1. Stimulus: Something causing a physical or psychological reaction.
2. Neuron: A nerve cell; composed of dendrite(s), a cell body, and axon.
3. Axon: The usually long process of a nerve fiber that generally conducts impulses away from the body of the nerve cell
4. Dendrite: A branched protoplasmic extension of a nerve cell that conducts impulses from adjacent cells inward toward the cell body.
5. Cell body: The portion of a nerve cell that contains the nucleus but does not incorporate the dendrites or axon.
6. Schwann cell: Any of the cells that cover the nerve fibers in the peripheral nervous system and form the myelin sheath.
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.
8. Node of Ranvier: A constriction in the myelin sheath, occurring at varying intervals along the length of a nerve fiber.
9. Sensory neuron: a neuron conducting impulses inwards to the brain or spinal cord.
10. Interneuron(associative neuron): A nerve cell found entirely within the central nervous system that acts as a link between sensory neurons and motor neurons.
11. Motor neuron: A neuron that conveys impulses from the central nervous system to a muscle, gland, or other effector tissue.
1. The various systems that help an organism respond to external stimuli are the neurotransmitters like acetylcholine and norepinephrine.
2. The evolution of the nervous system is as follows: hydras have a simple nervous system; bilaterally symmetrical planarians have the rudiments of a central nervous system and a peripheral nervous system; annelids and arthropods have a typical invertebrate nervous system; and vertebrates have a much larger brain with a dorsal hollow nerve cord.