Glossary

Action potential (AP). An all-or-none electrochemical potential transmitted through the neuron’s axon at constant speed and amplitude.

Brain. An anterior enlargement of the CNS. In vertebrates, the size (weight or volume) of the brain can, in some cases, be measured or estimated from fossil crania, an important source of data for an understanding of brain evolution.

Central and peripheral nervous system. The central nervous system (CNS) includes the neural tissue located in the ganglia (invertebrates) or in the brain and spinal cord (vertebrates). It is distinguished from the peripheral nervous system (PNS), a set of nerves connecting the CNS with receptors, organs, and glands in the periphery of the body.

Chemical synapse. In a chemical synapse, the arrival of an action potential at the axon terminal causes the release of a neurotransmitter.

Central pattern generator (CPG). A neural network that controls a sequentially organized series of behaviors without relying on sensory feedback from each individual component.

Collagen. A fibrous protein found in the intercellular space of all animals.

Command neuron. A unit, or a small network, responsible for activating a pattern of behavior.

Cotransmission. The release of more than one neurotransmitter or neuromodulator at the axon terminal by one neuron.

Deafferentation. Surgical elimination of sensory inputs (afferents) to the CNS.

Depolarization. A change in the resting potential of a neuron that makes the interior of the cell electrically less negative or positive, in relation to the extracellular space.

Diffuse nervous system. The nervous system of cnidarians is dispersed in the body without a clear differentiation between central and peripheral systems.

Electrical synapse. A synapse in which the electrochemical potential crosses directly from the presyn-aptic to the postsynaptic neuron, without neurotransmitter release.

Excitatory and inhibitory postsynaptic potentials (EPSP and IPSP). A depolarizing (EPSP) or hyperpolarizing (IPSP) gradual potential induced in the postsynaptic neuron by the release of neurotransmitter by the presy nap tic neuron.

Functional disconnection. A reduction in the efficacy of a synapse as a result of habituation training. Ganglion (plural: Ganglia). A cluster of neurons in the CNS of invertebrates.

Giant neurons. Neurons of large size that have served as models to understand the neuron’s electrochemical properties (e.g., the squid’s giant axon).

Graded potentials. A change in the electrochemical potential of a neuron proportional to the stimulus that provoked it.

Habituation. A decrease in the response to a stimulus that results from its repetitive presentation. The short-term version of this effect is considered to be an example of nonassociative learning. Long-term habituation may involve associative learning between the habituating stimulus and the contextual cues present at the time of training.

Heterosynaptic facilitation. The facilitation of transmission through a synapse as a result of a third neuron modifying the excitability of the presynaptic neuron via an axo-axonic synapse. Heterosynaptic facilitation has been proposed as the mechanism underlying some instances of dishabituation.

Homosynaptic depression. A decrease in the excitability of a synapse. Homosynaptic depression has been proposed as a mechanism underlying some instances of short-term habituation.

Hyper polarization. Action potentials that cause the interior of the postsynaptic neuron to become more negative than it is during a resting potential.

Increased excitability. An ability exhibited by some neurons to increase the amount of neurotransmitter released in the synapse by lengthening the time during which calcium channels remain open. Increased excitability has been proposed as a mechanism underlying some instances of dishabituation.

Learning (associative and nonassociative). Acquiring, storing, and retrieving information that results from experience. These processes occur in the CNS. Associative learning results from a single occurrence or from repeated experience with two or more events that occur in temporal or spatial contiguity. Nonassociative learning occurs when organisms modify their response to a single event after repeated presentations.

Learning-performance dichotomy. Learning cannot be equated with performance or behavior. This is particularly important when dealing with presumed instances of learning failure. The absence of a behavior does not imply the absence of learning.

Long-term habituation. Habituation that persists for at least 24 h.

Long-term sensitization. Sensitization that persists for at least 24 h.

Modal action pattern (MAP). Sequentially organized behaviors that, once activated, develop in a more or less ballistic manner. MAPs are said to be species-typical behaviors.

Monopolar neurons. A neuron with dendrites emanating from the part of the axon closest to the soma. Monosynaptic reflex. A reflex arc based on two neurons and a single synapse (a sensory-motor synapse).

Muscular fatigue. Exhaustion of the muscles (after repeated stimulation) resulting in the inability to perform a response.

Nematocyst. A coiled structure present in cnidarians (e.g., jellyfish) that can be released to seize a prey. Neurotransmitter. A chemical substance released at the axon’s terminals by an incoming action potential. Neurotransmitters diffuse in the synapse and reach the membrane of the postsynaptic neuron, where they activate specific receptors. Such activation may induce gradual potentials in the postsynaptic neuron.

Peripheral reflex arc. A reflex arc completely organized at the periphery, capable of functioning even when separated from the CNS.

Phylum. A taxonomic category applied to groups of animals exhibiting a common body plan.

Polysynaptic reflex. A reflex arc that includes sensory neurons, motor neurons, and interneurons.

Reflex arc. A basic unit of behavior involving a sensory input, a processing section in the CNS, and an output.

Reflex chain. A neural network that controls a sequence of responses based on the sensory feedback provided by each element in the sequence.

Resting potential. The electrochemical potential of a nonstimulated neuron.

Sensitization. An increase in the strength of a response as a result of the repeated presentation of a relatively intense stimulus.

Sensory adaptation. As a result of repeated stimulation, a sensory receptor may decrease or even stop firing in response to a stimulus.

Short-term habituation. Habituation that lasts for a period of time in the order of minutes to a few hours.

Short-term sensitization. Sensitization that persists for a period of minutes to a few hours.

Simple systems approach. A research strategy based on the selection of a relatively simple instance of behavior and a simple neural circuit to understand the cellular basis of behavior.

Spike broadening. The slowing down of potassium outflow by some neurons that lengthens the time during which a neurotransmitter is released after an action potential. Spike broadening has been proposed as a mechanism underlying some instances of sensitization.

Synaptic plasticity. The capacity of some neurons to modify their synaptic functioning as a result of use and disuse.

Varicosities. Sites where neurons establish synaptic connections.