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Neuron Cell that carries messages throughout the nervous system. Dendrite Extension of the cell body of a neuron that carries impulses from the environment or from other neurons toward the cell body. Myelin Sheath Insulating membrane surrounding the axon in some neurons. Resting Potential Electrical charge across the cell membrane of a resting neuron. Action Potential Reversal of charges across the cell membrane of a neuron; also called a nerve impulse. Threshold Minimum level of a stimulus required to activate a neuron. Synapse Location at which a neuron can transfer an impulse to another cell. Neurotransmitter Chemical used by a neuron to transmit an impulse across a synapse to another cell. Cell Body Largest part of a typical neuron; conatins the nucleus and much of the cytoplasm. Axon Long fiber that carries impulses away from the cell body of a neuron. Specialized Cell Cell that is uniquely suited to performing a particular function. Epithelial Tissue Tissue that covers the surface of the body and lines internal organs. Connective Tissue Tissue that holds organs in place and binds different parts of the body together. Nervous Tissue Tissue that receives messages from the body's external and internal environment, analyzes the data, and directs the response. Muscle Tissue Tissue that controls the internal movement of materials in the body, as well as external movement. Homeostasis Process by which organisms maintain a relatively stable environment. Feedback Inhibition Process in which the product or result stops or limits the process. Meninges Three layers of connective tissue in which the brain and spinal cord are wrapped. Cerebrospinal Fluid Fluid in the space between the meninges that acts as a shock absorber that protects the central nervous system. Cerebrum Area of the brain responsible for all voluntary activities of the body. Cerebellum Region of the brain that coordinates body movements. Brain Stem Structure that connects the brain and spinal cord; includes the medulla oblongata and the pons. Thalamus Brain structure that receives messages from the sense organs and relays the information to the proper region of the cerebrum for further processing. Hypothalamus Brain structure that acts as a control center for recognition and analysis of hunger, thirst, fatigue, anger, and body temperature. Reflex Quick automatic response to a stimulus. Reflex Arc Sensory receptor, sensory neuron, motor neuron, and effector that are involved in a quick response to a stimulus. Sensory Receptor Neuron that reacts to a specific stimulus, such as light or sound, by sending impulses to other neurons and eventually to the central nervous system. Pupil Small opening in the middle of the iris through which light enters the eye. Lens Transparent object behind the iris that changes shape to help adjust the eye's focus to see near or distant objects. Retina Innermost layer of the eye; contains photoreceptors. Rod Photoreceptor in eye that is sensitive to light but not to colors. Cone A photoreceptor that responds to the light of different colors, producing color vision. Cochlea Fluid-filled part of the inner ear; sends nerve impulses to the brain through the cochlear nerve. Semicircular Canal One of three structures within the inner ear that help monitor the position of the body. Taste Bud Sense organ that detects the flavor of a substance. Drug Any substance, other than food, that causes a change in the structure or function of the body. Stimulant A signal to which an organism responds. Depressant Drug that decreases the rate of functions regulated by the brain. Fetal Alcohol Syndrome Group of birth defects caused by the effects of alcohol on a fetus. Drug Abuse Intentional misuse of any drug for nonmedical purposes. Addiction Uncontrollable dependence on a drug. Levels of organization in multicellular organisms cell, tissue, organ, organ system Nervous system coordinates the body's response to changes in its internal and external environments. Integumentary System skin etc., serves as a barrier, regulates temp, protects against uv radiation Respiratory system provides oxygen and removes carbon dioxide from the body Digestive System Converts food into simpler molecules that can be used by cells of the body, absorbs food. Excretory system eliminates waste products of metabolism from the body, maintains homeostasis Skeletal System supports the body, protects internal organs, allows movement, stores mineral reserves, provides a site for blod cell formation Muscular system works with skeletal system to produce voluntary movement, help sto circulate blood and move food thru the digestive system Endocrine system controls growth, development, and reproduction. Reproductive system produces reproductive cells, in females nurtures and protects developing embryo Lymphatic system helps protect the body from disease, collects fluid lost from blood vessels and returns it to circulatory system Glands made from epithelial tissue - is a structure that makes and secretes a particular product Sensory neurons carry impulses from the sense organs to the spinal chord and brain. Motor neurons carry impulses from the brain and the spinal chord to muscles and glands. Interneurons connect sensory and motor neurons and carry impulses between them. Axon terminals axon ends in a small series of swellings Nerve impulse begins when a neuron is stimulated by another neuron or by its environment. Corpus callosum band of tissue that connects the hemispheres of the brain. Gray matter consists of desnsely packed nerve cell bodies. Cerebral cortex outer surface of cerebrum. Peripheral nervous system Divided into sensory and motor divisions. Central nervous system relays messages, processes info, analyzes info. Consists of the brain and spinal chord. Somatic nervous system regulates conscious activities. Also involved with reflex arcs. Autonomic nervous system regulates involuntary activities like hearbeats. Sympathetic and parasympathetic nervous systems both control autonomic nervous system responsibilities oppositely. Sensory division transmits impulses from sense organs to the c.n.s. Motor division transmits imuplses from the central nervous system to muscles or glands. Divided into somatic nervous system and autonomic nervous system.