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CONTRACTION IN ACTION

(This activity is entirely orginal. It is not adapted from any preexisting material.)

By Michael J. Vieira Lazaroff



Type of Entry:

  • lesson/class activity

Type of Activity:

  • hands-on activity
  • simulation
  • authentic assessment
  • group/cooperative learning
  • review/reinforcement

Target Audience:

  • Anatomy and Physiology
  • Advanced/AP Biology
  • Biology

Notes to Teacher:

Depending upon the level of the class, one may opt to concentrate less on the role of the nerves and the role of active transport in the contraction of muscle tissue. For a complete lesson, one must provide the following information to the student in one form or another. (I have provided some of my favorite analogies/demos.):
  1. Muscle contractions are instigated by nerve impulses.
    • Electricity is used to power a razor. The switch itself, which should preferably be at the beginning of the cord, takes the place of the brain choosing to send the message. Have a razor, or other appliance with a cord handy, and ask the students to fill in the analogy.


  2. The place where motor neurons and muscles meet is called the neuromuscular junction (NMJ). Instead of the two touching, there is a space called a synapse across which the message travels.

  3. At the axon terminal (the end of the motor neuron) there are vesicles filled with a neurotransmitter, for example: Acetylcholine (ACh). When the nerve impulse is sent, the vesicle fuses with the cell membrane and releases the ACh by exocytosis.
    • I usually demonstrate this by having four students hold hands in a circle (the vesicle) and one student inside (the ACh). All of these are within a larger circle (the outer cell membrane). The vesicle then fuses with the outer circle, students change to holding hands with members of the outer circle, and the ACh is seen to move outward. A similar technique can be used to demonstrate phagocytosis, and the formation of the food vacuole.


  4. Active transport, in the form of the Na+/K+ pump, is used by the myofibers (muscle cells) to maintain a higher concentration of Na+ outside the cell membrane.
    • It is good to take a large piece of cardboard at an angle, and pour packing material (preferably starch-based, but Styrofoam is OK) down the incline to show the idea of a concentration gradient. If one has a student attempt to keep the material from rolling downhill, that dramatically shows the effort and energy needed to maintain active transport (given the energy needed, ask the class what organelles are likely to be nearby - mitochondria!), as well as the rapidity of the change in concentration when active transport is suddenly stopped.


  5. On the sarcolemma (muscle cell membrane) there are ACh receptors (30 to 40 million at a typical NMJ!) that will open up a channel when ACh attaches, allowing the Na+ to flood into the cell through facilitated diffusion.
    • A comparison of the slow leak of Helium from a balloon versus facilitating the leak by poking a hole in the balloon is useful here.


  6. Inside the myofibers are sarcoplasmic reticula or SR (a muscle cell's version of the endoplasmic reticulum or ER) that store Ca2+ ions by active transport. The change in Na+ concentration due to the ACh causes the SR to release Ca2+ in the same flood-like fashion.
    • Given the amount of energy needed to keep a muscle in a "relaxed" state, I usually liken this state of affairs to a bow and arrow, with the arrow pulled back, a catapult pulled back, or any coiled spring mechanism. The idea is to emphasize the concepts of energy use and the quick response of the muscle's contraction. This also provides a good reinforcement for the ideas of potential and kinetic energy, especially as regards molecules along a concentration gradient.


  7. The contraction of the sarcomere involves the binding of the proteins Actin (thin filament) and Myosin (thick filament). Given that these will bind automatically due to their shapes, there must be a way to prevent contraction when necessary. Contraction is prevented by the protein molecules Troponin and Tropomyosin, which wrap around the Actin molecules, covering the Myosin-binding site on the Actin molecule.
    • I use the idea of a locking gas cap (or a padlock on a locker) preventing the gas pump from being used) (or the locker from being opened).


  8. The Ca2+ ions which were released earlier bind to the Troponin molecules, thus causing them to change shape, and in turn cause the Tropomyosin to change shape, thus exposing the Myosin-binding sites. The Ca2+ ion binding to the Troponin acts as a key to the gas cap (padlock), thus opening up access to the tank (locker).

  9. Heads on the Myosin molecule, appropriately called Myosin heads, then bind with the Actin. This is called a cross bridge and causes the rigidity of the muscles. The binding also changes the shape of the molecules thus moving the thin filament closer to the center of the sarcomere.
    • Rigor mortis after death involves the release of Ca2+ ions from the SR once the active transport stops due to lack of ATP; rigor mortis only stops when the proteins themselves break down.


  10. When ATP is present it is needed to break the bond between the Actin and the Myosin filaments. The Myosin heads contain the enzyme ATPase, which breaks ATP down into ADP and a phosphate, and releases energy. Due to the change in shape, this break forms a power-stroke that moves the thin filament closer to the center and allows the Myosin-binding site to bind to the next Myosin head.
    • I use the analogy of a treadmill, which moves on its own, powered by the motion of the previous footfall.


