Sense organs (comparative anatomy)

Plants can form complex organic compounds for nutrition from simple molecules, such as carbon dioxide and water, via the process of photosynthesis. Animals, on the other hand, rely upon obtaining complex organic compounds already formed by other organisms to meet their nutritional needs. Since such sources generally take the form of other organisms, these must be located and consumed by the animal. In short, animals, unlike plants, must use behavior—a set of responses to internal and external events occurring in their environment—to survive. It is the need for behavior that has formed the basis of virtually all animal evolution.

The first and most vital element of behavior is the detection of events occurring both within the body of the animal and in the surrounding external environment. The role of the sense organs is to detect these events (which are called stimuli) and translate them into the complex series of electrical and chemical signals that are the language of the brain. It is important to understand that the stimuli (such as sound, light, or heat) that animals detect and which may possess behavioral significance are meaningless to the brain. Brains are capable of interpreting only those signals that are in its language of electrical impulses and chemical interactions. Thus, sense organs have two vital functions: the detection of environmental stimuli and the translation of that stimulus into the language that is meaningful to the nerve cells of the brain. This latter process is called transduction, and it is the ultimate role of sensory organs.

Receptor Cells and Transduction

Sensory organs typically consist of several different types of cells. Receptor cells are directly responsible for transducing the stimulus into the electrical language of the nervous system. Supporting cells play various roles and, in some cases, may themselves become receptor cells. For example, in mammalian taste buds, the receptor cells routinely die after ten to fourteen days and are replaced by supporting cells that transform to become receptor cells. Accessory structures assist in the process of transduction, such as the lens of the eye. Finally, sensory nerve fibers are stimulated chemically by the receptor cells and send information concerning the presence of a stimulus into the central nervous system.

The process of transduction occurs when a stimulus interacts physically with a sense organ, causing a change in the distribution of electrical charge across the cell membrane of the receptor cell. This change in transmembrane voltage is referred to as a receptor potential, and the size of the receptor potential corresponds directly to the intensity of the stimulus applied to the receptor cell. There must, therefore, be a minimum intensity of the stimulus required to generate a receptor potential, referred to as the sensory threshold. In general, the sensory threshold corresponds to the smallest stimulus intensity that an animal can detect. At the same time, there is a maximum receptor potential that can be generated by a receptor cell, no matter how intense the stimulus. The intensity at which this occurs is known as receptor saturation. Above this level, when the receptor is saturated, it is impossible for the animal to discriminate whether one stimulus is more intense than another. Between the upper and lower limits of threshold and saturation, a change in the intensity of the stimulus will result in a corresponding change in the magnitude of the receptor potential. This range of intensities is known as the dynamic range of the sensory organ, and within this range of intensities, animals can discriminate between stimuli of different intensities.

When a continual stimulus is applied, and a receptor potential is generated across the membrane of the receptor cell, the nature of the receptor potential may change with time. If the receptor potential decreases with time, even though the applied stimulus remains constant, adaptation is said to occur. If one jumps into a pool of cool water on a warm summer day, the initial sensation is that of coolness against the skin. However, this perception disappears with time as the temperature receptors in the skin adapt until the water has no perceptible temperature. There are limits to adaptation, however; immersing one’s hand in very hot water does not result in the eventual disappearance of the perception of heat. Some senses exhibit sensitization—the opposite of adaptation—in which progressively less stimulus is required to elicit a sensation with increasing time. The responses to certain types of painful stimuli demonstrate sensitization.

The rates and degrees of receptor adaptation vary among different types of receptors and the specific types of information they detect. Some receptors adapt very quickly to an applied stimulus; after a short period of steadily applied stimulation, the receptor potential disappears. Such receptors are said to be phasic receptors. For example, if one carefully deflects a hair on the back of one’s arm with a pencil point and then holds it steadily in the deflected position, the sensation generated by the deflection quickly disappears. Other receptors show very little adaptation over time; such receptors are said to be tonic receptors. Many sorts of pain receptors are tonic, as anyone who has experienced a toothache may attest. Phasic and tonic receptors represent the extreme ends of a continuum. Most receptors can be said to be phasic-tonic receptors, which exhibit a greater or lesser degree of adaptation to continuous stimulation. Phasic and tonic receptors send different types of information concerning the nature of the stimulus to the brain. Tonic receptors send precise information concerning the duration and the intensity of a stimulus to the central nervous system. Such information may be useful, for instance, in determining the degree to which a limb is flexed. Phasic receptors, on the other hand, relay precise information about changes in the stimulus rather than its duration. Since animals live in a dynamic world, detecting small changes in the environment caused by the presence of a predator or prey may be of first importance. Both types of information are crucial to survival.

The Senses

Classically, there are five senses (vision, hearing, touch, taste, and smell), but in reality, there are many senses which can be further divided into subsenses. For example, the sense of temperature actually consists of two different sensory systems, one that detects heat and another that detects cold stimuli. Furthermore, these are both linked, at some level, with the detection of pain (intensely hot stimuli are also painful, but mild heat is not). Neurobiologists refer to a type of sense as a modality (such as taste) and the detection of variations within that modality as a quality (such as sweet versus sour). There are numerous different modalities, and more are being discovered every year. The ability of animals to detect and use many different types of complex information from their environment is a fascinating and continually unfolding story.

