In Search of Memory: The Emergence of a New Science of Mind (53 page)

motor system:
The part of the nervous system that mediates movement and other active functions, as opposed to the sensory system, which receives and processes stimuli.

nerve:
A bundle of axons.

nerve cell:
See
neuron.

neural analog of learning:
The attempt to simulate the sensory stimuli used in learning experiments by electrically stimulating axons that end on a target nerve cell in an isolated ganglion.

neural circuit
: A group of several neurons that are interconnected to and communicate with one another.

neural correlate of consciousness
: A process that occurs in neurons while a person is engaging in an activity that requires conscious attention.

neural map:
The orderly topographical arrangement of neurons in the central nervous system that reflects the spatial relationships of neurons in the primary sense organ. The brain contains a similarly ordered motor map for movement.

neurology:
The classic field of medicine concerned with the nervous system, both normal and diseased. Clinical neurology is concerned with the diagnosis and treatment of nervous system disorders, which usually do not primarily affect mental processes. Relevant disorders include strokes, seizures, Huntington’s disease, Alzheimer’s disease, and Parkinson’s disease. Neurology has posed many of the critical questions that cognitive neural science has attempted to address. In contrast, psychiatry attempts to address disorders of the brain that affect mental processes.

neuron
: The fundamental unit of any nervous system. The human brain contains about 100 billion neurons, each of which forms about 1000 synapses. Neurons are similar to other cells in having common molecular machinery for cellular function, but they have the unique ability to communicate rapidly with one another over great distances and with great precision.

neuron doctrine:
The theory that individual neurons are the fundamental signaling elements of the nervous system.

neurotransmitter:
A chemical substance that is released by one neuron and binds to receptors on another neuron, altering the flow of electrical current or internal biochemical events in the second cell. The specific action of a neurotransmitter depends on the properties of the receptor. There may be many different kinds of receptors for a single neurotransmitter.

NMDA receptor
(N-methyl-D-aspartate): One of two types of postsynaptic receptors for glutamate that are discussed in this book. The NMDA receptor plays a critical role in long-term potentiation. (Compare
AMPA receptor
.)

nongated channel:
A channel in the membrane of nerve cells that passively conducts ions (most often potassium ions) across the cell membrane. The flow of ions through these channels is responsible for the resting membrane potential of the cell. Also known as a resting channel. (Compare
gated channel
.)

nucleotide base:
The basic building block of DNA or RNA. There are typically four types that in combination code for genes. In DNA the four bases are thymine, adenine, cytosine, and guanine. In RNA uracil replaces thymine.

nucleus (pl. nuclei):
(1) The processing center of a cell, where all of the genetic material is found. The nucleus is surrounded by a membrane that separates it from the cytoplasm. (2) A cluster of functionally related neuron cell bodies in the central nervous system. In the peripheral nervous system, or in the central nervous system of invertebrate animals, groups of neurons are arranged in ganglia. (See
cell body
; compare
cytoplasm.
)

occipital lobe:
One of the four lobes of the cerebral cortex. Found at the rear of the cortex, the occipital lobe is important for vision. (Compare
frontal lobe; parietal lobe; temporal lobe.
)

operant conditioning
: A form of implicit associative learning in which a subject learns through reward or punishment to perform or not to perform an action (one that is not a preexisting reflex) in response to a previously neutral conditioned stimulus. Also called instrumental conditioning or operant conditioning.

organic ions:
Molecules containing carbon atoms and carrying an electrical charge (including some amino acids and proteins) that are involved in biological processes.

parietal lobe:
One of the four lobes of the cerebral cortex, it is located between the frontal and occipital lobes. The parietal lobe processes sensations such as touch, pressure, and pain and is important in integrating multiple sensations into a single experience. (Compare
frontal lobe; occipital lobe; temporal lobe.
)

peripheral nervous system:
The portion of the nervous system, including the autonomic nervous system, where motor or autonomic activities are mediated by neurons that lie outside the spinal cord and brain stem. The peripheral nervous system is functionally connected with the central nervous system. (Compare
central nervous system.
)

phosphorylation
: The addition of a phosphate group to a protein, thereby changing the structure, charge, or activity of the protein. Phosphorylation is carried out by a special class of enzymes called protein kinases.

phrenology:
A theory popular during the nineteenth century that posited a correlation between personality traits and the shape of the skull. It was thought that frequent use of underlying brain structures would lead to enlargement of those structures, which would be reflected in bumps on the skull.

place cells:
Neurons of the hippocampus that fire only when an animal is in a particular location within its environment, together forming a cognitive map of that environment. When the animal moves to a different location, different place cells become active.

plasticity:
The ability of synapses, neurons, or regions of the brain to change their properties in response to usage or different patterns of stimulation. Also known as plastic change.

positron-emission tomography (PET scan):
A computerized tomography technique for imaging brain functions in living organisms. The technique, while conceptually similar to functional magnetic resonance imaging, employs radioactive molecules to probe specific brain activities, such as blood flow and metabolism. (See
functional magnetic resonance imaging
.)

