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.
— Ginger Campbell, Are You Sure? The Unconscious Origins of Certainty

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)


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)

Neural Pathway(s): neural “tracts” connecting one part of the nervous system with another. (MeSH) Groups of neurons that are connected to one another and working together. (Doidge, 9) Neurons that are linked together by “synaptic” connections. (LeDoux, 49) 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) “Signals” in a neuronal circuit travel along a 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,’ 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.’

Peripheral Nervous System: the portion of the nervous system outside the brain and spinal cord. It includes the nerves of the trunk and limbs. (Fields, 318) Brings messages from the “somatosensory receptors” to the spinal cord and brain. Carries messages from the brain and spinal cord to the muscles and glands. This system was long known to be plastic, if you cut a nerve in your hand, it can “regenerate” or heal itself. (Doidge, 53) Has autonomic and somatic divisions. The “autonomic nervous system” includes the “enteric,”  “parasympathetic,” and “sympathetic” subdivisions. The “somatic nervous system” includes the “cranial” and “spinal nerves” and their “ganglia,” and the “sensory receptors.” (MeSH)

Autonomic Nervous System: part of the nervous system serving organs, which control the normally involuntary functions of the body. (Oxford) Responsible for regulating organ function and metabolism. Responsible for “homeostasis.” (Blakeslee, 182, 211) The “heart,” the “stomach,” and (the) “intestines” are regulated by the autonomic nervous system (Chudler, 33) Regulates involuntary functions, such as “heartbeat,”  “blood pressure,”  “breathing,” and digestion. These processes occur with little or no conscious involvement. Emotions and mental imagery also influence (the) autonomic nervous system. A peaceful mental image can lower many autonomic functions. (Hockenbury, 52-53) (Cells in this system) perform their functions below the conscious level. Two neurons are required to effect a “contraction” of smooth muscle or cardiac muscle, or to elicit secretion from the cell of a gland. The “cell body” of the first neuron is in the central nervous system, whereas the cell body of the second neuron is in the peripheral nervous system. (Patestas, 118)

Enteric Nervous System: situated completely within the wall of the digestive tract and controls the entire process of digestion. (Patestas, 9) (Includes) a neural "plexus," lying within the walls of the gut, that is involved in controlling ‘peristalsis’ (muscular contractions) and “gastrointestinal” “secretions.” (Blumenfeld, 24) ‘Innervates’ (stimulates with nerve impulses) the gastrointestinal tract, the pancreas, and the gallbladder. It contains sensory neurons, interneurons, and motor neurons. Thus the circuitry can autonomously sense the tension and the chemical environment in the gut and regulate blood vessel tone, (motion), secretions, and fluid transport. The system is itself governed by the central nervous system and receives both parasympathetic and sympathetic innervation. (MeSH)

Parasympathetic Nervous System: the ‘rest and digest’ part of the autonomic nervous system. (Blakeslee, 182) Generally acts to conserve resources and restore homeostasis. (MeSH) In times when there is no emergency, it works to save energy - blood pressure decreases, heartbeat slows, and digestion can start. (Chudler, 35) Helps the body conserve energy. (Hockenbury, 53)

Sympathetic Nervous System: the "fight or flight" part of the autonomic nervous system which gets the body revved up and ready for violence or intense physical (or emotional) activity. (Blakeslee, 215) The part of the nervous system that increases heart rate, blood pressure, breathing rate, and pupil size. It also causes blood vessels to narrow and decreases digestive juices. (NCIt) When you feel threatened the following happens: heart beats faster; breathing quickens; blood is diverted to your muscles and lungs; mouth dries up; pupils contract; senses focus outward. (Blakeslee, 182) Arouses the body to expend energy. (Hockenbury, 53)

Somatic Nervous System: responsible for voluntary muscle control, touch, and “propriorception.” Relays motor commands from the brain to the muscles. Sends feedback information from somatosensory receptors back to the brain. (Blakeslee, 182) Communicates sensory information to the central nervous system and carries motor messages from the central nervous system to the muscles. (Hockenbury, 52) In this system, only a single neuron is required to relay the signal for muscle contraction. (Patestas, 118)