For example, glia:. The brain is made up of many networks of communicating neurons and glia. The nervous system has two main parts: The central nervous system is made up of the brain and spinal cord.
The peripheral nervous system is made up of nerves that branch off from the spinal cord and extend to all parts of the body. For example, glia: Help support and hold neurons in place Protect neurons Create insulation called myelin, which helps move nerve impulses Repair neurons and help restore neuron function Trim out dead neurons Regulate neurotransmitters The brain is made up of many networks of communicating neurons and glia.
Brain facts. Washington, DC. Brain basics: Know your brain. Motor neurons carry messages away from the brain to the rest of the body. All neurons, however, relay information to each other through a complex electrochemical process, making connections that affect the way we think, learn, move, and behave. Intelligence, learning, and memory.
As we grow and learn, messages travel from one neuron to another over and over, creating connections, or pathways, in the brain. It's why driving takes so much concentration when someone first learns it, but later is second nature: The pathway became established. In young children, the brain is highly adaptable. In fact, when one part of a young child's brain is injured, another part often can learn to take over some of the lost function.
But as we age, the brain has to work harder to make new neural pathways, making it harder to master new tasks or change set behavior patterns. That's why many scientists believe it's important to keep challenging the brain to learn new things and make new connections — it helps keeps the brain active over the course of a lifetime.
Memory is another complex function of the brain. The things we've done, learned, and seen are first processed in the cortex. Then, if we sense that this information is important enough to remember permanently, it's passed inward to other regions of the brain such as the hippocampus and amygdala for long-term storage and retrieval.
As these messages travel through the brain, they too create pathways that serve as the basis of memory. Different parts of the cerebrum move different body parts.
The left side of the brain controls the movements of the right side of the body, and the right side of the brain controls the movements of the left side of the body. When you press your car's accelerator with your right foot, for example, it's the left side of your brain that sends the message allowing you to do it. Basic body functions. A part of the peripheral nervous system called the autonomic nervous system controls many of the body processes we almost never need to think about, like breathing, digestion, sweating, and shivering.
The autonomic nervous system has two parts: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system prepares the body for sudden stress, like if you witness a robbery. When something frightening happens, the sympathetic nervous system makes the heart beat faster so that it sends blood quickly to the different body parts that might need it.
It also causes the adrenal glands at the top of the kidneys to release adrenaline, a hormone that helps give extra power to the muscles for a quick getaway. This process is known as the body's "fight or flight" response.
The parasympathetic nervous system does the exact opposite: It prepares the body for rest. It also helps the digestive tract move along so our bodies can efficiently take in nutrients from the food we eat. Sight probably tells us more about the world than any other sense.
Light entering the eye forms an upside-down image on the retina. The retina transforms the light into nerve signals for the brain. The brain then turns the image right-side up and tells us what we are seeing. Every sound we hear is the result of sound waves entering our ears and making our eardrums vibrate.
These vibrations then move along the tiny bones of the middle ear and turned into nerve signals. The cortex processes these signals, telling us what we're hearing. The tongue contains small groups of sensory cells called taste buds that react to chemicals in foods. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients.
Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by Geoffroy Saint-Hilaire for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline Lichtneckert and Reichert, Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way.
As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected to each other by transverse nerves resembling the rungs of a ladder. These transverse nerves help coordinate movement of the two sides of the animal.
Two ganglia at the head end function as a simple brain. Photoreceptors in the animal's eyespots provide sensory information on light and dark Adey, WR. The nervous system of one particular type of nematode, the tiny roundworm Caenorhabditis elegans , has been mapped out down to the synaptic level. This has been possible because in this species, every individual worm ignoring mutations and sex differences has an identical set of neurons, with the same locations and chemical features, and the same connections to other cells.
Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are mapped. The nervous system of C. Males have exactly neurons, while hermaphrodites have exactly neurons Hobert, , an unusual feature called eutely. Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a pair of ventral nerve cords running along the length of the abdomen Chapman, Most body segments have one ganglion on each side, but some are fused to form the brain and other large ganglia.
