NERVOUS SYSTEM

Amphibians

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The amphibian nervous system has been well studied for decades in a laboratory setting. As with all vertebrates a central and a peripheral nervous system exists.

There is considerable debate over whether there are 10 or 12 pairs of cranial nerves (CN), with those that are pro­ponents of the lesser number classifying the spinal acces­sory nerve (CN XI) and the hypoglossal nerve (CN XII) as

Amphibians

12

spinal nerves instead (Duellman & Trueb 1986; Goin et al. 1978; Mitchell et al. 1988). The spinal cord of caecilians and urodeles extends to the tip of the tail, while in anurans it ends in the lumbar region, with bundles of spinal nerves continuing through the spinal canal to form a cauda equina. As with higher vertebrates, there are enlargements of the spinal cord in the caudal cervical and lumbar regions, asso­ciated with limb movement, and development of brachial and inguinal plexi in amphibians with well-developed limbs (Goin et al. 1978; Wright 2001c).

The larval stages and aquatic adult forms of amphibians possess a lateral line system, which is absent in terrestrial amphibians. Lateral line nerves, derived from the cranial nerves, innervate this series of pressure-sensitive receptors on the head and along the sides of the body. The lateral line is responsible for perception of low-frequency vibrations and functions to detect stationary or moving objects by wave reflec­tion (Goin et al.

Senses

Hearing

Auditory structures vary greatly among amphibians, and in particular, the anurans have very well developed ear struc­tures. An outer ear is lacking, and the tympanic membrane is responsible for transmission of high-frequency sounds to the bony columella in the middle ear, which then transfers it to the sensory patches in the membranous labyrinth of the inner ear. In many amphibian species, low-frequency sounds are transmitted to the inner ear by an opercular bone that receives the vibrations from the forelimbs (Goin et al. 1978; Mitchell et al. 1988; Wright 1996).

Sight

Ocular structures are well developed in amphibians, with the exception of caecilians and many cave-dwelling sala­manders, and there has been further evolutionary develop­ment of tear glands and eyelids in terrestrial species. In order to accommodate, the lens is moved toward or away from the cornea, rather than changing the shape of the lens as in mammals. Pupillary diameter adapts to changes in environmental light; however, the iris is composed of striated muscle under voluntary control, which makes assessment of pupillary light responses problematic for the clinician. The retina of most terrestrial amphibians is complex, but vision in most amphibians is based on pattern recognition in the visual field rather than visual acuity. Several types of retinal ganglion cells respond to different features in the visual field, allowing the amphibian to construct a crude but useful picture of its surroundings. Approximately 90% of the visual information is processed in the retina, while only 10% is passed on to the optic lobes' reflex centers. This well-developed retina is thought to compensate for the relatively simple brain (Goin et al. 1978; Mitchell et al. 1988; Whitaker et al. 1999; Wright 1996).

Taste, touch, olfaction

These senses are well developed in amphibians. Taste buds occur on the tongue, roof of the mouth, and in the mucous membranes of the mandible and maxilla. Tactile receptors are scattered throughout the dermis. In addition to the specialized olfactory epithelium that lines the nasal cavity, amphibians also possess a sense organ known as Jacobson's organ. It consists of a pair of epithelial-lined blind-ended sacs connected by ducts to the nasal cavity and is inner­vated by a branch of the olfactory nerve. This organ is responsible for the detection of airborne chemicals, such as pheromones, and is thought to be important in regulating behavior rather than just food recognition (Goin et al. 1978; Wright 1996, 2001c).