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Neurons are the principal cells in the brain that process information. There is a great diversity of neuronal cell types based on morphology, chemistry, location, and connections.
The nucleus and major cytoplasmic organelles in the cell body of neurons synthesize and process proteins, which are subsequently transported to their appropriate locations within the neuron.
The axon transports molecules and conducts action potentials to presynaptic terminals to initiate communication with other neurons, which occurs at synapses.
Dendrites, multiple fine processes that extend from the neuronal cell body, together with the cell body, serve as the primary structure for the reception of synaptic contacts from other neurons.
The cytoskeleton—the inner scaffold of a neuron formed by a system of interconnected protein filaments called microtubules, intermediate filaments, and actin filaments—plays a key role in the structure of neurons and in the transport of various proteins and organelles from the cell body to axonal and dendritic processes.
Three major classes of glia—astrocytes, oligodendrocytes, and microglia—play important roles in brain function.
The blood–brain barrier, formed by tight junctions between endothelial cells of capillaries in cerebral vascular beds, allows only small lipophilic substances to enter the brain from the general circulation.
In their resting state, neurons maintain a negative electrical potential in relation to the extracellular environment. This results from differences between the intracellular and extracellular concentrations of K+, Na+, and Cl− and the relative permeability of the cell membrane to these and other ions. The energy-consuming Na+/K+ pump helps to maintain appropriate ionic gradients across the membrane.
The generation of all-or-none action potentials relies on the activities of voltage-dependent ion channels, highly specialized proteins that allow the flow of a specific ion (K+, Na+, or Ca2+) across neuronal membranes in response to changes in neuronal membrane potential.
Sodium channels are the targets of many important drugs including local anesthetics and some antiseizure medications.
The three general classes of potassium channels include voltage-gated potassium channels, calcium-activated potassium channels, and inward rectifiers.
Entry of calcium into neurons through voltage-dependent calcium channels, of which there are five major classes—L-type, N-type, T-type, P/Q-type, and R-type—is important for neurotransmitter release and activation of intracellular signaling cascades. L-type calcium channel blockers are used to treat ischemic heart disease and hypertension.
Mutations in ion channels are the cause of several neurologic disorders, including certain inherited neuromuscular disorders, epilepsy, and migraine syndromes.
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It has been estimated that there are at least 100 billion neurons in the human brain, although this number in itself reveals little of the brain’s complexity. Unlike other organs, the brain contains an enormous diversity of cell types. Depending on the definition of a neuronal cell type, there may be thousands in the brain—each with their own structural, biochemical, and functional properties. Yet, the complexity of the brain as an information-processing organ extends beyond its cellular diversity. ...