Animals receive information from their environment via networks of neurons or nerve cells, which transmit information to each other. The general structure of these cells is shown in . A chemical signal is picked up by receptors on the dendrites and passed through the cell body and down the axon as an electrical signal. When the signal reaches an axon terminal, it must pass over the space, called a synapse, between the axon terminal and the next neuron's dendrites. The axon terminal releases a neurotransmitter chemical which travels across the synapse and delivers the signals to the next set of dendrites to trigger the next neuron, a process commonly called "firing".
Individual neurons can communicate only a limited amount of information because each cell can only fire or not fire, much like a light switch can only be on or off. But a group of neurons may transmit information to a single, more complex neuron, which itself reports to yet more complex neurons. In this manner, patterns of excitation and inhibition of individual neurons is translated into a degree of complex neuron firing which in turn translates into complex information.
Invertebrate animals vary greatly in the makeup of their nervous systems. Single celled animals cannot posses a multicellular network, but rather have an intracellular response mechanism. Cnidarians (Hydra, jellyfish) have a nerve net that conducts signals throughout the body. Flatworms (Platyhelminthes) have an anterior ganglion that together with the nerve cords forms a central nervous system. Networks of nerve fibers stemming from these cords form the peripheral nervous system. Other invertebrates have this basic form with various advancements. Vertebrates are characterized by a dorsal hollow nerve cord; in some vertebrates this forms the spinal cord. Vertebrate animals have a true anterior brain that together with the spinal cord forms the central nervous system. The autonomic nervous system enervates internal organs via two pathways: 1) the sympathetic nerve pathways activate during exertion or extreme emotion and accelerate heart rate, dilate air passages, and increase blood supply to the muscles and brain; 2) the parasympathetic pathways have a recuperative function, returning the blood supply to normal and counteracting the sympathetic pathway effects. The somatic nervous system carries sensory information between the central nervous system and skeletal muscles.
Sensory receptors are special nerve cells specialized for transmitting information according to the different types of external stimuli they respond to. For example Mechanoreceptors fire when the cell membrane is deformed, and so transmit tactile information. Photoreceptors contain pigments which fire when chemically altered by light.
Chemoreception is the identification of chemical substances and their concentrations. This sensory mechanism is found in even the most primitive life forms. The mechanism is not completely understood, but it is known that receptor sites on cell membranes recognize specific molecules. Whether this occurs by chemical reaction, molecular shape, or some combination of the two is not known. There are two types of chemoreceptors. Exteroreceptors detect chemicals from the external environment while interoreceptors detect chemicals within the body. The familiar senses of taste and smell are both forms of chemoreception.
Many animals release pheromones to cause a specific response by the receiving organism. Pheromone release and reception constitutes one of the most primitive forms of communication and is widely used in nature for a variety of purposes. Silkworm moth females use pheromones to attract males, while Eusocial bees use pheromones to indicate the location of a food source. Even a single pheromone, such as the queen substance, may serve many functions. A gradual decline in the concentration of this pheromone will stimulate worker bees to build queen cells in which new queens may be reared. A sudden absence, such as would occur should the queen be absent from the hive for a long time or be killed, stimulates emergency queen rearing. A high concentration, indicating overcrowding, may stimulate the colony to split into two new colonies. (See more about eusociality or pheromones. There have been some recent controversial studies that indicate humans may release and respond to pheromones. Some women recognized T-shirts worn by men as smelling "sexier" than others. Other investigations have revealed that women living in close proximity may synchronize their menstrual cycles due to pheromones.
Thermoreception, the detection of temperature changes, is present in most animals, but has been little studied. Many insects have temperature-sensitive nerve endings, either on their legs to detect ground temperature, or on their antennae to detect air temperature. Fish have thermoreceptors on the skin, lateral line (which also detects electrical signals and vibrations), and in the brain. Birds are not known to have many thermoreceptors in the skin, but have them on the tongue and bill in some species. Mammals have distinct heat and cold receptors distributed throughout the skin. There are also thermoreceptors deep within the body that can cause shivering even when skin and brain receptors are detecting a constant temperature. Thermoreceptors in the spinal cord can influence shivering, panting, and changes in blood flow.
