“In the case of blind-sight, people have damaged the part of the brain that allows them to have conscious awareness of vision, but visual information goes to other parts of their brain and they are able to act in ways that show that this visual information is actually reaching those other parts of their brain, even though they have no conscious awareness of being able to see.”
Nervous System Development: the nervous system develops in segments similar to those of simpler animals, such as segmented worms. The segments in the head expand and fuse together, forming the “cerebral hemispheres” and “brainstem.” (Blumenfeld, 22)
In humans, as it other vertebrates, the "brain" begins as part of the “neural tube.” The generation of the cells that will eventually form the “cerebral cortex” begins about 7 weeks after conception and is largely complete by 20 weeks. (Kolb, 195) Knowledge-specific cortical regions mature at different rates. For example, the “fusiform area” of the “visual cortex,” which responds to faces, triples in size between ages 8 and 20, as does the “parahippocampal place area," which responds to location. The lateral “occipital cortex,” which responds to objects, changes very little (during this period). (Memory and Mind, John Gabrieli)
Nerve(s): a bundle of axons. (Kandel, 442) Bundle of conducting “nerve fibers” that transmit “impulses” from the brain or spinal cord to the muscles and glands, or inwards from the sense organs to the brain and spinal cord. (OxfordMed) Several axons running together outside the brain. (Kolb, 46) A bundle of fibers that receives and sends messages between the body and the brain. The messages are sent by chemical and electrical changes in the cells that make up the nerves. (NCIt)
Afferent Nerves: sensory nerves that propagate information toward the central nervous system. (Patestas, 21) Neurons which conduct nerve impulses to the central nervous system. (MeSH) Editor’s note - part of the “peripheral nervous system.” Also referred to as "afferent nerve fibers,” ‘afferent neurons,’ ‘afferents,’ and “sensory neurons.”
Association Fibers: project to cortical association areas in the (same) hemisphere. Consist of axons arising from small "pyramidal cells," primarily from cortical layers II and III. Vary in length from short to long. Association fibers that connect various cortical areas make up most of the subcortical “white matter.” (Patestas, 400-404) Also referred to as ‘arcurate fibers.’
Long Association Fibers: connect nonadjacent gyri. (Patestas, 77) Connect different “lobes” of the same (cerebral) hemisphere. (Patestas, 405) Also referred to as ‘long arcurate fibers.’
Short Association Fibers: connect adjacent “gyri.” Bridge the primary sensory areas with the adjacent cortical association areas. (Patestas, 405) Do not usually reach the subcortical white matter of the cerebral cortex. Most of them are confined to the cortical gray matter. (Patestas, 77) Also referred to as ‘U-fibers.’ and ‘short arcurate fibers.’
Commissural Fibers: project to the opposite cerebral hemisphere to synapse in the cerebral cortex. Form most of the “corpus callosum.” (Patestas, 400)
Cranial Nerves: any of the 12 paired nerves that originate in the brain stem. (NCIt) Arise directly from the brain and leave the “skull” though separate (holes). Conventionally given Roman numbers. They include ‘olfactory,’ ‘optic,’ ‘oculomotor,’ ‘trochlear,’ ‘trigeminal,’ ‘abducens,’ ‘facial,’ ‘vestibulocochlear,’ ‘glossopharyngeal,’ ‘vagus,’ ‘accessory,’ and ‘hypoglossal.’ (OxfordMed) Editor’s note - part of the peripheral nervous system.
Vagus Nerve: the tenth cranial nerve. This long nerve travels to various organs and glands to control sensory, motor, and "autonomic" functions such as "digestion" and "heart rate." (Chudler, 37) The vagus is a mixed nerve which contains afferents from skin in back of the ear, afferents from the “pharynx,” “larynx,” "thorax", and “abdomen,” parasympathetic efferents (to the thorax and abdomen), and efferents to striated muscle (of the larynx and pharynx). (MeSH)
Efferent Nerves: motor nerves that propagate information that goes away from the central nervous system to a muscle or gland. (Patestas, 20-22) A neuron that sends impulses from the central nervous system to skeletal muscles, glands, and visceral organs. (NCIt) Also referred to as ‘efferent nerve fibers,’ and ‘efferents.’
