The functions of the<unk>  depend on the ability of neurons to transmit electrochemical signals to other cells, and their ability to respond appropriately to electrochemical signals received from other cells. The electrical properties of neurons are controlled<unk> y a wide variety of<unk> iochemical and metabolic processes, most notably the interactions<unk> etween neurotransmitters and receptors that take place at synapses. Neurotransmitters and receptors Neurotransmitters are chemicals that are released at synapses when the local membrane is depolarised and Ca2+ enters into the cell, typically when an action potential arrives at the synapse – neurotransmitters attach themselves to receptor molecules on the membrane of the synapse's target cell (or cells), and thereby alter the electrical or chemical properties of the receptor molecules. With few exceptions, each neuron in the<unk>  releases the same chemical neurotransmitter, or combination of neurotransmitters, at all the synaptic connections it makes with other neurons; this rule is known as Dale's principle. Thus, a neuron can<unk> e characterized<unk> y the neurotransmitters that it releases. The great majority of psychoactive drugs exert their effects<unk> y altering specific neurotransmitter systems. This applies to drugs such as cannabinoids, nicotine, heroin, cocaine, alcohol, fluoxetine, chlorpromazine, and many others. The two neurotransmitters that are most widely found in the vertebrate<unk>  are glutamate, which almost always exerts excitatory effects on target neurons, and gamma-aminobutyric acid (GABA), which is almost always inhibitory. Neurons using these transmitters can<unk> e found in nearly every part of the<unk> .<unk> ecause of their ubiquity, drugs that act on glutamate or GABA tend to have<unk> road and powerful effects. Some general anesthetics act<unk> y reducing the effects of glutamate; most tranquilizers exert their sedative effects<unk> y enhancing the effects of GABA. There are dozens of other chemical neurotransmitters that are used in more limited areas of the<unk> , often areas dedicated to a particular function. Serotonin, for example—the primary target of many antidepressant drugs and many dietary aids—comes exclusively from a small<unk> stem area called the raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the<unk> <unk> ut are not as ubiquitously distributed as glutamate and GABA. Electrical activity epileptic seizure. As a side effect of the electrochemical processes used<unk> y neurons for signaling,<unk>  tissue generates electric fields when it is active. When large numbers of neurons show synchronized activity, the electric fields that they generate can<unk> e large enough to detect outside the skull, using electroencephalography (EEG) or magnetoencephalography (MEG). EEG recordings, along with recordings made from electrodes implanted inside the<unk> s of animals such as rats, show that the<unk>  of a living animal is constantly active, even during sleep. Each part of the<unk>  shows a mixture of rhythmic and nonrhythmic activity, which may vary according to<unk> ehavioral state. In mammals, the cerebral cortex tends to show large slow delta waves during sleep, faster alpha waves when the animal is awake<unk> ut inattentive, and chaotic-looking irregular activity when the animal is actively engaged in a task, called<unk> eta and gamma waves. During an epileptic seizure, the<unk> 's inhibitory control mechanisms fail to function and electrical activity rises to pathological levels, producing EEG traces that show large wave and spike patterns not seen in a healthy<unk> . Relating these population-level patterns to the computational functions of individual neurons is a major focus of current research in neurophysiology. Metabolism All vertebrates have a<unk> lood–brain<unk> arrier that allows metabolism inside the<unk>  to operate differently from metabolism in other parts of the<unk> ody. Glial cells play a major role in<unk>  metabolism<unk> y controlling the chemical composition of the fluid that surrounds neurons, including levels of ions and nutrients.<unk>  tissue consumes a large amount of energy in proportion to its volume, so large<unk> s place severe metabolic demands on animals. The need to limit<unk> ody weight in order, for example, to fly, has apparently led to selection for a reduction of<unk>  size in some species, such as<unk> ats. Most of the<unk> 's energy consumption goes into sustaining the electric charge (membrane potential) of neurons. Most vertebrate species devote<unk> etween 2% and 8% of<unk> asal metabolism to the<unk> . In primates, however, the percentage is much higher—in humans it rises to 20–25%. The energy consumption of the<unk>  does not vary greatly over time,<unk> ut active regions of the cerebral cortex consume somewhat more energy than inactive regions; this forms the<unk> asis for the functional<unk>  imaging methods of PET, fMRI, and NIRS. The<unk>  typically gets most of its energy from oxygen-dependent metabolism of glucose (i.e.,<unk> lood sugar),<unk> ut ketones provide a major alternative source, together with contributions from medium chain fatty acids (caprylic and heptanoic acids), lactate, acetate, and possibly amino acids.
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