Cross section of the olfactory<unk> ulb of a rat, stained in two different ways at the same time: one stain shows neuron cell<unk> odies, the other shows receptors for the neurotransmitter GABA.|262x262px The shape and size of the<unk>  varies greatly<unk> etween species, and identifying common features is often difficult. Nevertheless, there are a number of principles of<unk>  architecture that apply across a wide range of species. Some aspects of<unk>  structure are common to almost the entire range of animal species; others distinguish "advanced"<unk> s from more primitive ones, or distinguish vertebrates from invertebrates. The simplest way to gain information about<unk>  anatomy is<unk> y visual inspection,<unk> ut many more sophisticated techniques have<unk> een developed.<unk>  tissue in its natural state is too soft to work with,<unk> ut it can<unk> e hardened<unk> y immersion in alcohol or other fixatives, and then sliced apart for examination of the interior. Visually, the interior of the<unk>  consists of areas of so-called grey matter, with a dark color, separated<unk> y areas of white matter, with a lighter color. Further information can<unk> e gained<unk> y staining slices of<unk>  tissue with a variety of chemicals that<unk> g out areas where specific types of molecules are present in high concentrations. It is also possible to examine the microstructure of<unk>  tissue using a microscope, and to trace the pattern of connections from one<unk>  area to another. Cellular structure Neurons generate electrical signals that travel along their axons. When a pulse of electricity reaches a junction called a synapse, it causes a neurotransmitter chemical to<unk> e released, which<unk> inds to receptors on other cells and thereby alters their electrical activity. The<unk> s of all species are composed primarily of two<unk> road classes of cells: neurons and glial cells. Glial cells (also known as glia or neuroglia) come in several types, and perform a number of critical functions, including structural support, metabolic support, insulation, and guidance of development. Neurons, however, are usually considered the most important cells in the<unk> . The property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals<unk> y means of an axon, which is a thin protoplasmic fiber that extends from the cell<unk> ody and projects, usually with numerous<unk> ches, to other areas, sometimes nearby, sometimes in distant parts of the<unk>  or<unk> ody. The length of an axon can<unk> e extraordinary: for example, if a pyramidal cell (an excitatory neuron) of the cerebral cortex were magnified so that its cell<unk> ody<unk> ecame the size of a human<unk> ody, its axon, equally magnified, would<unk> ecome a cable a few centimeters in diameter, extending more than a kilometer. These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials constantly, at rates of 10–100 per second, usually in irregular patterns; other neurons are quiet most of the time,<unk> ut occasionally emit a<unk> urst of action potentials. Axons transmit signals to other neurons<unk> y means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells. When an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to<unk> e released. The neurotransmitter<unk> inds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the<unk> . The essential function of the<unk>  is cell-to-cell communication, and synapses are the points at which communication occurs. The human<unk>  has<unk> een estimated to contain approximately 100 trillion synapses; even the<unk>  of a fruit fly contains several million. The functions of these synapses are very diverse: some are excitatory (exciting the target cell); others are inhibitory; others work<unk> y activating second messenger systems that change the internal chemistry of their target cells in complex ways. A large number of synapses are dynamically modifiable; that is, they are capable of changing strength in a way that is controlled<unk> y the patterns of signals that pass through them. It is widely<unk> elieved that activity-dependent modification of synapses is the<unk> 's primary mechanism for learning and memory. Most of the space in the<unk>  is taken up<unk> y axons, which are often<unk> undled together in what are called nerve fiber tracts. A myelinated axon is wrapped in a fatty insulating sheath of myelin, which serves to greatly increase the speed of signal propagation. (There are also unmyelinated axons). Myelin is white, making parts of the<unk>  filled exclusively with nerve fibers appear as light-colored white matter, in contrast to the darker-colored grey matter that marks areas with high densities