Listed earlier were the types of sensory information that could be detected by animals. What follows is a brief elaboration, detailing what types of sensation within these general groupings can be detected by animals.
Chemoreception is the detection of specific chemicals by a sensory receptor. Interaction of the chemical with its receptor results in the generation of receptor potentials, and, consequently, action potentials. Under the heading of chemoreception are included the senses of taste and smell, together with less obvious aspects such as the monitoring of O2 and CO2 levels in body fluids.
Chemoreception is a sense which has been demonstrated to exist in both invertebrates and vertebrates. Indeed, the chemoreceptor senses of invertebrates have quite often provided useful models for studying the physiological processes involved in the functioning of this sensory modality in higher animals. For example, the antennae of insects are easy structures to utilize in neurophysiological studies. Their ease of access means they can easily be stimulated and their response to such stimulation is easy to record. Taste is a common sensation throughout the animal kingdom. In particular, the ability to taste bitter substances is almost universal in the animal kingdom, as it serves a protective purpose by warning against potentially toxic substances. The role of smell is also important in many animals, pheromones are a good example of this. These are volatile chemicals released into the atmosphere which are used as signals to other animals. For example, the female silk worm moth, Bombyx mori, releases a chemical called bombykol which is used to attract a male mate. The male has receptors for bombykol in its antennae. It is thought that a single molecule of bombykol in 1015 molecules of air will evoke a response in the male to begin searching for a female. This obvious and directed movement towards a chemical substance is called chemotaxis. Similar mechanisms also exist in the vertebrates. For example, the sense of olfaction is thought to be involved in the process by which some fish, for example, salmon, return to their home waters to reproduce. In snakes and some lizards, there is a specialized olfactory structure called Jacobson’s organ, which is located in a little outgrowth of the nasal cavity. The characteristic trait of many snakes is the continual movement of the tongue back and forth into the mouth. What they are actually doing is transferring environmental air samples onto their Jacobson’s organ for analysis ― essentially, they are tasting the air. Humans have the ability to distinguish between many thousands of smells. However, just as there are only three primary colors from which all other colors are produced, there are also thought to be only a small number of ‘primary’ odors.
Mechanoreception is the measurement of force (pressure) and displacement. The receptors which monitor this can vary from simple dendritic endings of neurons, e.g. receptors in the skin which monitor pressure, through to highly complicated, specialized receptors, e.g. the inner ear of vertebrates. Although a common sensation, the actual physiological process whereby signals are transduced is far from clear. The simplest mechanism proposed is that stretch-activated ion channels exist in the membrane of the receptor components. At rest, the ion channels are closed. However, the application of pressure produces a conformational change in the protein that constitutes the ion channel, and this leads to depolarization and the generation of local currents.
Mechanoreception is seen to be present in both invertebrate and vertebrate animals. In invertebrates, it has been shown that receptors exist for pressure, sound and movement detection. For example, insects have surface receptors which provide information about wind direction, the orientation of the body in space, velocity of movement and sound. In a similar manner, the mechanoreceptors of vertebrates are equally varied. They range from receptors which can monitor the length of a muscle (specialized muscle fibers called muscle spindles) through to the organs of hearing and balance (inner ear structures). In the case of the latter, the sensory receptors are ciliated (), and it is the movement of the cilia which is responsible for the production of action potentials. Perhaps the simplest organization of this kind is the lateral line system of fish. The lateral line () provides information about the movement of the animal itself, as well as information about other movements nearby. Further evolutionary development of the lateral line system gave rise to the inner ear of vertebrates. It is beyond the scope of the present text to discuss auditory mechanisms in detail. However, many variations of the basic hearing process have developed in animals. Microchiropteran bats, for example, depend on echolocation ― they hear the echoes of sounds that they produce themselves which are reflected back by other objects. Given that echolocation is used by animals active at night, it can be used for both prey detection and the avoidance of objects. Echolocation is also used by other animals, such as cave-dwelling birds, whales and dolphins.
Thermoreception is the detection of heat and cold and changes in temperature in the environment (either external or internal). It is an important parameter to be able to measure since it can have a profound effect upon the functioning of the animal. For example, consider the consequences of increased temperature upon protein structure and function and the effects this would have on metabolic processes. At increased temperatures, proteins and, therefore, enzymes would cease to function and the necessary metabolic reactions essential for life would be unable to occur. Thermoreception occurs throughout the animal kingdom. Virtually all animals have an optimum temperature within their environment; for example, under experimental conditions, parasitic worms will move to regions of higher temperature rather than move to regions of lower temperature. The location of thermoreceptors varies between animals. Insects, for example have thermoreceptors on their antennae and legs for monitoring air temperature and ground temperature, respectively. Mammals, on the other hand, have peripheral and core thermoreceptors located in the skin and hypothalamus, respectively. This is essential if a constant core body temperature is to be maintained, in this case 37°C. The system would not work if there were only central receptors. Consider what would happen if there was a dramatic rise in the temperature of the external environment. By the time such an increase had been transmitted through to core thermoreceptors, serious heat-induced damage may have already occurred to the animal in the periphery.
