In Search of Memory: The Emergence of a New Science of Mind (51 page)

acetylcholine:
A chemical neurotransmitter released by motor neurons at synapses with muscle cells as well as at synapses between neurons.

action potential
: A large transient electrical signal about 1/10 of a volt in amplitude and 1 to 2 milliseconds in duration that propagates along the axon to the neuron’s presynaptic terminal without failure or flagging. At the presynaptic terminal, the action potential triggers the release of neurotransmitter onto target neurons.

agnosia
: Loss of knowledge; the inability to consciously recognize objects through otherwise normally functioning sensory pathways, e.g., depth agnosia, movement agnosia, color agnosia, and prosopagnosia (impairment of face recognition).

AMPA receptor (A-amino-3-hydroxy-5-methylisoxazole-4-pro-prionic acid)
: One of two types of postsynaptic receptors for glutamate. It is active in response to normal synaptic transmission. (Compare
NMDA receptor
.)

amygdala:
The region of the brain most specifically concerned with emotions, such as fear. It coordinates autonomic and endocrine responses in conjunction with emotional states, and it underlies emotional memory. The amygdala is itself a collection of several nuclei that lie deep in the temporal lobes of the cerebral hemispheres.

aphasia:
A category of language disorders resulting from lesions to specific structures in the brain. Such disorders can cause an inability to understand language (Wernicke’s aphasia), express language (Broca’s aphasia), or both.

associative learning:
A process in which the subject of an experiment (a person or an experimental animal) learns about the relationship between two stimuli or between a stimulus and a behavioral response.

autonomic nervous system
: One of two major subdivisions of the peripheral nervous system. It controls the viscera, smooth muscles, and exocrine glands, and it mediates involuntary control of heart rate, blood pressure, and respiration.

axon
: The long output fiber of the neuron that ends as presynaptic terminals and sends signals to other cells.

basal ganglia:
A group of brain structures lying deep within both cerebral hemispheres that helps to regulate motor activity and cognition. The basal ganglia include the putamen, caudate, globus pallidus, and substantia nigra. Together, the putamen and caudate are called the
striatum
.

behaviorism:
A theory, first developed at the beginning of the twentieth century, which holds that the only appropriate approach to the study of behavior is through direct observation of a subject’s actions. “Mental function” is regarded as unobservable. Behaviorism contrasts with cognitive approaches to the study of behavior, which have dominated psychological research in recent decades.

benzodiazepines:
A class of anti-anxiety drugs and muscle relaxants that includes diazepam (Valium) and lorazepam (Ativan). Benzodiazepines dampen synaptic transmission by binding to receptors for the inhibitory neurotransmitter
GABA
and enhancing GABA’s effect on neurons.

biochemistry
: A field in biology that attempts to understand life processes by studying the various chemical pathways and reactions that occur in living organisms, particularly the role played by proteins.

brain:
The organ that mediates all mental functions and all behavior. Conventionally subdivided into several main parts: the brain stem, hypothalamus and thalamus, cerebellum, and two cerebral hemispheres.

brain stem:
A collective term for three anatomical structures—the medulla, pons, and midbrain—all located at the bottom of the brain, above the spinal cord. The brain stem processes sensation from the skin and joints in the head, neck, and face, as well as specialized senses, such as hearing, taste, and balance. In addition, it mediates certain life-support functions, such as breathing, heart rate, and digestion. The sensory input and motor output of the brain stem are carried by the cranial nerves. (See
brain
.)

Broca’s area:
A region in the posterior part of the left frontal cortex that is critically involved in the expression of language. (Compare
Wernicke’s area
.)

calcium
(
Ca
²⁺
): The positively charged calcium ion is essential to the release of neurotransmitter. An influx of calcium ions, which is controlled by voltage-gated calcium channels in the nerve cell membrane, triggers this release of neurotransmitter.

cell biology:
A field in biology that attempts to understand life processes, such as growth, development, adaptation, and reproduction, within the context of the cell, its subcellular structures, and its physiological processes.

cell body:
The metabolic center of the neuron. It contains the nucleus with its chromosomes. It gives rise to two types of processes, the axon and the dendrites, both of which conduct electrical signals.

cell culture
: The growth of cells taken from an animal and then placed into a petri dish under controlled conditions in the laboratory.

cell theory:
The idea, proposed in the 1830s by anatomists Jakob Schleiden and Theodore Schwann, that all living tissues and organs in the bodies of all animals share a common structural and functional unit, the cell, and that all cells come from other cells.

central nervous system
: One of the two divisions of the nervous system, the other being the
peripheral nervous system
. The central nervous system includes the brain and the spinal cord. Although anatomically distinct, the central and peripheral nervous systems are functionally interconnected.

cerebellum:
One of the major parts of the brain involved in motor control. It modulates the force and range of motion and is involved in motor coordination and the learning of motor skills. (See
brain
.)

cerebral cortex
: The outer covering of the cerebral hemispheres. It is divided into four lobes (frontal, parietal, temporal, occipital).

cerebral hemisphere:
The cerebral hemispheres lie on either side of the brain and are connected by a large collection of axons called the corpus callosum which assures the unity of conscious experience. The cerebral hemispheres comprise the cerebral cortex and three deep-lying structures: the basal ganglia, hippocampus, and amygdala. (See
brain
.)