Projection Fibers: leave the cortex and project to other regions of the “central nervous system,” such as the “thalamus,” “striatum,” “brainstem,” or “spinal cord.” (Patestas, 400)
Nervous System: the network of nerves within the body. (Oxford) Up to one trillion neurons linked throughout the body in a complex, organized communication network. (Hockenbury, 51) It has two (morphological) divisions – the “central nervous system” and the “peripheral nervous system.” (Kandel, 433) Includes two other functional components, “sensory” and “motor.” The sensory component collects information and transmits it to the central nervous system where the information is sorted, analyzed, and processed. The motor component delivers the results of the analysis to the “muscles” and “glands.” (Patestas, 3)
Neural Pathway(s): groups of neurons that are connected to one another and working together. (Doidge, 9) Neural “tracts” connecting one part of the nervous system with another. (MeSH) Patterned connections (between) brain structures. Circuits pass information back and forth and in repeating loops, and allow brain structures to work together to create sophisticated “perceptions,” thoughts, and behaviors. (RamachandranTTB, 14) Immensely interconnected and highly dynamic cellular networks. (Nicolelis, 5) Some of the pathways between neurons are local, branching within their immediate ‘neighborhoods.’ But others are long, interconnecting distant neural structures. (Goldberg2, 27) Neurons…. are linked together by “synaptic” connections. (LeDoux, 49) A principle of general brain function (is that) when one neural pathway is turned off, another pathway may turn on. Why? Because the inactivated pathway normally “inhibits” the other pathway. (Kandel4, 128) “Signals” in a neuronal circuit travel along (the) pathway in a predictable pattern and only in one direction. Circuits contain three major classes of neurons, each with a specialized function – “sensory neurons,” “motor neurons,” and “interneurons.” (Kandel, 65-66) In addition to sensations of "touch," distinct neural pathways 'mediate' (convey) sensations of warmth, cold, and “pain” originating on the skin’s surface. (Ramachandran, 33) Many sensory, motor, and “cognitive” functions are served by more than on neural pathway - the same information is processed simultaneously and in parallel in different regions of the brain. (Kandel, 124) A rapid train of action potentials down a particular neural pathway causes a movement of our hands rather than a perception of colored lights, because that pathway is connected to our fingertips, not to our retinas. (Kandel, 79) Also referred to as ‘pathway,‘ ‘neuronal pathway,’ ‘circuit,’ ‘brain circuit,’ ‘neural circuit,’ ‘neuronal circuit,’ ‘neural net,’ ‘neural tract’ and ‘neural network.’
Afferent Pathway: nerve structures through which impulses are conducted from a peripheral part toward a nerve center. (MeSH)
Arcuate Fasiculus: bundle of nerve fibers connecting “Wernicke’s Area” and “Broca’s Area”. (The Brain-Norman Geschwind, 111)
Crus Cerebri: one of two symmetrical nerve “tracts” situated between the “medulla oblongata” and the “cerebral hemispheres.” (OxfordMed)
Efferent Pathway: nerve structures through which impulses are conducted from a nerve center toward a peripheral site. (MeSH)
Lemniscus: a ribbon-like tract of nerve tissue conveying information from the spinal cord and brainstem upwards through the midbrain to the higher centers. (OxfordMed)
Lateral Lemniscus: commences higher up above the spinal cord and is mainly concerned with hearing. (OxfordMed)
Medial Lemniscus: acts as a pathway from the spinal cord. (OxfordMed)
Secondary Pathway: older pathways that the brain uses if the primary pathways are blocked by damage or disorder. When blockage occurs in the primary pathways, the secondary pathways are exposed, or “unmasked.” (Doidge, 9)
Unmasking: the process in which, after a primary neuronal pathway is blocked, a secondary pathway is exposed, and with use, strengthened. This unmasking is generally thought to be one of the main ways the “plastic” brain reorganizes itself. Key evidence of “neuroplasticity.” (Doidge, 9)
Tract: a bundle of associated nerve fibers in the brain or spinal cord. (Oxford) Large collection of axons coursing together inside the brain. Several axons running together inside the brain. (Kolb, 46-47) A group of muscle or nerve fibers situated close together and running in the same direction. (OxfordMed) Supported by “glia,” (they) ferry information destined for the cerebral cortex, and the cerebral cortex “responses” to other regions of the central nervous system. (Patestas, 69) Also referred to as ‘bundle,’ ‘fascile,’ and ‘fasciculus.’
