Lab 3 – skeletal muscle Physiology
Introduction
Skeletal muscles are composed of hundreds to thousands of individual cells, each doing their share of work in the production of force. As their name suggests, skeletal muscles move the skeleton. Skeletal muscles are remarkable machines; while allowing us the manual dexterity to create magnificent works of art, they are also capable of generating the brute force needed to lift a 100-lb. sack of concrete. When a skeletal muscle from an experimental animal is electrically stimulated, it behaves in the same way as a stimulated muscle in the intact body, that is, in vivo. Hence, such an experiment gives us valuable insight into muscle behavior.
The Motor Unit and Muscle Contraction
A motor unit consists of a motor neuron and all of the muscle fibers it innervates. Motor neurons direct muscles when and when not to contract. A motor neuron and a muscle cell intersect at what is called the neuromuscular junction. Specifically, the neuromuscular junction is where the axon terminal of the neuron meets a specialized region of the muscle cell’s plasma membrane. This specialized region is called the motor end-plate. An action potential (depolarization) in a motor neuron triggers the release of acetylcholine, which diffuses into the muscle plasma membrane (also known as the sarcolemma).
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The acetylcholine binds to receptors on the muscle cell, initiating a change in ion permeability that results in depolarization of the muscle plasma membrane, called an end-plate potential.
The end-plate potential, in turn, triggers a series of events that results in the contraction of a muscle cell. This entire process is called excitation-contraction coupling. We will be simulating this process in the following activities, only instead of using acetylcholine to trigger action potentials, we will be using electrical shocks. The shocks will be administered by an electrical stimulator that can be set for the precise voltage, frequency, and duration of shock desired. When applied to a muscle that has been surgically removed from an animal, a single electrical stimulus will result in a muscle twitch – the mechanical response to a single action potential. A twitch has three phases: the latent period, which is the period of time that elapses between the generation of an action potential in a muscle cell and the start of muscle contraction; the contraction phase, which starts at the end of the latent period and ends when muscle tension peaks; and the relaxation phase, which is the period of time from peak tension until the end of the muscle contraction (Figure 2.1).
Figure 2.1 The muscle twitch: Myogram of an isometric muscle contraction
Objectives
At the end of this lab exercise, students should be able to: 1. Define the following terms: motor unit, latent period, threshold, summation, fatigue, isometric contraction, isotonic contraction and tetanus. 2. Understand how nerve impulses trigger muscle movement.
3. Describe the phases of a muscle twitch.
4. Understand the effect of an increase in stimulus intensity and on a muscle. 5. Understand muscle fatigue.
6. Explain the differences between isometric and isotonic muscle contraction.
Procedure
Single stimulus
From the drop-down menu, select Exercise 2: Skeletal Muscle Physiology and click GO. Then click Single Stimulus. You will see the opening screen for the Single Stimulus activity. On the left side of the screen is a muscle suspended in a metal holder that is designed to measure any force produced by the muscle. To the right of the metal holder are three pieces of equipment. The top piece of equipment is an oscilloscope screen. When you apply an electrical stimulus to the muscle, the muscle’s reaction will be graphically displayed on this screen. Elapsed time, in milliseconds, is measured along the X axis of this screen, while any force generated by the muscle is measured along the Y axis. In the lower right hand corner of the oscilloscope is a Clear Tracings button; clicking the button will remove any tracings from the screen. Beneath the oscilloscope screen is the electrical stimulator you will use to stimulate the muscle. Note the electrode from the stimulator that rests on the muscle. Next to the Voltage display on the left side of the stimulator are (+) and (-) buttons, which you may click to set the desired voltage.
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When you click on the Stimulate button, you will electrically stimulate the muscle at the set voltage. In the middle of the stimulator are display fields for active force, passive force, and total force. Muscle contraction produces active force. Passive force is generated from the muscle being stretched. The sum of active force and passive force is the total force. Also notice a Measure button on the stimulator. Clicking this button after administering a stimulus will cause a yellow vertical line to appear. Clicking the (+) or (-) buttons under Time (msec) will then allow you to move the yellow line along the X axis and view the active, passive, or total force generated at a specific point in time. Beneath the stimulator is the data collection box. Clicking on Record Data after an experimental run will allow you to record the data in this box. To delete a line of data, click on the data to highlight it and then click Delete Line. You may also delete the entire table by clicking Clear Table.
Activity 1 – Identifying the Latent Period
Recall that the latent period is the period of time that elapses between the generation of an action potential in a muscle cell and the start of muscle contraction. 1. Set the Voltage to 6.0 volts by clicking the (+) button on the stimulator until the voltage display reads 6.0. 2. Click Stimulate and observe the tracing that results. Notice that the trace starts at the left side of the screen and stays flat for a short period of time. Remember that the X axis displays elapsed time. 3. Click on the Measure button on the stimulator. Note that a thin, vertical yellow line appears at the far left side of the oscilloscope screen. 4. Click on the (>) button underneath Time (msec).
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You will see the vertical yellow line start to move across the screen. Watch what happens in the Time (msec) display as the line moves across the screen. Keep clicking the (>) button until the yellow line reaches the point in the tracing where the graph stops being a flat line and begins to rise (this is the point at which muscle contraction starts.)