channel:
A membrane-spanning protein that mediates the flow of ions into and out of cells. In nerve cells, some channels are responsible for the resting potential and others trigger the changes in membrane potential that generate the
action potential
, whereas still others change the excitability of the nerve cells. Ion channels may be opened or closed by changes in membrane potential (voltage-gated) or by the binding of chemical messengers (transmitter-gated), or they may passively conduct ions (nongated, or resting). (Compare
nongated channel; transmitter-gated channel; voltage-gated channel
.)

chemical synapse:
A site at which one neuron releases a chemical signal (neurotransmitter) that binds to receptors on an abutting neuron, thus exciting or inhibiting the cell that receives the signal. (Compare
electrical synapse
.)

chemical theory of synaptic transmission:
A theory that implicates certain chemicals called neurotransmitters as the mediators of synaptic transmission between two neurons.

chloride (Cl-):
The negatively charged chlorine ion mediates the inhibition of neurons by
GABA
.

chromosome:
A structure that contains the genetic material of an organism, usually in the form of a tightly coiled, double-stranded DNA molecule intertwined with various proteins. Chromosomes replicate themselves, thereby enabling cells to reproduce and pass on their genetic material to ensuing generations. (See
DNA
.)

classical conditioning:
A form of implicit learning discovered by Ivan Pavlov in which a subject learns to associate a previously neutral conditioned stimulus with an unconditioned stimulus that typically elicits a reflex action. In experiments with dogs, for example, presentation of food (the unconditioned stimulus) normally elicits salivation. Pavlov found that if the sound of a bell (the previously neutral conditioned stimulus) was consistently paired with food, the dog would learn to associate the sound of the bell with food and thus would salivate whenever it heard the bell, regardless of whether food was present. Conversely, if the sound is paired with a shock to the leg that causes the animal to lift its leg, the dog will soon lift its leg in response to the tone alone.

cognitive map:
A representation in the brain of a particular external physical space. An example is a
spatial map
evident in the hippocampus.

cognitive neuroscience:
A combination of the concept and methods of cognitive psychology designed to study mental processes with those of neuroscience that study the brain. The methods involved in this combined discipline include neural science, cognitive psychology, behavioral neurology, and computer science.

conditioned response:
The response elicited by the conditioned stimulus after classical conditioning. This response is similar to the response originally elicited by the unconditioned stimulus. (See
classical conditioning
.)

conditioned stimulus:
A neutral stimulus that, before training, produces no overt response; it can be associated with an unconditioned stimulus through classical conditioning. (See
classical conditioning
.)

connection specificity:
The principle formulated by Cajal according to which neurons form specific functional interconnections, based on three anatomical observations: (1) neurons, like other cells, are separated from one another by a cell membrane; (2) neurons do not connect indiscriminately to one another or form random networks; and (3) each neuron communicates only with specific postsynaptic cells, and only at specialized sites (synapses).

CPEB (Cytoplasmic Polyadenlyation Element-binding Protein):
A regulator of translation at the synapse. CPEB is thought to contribute to the stabilization of long-term memory.

CREB (Cyclic AMP Response Element-binding Protein):
A gene regulator protein which is activated by the cyclic AMP and protein kinase A pathway. CREB activates the genes responsible for long-term memory. (See
cyclic AMP, protein kinase A
.)

cyclic AMP (cyclic adenosine-3’, 5’-monophosphate):
A molecule that acts as a second messenger in the cell, triggering changes in protein structure and function. Cyclic AMP activates an enzyme called the cyclic AMP-dependent protein kinase, which acts on and modifies the function of many proteins, including ion channels and the proteins that regulate the transcription of DNA into RNA. (See
phosphorylation; protein kinase A; second messenger; transcription
.)

cytoplasm:
All of the material inside the cell except the nucleus. The machinery for making proteins is located here.

dendrite:
The branched structures on most nerve cells where the neuron receives signals from other neurons.

dentate gyrus:
See
gyrus
.

depolarization:
A change in the membrane potential of the cell toward more positive values and therefore toward the threshold for firing an action potential. Depolarization increases the likelihood that a neuron will generate an action potential and is therefore excitatory. (Compare
hyperpolarization
.)

DNA (deoxyribonucleic acid):
The material of which genes are made. DNA which is made up of four subunits called nucleotides contains the instructions needed for the synthesis of proteins. A greater portion of the total genetic information encoded in DNA is expressed in the brain than in any other organ of the body. (See
chromosome
.)

dopamine
: A neurotransmitter in the brain that plays a major role in long-term potentiation, the control of attention, voluntary movement and cognition, and in the action of many stimulants (e.g., cocaine). Dopamine deficiency results in Parkinson’s disease; dopamine excess contributes to the positive symptoms of schizophrenia.

dynamic polarization:
The principle that information within a neuron flows in a single predictable and consistent direction.

electrical synapse
: A site at which one neuron connects with another, transmitting signals via an electrical current flowing through a junction between the two neurons. (Compare
chemical synapse
.)

electrode:
A sensing instrument made out of glass or metal and shaped like a needle. Glass electrodes are inserted into a neuron in order to record electrical activity across the surface membrane. Metal electrodes are used to record from outside the cell.