Neural Plate: flat sheet of (“tissue”) that will roll up to form the neural tube of a "vertebrate" "embryo." (SDBCoRe) The strip of “ectoderm” lying along the central axis of the early embryo that forms the neural tube and subsequently the “central nervous system.” (OxfordMed) Three weeks after conception, (this) primitive neural tissue occupies part of the outmost layer of embryonic cells. The neural plate first folds to form the ‘neural groove.’ The neural groove then curls to form the neural tube. (Kolb, 190-191)
Neural Tube: process whereby the embryo internalizes its developing nervous system. (Patestas, 12) The multistep embryological process responsible for initiating central nervous system formation. Occurs just after “gastrulation” and involves the formation of the neural tube from ectoderm located “dorsal” to the “notochord.” All "neurons," and their supporting cells in the central nervous system, originate from neural "precursor" cells derived from the neural tube. (Booker, 1116) By the twenty-eighth day, the neural tube seals up, completely isolating it from the outside world. (Bainbridge, 44)
Notochord: a strip of “mesodermal” tissue that develops along the dorsal surface of the early embryo, beneath the neural tube. It becomes almost entirely obliterated by the development of the “vertebrae,” persisting only as part of the "intervertebral discs." (OxfordMed) Cartilage-like rod which comprises the dorsal-most part of the mesoderm of a vertebrate embryo. (SBECoRe) A slender rod of cells of mesodermal origin running along the back of the early embryo and which directs formation of the neural tube. In vertebrates, it is replaced by the spinal column. (Lawrence)
Pre-Central Nervous System Development: (period during which) the neural tube is subdivided into three “morphologically” recognizable components. (Patestas, 21) By the thirtieth day in humans, the top end of the neural tube has swollen into three little hollow bulges, the "forebrain," "midbrain," and "hindbrain," the last of which connects to the spinal cord. Next, the brain starts to progress further into a five-bulge structure. Two lobes start to grow out of the left and right side of the forebrain bulge, drawing a part of the fluid-filled space inside the brain with them. They are referred to as the ‘endbrains’ or the “telencephalon.” Next, (the bulge that is the hindbrain) starts to divide into the upper “pons” and lower “medulla.” These two latter regions are also called the “metencephalon.” At this early stage the brain is growing faster than the head that imprisons it. Because of this it must fold and twist just to fit inside the head. (Bainbridge, 47-49) A globular structure soon starts to bulge out at the top end of the “fourth ventricle.” Because of its size and the convolutions that form on its surface, this bulge is called the “cerebellum,” or ‘little brain.’ (Bainbridge, 52)
Forebrain: the “anterior” of the three primitive (sacs) of the embryonic brain arising from the neural tube. It subdivides to form (the) “diencephalon” and (the) “telencephalon.” (MeSH) The furtherest forward division of the brain, consisting of the telencephalon and the diencephalon. (OxfordMed) By the thirtieth day in humans, appears as a little hollow bulge in the neural tube. (At this stage referred to as the ‘prosencephalon.’) (Bainbridge, 47-48) Sits on top of the midbrain, pons, and medulla, almost like a cauliflower on its stalk. (Blumenfeld, 15) Includes the “basal ganglia” and the “limbic system.” (Kolb, 52) The largest part of the brain. Credited with the highest intellectual functions (BrainFacts) Evolutionary studies have shown that all vertebrates probably share the same basic arrangement of the forebrain, but that different regions are emphasized in different groups (Bainbridge, 277) Also referred to as ‘prosencephalon.’
Hindbrain: the “posterior” of the three primitive (sacs) of an embryonic brain. It consists of (areas) which develop the major brain stem components, such as “medulla oblongata,” cerebellum, and pons, with an expanded cavity forming the fourth ventricle. (MeSH) Contains the structures that coordinate and control most voluntary and involuntary movements. Evolutionarily the oldest part of the brain. (Kolb, 49) There is no clear boundary between the pons and medulla, they merge imperceptibly into each another. (Bainbridge, 47-48) Also referred to as ‘rhombencephalon.’
Midbrain: region of the embryonic vertebrate brain that will become the 'optic tract.' (SBECoRe) The middle of the three primitive cerebral vesicles of the embryonic brain. Without further subdivision, the midbrain develops into a short, constricted portion connecting the pons and the diencephalon. (MeSH) With the pons and medulla, the midbrain is involved in many functions, including regulation of "heart rate," "respiration," pain perception, and movement. (BrainFacts) Central part of the brain that contains neural circuits (pathways) for hearing and seeing as well as orienting movements. (Kolb, 49) Also referred to as ‘mesencephalon.’