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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.
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