Procedure

1. Review a textbook section on cell membrane potential.

2. Complete Part A of Laboratory Report 26.

3. Obtain a live frog, and pith its brain and spinal cord as described in Procedure C of Laboratory Exercise 19.

ALTERNATIVE PROCEDURE

a n anesthetizing agent, tricaine methane sulfonate, can be used to prepare frogs for this lab. This procedure eliminates the need to pith frogs.

4. Place the pithed frog in a dissecting tray and remove the skin from its hindlimb; beginning at the waist, as described in Procedure C of Laboratory Exercise 19. (As the skin is removed, keep the exposed tissues moist by flooding them with frog Ringer's solution.)

5. Expose the frog's sciatic nerve. To do this, follow these steps:

a. Use a glass rod to separate the gastrocnemius muscle from the adjacent muscles.

b. Locate the calcaneal (Achilles) tendon at the distal end of the gastrocnemius, and cut it with scissors.

c. Place the frog ventral side down, and separate the muscles of the thigh to locate the sciatic nerve. The nerve will look like a silvery white thread passing through the thigh, dorsal to the femur (fig. 26.1).

d. Dissect the nerve to its origin in the spinal cord.

Figure 26.1 The sciatic nerve appears as a silvery white thread between the muscles of the thigh.

Figure 26.1 The sciatic nerve appears as a silvery white thread between the muscles of the thigh.

Human Cutting Thigh

e. Use scissors to cut the nerve at its origin, and carefully snip off all of the branch nerves in the thigh, leaving only its connection to the gastrocnemius muscle.

f. Use a scalpel to free the proximal end of the gastrocnemius.

g. Carefully remove the nerve and attached muscle, and transfer the preparation to a glass plate supported on the ring of a ring stand.

h. Use a glass rod to position the preparation so that the sciatic nerve is hanging over the edge of the glass plate. (Be sure to keep the preparation moistened with frog Ringer's solution at all times.)

6. Determine the threshold voltage and the voltage needed for maximal muscle contraction by using the electronic stimulator, as described in Procedure D of Laboratory Exercise 19.

7. Expose the cut end of the sciatic nerve to each of the following conditions, and observe the response of the gastrocnemius muscle. Add frog Ringer's solution after each of the experiments.

a. Firmly pinch the end of the nerve between two glass microscope slides or pinch using forceps.

b. Touch the cut end with a glass rod that is at room temperature.

c. Touch the cut end with a glass rod that has been cooled in ice water for 5 minutes.

d. Touch the cut end with a glass rod that has been heated in the flame of a Bunsen burner. Wear heat-resistant gloves for this procedure.

8. Complete Part B of the laboratory report.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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