The head segment contains the brain, also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles.
Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain. In arthropods, most neurons have cell bodies that are positioned at the edge of the brain and are electrically passive — the cell bodies serve only to provide metabolic support and do not participate in signalling.
A protoplasmic fiber, called the primary neurite, runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called "neuropil", in the interior Chapman, There are, however, important exceptions to this rule, including the mushroom bodies, which play a central role in learning and memory.
A neuron is called identified if it has properties that distinguish it from every other neuron in the same animal — such as location, neurotransmitter, gene expression pattern, and connectivity — and if every individual organism belonging to the same species has one and only one neuron with the same set of properties Hoyle and Wiersma, In vertebrate nervous systems very few neurons are "identified" in this sense — in humans, there are believed to be none — but in simpler nervous systems, some or all neurons may be thus unique.
As mentioned above, in the roundworm Caenorhabditis Elegans every neuron in the body is uniquely identifiable, with the same location and the same connections in every individual worm. The brains of many molluscs and insects also contain substantial numbers of identified neurons Hoyle and Wiersma, In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish Stein, Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right.
Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then traveling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape, then straightens, thereby propelling itself rapidly forward.
Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish — there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response.
Mauthner cells have been described as "command neurons". A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually Stein, , p.
Such neurons appear most commonly in the fast escape systems of various species — the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances Simmons and Young, The ultimate function of the nervous system is to control the body, especially its movement in the environment.
It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perceptual capabilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals.
In humans, the sophistication of the nervous system makes it possible to have language , abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.
At the most basic level, the nervous system sends signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells.
One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals — neurons project their axons to specific target areas and make synaptic connections with specific target cells. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed meters per second.
Most neurons send signals via their axons, although some types are capable of emitting signals from their dendrites. In fact, some types of neurons such as the amacrine cells of the retina have no axon, and communicate only via their dendrites.
Neural signals propagate along an axon in the form of electrochemical waves called action potentials , which emit cell-to-cell signals at points of contact called "synapses". Synapses may be electrical or chemical. Electrical synapses pass ions directly between neurons Hormuzdi et al.
At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic regions of contact are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles, packed with neurotransmitter chemicals.
When calcium enters the presynaptic terminal through voltage-gated calcium channels, an arrays of molecules embedded in the membrane are activated, and cause the contents of some vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the synaptic cleft.
The neurotransmitter then binds to chemical receptors embedded in the postsynaptic membrane, causing them to enter an activated state. Depending on the type of receptor, the effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell depolarizes the muscle cell and starts a series of events, which results in a contraction of the muscle cell.
The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer even indefinitely, in cases where the synaptic signal leads to the formation of a memory trace. There are literally hundreds of different types of synapses, even within a single species. In fact, there are over a hundred known neurotransmitter chemicals, and many of them activate multiple types of receptors.
Many synapses use more than one neurotransmitter — a common arrangement is for a synapse to use one fast-acting small-molecule neurotransmitter such as glutamate or GABA , along with one or more peptide neurotransmitters that play slower-acting modulatory roles. Neuroscientists generally divide receptors into two broad groups: ligand-gated ion channels and G-protein coupled receptors GPCRs that rely on second messenger signaling.
When a ligand-gated ion channel is activated, it opens a channel that allow specific types of ions to flow across the membrane. Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory by bringing the membrane potential closer or farther from threshold for triggering an action potential. When a GPCR is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription.
According to Dale's principle, which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses Strata and Harvey, This does not mean, though, that a neuron exerts the same effect on all of its targets, because the effect of a synapse depends not on the neurotransmitter, but on the receptors that it activates. Because different targets can and frequently do use different types of receptors, it is possible for a neuron to have excitatory effects on one set of target cells, inhibitory effects on others, and complex modulatory effects on others still.
Nevertheless, it happens that the two most widely used neurotransmitters, glutamate and gamma-Aminobutyric acid GABA , each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory.
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