Many arthropods have vibration sensitive hairs in their limb joints. Tactile sensations can be propagated via hairs or by the deformation of skin neurons. Such neurons are called mechanoreceptors. These receptors are also involved in hearing. Sound waves are propagated by vibrations of air or water molecules. Small changes in pressure that result from these vibrations are detected by mechanoreceptors that can rapidly adapt, and so are sensitive to sound vibrations.
The silkworm moth has one of the simplest types of auditory systems, which converts sound pressure waves into vibratory motion. These moths have two simple ears, each consisting of a tympanic membrane and two receptors imbedded in connective tissue. There are two tympanic membranes on either side of the thorax that transmit sound waves from the environment to the receptors; each receives different sound intensities. The A1 receptor cell detects low-intensity sounds. The frequency of the impulse from the A1 cell, or the rapidity with which it fires, is proportional to the volume of the sound, allowing the moth to determine if a predatory bat is approaching or is merely present in the area. The direction of the source is detected by the difference in both the time of arrival and intensity of the vibrations at the two ears. When the bat is above the moth, the sound of its cry will be interrupted by the beating of the moth's wings, but if the bat is below the moth, this will not happen. This is how the moth determines relative altitude. The A2 cells only detect high-intensity, or loud, sounds. It produces an emergency response only when the bat is nearby by disrupting the moth's flight control. In response, flight becomes erratic, an evasive maneuver which helps the moth to escape when the bat is within striking distance.
Most animals utilize far more complex auditory systems than that of the silkworm moth. A bullfrog call has many frequencies at various amplitudes within a single timeframe because many sounds are emitted at once. The auditory nerves of the receiver must respond to these variations. Nerve cells designed to receive a specific amplitude and frequency excite a more complex nerve, while other neurons inhibit it. The system would look something like that of . Receptors excited by a high amplitude wave (loud sound) excite a more complex neuron. Receptors that receive low amplitude sounds inhibit the same complex neuron. In this manner, distinctive sounds can be recognized, rather than just the intensity of sound that the silkworm moth detects.
Photoreceptors cells contain a pigment that is sensitive to light. Light reversibly changes the shape of the pigment molecules. This process leads to electrical changes in the receptor membrane that in turn lead to the propagation of a nerve signal. In some animals, such as the earthworm, photoreceptors are scattered over the skin. Usually, however, photoreceptors are clustered together to form an eye. Primitive eyes detect only the presence or absence of light. In the more advanced vertebrate eye, there are two types of receptors: rods and cones. Rods are elongated and sensitive to low levels of illumination. This vision is colorless and has poor definition. Rods are predominant in nocturnal animals, for which increased sensitivity is important. Cones are sensitive to high levels of illumination and produce a sharp picture. Unlike rods, cones contain more than one type of photopigment, each sensitive to different wavelengths of light. Cones provide color vision.
Toads, like many animals, detect their prey visually. A shape that is long in the horizontal direction looks like a worm, and so the toad's brain interprets that as food. A square shape elicits no reaction from the toad, and a tall, thin shape is seen by the toad as the "anti-worm."
No lateral inhibition
Assume that neurons fire when light hits them, and do not fire when there is no light. Edges would appear "fuzzy" because neurons near the edge of the shape's shadow would fire somewhat.
Now, let's look at a system that includes lateral inhibition, a process by which neurons that fire due to light inhibit their neighboring neurons. Neurons in the middle of a light-receiving area will be inhibited slightly by their neighboring neurons, which are also receiving light. However, neurons on a black/white edge fire more intensely than nearby neurons receiving exactly the same amount of light because neurons on this edge are not being inhibited by their nearest neighbors in the dark areas, which is not firing. This allows edges to be seen with greater definition. Look at a black shape against a white piece of paper. Does the white right at the edge of the shape look brighter than the rest of the paper? Well-defined shapes can be detected using complex systems based on this principle of light and dark patterns.