WEEK 4
SKELETONS AND MUSCLES

October 4-October 8, 2004

Learning objectives
  1. To compare and analyze the forearm bones of the human, bat, pigeon and cat.
  2. To be able to identify the structural equivalencies of bones in the forearm of the human, bat, pigeon and cat.
  3. To learn how to make and to interpret electromyography (EMG) recordings from your own muscles.
4.To understand the causal connection between motor unit recruitment, the magnitude of the EMG signal, and force generated during a skeletal muscle contraction.5.To learn how antagonistic muscles control the movement of bones around joints.
Discussion questions
  • What is the distinction between a homologous and analogous structure? What types of evidence are useful in judging homology?
  • Are the cat’s paw and the human hand structurally equivalent? Functionally equivalent?
  • What are the functional similarities and structural differences between the wings of the bat and the pigeon?
  • What is the source of the signal detected by the EMG electrodes?
  • What physiological process(es) underlies the observed relationship between EMG response and force output?
  • What does the term “motor unit recruitment” mean?
  • When you are holding a heavy cup of coffee in your hand, would you expect your biceps and triceps muscle to be contracting vigorously? Explain. Would you expect your triceps muscle to be activated vigorously as you lower the same coffee mug down to the surface of a table?

Readings: Campbell et al. 6th edition: pp. 438-440, 495-496, and 1080-1086.
MUSCLES AND SKELETONS
In this week’s recitation and laboratory, you will investigate the vertebrate muscular-skeletal system from two perspectives. First, you will study the forelimb of 4 species of vertebrate (human, bat, cat and bird), and attempt to identify the homologous bones (i.e., structural equivalents). Second, you will examine how skeletal muscles are able to generate graded forces and move skeletal elements around joints.
BEFORE CLASS
IN GROUPS
DURING LAB
ASSIGNMENT
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Prepare for this week’s lab and pre-lab quiz:
attend recitation
read the lab handout and the following pages from your Campbell textbook (6th edition: pp. 438-440, 495-496, 1080-1086)
  • Half of the class will begin with Part I of lab, focusing on the comparative anatomy of vertebrate forelimb skeletons. This group will also complete the worksheet on muscle histology.
  • Half of the class will begin with Part II of lab, focusing on electromyography (EMG).
  • Groups will switch activities halfway through lab.

Your group should turn in the following at the beginning of lab next week:
Comparative anatomy worksheets 1- 3 (drawings)
Comparative anatomy worksheet 4 (questions)
EMG worksheet 5 with data printout
EMG worksheet 6 with data printout
Individually, you will need to turn in your muscle histology worksheet.
similarity

homology analogy

common ancestry convergent evolution

e.g. bee wing/bat wing

different functionssimilar functions

divergent evolutionparallel evolution

e.g. chimp forelimb/bat forelimbe.g. bat forelimb/bird forelimb

Terrestrial vertebrates share many of the same skeletal features, despite looking quite different from one another. For example, the forelimbs of terrestrial vertebrates contain many of the same bones. This observation reflects that all vertebrates evolved from a common ancestor. Accordingly, the arms, forelegs, flippers and wings of different vertebrates are all variations on a common anatomical theme (i.e., they are all homologous, or structural equivalents that were passed on through common ancestry). As the forelimbs of each species assumed different functions (over evolutionary time), the bones were modified. In some cases, these modifications involved changes in the size or shape of specific bones; occasionally, specific bones were fused together or even lost.

In this exercise, you will compare the forelimb bones of 4 vertebrates: humans, cats, bats and birds. Your task will be to identify all the bones in the human forelimb and then to hypothesize which bones are homologous in the cat, bat and pigeon.

One reason it is difficult to judge homology is that sometimes structures have similar function. For example, both birds and bats can fly. It is possible — but not necessarily the case — that bird and bat forelimbs may be similar not because of common ancestry, but because of common function and similar natural selection. In other words, there may have been convergent evolution.

Ideally, you would judge homology by looking at a phylogeny, or family tree, of relationships among these organisms. When phylogeny is not available, one important clue for judging homology would be looking at bone development in each organism. When examining skeletons, however, one of the only tools available for judging homology is the structural equivalency of the bones. You can also use the general principle that during evolution it is more likely for a particular structure (i.e., a bone) to be lost than to evolve several different times. Also, common ancestry is a much more likely explanation for a complex, multi-part structure than convergent evolution.

Your textbook provides additional, excellent discussions of the principle of homology, including the relationship between analogy and homology. Be sure (as always) to read the suggested pages in Campbell before coming to lab.

