Muscle Contraction

Tomorrow I have to take my final for Biology 201-Anatomy and Physiology. I'm very nervous as I am tetering on a passing grade. However, I've made it this far and I'm pretty proud of myself. Here is one of my essay's from this semester that I managed to get 100% on where I discussed how a muscle contraction occurs...


Muscle contraction is a complicated series of reactions that begins with a nerve impulse also known as an action potential. An impulse travels from the brain and spinal cord to a motor neuron that interacts with the sarcolemma (plasma membrane) of a muscle fiber. Located at the end of the motor neuron is the axon terminal and the point at which it meets the muscle fiber is called the neuromuscular junction. Within the axon terminal, synaptic vesicles contain the necessary materials to ignite a reaction from the muscle fiber. Synaptic vesicles are membrane bound sacs which contain the neurotransmitter acetylcholine (ACh). Dividing the axon terminal and the sarcolemma is the synaptic cleft which provides a transitional path for acetylcholine to the muscle fiber. When the action potential reaches the axon terminal ACh is released into the synaptic cleft. At the surface of the sarcolemma are folded junctions called the motor end plate which receives ACh at receptor sites. Once ACh binds to the receptor site it triggers the opening of chemically gated ion channels which allows the passage of sodium ions into the muscle fiber. The entrance of sodium ions results in the action potential sweeping over the sarcolemma of the whole muscle. Depolarization of the sarcolemma occurs and travels along invaginations deep within the muscle fiber called t-tubules. Once voltage sensitive proteins among the t-tubules are stimulated by the current of the action potential it opens calcium ion channels located adjacent to the proteins. Open calcium channels release ions into the cytosol from storage sites of the muscle fiber, the sarcoplasmic reticulum (SR).

The presence of calcium is necessary to initiate movement of the contractile unit, or sarcomere, of each myofibril within a muscle fiber. Myosin and actin filaments responsible for contraction cannot interact without calcium because a blocking agent called tropomyosin prevents interaction. At the end of each tropomyosin rod is a complex called troponin, which binds to calcium as it enters the cell. When calcium and troponin join, troponin changes shape which causes tropomyosin to be released, leaving the binding myosin binding site available. The thick myosin heads are now able to attach and crawl along the thin actin filaments, drawing them towards the center of the sarcomere, this is the contractile movement of the fiber. The attraction of myosin heads to actin is referred to as a cross bridge formation. When the action potential ends calcium ions are returned to the SR via active transport, tropomyosin’s blocking characteristic is reestablished, disabling myosin binding sites and the muscle fiber returns to a relaxed state.

If a blocking agent such as metubine were present at the neuromuscular junction it would prevent acetylcholine from binding to receptor sites. Metubine, would act much like an enzyme inhibitor and compete for the binding site upon the motor end plate of the muscle fiber. If ACh is unable to bind and open the gated ion channel sodium will not be released and the depolarization of the rest of membrane will not occur. Without the wave of action potential contraction cannot take place.

Resources:
"Animations." Muscular Contraction. Thomson-Brooks/Cole, 1 Mar. 2011. Web. 11 Mar. 2011. .
Dugdale,III, M.D., David C. "Lambert-Eaton Syndrome - PubMed Health." PubMed Health. A.D.A.M, 27 Aug. 2010. Web. 12 Mar. 2011. .


How a Muscle Contraction Is Signalled - Animation. YouTube - Broadcast Yourself. 3 Feb. 2009. Web. 11 Mar. 2011. .
Muscular System, Neuromuscular Junction. YouTube - Broadcast Yourself. 8 Dec. 2007. Web. 12 Mar. 2011. .

Analogy

Yesterday, my mind was open. I was pondering and searching for answers to difficult questions. I encountered several events through out the day that seemed to apply directly to what I was feeling.

As I was getting gas a young man came up to me. He couldn't have been more than 27 or 28, but he was dirty looking. He asked for some money so I reached for some cash and a pass-along card. (A pass-along card is a small picture of Jesus or a Holy temple on one side and a phone number to call on the other that provides information about my church). He took it and replied "Oh, you're a Mormon. Me too! So is my family. I severed a mission in Vancouver, Canada." We didn't say much more and he was quickly on his way. Reaching for the cash was easy, but finding the courage to offer the card was much more difficult. It was hard to see that returned missionary dirty and alone. He was obviously in some sort of trouble, my guess was substance abuse. His eyes were red and he was jittery. Even though he was a stranger I understood him. I used to be him. I am married to him. He is some of my best friends. Unfortunately, it is a common story. But now I regret not doing more. I should have asked him questions. I should have offered him more. He is all of us. I should offer all that I have. I should have offered all of my knowledge. I should have shared my pain and my relief. I should have offered the peace and confidence I now enjoy. I should have tried harder.

Later that night I took Saylor (my two year old daughter) to Chick-fil-a to let her play, (it's clean and free...two of mommy's favorite things). The stairs to climb to the slide where off set on either side of a vertical tube. She couldn't coordinate herself to climb from one stair to the other. She kept asking for help, but I couldn't fit into the tiny child space I could instruct her, but she had to do it herself. A few minutes later another child, somebody on her level, came along and offered to help her. He climbed one step ahead of her and then took her by the hand and pulled her up. The little boy did this several times. It came naturally to him and he was happy to help her. He did something that neither I nor Saylor could do. I watched this and thought that he just mirrored so perfectly our responsibility to help others. We are our Heavenly Fathers hands. He wants to help us and he can prompt us, but he will work through others. I was touched by this young child's selfless act and the happiness he brought to my child. I caught a glimpse of how God must feel when we love and serve eachother willingly. I loved watching the little boy "lift" Saylor up when she could go no further. This is example was the answer I was searchiing for.

Unfolding

Climbing out the window, searching for the core.
Walk me through the back yard, I will search no more.
Inhale all the moments, where this flora stands.
Shining on forgiveness, broken in your hands.

Expectations building, a calm before the storm.
Empty packages glitter, turned cold what once was warm.
Dreams can be replenished, hope can be restored.
But I had already unlocked this, I've opened up the door.

Now my mind is racing, fear I'll never know,
Slowly thoughts are fading, darkness starts to show.
Words of other authors, spoke in reverance,
Grant me all security, and maybe another chance.

Still. Numbers on the wall.
Silence around mending, no memory at all.
Hold up the understanding, hold back the stinging debt.
Pressing palms embracing, higher as they met.