2. Neuron: Any of the impulse-conducting cells that constitute the brain, spinal column, and nerves, consisting of a nucleated cell body with one or more dendrites and a single axon. Also called nerve cell
3. Nervous system: The system of cells, tissues, and organs that regulates the body's responses to internal and external stimuli. In vertebrates it consists of the brain, spinal cord, nerves, ganglia, and parts of the receptor and effector organs.
4. Cell body: The portion of a nerve cell that contains the nucleus but does not incorporate the dendrites or axon. Also called soma1.
5. Dendrite: A branched protoplasmic extension of a nerve cell that conducts impulses from adjacent cells inward toward the cell body. A single nerve may possess many dendrites. Also called dendron.
6. Axon: The usually long process of a nerve fiber that generally conducts impulses away from the body of the nerve cell.
7. Myelin sheath: The insulating envelope of myelin that surrounds the core of a nerve fiber or axon and facilitates the transmission of nerve impulses. In the peripheral nervous system, the sheath is formed from the cell membrane of the Schwann cell and, in the central nervous system, from oligodendrocytes. Also called medullary sheath
8. Resting potential: the membrane potential of a cell that is not exhibiting the activity resulting from a stimulus
9. Action potential: A momentary change in electrical potential on the surface of a nerve or muscle cell that takes place when it is stimulated, especially by the transmission of a nerve impulse
10. Threshold: The point that must be exceeded to begin producing a given effect or result or to elicit a response
11. Synapse: The junction across which a nerve impulse passes from an axon terminal to a neuron, muscle cell, or gland cell
12. Neurotransmitter: A chemical substance, such as acetylcholine or dopamine, that transmits nerve impulses across a synapse.
13. Meninges: A membrane, especially one of the three membranes enclosing the brain and spinal cord in vertebrates.
14. Cerebrospinal fluid: The serumlike fluid that circulates through the ventricles of the brain, the cavity of the spinal cord, and the subarachnoid space, functioning in shock absorption.
15. Cerebrum: The large rounded structure of the brain occupying most of the cranial cavity, divided into two cerebral hemispheres that are joined at the bottom by the corpus callosum. It controls and integrates motor, sensory, and higher mental functions, such as thought, reason, emotion, and memory.
16. Cerebellum: The trilobed structure of the brain, lying posterior to the pons and medulla oblongata and inferior to the occipital lobes of the cerebral hemispheres, that is responsible for the regulation and coordination of complex voluntary muscular movement as well as the maintenance of posture and balance
17. Brain stem: The portion of the brain, consisting of the medulla oblongata, pons Varolii, and midbrain, that connects the spinal cord to the forebrain and cerebrum
18. Thalamus: A large ovoid mass of gray matter situated in the posterior part of the forebrain that relays sensory impulses to the cerebral cortex
19. Hypothalamus: The part of the brain that lies below the thalamus, forming the major portion of the ventral region of the diencephalon and functioning to regulate bodily temperature, certain metabolic processes, and other autonomic activities.
20. Reflex: Being an involuntary action or response, such as a sneeze, blink, or hiccup.
21. Reflex arc: The neural path of a reflex
22. Sensory receptor: an organ having nerve endings (in the skin or viscera or eye or ear or nose or mouth) that respond to stimulation.
23. Pupil: The apparently black circular opening in the center of the iris of the eye, through which light passes to the retina.
24. Lens: transparent, biconvex body of the eye between the iris and the vitreous humor that focuses light rays entering through the pupil to form an image on the retina.
25. Retina: A delicate, multilayered, light-sensitive membrane lining the inner eyeball and connected by the optic nerve to the brain
26. Rod: Any of various rod-shaped cells in the retina that respond to dim light
27. Cone: One of the photoreceptors in the retina of the eye that is responsible for daylight and color vision. These photoreceptors are most densely concentrated in the fovea centralis, creating the area of greatest visual acuity
28. Cochlea: A spiral-shaped cavity of the inner ear that resembles a snail shell and contains nerve endings essential for hearing.
29. Semicircular canal: Any of three tubular and looped structures of the inner ear, together functioning in maintenance of the sense of balance in the body.
30. Taste bud: Any of numerous spherical or ovoid clusters of receptor cells found mainly in the epithelium of the tongue and constituting the end organs of the sense of taste.
31. Drug: chemical substance, such as a narcotic or hallucinogen, that affects the central nervous system, causing changes in behavior and often addiction.
32. Stimulant: An agent, especially a chemical agent such as caffeine, that temporarily arouses or accelerates physiological or organic activity
33. Depressant: An agent, especially a drug, that decreases the rate of vital physiological activities
34. Drug abuse: Habitual use of drugs to alter one's mood, emotion, or state of consciousness.