Skeletal Twitch Lab Report

LAB EXERCISE 2
THE PHYSIOLOGY OF SKELETAL MUSCLE
(Martini p. 306-316)

Work in groups of 4

Objectives:
Name and describe the phases of a muscle twitch.
Explain why the "all or none" law is reflected in the activity of a single muscle cell but not in an intact skeletal muscle.
Understand that the graded response of skeletal muscle is a function of the number of muscle fibers stimulated and the frequency of the stimulus.
Define and explain the physiological basis of the following: - subminimal, minimal, maximal, supramaximal stimuli; - latent period; - wave summation; - tetanus; - muscle fatigue.
Explain the effect of load on skeletal muscle.

SAFETY:
Students handling the frog must wear gloves.
At the end of the lab:
- discard the frogs
Keep your recording. You will have to include it (or a photocopy of it) in your lab report. Label the chart clearly, and record chart speed and direction, as well as the intensity of the stimuli applied to the muscle. Indicate on the chart recording the subminimal, minimal, maximal and supramaximal stimuli.

Table 1: Values of the minimal and maximal stimuli (in volts)
Minimal stimulus
Maximal stimulus

III. ISOLATED MUSCLE CONTRACTION (= MUSCLE TWITCH)

The response of a muscle to a single brief threshold stimulus is called a muscle twitch. There are three phases in a muscle twitch:
1) the latent period: it follows the stimulation, the excitation-contraction coupling is occurring and no response is seen on the myogram;
2) the period of contraction; and
3) the period of relaxation.

A twitch may be strong or weak depending on the number of motor units activated. A twitch may be fast or slow depending of which muscle is stimulated: for example the extraocular muscle has a fast twitch, but the soleus has a slow twitch.

EXERCISE B.
1. Set stimulator mode switch to single shot, and duration control to 1.2 msec.

2. Set chart recorder speed to 50 mm/sec and make sure that the pen touches the paper.

3. Set the voltage dial of the stimulator above the maximal stimulus found in exercise A and deliver one single stimulus to the muscle by pressing the pulse
A. GRADED RESPONSE OF MUSCLE (Exercise A)
- Include a fully labeled recording of the graded response of your skeletal muscle.
Label subthreshold, threshold (minimal), maximal, and supramaximal stimuli
Label pen direction, stimuli voltages and chart speed
- In no more than five lines describe your results. Clearly explain what about the tracing enables you to label the above stimuli.

B. ISOLATED MUSCLE CONTRACTION (= MUSCLE TWITCH) (Exercise B)
- Include a fully labeled recording of the graded response of your skeletal muscle
Label the latent period, the period of contraction, the period of relaxation
Label the stimulus voltage, pen direction, chart speed
- Create a table to show the duration of the latent period, contraction period and relaxation period (in seconds).

C. WAVE SUMMATION, TETANUS and FATIGUE (Exercise C)
- Include a fully labeled recording showing an example each of individual twitches, wave summation, incomplete tetanus, tetanus and fatigue of your skeletal muscle
Label the stimulus frequency, stimulus voltage, pen direction, chart speed
- In five lines describe your results. Clearly explain what about the tracing enables you to identify each. If you are missing an example tracing, explain what you would have expected to