postsynaptic cell; postsynaptic neuron:
The neuron that receives signals (electrical or chemical) from another neuron at a synapse. The signals affect the excitability of the postsynaptic cell.

postsynaptic receptor:
See
receptor
.

potassium
(
K
⁺
): A positively charged ion that is essential for nervous system function. Concentrations of potassium inside the resting neuron are higher than those outside the cell.

potentiation:
The process by which activity in one neuron causes an enhancement of the strength of the synaptic connection with its target. Long-term potentiation is a persistent increase (lasting hours to days) in the synaptic response of a postsynaptic neuron following repeated stimulation of the presynaptic neuron.

prefrontal cortex:
The forwardmost portion of the frontal cortex, it is associated with planning, decision making, higher-level cognition, attention, and aspects of motor function.

presynaptic cell:
The neuron that sends signals (electrical or chemical) to another neuron at the synapse.

presynaptic terminal:
The terminal area at the end of the axon of the presynaptic neuron from which synaptic vesicles with neurotransmitters are released onto the postsynaptic cell (chemical synapses) or that connects via electrical junctions to the postsynaptic cell (electrical synapses).

prion (proteinaceous infectious agent):
A very small class of infectious proteins that can take on two functionally distinct shapes, the recessive form which is inactive or has a conventional, physiological role, and the dominant form which is self-perpetuating and toxic to nerve cells. In the dominant form, prions can cause degenerative diseases of the nervous system such as mad cow disease (bovine spongiform encephaly) and, in humans, Creutzfeldt-Jakob disease.

procedural memory:
See
implicit memory
.

processes:
In a neuron, protrusions where synapses can or will develop. (See
axon; dendrite
.)

promoter:
A specific site for each gene located on the DNA to which regulatory proteins bind, thus turning the gene on or off.

propagation
: (1) A process by which nerve impulses travel down the neuron. (2) In prions, a process by which one form of the prion perpetuates itself.

protein
: A large molecule made up of one or more chains of amino acids, held together in a complex three-dimensional structure. Proteins play regulatory, structural, and catalytic roles in living systems.

protein kinase
: An enzyme that catalyzes the phosphorylation of other proteins, thereby modifying their function.

protein kinase A:
The target of cyclic AMP and the enzyme that phosphorylates target proteins. It is comprised of four subunits, two regulatory subunits which inhibit the two catalytic subunits. The catalytic subunit phosphorylates other enzymes.

psychiatry:
The field of medicine concerned with normal and abnormal mental functions. Clinical psychiatry deals with such disorders as schizophrenia, depression, anxiety, and drug abuse.

pyramidal cells:
A particular type of neuron, typically excitatory and found in the cerebral cortex, that is shaped roughly like a pyramid. Pyramidal cells are the major class of neurons in the hippocampus, where they encode place. (See
place cells.
)

quantum (pl. quanta):
A small packet containing about 5000 molecules of neurotransmitter, that is released from the presynaptic terminal of the axon. Quanta are packaged in synaptic vesicles. (See
synaptic transmission; synaptic vesicle.
)

receptive field
: The portion of the total sensory world that activates a particular sensory neuron. For example, the receptive field of one sensory neuron in the retina may respond to a spot of light shone on the upper left portion of a visual field.

receptor:
A specialized protein in the postsynaptic cell that recognizes and binds the neurotransmitter released by the presynaptic cell. All receptors for chemical transmitters have two functions: they recognize transmitters and they carry out an effector function within the cell. For example, they can be involved in gating ion channels or in activating second messengers. Based on these gating or activation functions, receptors fall into two major categories: ionotropic and metabotropic. (See
ionotropic receptor; metabotropic receptor.
)

receptor cell
: A sensory cell specialized to respond to a particular physical property, such as touch, light, or temperature.

recombinant DNA
: A molecule of DNA formed by combining strands from two originally separate DNA molecules.

recruitment:
The process by which various components necessary for a certain biochemical pathway are gathered together so that the requisite chemical reactions can occur in sequence.

reductionist analysis; reductionism:
A scientific approach that seeks to eliminate features of the process studied that are not essential to its function, thereby isolating the most important features. This may involve creating a simple model for a more complicated process, as the more complicated process may be too complex to study effectively.

reflex:
An unlearned, involuntary response to a stimulus. In the case of spinal reflexes, these responses are mediated by the spinal cord and do not require that messages be sent to the brain. (Compare
voluntary attention
.)

refractory period:
The time during which the neuron has a higher threshold for generating further action potentials after its firing of one action potential.

replication:
The formation of copies of double-stranded DNA. The two DNA strands divide and each serves as a template, or parent strand, and is copied. The new strands, or daughter strands, carry the complement.

repressor:
A regulatory protein that binds to the promoter and prevents a gene from being turned on.

resting membrane potential:
The difference in electrical charge between the inside and outside surfaces of a nerve cell membrane, resulting from the uneven distribution of sodium, potassium, and chloride ions. The resting potential is about–60 to–70 millivolts in most mammalian nerve cells.

reverse genetics:
Genetic technique by which a gene is either removed or introduced into a mouse’s genome and the effect of the genetic alteration is tested in order to evaluate a specific hypothesis.