  11. The repeated breaking and forming of bonds is the contraction of the sarcomere, and it necessitates a great deal of ATP.
    • As such it is good to ask the students not only the organelle needed for this - mitochondria - but what else is needed - O2 and glucose. At this point it is often useful to ask the students to describe the physical changes that occur in the body during exercise (heavy breathing and rapid heart rate), and why they are necessary (getting O2, and distributing it and glucose to the body).


  12. Relaxation is, in essence, a reversal of much of contraction. Given that the ACh starts the whole process, its removal from the synapse is one of the first steps of relaxation. Acetylcholinesterase (AChE) in the synaptic cleft breaks down the ACh, thus closing the ACh receptors and allowing the high concentration of Na+ ions to be reestablished by active transport and remain on the outside of the sarcolemma.

  13. Active transport pumps in the SR then work toward keeping the concentration of Ca2+ ions 10,000 times higher in the SR than in the sarcoplasm (myofiber cytoplasm). This uptake of Ca2+ ions is facilitated by the binding of Ca2+ ions to a protein called calsequestrin in the SR.

  14. The lack of Ca2+ ions in the sarcomere causes the Troponin and Tropomyosin to block the Myosin-binding sites on the Actin, thus preventing the cross bridges from forming.
    • It is often useful here to ask the student to explain why it takes less time for the muscle to contract following the nerve stimulation (latent period = 2 msec) than it does for it to prepare itself for the next contraction (relaxation period = 10 to 100 msec) - facilitated diffusion vs. active transport.


  15. It is important for the students to know that relaxation alone will not return a muscle to its previous length, but that it requires the contraction of an antagonistic muscle (i.e. biceps and triceps).
    • At this point I often have students measure the diameter of a flexed biceps, and then a relaxed biceps with the elbow still bent. I often branch off at this point to isotonic and isometric contractions, refractory period, wave summation, treppe, etc.

Required of students:

The students will be put in groups of 6 to 8, and they are given 20 to 30 minutes to write, design, direct, and block out (determine location and movement) a skit that illustrates the function of a sarcomere, and all the steps involved in its contraction and relaxation. Each student must have a speaking part, with no one person dominating the performance. The performance must be scientifically accurate, and be a predominantly VISUAL representation of a sarcomere. Students are limited to whatever is in the room at the time for their props (coats, books, pens, paper, scissors, tape, etc.). At the end of the period the groups will perform their skits in front of the class.

Preparation time needed:

Given the lack of specific materials, the only time is that needed to familiarize oneself with a working sarcomere. Should one want to include specific props, one would need time to gather them. I prefer to leave props up to the students' own ingenuity.

Class time needed:

Approximately 1 hour for the lesson, and 1/2 to 1 hour for the activity, depending on the number of groups, and the amount of time needed for the groups to design their performances. Performances themselves are usually only 2-5 minutes each, but one may want to repeat them, or ask students to answer questions pertaining to any gaps.


Abstract

The contraction of a sarcomere involves the integrated effort of motor neurons, neurotransmitters, active transport, calcium ions, ATP, and several proteins, that work with great speed. This activity is designed to help the students visualize not only the structure of a sarcomere, but the actual physical action, and the rapidity, of its contraction and relaxation. It is intended as a follow-up to a discussion of the material. The activity, in turn, may be followed up by a student-built three dimensional model.

Students are placed into groups of 6 to 8, and they must divide themselves into separate roles and cooperatively design a skit that shows both the anatomical and physiological nature of a sarcomere contracting and relaxing, as well as the neural stimulation that preceded it. Emphasis is given to the visual, rather than merely the verbal, imparting of information. Ideal performances include equality of roles, drama, the use of props, music (!), and, of course, scientific accuracy.

Should one not actually plan on emphasizing muscle contraction in one's curriculum, this activity is a good way of integrating the concepts of active transport, facilitated diffusion, exocytosis, oxygen and energy usage, etc., into a coherent whole. This is an excellent summative activity for the information is applied in a real life situation.


Background

What questions does this activity help students to answer:

  1. How does the message from motor neurons cause the muscles to contract?
  2. How do the parts of a sarcomere work together to cause a muscle to contract?
  3. How is calcium important to muscle contraction?
  4. How is energy (ATP) used in muscle contractions?


Project

Materials needed:

To help the students with the initial information, and as a reference for use during the activity, I recommend a good textbook description of the sliding filament model that includes a description of the neuromuscular junction, each of the relevant proteins, the binding sites, Calcium ions, and the sarcoplasmic reticulum. A drawing of a contracted and relaxed sarcomere, which is often a part of the textbook description, is also helpful. Anatomy and Physiology texts usually have more detailed information than do general Biology or AP Biology textbooks.