Comparative Anatomy of Forelimb Procedure

Each lab group should obtain the complete skeleton of all four species. Each worksheet has a drawing of the human arm and hand bones (complete with labels) on the left side and an empty space on the right side. You should sketch the forelimb bones of the cat, bat and pigeon in this empty space (each on separate work sheets). [Note: you will lose points for sloppy sketches.] Then, you should identify the structurally equivalent bones in the human arm and the forelimb of the other species. A fourth worksheet asks you some questions about homology, structural equivalence, and function in the vertebrate forelimb. You should discuss and answer these questions as a group. Your group should hand in all 4 worksheets at the beginning of next week’s laboratory period.

Muscle Function

In the following lessons, you will investigate some properties of skeletal muscle. The human body contains three kinds of muscle tissue and each performs specific tasks to maintain homeostasis: cardiac muscle, smooth muscle, and skeletal muscle. Cardiac muscle is found only in the heart. When it contracts, blood circulates, delivering nutrients to cells and removing wastes. Smooth muscle is located in the walls of hollow organs, such as the intestines, blood vessels, or lungs. Contraction of smooth muscle changes the internal diameter of hollow organs, and is thereby used to regulate the passage of material through the digestive tract, control blood pressure and flow, or regulate airflow during the respiratory cycle.

Skeletal muscle derives its name from the fact that it is usually attached to the skeleton. Contraction of skeletal muscle moves one part of the body with respect to another part, as in flexing the forearm. Contraction of several skeletal muscles in a coordinated manner moves the entire body in its environment, as in walking or swimming.

The primary function of muscle, regardless of the kind, is to convert chemical energy to mechanical work, and in so doing, the muscle shortens, or contracts.

Human skeletal muscle consists of hundreds of individually cylindrically shaped cells (called fibers) bound together by connective tissue. In the body, skeletal muscles are stimulated to contract by somatic motor nerves that carry signals in the form of nerve impulses from the brain or spinal cord to the skeletal muscles. Axons (or nerve fibers) are long cylindrical extensions of the neurons. Axons leave the spinal cord via spinal nerves and the brain via cranial nerves, and are distributed to appropriate skeletal muscles in the form of a peripheral nerve, which is a cable-like collection of individual nerve fibers. Upon reaching the muscle, each nerve fiber branches and innervates several individual muscle fibers.

Although a single motor neuron can innervate several muscle fibers, each muscle fiber is innervated by only one motor neuron. The combination of a single motor neuron and all of the muscle fibers it controls is called a motor unit (see figure 49.38 in Campbell).

When a somatic motor neuron is activated, all of the muscle fibers it innervates respond to the neuron’s impulses by generating their own electrical signals that lead to contraction of the activated muscle.

The size of the motor unit arrangement of a skeletal muscle (e.g., 1:10, 1:50, or 1:3000) is determined by its function (flexion, extension, etc.) and location in the body. The smaller the size of a muscle’s motor units, the greater the number of neurons needed for control of the muscle, and the greater degree of control the brain has over the extent of shortening. For example, muscles that move the fingers have very small motor units to allow for precise control, as when operating a computer keyboard. Muscles that maintain the posture of the spine have very large motor units, since precise control over the extent of shortening is not necessary.

Physiologically, the degree of skeletal muscle contraction is controlled by:

1. Activating a desired number of motor units within the muscle, and

2. Controlling the frequency of motor neuron impulses in each motor unit.

When an increase in the strength of a muscle’s contraction is necessary to perform a task, the brain increases the number of simultaneously active motor units within the muscle. This process is known as motor unit recruitment.

When a motor unit is recruited (or activate), the component muscle fibers generate and conduct their own electrical impulses that ultimately result in contraction of the fibers. Although the electrical impulse generated and conducted by each fiber is very weak (less than 100 microvolts), many fibers conducting simultaneously induce voltage differences in the overlying skin that are large enough to be detected by a pair of surface electrodes. The detection, amplification, and recording of changes in skin voltage produced by underlying skeletal muscle contraction are called electromyography.

Once you have lifted a light object, the brain recruits approximately the same number of motor units to keep the object up, but cycles between different motor units. The muscle fibers consume stored energy available in the muscle and generate a force by contracting. As the muscle fibers deplete this fuel source, more energy must be mobilized in order to continue contracting. By recruiting different motor units, motor units can relax and replenish their fuel sources.

Typical EMG results: the greater magnitude on the right is the result of higher motor unit recruitment.

This part of the lab studies the contractile properties of the tissues that move the skeleton — i.e., the skeletal muscles. You will study the cellular histology of skeletal muscle and compare it to cardiac muscle and smooth muscle tissue.

You will also use a noninvasive technique, electromyography (EMG), to monitor neural activity of muscles. We will use EMG to study how the brain (a) generates graded amounts of force in the muscles, and (b) causes bones to move around a joint.

Before discussing the methodological details of the exercises, we need to review three features of muscle anatomy and physiology. See your textbook (6th edition pp. 1084-1086) for more details.

