The two major divisions of the nervous system are the [1] central nervous system, consisting of the brain and spinal cord, and the [2] peripheral nervous system, consisting of nerves and ganglia. Neurons that conduct signals from receptors to division 1 are called [3] sensory, or afferent neurons; those that conduct signals from division 1 to muscles and glands are called [4] motor, or efferent neurons; and those contained entirely within 1 and concerned mainly with neural integration are called [5] interneurons . To carry out their functions, neurons must have the properties of [6] excitability, or irritability , or responsiveness to stimuli; [7] conductivity , the ability to transmit signals over long distances; and [8] secretion , the ability to release chemicals that stimulate another cell.

The nervous system has about 1 trillion neurons and up to 50 trillion [9] neuroglia (glial cells) . Among the latter are [10] Schwann cells, which form wrappings around peripheral nerve fibers; [11] oligodendrocytes , which do the same in the CNS; and [12] astrocytes , which help form the blood-brain barrier and repair damaged nervous tissue. A typical neuron has a cell body, one or more short processes called [13] dendrites that receive signals and conduct them to the cell body, and one long process called the nerve fiber, or [14] axon , which conducts signals away from the cell body. The nerve fiber usually branches extensively at its distal end, and each branch ends in a little swelling called a [15] synaptic knob , where the neuron releases its neurotransmitter. Chemicals and organelles move up and down the nerve fiber by a process called [16] axonal transport . Neurons are outnumbered as much as 50:1 by six kinds of [17] glial cells. Two of these six types often produce an insulating [18] myelin sheath of lipid around nerve fibers: oligodendrocytes in the central nervous system and [19] Schwann cells in the peripheral nervous system. The sheath is segmented, with a gap called a [20] node of Ranvier every 0.2 to 1.0 mm along its length. Nerve fibers without this sheath are called [21] unmyelinated fibers and usually conduct signals more slowly than those with a sheath. In the peripheral nervous system, the cells that form this sheath also help to form a [22] regeneration tube when a nerve fiber is damaged. As the damaged nerve fiber regrows, this structure guides it back to its original target cell. Other non-neuronal cells in the central nervous system include [23] astrocytes , which cover the brain surface, stimulate the formation of a blood-brain barrier, and form scar tissue when the brain is injured, and [24] ependymal cells, which line the internal cavities of the brain and spinal cord.

When a dendrite or the soma of a neuron is stimulated, it exhibits a small voltage change across its plasma membrane called the [25] local potential . This results mainly from the inflow of [26] sodium (Na+) ions through open channels in the membrane. As these ions spread out under the plasma membrane, they make the interior of the cell less [27] negative in charge. If this effect is still strong enough by the time it reaches the trigger zone of the neuron, it can stimulate a more dramatic voltage change, the [28] action potential , there. This change is set off only if the voltage change reaches a critical level called the [29] threshold . If it does so, then [30] sodium ions rush into the cell and the voltage rises to about +30 mV. At that point, the inflow of these ions ceases, and the overriding outflow of [31] potassium ions causes the voltage to drop again to slightly lower than –70 mV. The voltage change at 28 always rises to the same level, according to the [32] all-or-none law. While this change is underway, the neuron exhibits an absolute and then relative [33] refractory period in which it is impossible or difficult to restimulate it. In an unmyelinated fiber, a/an 28 stimulates [34] voltage-gated sodium channels immediately distal to this, causing them to open and produce a new 28. This process repeats itself down the entire nerve fiber, resulting in a nerve signal that travels from the soma to the end of the fiber. In a myelinated fiber, however, this process can only occur at the gaps called the [35] nodes of Ranvier . From one of these gaps to the next, the nerve signal travels by the diffusion of ions under the plasma membrane, but since this is a very fast process, signal conduction is much faster in this type of fiber than in unmyelinated fibers. Since the 28s seem to jump from one gap to the next, this type of signal transmission is called [36] saltatory conduction .

Nerve signals arrive at a synapse by way of the [37] presynaptic neuron, which must communicate across the synaptic cleft with the [38] postsynaptic neuron. It does this by secreting chemicals collectively known as [39] neurotransmitters , though neurons also secrete amino acid chains called [40] neuropeptides to modify the response of the second cell to 39s. A very common 39, and the first one discovered, is [41] acetylcholine . Neurons that release this chemical are called [42] cholinergic neurons. The effect of this chemical is to bind to the second neuron and create a local voltage change called a [43] postsynaptic potential . Some other neuronal chemicals, however, act by causing the second cell to produce a second messenger such as cAMP. This is how the [44] catecholamines commonly work—a class of neurotransmitters that includes norepinephrine and dopamine. To stop the transmission across a synapse, the chemical may diffuse out of the synaptic cleft, it may be broken down by an enzyme, or it may exhibit [45] reuptake into the neuron that secreted it. It has recently been discovered that some 38 neurons release gases such as carbon monoxide or [46] nitric oxide , which stimulate the 37 neuron and alter synaptic transmission.

Synapses slow down the transmission of information but have the overriding advantage of being the site where the nervous system makes [47] decisions . It does this by means of a balance between voltage changes called [48] excitatory postsynaptic potentials (EPSPs) , which bring a postsynaptic cell closer to firing, and [49] inhibitory postsynaptic potentials (IPSPs) , which make the postsynaptic cell less likely to fire. The former effect [50] depolarizes the plasma membrane, which means to make its membrane potential less negative. The latter effect [51] hyperpolarizes the membrane, which means to make it even more negative than the resting membrane potential. In the process of [52] summation , a postsynaptic neuron adds up incoming, often contradictory, information and either fires or doesn’t fire, depending on the net effect of this input. There are two forms of 52—[53] temporal summation , in which a postsynaptic neuron fires because many 48s occur at short time intervals and bring the cell to threshold, and [54] spatial summation , in which several different input neurons contribute 48s and collectively depolarize it to threshold. [55] Neural coding is the process in which the nervous system converts stimulus information to a simple but meaningful pattern of action potentials. For example, a stronger stimulus may excite different [56] fibers in the sensory nerves, thus a different pathway is established. Neurons of the CNS commonly function in groups called [57] neuronal pools that are dedicated to a particular function. Within these groups, neurons have certain connection pathways called neuronal circuits. As an example of this, if a signal arriving through one input neuron results in output through multiple output neurons, we call the pathway a [58] diverging circuit . If a neuron late in the pathway restimulates one early in the pathway, so that the group produces long-term, repetitive output, we call the group a [59] reverberating circuit . The repeated use of a synapse may make it easier for future signals to travel that route; this effect, called [60] synaptic potentiation , is one basis for memory.