Because changes in electrical and chemical activity are the only language understood by the central nervous system, information arriving in the brain from different sensory systems must be kept segregated from each other to avoid confusion. Thus, any activity arriving from the eyes via the optic nerve is interpreted by the brain as “light,” whether light is actually present. Electrical or physical stimulation of the optic nerve in the absence of actual light will also be perceived by the animal as “light.” The sensory systems within the brain are thus organized into a series of labeled lines, each dedicated to a specific sensory modality. Any electrical activity in a labeled line is interpreted by the brain as the presence of that modality.

Sensory organs can generally be broadly classified by the nature of the events that they can detect and transduce. Mechanoreceptors detect mechanical forces applied directly or indirectly to the body. The sense of touch is the most familiar sense employing mechanoreceptors, but there are others, such as hearing and balance, that are equally important. Chemoreceptors detect signals that occur when various chemicals contact sensory organs (such as in taste or smell). Electromagnetic receptors detect energy contained in the electromagnetic spectrum. The most familiar, and one most heavily relied on in most mammals, is vision. Visual sensory organs (eyes) detect the energy contained within a limited range of frequencies within the electromagnetic spectrum, commonly referred to as visible light. Other familiar electromagnetoreceptors detect heat (infrared radiation) or its lack (cold). Other animals detect other portions of the electromagnetic spectrum and, in some cases, are capable of directly detecting electrical and magnetic fields. These three categories are not absolutely rigid, however. Some types of receptors, such as hygroreceptors (that detect the water content of air), seem to not fall conveniently into any one category. Others, such as nociceptors (pain receptors), straddle several categories and may respond to mechanical, chemical, or thermal stimuli. Similarly, the sense of balance employs information from several kinds of sense organs responding to a number of discrete stimuli, such as the direction of Earth’s gravitational pull, rotational acceleration of the head, and body position.

Exteroceptors and Endoreceptors

Sensory receptors of all types may be used to detect either stimuli originating in sources outside the body (exteroreceptors) or stimuli originating within the body itself (endoreceptors). These latter receptors play a vital role in the maintenance of a constant internal chemical environment and temperature (homeostasis). If the internal environment varies outside of a narrow set of parameters, the organism may die. Endoreceptors must detect fluctuations in the internal environment and signal the body’s involuntary control mechanisms (the autonomic nervous system) to effect corrections. For example, the detection of a drop in the core body temperature in mammals can result in a variety of responses, including the shunting of blood away from the skin surface (to minimize loss of heat to the environment), erection of hair or fur on the skin (what humans experience as goosebumps) to trap a layer of warmed air next to the skin, and shivering, which generates heat via muscular contractions. There are many other endoreceptors in the body that detect stimuli, such as the amount of dissolved gases in the blood like oxygen and carbon dioxide (chemoreceptors), sugars and salts (Osmoreceptors), and blood pressure changes (Baroreceptors).

Another important role of endoreceptors is proprioceptionthe detection of the relative positions of the body’s parts in relation to one another. It is this sense that allows an individual to touch their nose with the tip of their finger when their eyes are closed. “Muscle sense,” or kinesthesia, and the detection of the amount of flexion of the joints are included as parts of the modality of proprioception.

There are many types of exteroreceptors that comprise all general classes of receptors. Exteroreceptors are dedicated to detecting external events that impinge in some manner upon the body. Some exteroreceptors are scattered across the body's surface of the body, such as those that detect touch pressure or vibration (mechanoreceptors) and those that detect external temperature changes (thermoreceptors). Others are confined within specialized structures, such as the eyes or ears. The twofold function of these receptors is essentially the sameexteroceptors relay information about the nature of a stimulus and its location with respect to the animal (localization). This latter purpose is crucial; it may be important to know that a given sound indicates the presence of a predator, but it is equally critical to know from whence the threat originates so that appropriate behaviors can then be generated.

All sensory organs typically possess a receptive field, that area of space on or around the body which, when stimulated, results in the generation of a response in the receptor. The size of the receptive field may vary widely among different types of receptors, and it is the receptive field size, together with the density of the receptors in a given area, that determines the acuity, or spatial resolution, of the sense system. For example, in humans, the skin of the fingers and lips contains a very large number of tactile (touch) receptors, most of which possess very small receptive fields. This allows individuals to discern to a very high degree precisely where on the skin a stimulus is occurring. In other areas of the body, like the back of the neck, the density of receptors in the skin is much lower, and it is more difficult to localize exactly where a stimulus is being applied. The density of the receptor cells (rods and cones) in the eye decreases from the central portion of the retina toward the edges. That is why visual acuity is greatest when looking directly at an object and why it is very difficult to read using one’s peripheral vision.

The localization of a given stimulus is critical to the organization of an appropriate response. The location of a given sense organ in the body corresponds to a location within the central nervous system, and adjacent receptors are represented by adjacent locations within the brain. Thus, there is a sensory map of the body within the brain, and it is via this topographic organization that information about stimulus location is maintained. There are many sensory maps within the brain, and they play a prominent role in the organization of behaviors.

