Biology

Experiment 2: The Neuromuscular Junction

Each skeletal muscle is connected to the nervous system be motor end units. When the neurotransmitter acetylcholine is release at this site, the muscle fiber depolarizes. Calcium ions are released from stores in the sarcoplasmic reticulum. The presence of Ca2+ triggers the ratcheting of actin and myosin filaments and the contraction of the myofiber as all of the myofibrils contract simultaneously.

Materials Neuromuscular Junction Digital Slide Images Neuromuscular Junction (Longitudinal Section) Digital Slide Images

Procedure

1. Examine the digital slide images of the neuromuscular junction.

Neuromuscular junction 100X.

Neuromuscular Junction (Longitudinal Section) 1000X.

Post-Lab Questions

1. Identify the axon, terminal branches, and muscle fibers in the slide image below. If possible, trace the fiber to its terminus. 

2. Are there few or many nuclei at the end plate?

3. What is a motor unit?

4. How is a greater force generated (in terms or motor unit recruitment)?

5. What types of sensors are present within the muscle to identify how much force is generated?

Experiment 3: Muscle Fatigue

Muscle contractions are essential for muscles to function properly. The inability of a muscle to maintain tension is muscle fatigue. Failure to contract may occur because of the accumulation of lactic acid, a lack of ATP, or decreased blood flow. In this exercise, you will investigate the correlation between repeated movements and muscle fatigue.

Materials Rubber Band

Stopwatch

Note: If you suffer from a medical condition that does not permit you to perform this activity, please ask a partner to volunteer for you.

Procedure

1. Hypothesize how many times you can stretch a rubber band between your thumb and pinky finger in 20 seconds. Record your predictions in the table below.

2. Using your dominant hand, count the number of times you can completely stretch a rubber band between the thumb and pinky finger in 20 seconds. Be sure to stretch the rubber band as far as possible each time and do not take a break in between trials.

3. Record your count for each trial in Table 1.

Table 1: Experimental Counts
	Trial 1	Trial 2	Trial 3	Trial 4	Trial 5
Predicted Value
Actual Value

Post-Lab Questions

1. How did the predicted results compare to the actual results?

2. Did you notice any changes in the number of repetitions you could perform, or how your hand felt after each of the trials?

3. Explain the actions that were occurring at the cellular level to produce this movement. Include sources of energy and any possible effect of muscle fatigue.

4. Hypothesize what would happen if blood flow was restricted to the hand when this experiment is performed.

Experiment 4: Gross Anatomy of the Muscular System

Muscle actions are often described as a departure of the from the anatomical position of the body. In performing the next exercise, you will understand how muscles act to affect motion.

Materials *Participant (can be yourself) *Heavy Object (approximately 5 pounds)

*You must provide

Procedure

1. Begin by examining muscles found in the upper limbs. First, extend your forearm so you have a clear view of your hand. What muscle is required to perform this extension? Extend your fingers out so they are straight and splayed apart. Then, retract your fingers into a tight fist. Repeat this motion several times, observing the wrist and hand muscles as the flex and relax. What muscles are used to complete this action? Record your observations in Table 2.

Note: It is helpful to palpate the area being flexed to better identify which muscles are being used.

2. Moving up the limb, extend your forearm out until it is parallel to the ground. Have a partner press down on your forearm. Flex your forearm to provide resistance as your partner pushes down. Observe the forearm and identify which muscles are being used. Record your observations in Table 2.

3. The partner should stop pushing down on your forearm, but keep it extended. Curl the forearm upward, creating a bend at the elbow. Observe which muscles are being used to complete this action, and record your observations in Table 2.

4. Find a heavy object, and pick it up. Keeping your arm straight, raise the object out to the side until it is parallel to the ground. What muscles does this require? Continue holding the object, and extend your arm back behind you. What muscles does this activate? Record your observations in Table 2.

Note: Be sure the heavy object is not too heavy before you lift it. This item should be approximately 5 – 10 pounds. This object should never be raised above your head!

5. Move down to the lower limbs, and determine what motions are needed to view the lower limb muscles in action. For example, you may want to walk, jump, sit, point your toes, etc. Engage at leave seven different muscles and indicate what motion was used to engage each muscle in Table 2.

Table 2: Gross Anatomy Data
Movement	Muscle(s) Activated	Action(s) of Muscle(s)
1. Forearm Extended (Step 1)
2. Fingers Extended and  Splayed (Step 1)
3. Fingers Retracted (Step 1)
4.  Forearm Pressed Down Upon (Step 2)
5. Elbow Bent (Step 3)
6. Arm Raised to Side with Heavy Object (Step 4)
7. Arm Extended Back with Heavy Object (Step 4)
8. (lower limbs; student selects action…)
9. (lower limbs; student selects action…)
10. (lower limbs; student selects action…)
11. (lower limbs; student selects action…)
12. (lower limbs; student selects action…)
13. (lower limbs; student selects action…)
14. (lower limbs; student selects action…)

Post-Lab Questions

1. Label the human muscle diagram.

2. Which muscle(s) were used to extend your arm backwards?

3. Which muscle(s) were used to extend and splay your fingers outward?

Experiment 5: ATP and Muscle Fatigue

Muscles require energy to contract. Energy is released when biomolecules such as sugars and fats are broken down, and is stored in the form of ATP. ATP enables muscle contraction, but can only be stored in relatively small amounts. For this reason, the body must continually metabolize new ATP molecules.

Muscle fatigue occurs if the local ATP reservoir for a muscle becomes depleted. This is a common result of strenuous exercise in which ATP is consumed at a faster rate than it is produced. At this point, muscles may fail to contract and the intensity of an exercise must decrease. In this experiment, you will test how long it takes your muscles to fatigue.

Materials Stopwatch *Participant *Sturdy Wall to Stand Against

*You must provide

Procedure

1. To begin, find a wall that is strong enough for you to push against. A temporary wall (such as a partition panel) is not suitable.

2. Find the stopwatch and adjust the settings so it is ready to operate.

3. Stand with your back pressed up against the wall, and lower yourself into a “wall-sit”. To do this:

a. Align the backs of your heels, hips, and shoulders with the wall.

b. Keeping your back pressed against the wall, take a few small steps forward (your upper half will lower as you walk your feet out).

c. Lower yourself into a sitting position, keeping your back flat against the wall, until your knees form a 90 degree angle.

d. Steady this position by focusing the majority of your weight in your heels. Do not allow your lower back to pull away from the wall.

4. Start the stopwatch and time how long you are able to hold the wall-sit position. The amount of time will vary, but will likely fall within approximately 30 – 120 seconds. When you are tired, check the time on the stopwatch, and move out of the position by slowly lowering your body down to the floor or standing up.

5. Record how long you were able to hold the wall-sit in Table 3.

6. Allow your muscles to rest for approximately two minutes, reset the stopwatch, and repeat Steps 3 – 5.

7. Again, allow your muscles to rest for approximately two minutes, reset the stopwatch, and repeat Steps 3 – 5. You should have three trials of data.

Table 3: Muscle Fatigue Data
Trial	Time (seconds)
Trial 1
Trial 2
Trial 3

 

Post-Lab Questions

1. What happened to the time intervals between Trial 1 and Trial 3? What caused this change?

2. Identify three muscles which were engaged during the wall-sit.

3. Explain the biochemical reasoning behind muscle fatigue.

 

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