In Search of Memory: The Emergence of a New Science of Mind (52 page)

endocrine
: A class of glands that secretes chemicals called hormones directly into the bloodstream. The hormones then travel to target tissues and exert their effect.

ethology
: the study of animal behavior in its natural environment.

excitability change:
The change in the threshold of a nerve cell that follows activity.

excitation:
The depolarization of a postsynaptic cell, increasing the likelihood that an action potential will be generated.

excitatory
: Indicates a neuron or synapse that depolarizes its target, increasing the chance that the neuron will fire an action potential. (Compare
inhibitory
.)

explicit learning:
A class of learning that requires conscious participation and is concerned with acquiring information about people, places, and things. Also known as declarative learning. (Compare
implicit learning
.)

explicit memory:
The storage of information about people, places, and things that requires conscious attention for recall. Such memories can be described in words. Explicit memory is what most people refer to when they speak of memory. Also known as declarative memory. (Compare
implicit memory
.)

expression:
See
gene expression.

facilitation:
The process by which the strength of the synaptic connection between two cells is strengthened.

fiber:
An axon.

first messenger:
The neurotransmitter or hormone that binds to a receptor on the cell surface and activates a chemical (the
second messenger
) inside the cell.

fornix:
A bundle of axons that carries information into and out of the hippocampus.

forward genetics:
A genetic technique which usually employs a chemical to produce random mutations in a single gene. These mutants are then selected for a specific phenotype.

frontal lobe:
One of the four lobes of the cerebral cortex. The frontal lobe is primarily concerned with executive function, working memory, reasoning, planning, speech, and movement. The frontal lobes are disordered in schizophrenia. (Compare
occipital lobe; parietal lobe; temporal lobe
.)

functional magnetic resonance imaging (fMRI):
A noninvasive biomedical imaging technique that employs a large magnet to detect changes in blood flow and oxygen consumption in the brain. Blood flow and oxygen utilization increases in regions where neurons are more active, such as during the performance of a cognitive task.

GABA (gamma-aminobutyric acid)
: The primary inhibitory neurotransmitter in the brain, capable of causing, among other effects, sleep, muscle relaxation, and decreased emotional activity.

ganglion (pl. ganglia)
: A cluster of functionally related neuron cell bodies in the peripheral nervous system of vertebrates and in the central nervous system of
Aplysia
and other invertebrate animals.

gated channel
: An ion channel that opens and closes in response to a particular type of signal. (See
transmitter-gated channel; voltage-gated channel
.)

gene
: A specific sequence of DNA that is located at a certain point on a chromosome and contains the instructions for synthesizing a particular protein.

gene expression
: The production of proteins based upon the specific genetic information encoded in the DNA of an organism.

Gestalt psychology:
A school of psychology that focused particularly on visual perception and emphasized the fact that perception occurs as a reconstruction of sensory information in the brain based on an analysis of the relationship between an object and its surroundings.

glutamate:
A common amino acid that functions as the major excitatory neurotransmitter in the brain and spinal cord.

gyrus (pl. gyri):
The crest of a convolution on the outside of the cerebral cortex. Many of the gyri are invariant in location and help identify regions of the cortex. The groove between two gyri is called a sulcus. The
dentate gyrus
is part of the hippocampal formation and sends information to the hippocampus.

habituation:
A simple, nonassociative form of learning in which a subject learns about the properties of a single, innocuous stimulus. The subject learns to ignore the stimulus, resulting in decreased neuronal response to it.

heterosynaptic facilitation:
A neural mechanism whereby sensitization occurs. In heterosynaptic facilitation, enhanced strength of synaptic connections between two nerve cells is brought about by the activity of another cell or group of cells.

heterosynaptic plasticity
: Any change in the strength (be it enhancement or depression) of a synaptic connection between two cells brought about by activity in a third cell or a group of cells.

higher-order cortex
: Any of several regions of the cerebral cortex that process information from a primary sensory or motor area of the brain.

higher-order mental processing
: Neuronal processing that occurs beyond the primary sensory or motor area of the brain.

hippocampus:
The hippocampus is required for the storage of explicit memory. A structure lying deep in the temporal lobe of the cerebral hemispheres. The hippocampus, the dentate gyrus, and the subiculum constitute the hippocampal formation.

homosynaptic depression:
A neural mechanism by which habituation occurs. In homosynaptic depression, the strength of the synaptic connection between two cells is decreased as a result of activity in one, the other, or both cells. This abated response occurs within the same pathway that is repeatedly stimulated.

homosynaptic plasticity
: A change in the strength of the synaptic connection between two cells (be it enhancement or depression) brought on by activity in one, or the other, or both of these cells.

hormone:
A chemical produced by endocrine glands in the body that serves as a messenger; hormones are secreted most often by endocrine glands directly into the bloodstream, through which they travel to their target. (See
endocrine
.)