Responding to Sensory Data

Sense organs may be sensitive to a wide array of different qualities and provide the animal with a general sensory scene of the surrounding environment (as in mammalian vision) or may be restricted to a narrow range of stimuli that serve as channels of communication between animals of the same species. This latter case is particularly true for special chemical senses that detect chemicals that have specific behavioral meanings for members of the same species (pheromones). Often, the sensitivity of sensory systems lies between these two extremes, with the system showing the greatest sensitivity to ranges of stimuli that have greater behavioral significance to the animal. Dolphins, for example, locate objects underwater by echolocation, emitting a high-frequency call and then listening for the returning echoes. The greatest sensitivity of the dolphin auditory system is to the range of sound frequencies that are reflected as echoes, although dolphins can hear other sounds as well.

As animals and their behaviors have evolved, so have their sense organs, providing the animals with the competitive advantages that allow them to survive and reproduce. Furthermore, natural selection frequently results in the evolution of very similar sense organs in widely divergent animals. The eyes of mammals closely resemble those of octopuses, even though these animals are not at all closely related, and their common ancestor lacked complex eyes. Such convergent evolution of sense organs has resulted from the adaptive pressures on both animal groups that depend strongly upon vision to organize behavior.

Sometimes, the sensory systems of different animal species evolve in tandem. This coevolution of sensory systems is a direct result of the interactions of the species. Bats hunt flying moths by echolocation, emitting ultrasonic calls and homing in on the echo reflected by the moth. Many moth species, in response, have evolved “ears,” located on either side of their abdomen, that are specialized to detect the calls of hunting bats. Depending on the intensity of the detected call (and, thus, the nearness of the bat), the moths display different behaviors. If the intensity is low, indicating the bat is still at a distance, the moth will fly away from the side of the body upon which the call is loudest. If the intensity of the bat’s call reaches a certain level, however, the moth will execute an erratic, fluttering crash dive toward the ground in a final attempt to escape. In an additional twist, the dogbane tiger moth emits ultrasonic pulses of its own when it detects the calls of an approaching bat. Such calls may jam the bat’s echolocation by interfering with the detection of the returning echoes. Bats also have geomagnetic and polarization senses, which some consider a seventh and eighth sense for the species.

Among other animals with notable sense organs are groundhogs, which have excellent hearing, vision, and sense of smell from sensory organs on top of their heads. Bees and elephants have extraordinary senses of smell. The mantis shrimp has the most complex vision system of any animal, and the catfish has more than 175,000 tastebuds to help them find food on the bottom of lakes and ponds.

A review of all known types of sense organs in animals and the way they are used to organize and shape behaviors would fill an entire volume, and more are continually discovered. For example, the sensory organ in the skin called the nociceptive glio-neural complex that helps detect pain was described in 2019, and the rhinarium organ in dogs' noses senses weak thermal radiation that was discovered in 2020. Sense organs are, in a very real sense, the keys to our individual understanding of the world. They provide the information upon which the daily understanding of reality is entirely based.

Principal Terms

Adaptation: The decrease in the size of the response of a sense organ following continuous application of a constant stimulus

Modality: A specific type of sensory stimulus or perception, such as taste, vision, or hearing

Phasic Receptors: Receptors that adapt quickly to a stimulus

Receptive Field: The area upon or surrounding the body of an animal that, when stimulated, results in the generation of a response in the sense organ

Receptor Cells: Sensory cells within sense organs that are directly responsible for detecting stimuli.

Receptor Potential: A change in the distribution of electric charge across the membrane of a receptor cell in response to the presentation of a stimulus

Tonic Receptors: Receptors that typically show little or no adaptation to a continuously applied stimulus

Transduction: The translation of a stimulus’s energy into the electrical and chemical signals that are meaningful to the nervous system

Bibliography

"Animal Sensory Systems." Georgia Tech Biological Sciences, organismalbio.biosci.gatech.edu/chemical-and-electrical-signals/sensory-systems-i. Accessed 10 Sept. 2024.

Downer, J. Supernature: The Unseen Powers of Animals. Sterling, 2000.

Gregory, R. L. The Oxford Companion to the Mind. 2nd ed. Oxford, Oxford University Press, 2006.

Halliday, Tim, ed. The Senses and Communication. Berlin, Springer-Verlag, 1998.

McFarland, D. The Oxford Companion to Animal Behavior. Reprinted and corrected ed. Oxford, Oxford University Press, 1987.

Schlosser, Gerhard. "A Short History of Nearly Every Sense—The Evolutionary History of Vertebrate Sensory Cell Types." Integrative and Comparative Biology, vol. 58, no. 2, Aug. 2018, pp. 301–16, doi.org/10.1093/icb/icy024.

Wnuk, Alexis. "What Can Animals Sense That We Can't?" Society for Neuroscience: Brain Facts, 2017, www.brainfacts.org/thinking-sensing-and-behaving/vision/2017/what-can-animals-sense-that-we-cant-071317. Accessed 10 July 2023.