  • Each muscle consists of thousands of cylindrically shaped cells (called muscle fibers) that are bound together by connective tissue. A muscle fiber contracts when it receives the appropriate amount of electrical stimulation. This electrical stimulation is usually provided by a motor neuron, which carries electrical signals from the brain, through the spinal cord, to each muscle fiber. Upon reaching the muscle, each motor neuron branches and innervates several different muscle fibers. The combination of a single motor neuron and all of the muscle fibers that it innervates is called a motor unit. Each muscle consists of many motor units.
  • It is important to realize that muscle fibers, like neurons, respond to electrical stimulation in an all-or-none manner. That is to say, individual muscle fibers cannot exhibit a graded response—whenever one receives the critical level of electrical stimulation, it contracts maximally. Given this observation, you should be wondering at this point how a whole muscle (e.g., the biceps) can generate graded amounts of force. After all, shouldn’t the muscle contract maximally every time it is stimulated?
  • The resolution to this conundrum is that each muscle consists of many motor units that differ greatly in size. When the brain wants a particular muscle to contract weakly, it causes a limited number of small motor units to contract. When the brain wants a muscle to contract more forcefully, it causes a greater number of motor units to contract. This process, called motor unit recruitment, enables muscle to generate graded amounts of force.
  • When the muscle fibers within a motor unit are stimulated by a motor neuron, they respond by generating their own electrical signal. These signals propagate across the surface of the muscle fiber, and then down into the center of the muscle fiber (via the transverse-tubules), where they activate the contractile machinery. You can record this electrical activity with surface electrodes (i.e., electrodes placed on the surface of the skin) coupled to recording equipment. You should realize, however, that your EMG will record neural activity from several motor units, and in some cases, from several muscles. Thus, the EMG signal should be viewed as the sum of all neural activity occurring in the muscles immediately below the surface electrodes.

EMG ACTIVITY 1

Relationship between the size of the EMG signal and force output

This exercise examines the relationship between the magnitude of the EMG signal from your forearm flexors and the amount of force generated while clenching an object. The forearm flexors are the muscles on the inside of your forearm. Contraction of these muscles causes your fingers to curl and, hence, clench an object in your hand. To release your grip on the same object, you would relax your forearm flexors and contract your forearm extensors (i.e., the muscles located on the outside of your forearm).

Hypothesis generation

At this point, your group should generate a hypothesis about the relationship between the magnitude of the EMG signal and the force output of your forearm flexors (see worksheet 5). Your prediction should be stated graphically—that is, construct a bivariate (x-y) plot with the magnitude of the EMG signal on the Y-axis and the amount of force exerted on the X-axis. Indicate your prediction by drawing a line on this graph.

  • If you predict a direct linear relationship between the two variables, then draw a straight line with a positive slope.
  • If you predict an inverse linear relationship, then draw a straight line with a negative slope.
  • If you predict an asymptotic relationship, then draw such a line.
  • If you predict no relationship, then draw a horizontal line with no slope.

In addition to stating your hypothesis graphically, briefly write out your hypothesis. After you have completed these steps, test your hypothesis by performing an experiment.

Procedure

Electrode placement on dominant forearm

Attach 3 surface electrodes, and their associated leads, to the ventral (inside) surface of the subject’s forearm. To this end, ask the subject to rotate the forearm of her dominant arm so that the palm of her hand is facing up. Then, attach the surface electrodes at the locations indicated in Fig 2. Next, attach the 3 leads (via the pinch connectors) to each of the surface electrodes in the pattern shown in Figure 3.

Equipment set-up

Click on the desktop icon labeled “ECG Activity 1 and EMG Activity 1.” A menu will appear asking you to choose among several lessons. Select the lesson titled L01-EMG-1. If you wish, type in the name of the subject in the box that appears (this is not necessary for the program to work). If someone with the same name has already used the program, click to reuse the name.

Next, a new screen should appear containing an empty window and a button labeled “CALIBRATE” in the upper left corner. Click on this button and follow the instructions carefully. This calibration procedure lasts 8 seconds and is critical for optimum performance. The computer needs to take an EMG reading while you clench your fist (i.e., activate your forearm flexors) maximally to set internal parameters of the program. You should see an EMG signal appear as soon as the subject begins clenching her fist—it should look like a cluster of vertical lines. Once this calibration procedure is completed, your group will be ready to run the experiment.

Experimental procedure

Before beginning the experiment, your subject should hold the dynamometer in her hand and place her forearm flat on the lab bench. Have the subject practice clenching her fist to varying degrees of intensity. She should become proficient at generating 7 different levels of clench intensity. One member of your group should become proficient at reading the hand dynamometer to quantify the amount of force she generates with each clench. After becoming proficient with these skills, your group will be ready to collect data.

Begin by clicking on the RECORD button. At this point, the subject should make the weakest fist clench for approximately 1.0 second. After the clench has been released, click on the SUSPEND button and record the amount of force (in kg) from the dynamometer in the appropriate place on Worksheet 5.