Nervous system and sensory organs

The reptilian nervous system is organized into identifiable regions based on structure and function. The central nervous system consists of the brain and spinal cord and contains most of the body's nerve cells or neurons. Collections of neurons that have similar function are called nuclei, and the bundles of axons that extend from the cell bodies and transmit messages are called tracts. The peripheral nervous system consists of sensory and motor nerves that communicate between the brain or spinal cord and various other parts of the body. The motor neurons that control contraction of skeletal muscles compose the voluntary nervous system. The autonomic nervous system, consisting of sympathetic and parasympa-thetic branches, provides unconscious control of the heart and lungs and activity of smooth muscles and various glands.

The spinal cord is enclosed and protected by the vertebral column. It provides reflex actions that occur independently of the brain but also receives input from higher brain centers. The spinal cord has a segmental organization that is largely lost in the brain, although bilaterally paired cranial nerves connect centers of the brain with structures in the head and body and represent a vestige of segmental organization. The cranial nerves carry both sensory and motor information.

Chemical senses

Receptors that sense chemical stimuli are designated gustatory, or taste, receptors if they respond to dissolved molecules and olfactory, or smell, receptors if they respond to airborne particles. Taste receptors typically are present in the mouth, but these vary in occurrence, and many reptiles rely more on olfaction than on taste. Olfactory receptors are located in the nasal passages, where a stream of air flows over them during ventilation of the lungs or during pumping movements of the throat, which may have a "sniffing" function. Olfactory receptors are generally more varied than taste receptors, and many are highly specific for chemical stimuli. A second kind of olfactory chamber, the vomeronasal or Ja-cobson's organ, is a pair of blind-ended cavities opening into the oral cavity. These are well developed in snakes and some lizards that transfer odor molecules directly from their tongue to the sensory epithelium of the cavity. Jacobson's organs are lacking in crocodilians and are poorly developed in many other reptiles.

Snake Vomeronasal Organ
Jacobson's organ, a second kind of olfactory chamber, is well developed in snakes and some lizards. (Illustration by Dan Erickson) Grzimek's Animal Life Encyclopedia
Big Headed Turtles

Big-headed turtles (Platysternon megacephalum) have such large heads that they do not fit back into their shells. To protect their heads from predators, they have hard horny scutes on the top and sides of their heads and their skulls are solid bone. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

Big-headed turtles (Platysternon megacephalum) have such large heads that they do not fit back into their shells. To protect their heads from predators, they have hard horny scutes on the top and sides of their heads and their skulls are solid bone. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

Mechanoreceptors

A variety of mechanoreceptors are present in reptiles. These structures sense vibrations, sound, and other forms of mechanical stimuli. Specialized structures called muscle spindles are present in the skeletal muscles and detect, as well as control, the stretch of body muscles. Various nerve endings associated with the skin and joints detect a variety of vibrational stimuli. The scales on the body of many lizards and snakes, and on the head of crocodilians, bear small, circular sense organs that are thought to detect vibrations and other mechanical stimuli. Some of these organs appear as flattened plates; others bear complex hairlike structures. The most specialized of mechanoreceptors, however, are those involved in hearing. The inner ear contains collections of specialized receptors called hair cells, named for numerous tiny projections sensitive to movement. These are responsible for mediating the sense of equilibrium and for hearing.

Sound stimuli are conducted through tissues of the head to the inner ear, as in snakes, or impinge on a taut membrane associated with the external ear. In essence, vibrational waves in air bounce off this membrane and set it into vibrational motion in doing so. The vibrations reflect the frequency and intensity of the sound and are transmitted from the external membrane to fluid in the inner ear by means of a small connecting bone. The fluid of the inner ear bathes the hair cells and stimulates their response by means of vibrational waves in liquid. Sounds are an excellent monitor for conditions in the environment and serve primarily as a warning source. Reptiles respond to frequencies from a few tens to several hundred hertz (cycles per second, abbreviated Hz) in tortoises and squamates to more than 10,000 Hz in some species of geckos. Alligators can hear throughout the range of frequencies tested, from approximately 30 Hz to 10,000 Hz, both in air and beneath water. Contrary to popular belief, snakes not only sense vibrations in the substrate on which they rest but also can hear airborne vibrations of low frequencies (espe cially 150-500 Hz). Relatively few reptiles vocalize or use sounds for communication.

