Nervous System

Use the diagram in Figure 40-11 to find the main components of the amphibian nervous system. An amphibian's brain is about the same size as that of a fish of similar size. The olfactory lobes, which are the center of the sense of smell, are larger in amphibians than in fish, and they lie at the anterior end of the brain. Behind the olfactory lobes are the long lobes of the cerebrum, the area of the brain that integrates behavior and is responsible for learning.

The optic lobes, which process information from the eyes, lie behind the cerebrum. The cerebellum, a small band of tissue that lies at a right angle to the long axis of the brain, is the center of muscular coordination and is not as well developed in amphibians as it is in other tetrapods. The medulla oblongata lies at the back of the brain and joins the spinal cord. It controls some organ functions, such as heart rate and respiration rate.

There is continuous communication among most areas of the brain. Ten pairs of cranial nerves extend directly from the brain. The spinal cord conducts signals from all parts of the body to the brain and from the brain back to the body. Encased in protective bony vertebrae, the spinal cord extends down the back. As in fishes, the spinal nerves branch from the spinal cord to various parts of the body.

This diagram shows the frog's nervous system in ventral view. The brain of the frog is sufficiently developed to cope with both land and water environments.

This diagram shows the frog's nervous system in ventral view. The brain of the frog is sufficiently developed to cope with both land and water environments.

Sense Organs

Some sense organs work as well in air as in water, but others do not. For example, the lateral line system, used by fishes to detect disturbances in the water, works only in water. Thus, while larval amphibians have a lateral line, it is usually lost during metamorphosis. Only a few species of aquatic amphibians have a lateral line as adults.

The senses of sight, smell, and hearing are well developed in most amphibians. Visual information is often important in hunting and in avoiding predators. The eyes are covered by a transparent, movable membrane called a nictitating (nik-ti-tayt-eeng) membrane.

Sound is detected by the inner ear, which is embedded within the skull. Sounds are transmitted to this organ by the tympanic (tim-PAN-ik) membrane, or eardrum, and the columella (CAHL-yoo-mel-uh), a small bone that extends between the tympanic membrane and the inner ear. Sounds first strike the tympanic membrane, which is usually located on the side of the head, just behind the eye as shown in Figure 40-12. Vibrations of the tympanic membrane cause small movements in the columella that are transmitted to the fluid-filled inner ear. In the inner ear, the sound vibrations are converted to nervous impulses by sensitive hair cells. These impulses are transmitted to the brain through a nerve.

Frogs have a well-developed sense of sight, which is necessary for locating prey and avoiding predators. Frogs also have good hearing. Note the tympanic membrane (the circle below and to the left of the eye), which transmits sound to the inner ear.

Word Roots and Origins nictitating from the Latin nictare, meaning "to wink"

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