An Overview of the Nervous System

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

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Cells of the Nervous System

The nervous system has about 1 trillion neurons and up to 50 trillion [9] cells. Among the latter are [10] cells, which form wrappings around peripheral nerve fibers; [11], which do the same in the CNS; and [12], which help form the blood-brain barrier and repair damaged nervous tissue. A neuron typically consists of a cell body; numerous short, branched processes called [13] that conduct signals to the cell body; and a long, less branched [14] that conducts signals away from the cell body. The last of these branches extensively at its distal end, and each branch ends in a swelling called the [15]. Molecules and organelles move between the soma and the distal end of the axon by means of [16], which may be fast or slow, anterograde or [17]. All peripheral nerve fibers have a layer called the [18] around them, formed by the cells at 10. In many cases, these cells produce a [19] sheath between the nerve fiber and 18, composed of many spiral layers of their own plasma membrane. This sheath has gaps called [20] about every 1 mm. The region from the axon hillock of the cell body to the first segment of this sheath is called the [21] because nerve signals originate here. Fibers without such a sheath are said to be [22]. When a peripheral nerve fiber is damaged, it can often repair itself by growing into a [23] tube formed by the 10 cells and a thin connective tissue sheath, the [24], outside that.

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Electrophysiology of Neurons

When a dendrite or the soma of a neuron is stimulated, it exhibits a small voltage change across its plasma membrane called the [25]. This results mainly from the inflow of [26] ions through open channels in the membrane. As these ions spread out under the plasma membrane, they make the interior of the cell more [27] 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], there. This change is set off only if the voltage change reaches a critical level called the [29]. If it does so, then [30] 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] ions causes the voltage to drop again to slightly lower than -70 mV. The voltage change at 27 always rises to the same level, according to the [32] law. While this change is underway, the neuron exhibits an absolute and then relative [33] period in which it is impossible or difficult to restimulate it. In an unmyelinated fiber, a 28 stimulates [34] 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]. 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].

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Synapses

Nerve signals arrive at a synapse by way of the [37] neuron, which must communicate across the synaptic cleft with the [38] neuron. It does this by secreting chemicals collectively known as [39], though neurons also secrete amino acid chains called [40] to modify the response of the second cell to 39s. A very common 39, and the first one discovered, is [41]. Neurons that release this chemical are called [42] neurons. The effect of this chemical is to bind to the second neuron and create a local voltage change called a [43]. Some other neuronal chemicals, however, act by causing the second cell to produce a second messenger such as cAMP. This is how the [44] commonly work–a class of neurotransmitters than 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] into the neuron that secreted it. It has recently been discovered that some 38 neurons release gases such as carbon monoxide or [46], which stimulate the 37 neuron and alter synaptic transmission.

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Neural Integration

Synapses slow down the transmission of information but have the overriding advantage of being the site where the nervous system makes [47]. It does this by means of a balance between voltage changes called [48], which bring a postsynaptic cell closer to firing, and [49], which make the postsynaptic cell less likely to fire. The former effect [50] the plasma membrane, which means to make its membrane potential less negative. The latter effect [51] the membrane, which means to make it even more negative than the resting membrane potential. In the process of [52], 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], in which a postsynaptic neuron fires because many 48s occur at short time intervals and bring the cell to threshold, and [54], in which several different input neurons contribute 48s and collectively depolarize it to threshold. [55] 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 be represented by a higher firing [56] than a weaker stimulus. The repeated use of a synapse may make it easier for future signals to travel that route; this effect, called [57], is one basis for memory. Neurons of the CNS commonly function in groups called [58] 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 [59]. If a neuron late in the pathway restimulates one early in the pathway, so the group produces long-term, repetitive output, we call the group a [60].

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