin to expose the myosin binding sites on the actin.


After that, the cross-bridges perform their "oring" where they reach out, attach to the myosin binding sites, and row. It pulls the actin in and the fibers shorten.

For all of this to happen, a neuron needs to be going to the muscle cell to trigger a contraction.


Here is a picture of a neuron I made out of an egg, spaghetti, and another type of noodle. The egg is represented of a cell body that contains dendrites. The spaghetti is representing the axon and the other noodle, that was colored, is representative of the myelin sheath.



Here is a close up of the cell body.

A close up of the axon with Schwann cells.

What happens here is the transmission of messages. At the top of the neuron would be sensory receptors and the other end would have an axon terminal. The message would travel from the sensory receptors to the axon terminal via the axon. First, there is the action potential.



In this picture, we are inside and right outside of the axon. The blue papers are representative of sodium ions, the pink are potassium ions. In the resting stage, you will see the corresponding blue gate at the top of the axon. It is closed and there is more sodium on the inside than the outside, resulting in a -70mV. When the threshold of a transmitter has been fired, the sodium gates open.



What happens then is the sodium ions go out and potassium ions come in. This results in having a +40 mV. This is the action potential, where messages are going to their effectors. It then returns to the normal resting state.

During this process, is something called propagation. When an action potential is occurring, the message races down the axon. When it is myelinated, it is covered in sections with Schwann cells. The ions cannot pass through this, so they "jump" it. This makes the message move much faster and it does not expend as much energy. The picture below show this.




Conclusion: That concludes how a muscle works. We took a look at the outside, how and where the muscle is attached, which muscle is contracting while the other is relaxing. We then jumped into the muscle cell to see how it actually does contract by way of actin-myosin filaments. Lastly, we took a look at how the message gets sent so the actin-myosin filaments know they need to get moving. I hope you enjoyed!