Cut-the-head-off-the-frog Lab

MATERIALS NEEDED:

• Frogs (Rana pipiens). • Plastic containers to recapture the frogs. • Several aquaria or other suitable containers with enough water to allow

the frogs to swim. • Wood platforms to float within the containers. • Paper towels to cover frogs when not being observed. • 15 % acetic acid solution.

During this exercise you will determine the nervous system location of various reflexes in a frog. In brief, the behavior of a normal frog will be observed and then portions of the nervous system will be sequentially destroyed. If a behavior is still observed in animals with a lesioned system, then the portion that was destroyed does not have the nervous pathways for the reflex. PROCEDURE: 1. Work in groups of three or four and read through the entire procedure before beginning. Obtain a living frog and capture container from your instructor. Collect several paper towels and wet them. Use the paper towels to cover the animal when it is not being observed. Always wet your hands before handling the animal so as not to cause it undue discomfort. When carrying your frog, make sure that its legs are held in an extended position. If you allow it to assume a flexed leg position, it can push against the palm of your hand and will get away. Should the frog escape, do not stop the frog with your foot. Instead, confine it in the capture container provided by your instructor. 2. Frog Reflex Procedure. The frog will serve as it's own control in this experiment. Begin by observing the behaviors (listed below; step 5) in a living frog. When recording the behaviors you may find it helpful to make a few simple sketches. Move slowly and speak quietly when near the animal. Fast movements or loud noises will startle the frog, and you will spend much of your time retrieving escaped animals. In addition, a frightened animal is not as likely to perform all of the behaviors.3. After the frog's normal behavior has been recorded, take the animal to your instructor, and he or she will remove the cerebrum from the animal by cutting directly behind the eyes (Figure 1). If the procedure is quickly done, the animal will experience little or no discomfort, much as when you get a really bad burn or cut (it takes a few moments to experience the pain). Once the cerebral hemispheres are removed, the frog is dead and is termed a decerebrate frog. Although you may find it unsettling to watch a headless frog move about, keep in mind that the animal is dead and therefore feels no pain. In order to feel

FIRST CUT

(DECEREBRATE FROG)

OLFACTORY LOBE

CEREBRUM

OPTIC LOBE

MEDULLA

SPINAL CORD

REMOVAL OF MEDULLA(DEMEDULATED)

REMOVAL OF SPINALCORD (SPINAL FROG)

Figure 1. Procedure to create a decerebrate frog. pain, it must perceive the pain. Centers for perception of pain are located in the cerebrum, which has been removed. Cover the frog with moist towels and allow it to recover for a few minutes and then record the behavior of the decerebrate frog. Expect to see some decrease in the spontaneous movements of the animal (you may have to try for a response several times or prod it). When you have finished recording the behavior of the decerebrate frog, return it to your instructor for destruction of the medulla. The medulla is destroyed by inserting a dissecting needle into the opening created by the cut and then moving it back and forth. Care must be taken that the spinal cord is not damaged. These animals are demedullated frogs and are, of course, still dead. Again, cover the animal with moist towels, wait a few minutes, and re-record the behaviors. Finally, your instructor will destroy the spinal cord by inserting and twisting a dissecting needle down the hole previously occupied by the medulla. The procedure is finished when the hind legs straighten out completely and then go limp. Return to your station, cover the frog for several minutes, and rerecord the behaviors. Dispose of the frog as directed by your instructor. 4. As portions of the central nervous system are removed, behaviors that require those structures will drop out. Thus, if a behavior depends on the presence of the medulla, it will be seen in the normal and decerebrate frog, but will disappear when the medulla is removed. If a behavior is not immediately seen, retest the animal before continuing with the rest of the lesion experiment. Use your data to complete the Results Section table. 5. Behaviors to Record. Cover the animal with moist towels and allow it to adjust for a few minutes. Always moisten your hands before handling the animal.

• Carefully remove the towels and observe the resting position of your animal. (Careful! It may jump when the towels are removed.) How are the front and back legs positioned? Is the body parallel with the surface of the table or at an angle? If an experimental animal is not standing in a normal position, prod it at the front to try and make it stand.

• Surround the animal. One member of the group should gently prod the frog into jumping (this procedure is probably best done on the floor). Record the general form of the jumping behavior. Does the frog pull its legs in while in the air, or are they extended?

