Characteristics of the Central Nervous System

The largest part of the brain, divided into two hemispheres, is the [1]. It is marked by numerous twisted folds called [2] separated by shallow grooves called [3]. The second-largest part of the brain, the [4], is located at the rear of the cranial cavity beneath 1. If 1 and 4 are removed, there remains a stalklike [5]. The neurons of the central nervous system have their somas and dendrites concentrated in tissue called the [6], whereas the [7] gets its color from their myelinated axons. At 4 weeks, a human embryo shows a hollow channel called the [8] along the dorsal side. This channel then begins to dilate anteriorly and differentiate into major regions of the future brain. These include the [9], midbrain, and [10].

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Meninges, Cerebrospinal Fluid, Ventricles, and Blood Supply

The meninges are three fibrous membranes that envelop the CNS. The outermost meninx is the tough [11]. The middle layer is a more loosely organized [12], and the inner meninx is a thin, translucent [13] that closely follows the contours of the brain and spinal cord. The brain and spinal cord are hollow and contain a liquid called [14], which is secreted by capillary networks called the [15]. The four fluid-filled cavities of the brain are called the [16]. Fluid is secreted into these by 15 and eventually flows out of the [17] ventricle to bathe the external surfaces of the brain and spinal cord. From there, the fluid is absorbed by [18], which are extensions of the middle meninx. Although the brain is richly supplied with blood vessels, there is a [19] which strictly regulates what substances can exit the bloodstream and get into the brain tissue. This is lacking from regions called the [20], where such pathogens as HIV can invade the brain.

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The Spinal Cord

The spinal cord extends from the foramen magnum to vertebra [21]. It gives rise to 31 pairs of [22] at regular intervals along its length. The gray matter of the cord is arranged in a butterfly-like shape with two [23], which receive sensory input, and two [24] where motor output originates. The white matter is divided into six major regions called [25], and each of these is subdivided into several bundles of nerve fibers called [26]. Ascending 26s carry information up the cord to the [27]. Two of these are the [28] gracilis and 28 cuneatus, which form the entire dorsal column. The [29] 26s carry signals down the cord, and have names ending in -spinal.

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The Hindbrain and Midbrain

When the spinal cord enters the foramen magnum, it gives rise to part of the brain called the [30]. This contains nuclei for cardiovascular and respiratory control, gives rise to the last four cranial nerves, and conducts signals forward to the next part of the brainstem, the [31]. This region has nuclei that control sleep, posture, respiration, and urination, among other functions. This and the larger [32] are the only two structures derived from the embryonic metencephalon. The latter structure consists of two hemispheres connected by a wormlike [33]. It is concerned with motor skills and modulation of skeletal muscle contractions. Continuing forward from 30, the next part of the brainstem is the [34], which includes nuclei concerned with auditory and visual reflexes and other eye movements. The brainstem regions up to this point contain a group of more than 100 nuclei collectively called the [35], concerned with autonomic functions, pain modulation, alertness, and arousal.

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The Diencephalon

The largest part of the diencephalon is the [36], which bulges into the lateral ventricle under each cerebral hemisphere. It acts as a relay point for nearly all signals going from lower parts of the brain to their destinations in the [37]. Inferior to 36 is the funnellike [38], which forms the walls and floor of the [39] ventricle. Among other things, it contains a [40], which controls body temperature, and neurons called [41], which monitor the osmolarity of the blood. The pineal gland and habenula constitute a small superior part of the diencephalon called the [42].

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The Telencephalon (Cerebrum)

Most of the cerebrum is composed of white matter, which consists of three kinds of [43] that carry signals from one part of the brain to another. Those that communicate between the right and left cerebral hemispheres pass mainly through a large C-shaped [44]. The gray matter forms a surface layer called the cerebral [45]. This is divided into five lobes–a deep [46], which cannot be seen at the brain surface, and four others named for the bones overlying them. Of these four lobes, voluntary motor control and social judgment are located in the [47] lobe, visual signals are received in the [48] lobe, hearing is associated with the [49] lobe, and most other senses such as touch and pain throughout the body are processed in the [50] lobe. The most characteristic neurons of the gray matter are triangular-looking [51] cells. In regions called the [52], these neurons and glial cells form six histologically recognizable layers. Gray matter forms not only the surface of the cerebrum, but also a deep cluster of six [53] in each hemisphere, concerned mainly with motor control. Many primitive emotional functions reside in the [54], a ring of structures on the medial border of the temporal lobe and encircling the corpus callosum and thalamus.

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Functions of the Neocortex

Neurological disorders can often be diagnosed with the aid of a/an [55], a recording of the brain waves. These brain waves show predictable changes as one falls asleep, with delta waves dominating in the deepest stage, called [56] sleep. Most dreaming occurs during a stage called [57] because of flickering movements of the eyes that occur then. Voluntary muscle contractions originate with neurons in the [58] gyrus of the cerebrum, which forms the posterior limit of the frontal lobe. Somatic sensation resides in a corresponding [59] gyrus just posterior to this. Both of these gyri exhibit [60], an inverted "map" of the opposite side of the body. The special senses such as smell, vision, and hearing project to other specialized regions of the cerebral cortex. In each case, the primary sensory cortex is adjacent to an [61] area of neurons that interpret the sensory information. The two cerebral hemispheres look similar but are specialized for different functions. This division of labor between the right and left sides is called cerebral [62]. The comprehension of written and spoken language resides in the [63] area slightly anterior to the visual cortex. This area is also where one decides what to say. The [64] area of the inferior frontal cortex generates a motor program, or plan, to say what one has decided to say. Lesions to these areas produce various forms of language defects called [65]. Memory is a complex function involving multiple regions of the cerebrum. Two forms of long-term memory are [66], which is a memory for facts that one can state, as in taking an exam; and [67] memory, which is a memory for motor skills such as handwriting. The hippocampus and amygdaloid, two components of the [68] system, are important for the formation of memories.

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