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Absolute refractory period

The minimum length of time after an action potential during which another action potential cannot begin.

Action potential

A brief change in a neuron's electrical charge.


An inherited characteristic that increased in a population (through natural selection) because it helped solve a problem of survival or reproduction during the time it emerged.

Adoption studies

Research studies that assess hereditary influence by examining the resemblance between adopted children and both their biological and their adoptive parents.

Afferent nerve fibers

Axons that carry information inward to the central nervous system from the periphery of the body.


A chemical that mimics the action of a neurotransmitter.


A chemical that opposes the action of a neurotransmitter.

Autonomic nervous system (ANS)

The system of nerves that connect to the heart, blood vessels, smooth muscles, and glands.


A long, thin fiber that transmits signals away from the neuron cell body to other neurons, or to muscles or glands.

Behavioral genetics

An interdisciplinary field that studies the influence of genetic factors on behavioral traits.

Central nervous system (CNS)

The brain and the spinal cord.

Cerebral cortex

The wrinkled outer layer of the cerebrum.

Cerebral hemispheres

The right and left halves of the cerebrum.

Cerebrospinal fluid (CSF)

(CSF) A solution that fills the hollow cavities (ventricles) of the brain and circulates around the brain and spinal cord.


Threadlike strands of DNA (deoxyribonucleic acid) molecules that carry genetic information.

Corpus callosum

The structure that connects the two cerebral hemispheres.

Critical period

A limited time span in the development of an organism when it is optimal for certain capacities to emerge because the organism is especially responsive to certain experiences.


Branchlike parts of a neuron that are specialized to receive information.

Dominant gene

A gene that is expressed when paired genes are heterozygous (different).

Efferent nerve fibers

Axons that carry information outward from the central nervous system to the periphery of the body.

Electrical stimulation of the brain (ESB)

Sending a weak electric current into a brain structure to stimulate (activate) it.

Electroencephalograph (EEG)

A device that monitors the electrical activity of the brain over time by means of recording electrodes attached to the surface of the scalp.

Endocrine system

A group of glands that secrete chemicals into the bloodstream that help control bodily functioning.


The entire family of internally produced chemicals that resemble opiates in structure and effects.

Excitatory PSP

An electric potential that increases the likelihood that a postsynaptic neuron will fire action potentials.

Family studies

Scientific studies in which researchers assess hereditary influence by examining blood relatives to see how much they resemble each other on a specific trait.


The reproductive success (number of descendants) of an individual organism relative to the average reproductive success of the population.


The largest and most complicated region of the brain, encompassing a variety of structures, including the thalamus, hypothalamus, limbic system, and cerebrum.

Fraternal twins

Twins that result when two eggs are fertilized simultaneously by different sperm cells, forming two separate zygotes. Also called Dizygotic twins.


DNA segments that serve as the key functional units in hereditary transmission.

Genetic mapping

The process of determining the location and chemical sequence of specific genes on specific chromosomes.


A person's genetic makeup.

Heterozygous condition

The situation that occurs when two genes in a specific pair are different.


The part of the brain that includes the cerebellum and two structures found in the lower part of the brainstem: the medulla and the pons.

Homozygous condition

The situation that occurs when two genes in a specific pair are the same.


The chemical substances released by the endocrine glands.


A structure found near the base of the forebrain that is involved in the regulation of basic biological needs.

Identical twins

Twins that emerge from one zygote that splits for unknown reasons. Also called Monozygotic twins.

Inclusive fitness

The sum of an individual's own reproductive success plus the effects the organism has on the reproductive success of related others.

Inhibitory PSP

An electric potential that decreases the likelihood that a postsynaptic neuron will fire action potentials.


Destroying a piece of the brain.

Limbic system

A densely connected network of structures roughly located along the border between the cerebral cortex and deeper subcortical areas.


The segment of the brain stem that lies between the hindbrain and the forebrain.

Myelin sheath

Insulating material, derived from glial cells, that encases some axons of neurons.

Naturalistic observation

A descriptive research method in which the researcher engages in careful, usually prolonged, observation of behavior without intervening directly with the subjects.


Bundles of neuron fibers (axons) that are routed together in the peripheral nervous system.


Individual cells in the nervous system that receive, integrate, and transmit information.


Chemicals that transmit information from one neuron to another.

Parasympathetic division

The branch of the autonomic nervous system that generally conserves bodily resources.

Perceptual asymmetries

Left-right imbalances between the cerebral hemispheres in the speed of visual or auditory processing.

Peripheral nervous system

All those nerves that lie outside the brain and spinal cord.


The ways in which a person's genotype is manifested in observable characteristics.

Pituitary gland

The "master gland" of the endocrine system; it releases a great variety of hormones that fan out through the body, stimulating actions in the other endocrine glands.

Polygenic traits

Characteristics that are influenced by more than one pair of genes.

Postsynaptic potential (PSP)

A voltage change at the receptor site on a postsynaptic cell membrane.

Recessive gene

A gene whose influence is masked when paired genes are different (heterozygous).

Resting potential

The stable, negative charge of a neuron when it is inactive.


A process in which neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane.


The cell body of a neuron; it contains the nucleus and much of the chemical machinery common to most cells.

Somatic nervous system

The system of nerves that connect to voluntary skeletal muscles and to sensory receptors.

Split-brain surgery

A procedure in which the bundle of fibers that connects the cerebral hemispheres (the corpus callosum) is cut to reduce the severity of epileptic seizures.

Sympathetic division

The branch of the autonomic nervous system that mobilizes the body's resources for emergencies.


A junction where information is transmitted from one neuron to the next.

Synaptic cleft

A microscopic gap between the terminal button of a neuron and the cell membrane of another neuron.

Terminal buttons

Small knobs at the end of axons that secrete chemicals called neurotransmitters.


A structure in the forebrain through which all sensory information (except smell) must pass to get to the cerebral cortex.

Transcranial magnetic stimulation (TMS)

A new technique that permits scientists to temporarily enhance or depress activity in a specific area of the brain.

Twin studies

A research design in which hereditary influence is assessed by comparing the resemblance of identical twins and fraternal twins with respect to a trait.


A one-celled organism formed by the union of a sperm and an egg.

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