• Restrain the animal while one member of the group gently pulls on one of the hind legs to straighten it. When the leg is released, is it returned to the flexed position? This is the extension reflex.

• Turn over the animal and record the presence or absence of a "righting response." How are the front and hind legs used when the animal rights itself? This behavior is fairly rapid, and you'll probably have to try it several times to see the movements of the legs and body.

• Turn your frog over and hold it upside down. Moisten your finger and gently stroke its belly. Male frogs may clasp your finger with their forelegs. This is part of their sexual behavior and is termed the "clasping response." Don't worry; you won't get "frog stuff" on you. Females and males of low sexual tone may only be quieted by the stroking (the "quieting response").

• Take a small piece of paper towel (about 5 mm X 5 mm) and soak it in 15% acetic acid. Place the towel on the body of the frog near the rear of the animal. Depending on the condition of the animal, the frog may attempt to remove the towel with its hind leg. This "scratch reflex" may be seen in a different form when the frog's head is removed. The frog may reach forward toward its missing head with its forelegs. The animal is not searching for its head; the central nervous system is merely showing a scratch response to irritated nerve endings (thus, count this movement as a positive scratch reflex). If the animal does not show a scratch response within 30 seconds, remove the towel and wash the area with a few drops of water. The absence of a scratch reflex in a normal frog simply indicates that the animal is too agitated to show the behavior. In experimental frogs, the absence of a scratch reflex should be considered as being caused by the treatment.

• Take the animal over to a swimming container and drop it in to record the movements used in swimming. If the frog contacts the floating board, it may attempt to climb on the board. Move the board closer to the frog if it does not contact the board after a few minutes. For dead frogs, help them climb by placing the forelegs on the board. Describe the movements involved both in swimming and climbing. Turn the frog over and record the righting response in water.

• Remove the frog from the water and watch the motion of the frog's throat. Frogs force air into their lungs by gulping air and then swallowing it (similar to when you swallow air to force a burp). Count the number of throat motions over a one-minute period to determine the frog's respiration rate. Record for a second minute, average the results, and record this as the rate if respiration. Headless frogs will show less conspicuous throat movements and no movement of their sides.

• Take the frog to your instructor for the next part of the experiment and begin again with the first step if you have not finished recording with a spinal frog.

• When you have completed the spinal frog portion of the experiment, you should open your frog and look for signs of a heartbeat, identify the sex of the animal, and locate parasites (if any). The following illustrations will serve as an aid to the identification of frog structure and are included so that this lab follows Dr. Bill's Conservation of Meat Law.

INTERNAL CAROTID

EXTERNAL CAROTID

CAROTID

CAROTID BODY

SYSTEMIC ARCH

LIVER

STOMACH

HEPATIC ARTERY

GASTRIC ARTERY

MESENTERIC ARTERY

SMALL INTESTINE

RENAL ARTERIES

ILIAC ARTERYKIDNEY

OVARY

OVARIANARTERY

FAT BODY

DORSALAORTA

LUNG

PULMONARY

CUTANEOUS

SUBCLAVIAN

LINGUAL ARTERY

Frog Arteries

INTERNAL JUGULAR

SUBCLAVIAN

BRACHIAL

CUTANEOUS

LUNG

LIVER

HEPATIC PORTAL

SMALL INTESTINE

PELVIC VEIN

FEMORAL VEIN

SCIATIC VEIN

RENAL PORTALVEIN

KIDNEY

RENAL VEINS

SPERMATIC

TESTIS

FAT BODY

HEPATIC

VENA CAVA

PULMONARY

EXTERNALJUGULAR

MANDIBULAR

LINGUALINNOMINATE

VENTRAL ABDOMINAL

Frog Veins

STOMACH

PANCREAS

SPLEEN

SMALLINTESTINE

DORSAL AORTA

LEGS (YUM!)

COLON

BLADDER

VENTRALABDOMINALVEIN

KIDNEY

VENA CAVA

LIVER

VENTRICLE

LUNG

RIGHT

ARTERIOSUSTRUNCUS

LEFT ATRIUM

ATRIUMMESENTERICCOELIACO-

ARTERY

Frog Gutz

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