Vision

Most reptiles have well-developed eyes with properties that are generally similar to eyes of other vertebrates. Incoming light is bent by the curvature of a lens that focuses light on photoreceptors arranged on the retina within the eye. An opaque iris controls the entering light by means of a variable aperture called the pupil. Muscles control the diameter of the pupil and help to control the intensity of light entering the eye and the quality of the image on the retina. There are many other structural similarities to a camera.

Photoreceptors consist of rods and cones. These structures bear protein molecules that capture light energy and convert it to nerve signals. The rods function best in dim light, whereas the cones function best in bright light and provide higher resolution. Varying abilities to differentiate color depend on the possession of multiple visual pigments, each of which absorbs maximally at different wavelengths of light. Some reptiles, such as arboreal snakes, have "keyhole" pupils, which enhance binocular vision (similar images are formed simultaneously on both retinas of the two eyes), and a fovea, where high densities of cones on the retina provide high visual acuity. Slender head shape, especially an attenuated snout, confers considerable overlap of vision in the two eyes. The eyes of chameleons are unique among vertebrates in their degree of movement and ability to scan the environment. Each eye is located on a turret and moves independently of the other. The lens of the chameleon's eye focuses very rapidly and produces enlarged images on a comparatively large retina with high densities of cones, acting somewhat like a telephoto lens.

Most reptiles have movable eyelids and a transparent nictitating membrane. Snakes and some lizards lack movable eyelids and have instead a transparent window that covers the eye. Eyes are greatly reduced in snakes and lizards that burrow in soil.

In addition to eyes, most reptiles have a single photore-ceptive structure, a parietal organ, on the mid-dorsal aspect of the head and brain and associated with the pineal complex of the brain. Lizards and tuatara have a distinctive parietal organ called a third eye, which is equipped with a lens and a retina. These organs are thought to be ancient structures that evolved as accessory sensory systems sensitive to visible radiation. The parietal organs of living reptiles are photosensitive and appear to be involved with circadian or seasonal cycles and possibly with aspects of thermoregulation. Lightsensitive receptors are thought to be present on the skin of the tails of certain sea snakes.

Thermoreception

Sensory nerve endings in the skin and possibly other tissues are sensitive to temperature change. Extraordinarily sensitive receptors are present in the facial pits of pitvipers and the labial pits of boid snakes. A pit organ consists of a thin membrane stretched across an open cavity. The membrane is richly supplied with nerve endings that have remarkable sensitivity to temperature change. The nerve endings detect infrared or radiant heat energy, which affects the heat-sensitive membrane. The design allows rapid detection of temperature changes as small as 0.005°F (0.003°C). The temperature receptors lie deep within the facial pit and enable a snake to detect the direction as well as the intensity of a radiant heat source. The information is relayed to the brain via nerve tracts and assists the hunting and capture of prey under conditions of dim light.

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Responses

  • dodinas
    How are organs developed?
    4 years ago
  • robert
    Where are the sensory organs in the reptile?
    4 years ago
  • adriana buccho
    How do sense of organs and nervous system handle incoming sensory?
    3 years ago
  • Patrick
    What sensory organs do lizards have?
    3 years ago
  • meriadoc oldbuck
    What is nervous and sensory functions in reptile?
    4 months ago
  • anna
    What are the sensory and nervous function of reptiles?
    4 months ago
  • ines
    What are the nervous functions and sensory functions in reptiles?
    4 months ago
  • BREE RICHARDSON
    What is the function of the nervous system of reptiles?
    4 months ago

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