hyperpolarization:
A change in the membrane potential of a nerve cell toward a more negative value. Hyperpolarization decreases the likelihood that a neuron will generate an action potential and is therefore inhibitory. (Compare
depolarization
.)

hypothalamus:
A part of the brain that lies immediately below the thalamus and regulates autonomic, endocrine, and visceral functions. (See
brain
.)

implicit memory:
The storage of information that does not require conscious attention for recall—usually in the form of habits, perceptual or motor strategies, and associative and nonassociative conditioning. Also called procedural memory. (Compare
explicit memory
.)

inhibition:
A change of the membrane potential toward more negative values, preventing or reducing the likelihood of an action potential in that cell.

inhibitory:
Indicates a neuron or synapse that hyperpolarizes its target, decreasing the chance that the neuron will fire an action potential. (Compare
excitatory
.)

inhibitory feedback:
A circuit in which a neuron excites an inhibitory interneuron that, in turn, connects with the first neuron and inhibits its action. This type of circuit is a form of self-regulation.

instrumental conditioning
: See
operant conditioning
.

integration:
The process by which a neuron adds up all incoming excitatory and inhibitory signals and determines whether an action potential will be generated.

interneuron:
One of the three major functional types of neurons. These connect or regulate other neurons. Many interneurons are inhibitory. (Compare
motor neuron; sensory neuron
.)

involuntary attention:
Attention focused on a particular stimulus, whether internal or external, as a result of a reflexive response to some aspect of the stimulus, usually a powerful, noxious, or otherwise highly novel stimulus.

ion:
An atom or molecule having a net positive or negative charge. The major ions found on the inside or outside of the nerve cell membrane are potassium, sodium, chloride, calcium, and magnesium, as well as organic ions such as certain amino acids.

ion channel:
See
channel.

ionic hypothesis:
The theory developed by Hodgkin and Huxley that the movements of sodium and potassium ions through the nerve cell membrane are regulated independently and that they give rise to the action potential and the resting potential.

ionotropic receptor:
A protein that spans the cell surface membrane and contains a transmitter-binding site and channel through which ions can pass. The binding of the appropriate transmitter directly opens or closes the channel to the movement of ions. (See
transmitter-gated channel
; compare
metabotropic receptor
.)

localization:
A theory that specific functions are carried out by specialized parts of the nervous system. (Compare
mass action
.)

magnetic resonance imaging (MRI):
A noninvasive technique that uses a large magnet for imaging living subjects; used to visualize structures in the brain.

MAP kinase (Mitogen Activated Protein):
A kinase that often acts in conjunction with protein kinase A to initiate long-term memory. In
Aplysia
, it is thought to act on CREB-2 (the inhibitor of CREB-mediated transciption). (See
CREB; protein kinase A.
)

mass action:
The view, championed by Jean Pierre Flourens and by Karl Lashley during the first half of the twentieth century, that brain function is holistic rather than subdivided into specialized and localizable subunits. These theorists believed that loss of functionality due to brain damage would be directly proportional to the amount of tissue damaged rather than to the location of the damage. Also known as aggregate field theory. (Compare
localization.
)

mediating circuit:
The primary circuit involved in a reflex action; it comprises the motor neurons, sensory neurons, and interneurons directly involved in the reflex. (Compare
modulating circuit
.)

medulla:
One of the parts of the brain stem, it lies directly on top of the spinal cord. The medulla includes several centers responsible for such vital autonomic functions as digestion, breathing, and control of heart rate.

membrane hypothesis:
The concept that even in the resting state, there exists a steady voltage difference across the neuronal membrane.

membrane potential:
See
resting membrane potential.

memory:
The storage of learned information. Memory exists in at least two stages, short-term (minutes to hours) and long-term (days to weeks). It also has two forms: explicit and implicit. (See
explicit memory; implicit memory
.)

messenger RNA:
The form of ribonucleic acid (RNA) that carries the instructions for a particular protein from the DNA in the nucleus of a cell to the protein synthesis machinery in the cytoplasm. The process of messenger RNA production is called transcription. (See
translation, transcription
.)

metabotropic receptor:
A protein on the cell surface that binds a transmitter or hormone (the first messenger) and then activates a chemical inside the cell (the second messenger) that initiates a cell-wide response. (Compare
ionotropic receptor
.)

midbrain:
The uppermost part of the brain stem, it controls many sensory and motor functions, including eye movements and the coordination of visual and auditory reflexes.

modulating circuit:
The circuit for regulatory (nonreflex) processing, such as sensitization and classical conditioning, it modifies the function of the primary circuit involved in the behavior. (Compare
mediating circuit.
)

molecular biology:
A hybrid discipline of genetics and biochemistry that attempts to understand life processes at the level of the macromolecules of the cell and their structure and function.

motor neuron:
One of the three major functional types of neurons. Motor neurons form synapses with muscle cells, conveying information from the central nervous system and converting it into movement. (Compare
interneuron; sensory neuron.
)