of neuron cell<unk> odies. Evolution Generic<unk> ilaterian nervous system Nervous system of a generic<unk> ilaterian animal, in the form of a nerve cord with segmental enlargements, and a<unk> " at the front. Except for a few primitive organisms such as sponges (which have no nervous system) and cnidarians (which have a nervous system consisting of a diffuse nerve net), all living multicellular animals are<unk> ilaterians, meaning animals with a<unk> ilaterally symmetric<unk> ody shape (that is, left and right sides that are approximate mirror images of each other). All<unk> ilaterians are thought to have descended from a common ancestor that appeared late in the Cryogenian period, 700–650 million years ago, and it has<unk> een hypothesized that this common ancestor had the shape of a simple tubeworm with a segmented<unk> ody. At a schematic level, that<unk> asic worm-shape continues to<unk> e reflected in the<unk> ody and nervous system architecture of all modern<unk> ilaterians, including vertebrates. The fundamental<unk> ilateral<unk> ody form is a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a ganglion) for each<unk> ody segment, with an especially large ganglion at the front, called the<unk> . The<unk>  is small and simple in some species, such as nematode worms; in other species, including vertebrates, it is the most complex organ in the<unk> ody. Some types of worms, such as leeches, also have an enlarged ganglion at the<unk> ack end of the nerve cord, known as a "tail<unk> ". There are a few types of existing<unk> ilaterians that lack a recognizable<unk> , including echinoderms and tunicates. It has not<unk> een definitively established whether the existence of these<unk> less species indicates that the earliest<unk> ilaterians lacked a<unk> , or whether their ancestors evolved in a way that led to the disappearance of a previously existing<unk>  structure. Invertebrates Fruit flies (Drosophila) have<unk> een extensively studied to gain insight into the role of genes in<unk>  development. This category includes tardigrades, arthropods, molluscs, and numerous types of worms. The diversity of invertebrate<unk> ody plans is matched<unk> y an equal diversity in<unk>  structures. Two groups of invertebrates have notably complex<unk> s: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and similar molluscs). The<unk> s of arthropods and cephalopods arise from twin parallel nerve cords that extend through the<unk> ody of the animal. Arthropods have a central<unk> , the supraesophageal ganglion, with three divisions and large optical lobes<unk> ehind each eye for visual processing. Cephalopods such as the octopus and squid have the largest<unk> s of any invertebrates. There are several invertebrate species whose<unk> s have<unk> een studied intensively<unk> ecause they have properties that make them convenient for experimental work: * Fruit flies (Drosophila),<unk> ecause of the large array of techniques available for studying their genetics, have<unk> een a natural subject for studying the role of genes in<unk>  development. In spite of the large evolutionary distance<unk> etween insects and mammals, many aspects of Drosophila neurogenetics have<unk> een shown to<unk> e relevant to humans. The first<unk> iological clock genes, for example, were identified<unk> y examining Drosophila mutants that showed disrupted daily activity cycles. A search in the genomes of vertebrates revealed a set of analogous genes, which were found to play similar roles in the mouse<unk> iological clock—and therefore almost certainly in the human<unk> iological clock as well. Studies done on Drosophila, also show that most neuropil regions of the<unk>  are continuously reorganized throughout life in response to specific living conditions. * The nematode worm Caenorhabditis elegans, like Drosophila, has<unk> een studied largely<unk> ecause of its importance in genetics. In the early 1970s, Sydney<unk> renner chose it as a model organism for studying the way that genes control development. One of the advantages of working with this worm is that the<unk> ody plan is very stereotyped: the nervous system of the hermaphrodite contains exactly 302 neurons, always in the same places, making identical synaptic connections in every worm.<unk> renner's team sliced worms into thousands of ultrathin sections and photographed each one under an electron microscope, then visually matched fibers from section to section, to map out every neuron and synapse in the entire<unk> ody. The complete neuronal wiring diagram of C.elegans – its connectome was
Brain
Heart
Central nervous system
Peripheral nervous system
Head
Liver
Skin
Kidney