Activity:

Once the actual structure and function of the sarcomere has been taught, (and one should always make sure to get the students out of their seats as much as possible during this - see notes to the teacher), one can actually begin this activity. One of the keys to this activity is the role of the student-as-teacher. This is, in addition to being an opportunity for review and reinforcement, in many ways a bit of a discovery activity for them as well. The act of physically blocking out the proteins and demonstrating their movement reveals a great deal to them about the way the whole process works.

One of the first things to give the students is a list of roles that are needed, with the understanding that the size of the group necessitates some students playing more than one role. These roles include, but are not necessarily limited to: ACh, AChE, ACh receptor, Na+ ions, SR, Ca2+ ions, Myosin and Myosin heads, Actin and Myosin-binding sites, Troponin, Tropomyosin, ATP, Mitochondria. Given the need for the students to come into physical contact, one must put certain rules into place as to what parts of the body are O.K. (e.g. holding hands, feet touching) and what parts are off limits. Such rules are essential to establishing the level of comfort necessary to good cooperative learning.

The students are then divided into groups of 6 to 8, using whatever method you prefer (One of my favorites in a class of 24 is to have the students count off from 1 to 3, and then dividing all the 1's, 2's, etc. into groups of 8). I assign no specific roles, and no one person is chosen to be the leader (that is something that usually develops naturally). The parameters of the performance are given to the students as stated in the section "Required of students." I usually start off giving the students 15 minutes to plan their skit, but they often need more time, and I reward their industry with more preparation time. The goal is always a coherent performance in front of the entire class, with everyone not only preparing a speaking part, but also a visual performance.

It is important to always be accessible to the students during this activity, for the students are bound to have questions. I try to limit my students to factual questions (e.g. Why is the calcium needed?), rather than questions such as "How can I show this?" If the students are merely left alone, some may take the assignment as too overwhelming and retreat from it. One's presence and encouragement can lead the students to great things, yet one must make sure not to take over and make it into the teacher's own skit.

I try to be fairly inconspicuous during their planning stages, but it is necessary from time to time to point out areas that were overlooked ("I noticed that you formed the cross bridges here, but how does this help the sarcomere to shorten?" and following their answer, "How could you change your skit to show that?") This is a marvelous way of gauging the level of the students' understanding and finding areas in need of micro teaching (on the spot elaboration). Whenever a group makes an error in their rehearsals I ask my students to explain what is happening in scientific terms. This is often the point where the students themselves discover their error, and then create their own solution.

It is important to have the students include rehearsals in their planning stages, for that allows them to not only spot gaps in the information, but it also helps promote ideas for improvement. This also helps eliminate the hazard of having one student plan and the others merely follow; this method encourages teamwork and collaboration.

I generally don't provide props because it tends to stifle the student's creativity. The students are instructed to use any means of delivery, as long as they are not insulting or offensive to anyone. Students can use song, dance, high drama, etc. to further their performance.


Method of Assessment/Evaluation

Since each of the groups will perform in front of the class, I not only give a group grade, but I also have the students "vote" to grade the other groups by "secret ballot." I then average the students' grade votes for each group. This grade becomes 25% of the groups' final grade. If a supportive and honest atmosphere exists, the students tend to grade each other quite fairly and honestly. Part of my grade also includes an assessment of their cooperation as a group during the planning stages.

Some of the criteria I use are:

  • scientific accuracy
  • visualization over verbalization
  • brevity of narrative (supportive only)
  • teamwork and equality of planning and roles
  • creativity and enthusiasm

I tend to aim for a holistic score over an actual breakdown into categories, as a point by point breakdown tends to distract me from the subtlety of the performances. Individual teachers may prefer to choose their own grading methods and/or criteria.

In some cases I will assign certain students a slightly higher grade (B+ to A-) if they take a more active role in the construction of the performance, or a slightly lower grade (B+ to B) if they contributed less. One has to be careful about this practice, for it can ultimately defeat the purpose of this as a GROUP activity based on the equality of the roles. On the other hand, I have seen some students shine in this type of activity who often have trouble with science and, as such, they should be rewarded.


Extension/Reinforcement/Additional Ideas

Following this activity I traditionally ask my students to create a three dimensional model of a sarcomere. This model must be movable in order to demonstrate the "sliding filament" model. In order to prevent any students taking advantage of greater access to monetary resources, I always emphasize the word INEXPENSIVE! The students are instructed to limit themselves to simple materials that can be found at home. This limitation actually has the effect of increasing the students' creativity (necessity is the mother of invention!) rather than stifling it. Other important limitations include prohibiting the use of food, with the exception of dry pasta or dry rice (any other food tends to create an ant problem, not to mention spoilage), and prohibiting the use of living things (I'd rather not have things die in the name of expanding our understanding of life). These limitations apply to all models I assign.


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