The ability to detect light is an almost universal phenomenon across the animal kingdom. Although some organisms, e.g. Amoeba, may be able to detect the presence of light without the aid of any specialized structures (how this is achieved is far from clear), it is more usual for an animal to possess a specialized photoreceptor. This may vary from the light-sensitive cytoplasmic region (eye-spot) of Euglena to the complicated neural organization of the vertebrate eye. In all cases, though, the operation of photoreceptors is essentially the same. Highly folded photoreceptor cells contain pigments, the most common of which is rhodopsin, which are chemically changed in the presence of light through a series of intricate reactions that result in alteration in the membrane potential of the receptor. The physiological processes which are occurring at photoreceptors, and the way that neural information generated from them is dealt with by the nervous system, is far from fully understood. Flatworms have so-called cup-shaped eyes () which face in opposite directions and are used essentially to obtain directional information. The animal will tend to move to darker regions, thereby minimizing the risk of attack by predators. The arthropods (insects, crustaceans and spiders) have compound eyes (). Each compound eye is made from many smaller optical units called ommatidia. The quality of the representation of the world varies depending on the number of ommatidia ― the greater the number, the better the definition of the visual experience. A simple analogy here would be the printed output from a dot-matrix printer. The greater the number of dots which make up the print, the greater the clarity of the image. Vertebrates and cephalopod molluscs have developed vesicular eyes () which have the ability to form high definition images of the external world. The iris is able to alter the amount of light entering the eye and the lens focuses the light on the retina, the layer of photoreceptors at the back of the eye. The lens operates differently in different vertebrates. For example, fish, amphibians and reptiles move the lens nearer or further away from the retina in order to focus on a particular object. Birds and mammals on the other hand alter the shape of their lens so that light is focused on the retina.
Some animals, particularly sharks, rays and catfish, have the ability to detect the minute electric fields generated by other animals. Such electric fields are generated by muscular activity, for example. This represents one way by which prey may be detected. The electrorecepors, the best known example of which is the ampulla of Lorenzini (in the sharks and rays), are located mainly in the head and along the lateral line. The dogfish, for example, can detect electric fields as small as 10 nV per cm. Some fish, the so-called electric fish, produce a continuous electric field around them. Disruptions to these fields by other fish or inanimate objects in their environment are detected by the electroreceptors. Living and nonliving objects are distinguished by the way they disrupt the electric field.
Many animals display the ability to orientate to the Earth’s magnetic field. Such an ability allows animals to distinguish a north-south axis, which is potentially useful as a navigation aid. However, it is a sensory modality which is not widespread throughout the animal kingdom. In invertebrates, it is known that honey bees utilize the Earth’s magnetic field in order to communicate. When bees arrive at their hive having found a new food source, they pass on information about the direction of the source to other bees by performing a waggle dance. In this way they communicate the angle between the sun and the food source, together with whether the source is away from or towards the sun. In fish, the electroreceptors are thought to be sensitive enough to detect a magnetic field. Homing birds are also thought to use magnetic fields as one of many cues that allow them to return home. The actual mechanism that allows for magnetoreception is far from clear. Some sharks, honeybees and birds have been shown to contain the magnetic substance magnetite. One theory is that the magnetite allows these animals to orientate to the Earth’s geomagnetic field. This provides them with directional clues and enables them to find their way around. It is interesting to note that bacteria have also been found which contain magnetite, although the implications of this finding are unclear.
Before discussing movement of whole animals, it is worth considering the movement of substances within cells. All animal cells are dynamic structures, with organelles and other substances continually moving. However, it is worth remembering that order is superimposed upon this dynamic structure. This order comes from the cytoskeleton, which, as the name implies, is the internal skeleton of all animal cells. The cytoskeleton, which contributes to the shape of the cell, its movement, and movement within the cell is composed of three types of fibre: microfilaments, microtubules and intermediate filaments. Microtubules are polymers of a protein called tubulin and these elements make up the cilia and flagella which can be found in animal cells. Microfilaments are also proteinacious structures, formed from proteins called actins, sometimes together with other protein molecules, such as myosin. As will be seen later, these structures are important in making cells shorten ― the type of response which occurs when muscles contract. The final group are the intermediate filaments. These structures play a less dynamic role in cell movement and are probably important in stabilizing cell structure and resisting movement.
In virtually all cells, the cytoplasm of the cell is continually moving, a phenomenon called cytoplasmic streaming. It should be remembered that this is a directed movement, i.e. it does not occur randomly. A good example of this cytoplasmic movement is seen in axonal transport. Here, microtubules run along the entire length of the axon from the cell body to the axon terminals. Movement of substances occurs in both directions, from the cell body to the axon terminals and vice versa. Some neurotransmitters are transported from the cell body to the axon, known as anterograde transport. This may reach speeds of up to 40 cm day-1. Flow in the opposite direction, retrograde flow, carries material from the axon terminal back to the cell body for reprocessing and occurs at a much slower rate of about 8 cm day-1.
Amoeboid movement is a characteristic movement of both unicellular animals, such as Amoeba, and also the cells of multicellular animals. For example, white blood cells in vertebrates use amoeboid movement to leave the circulation and enter the tissues where they may become involved in inflammatory reactions, and during the early development of animals, many cells move to their final destination by such movement. Amoeboid movement by single-celled animals involves the animal extending the cell to form a pseudopodium (). The precise mechanism behind this movement is unclear, but it is thought to involve changes in the physical nature of the cell cytoplasm. The general cytoplasm of the cell is liquid in its nature and is known as plasmasol. However, around the periphery of the cell it is much more viscous in nature, and here it is called plasmagel. Pseudopodia are formed by the breakdown of the plasmagel cytoplasm in a particular region of the cell membrane. Positive pressure is generated in the main part of the cell by the interaction of microfilaments. There are two types of microfilament which form a network in the cytoplasm ― these are actin and myosin. Actin and myosin interact with each other in a similar manner to the interaction of actin and myosin in muscle contraction, described in site. This interaction forces the plasmasol past the breakdown of the plasmagel, which causes the cell well to bulge outwards at that point to form a pseudopodium. As the plasmasol enters the pseudopodium it changes into plasmagel and the pseudopodium is thus prevented from forming any further.